Gene_locus Report for: human-CES1

The angiotensin-converting enzyme inhibitor enalapril is hydrolysed to an active metabolite, enalaprilat, in the liver via carboxylesterase 1 (CES1). Previous studies show that variant rs71647871 in the CES1 gene affects the pharmacokinetics of enalapril on liver samples as well as healthy volunteers. This study included 286 Caucasian patients with arterial hypertension who received enalapril. The concentrations of enalapril and enalaprilat were determined before subsequent intake of the drug and 4 h after it with high-performance liquid chromatography (HPLC) and mass spectrometric detection. The study included genetic markers as follows: rs2244613, rs71647871 (c.428G>A, p.G143E) and three SNPs indicating the presence of a subtype CES1A1c (rs12149368, rs111604615 and rs201577108). Mean peak and trough enalaprilat concentrations, adjusted by clinical variables, were significantly lower in CES1 rs2244613 heterozygotes (by 16.6% and 19.6%) and in CC homozygotes (by 32.7% and 41.4%) vs. the AA genotype. In CES1A1c homozygotes, adjusted mean enalaprilat concentrations were 75% lower vs. heterozygotes and wild-type (WT) homozygotes. Pharmacogenetic markers of the CES1 gene may be a promising predictor for individualisation when prescribing enalapril.

Remdesivir, an intravenous nucleotide prodrug, has been approved for treating COVID-19 in hospitalized adults and pediatric patients. Upon administration, remdesivir can be readily hydrolyzed to form its active form GS-441524, while the cleavage of the carboxylic ester into GS-704277 is the first step for remdesivir activation. This study aims to assign the key enzymes responsible for remdesivir hydrolysis in humans, as well as to investigate the kinetics of remdesivir hydrolysis in various enzyme sources. The results showed that remdesivir could be hydrolyzed to form GS-704277 in human plasma and the microsomes from human liver (HLMs), lung (HLuMs) and kidney (HKMs), while the hydrolytic rate of remdesivir in HLMs was the fastest. Chemical inhibition and reaction phenotyping assays suggested that human carboxylesterase 1 (hCES1A) played a predominant role in remdesivir hydrolysis, while cathepsin A (CTSA), acetylcholinesterase (AchE) and butyrylcholinesterase (BchE) contributed to a lesser extent. Enzymatic kinetic analyses demonstrated that remdesivir hydrolysis in hCES1A (SHUTCM) and HLMs showed similar kinetic plots and much closed K(m) values to each other. Meanwhile, GS-704277 formation rates were strongly correlated with the CES1A activities in HLM samples from different individual donors. Further investigation revealed that simvastatin (a therapeutic agent for adjuvant treating COVID-19) strongly inhibited remdesivir hydrolysis in both recombinant hCES1A and HLMs. Collectively, our findings reveal that hCES1A plays a predominant role in remdesivir hydrolysis in humans, which are very helpful for predicting inter-individual variability in response to remdesivir and for guiding the rational use of this anti-COVID-19 agent in clinical settings.

Egasyn-beta-glucuronidase complex is located at the luminal site of liver microsomal endoplasmic reticulum. When organophosphorus insecticides (OP) are incorporated into the liver microsomes, they become tightly bound to egasyn, a carboxylesterase isozyme, and subsequently, beta-glucuronidase (BG) is dissociated and released into blood. Consequently, the increase in plasma BG activity becomes a good biomarker of OP exposure. Thus, the single administration of EPN (O-ethyl O-p-nitrophenylphenylphosphonothioate), acephate and chlorpyrifos increased plasma BG activity in approximately 100-fold the control level in rats. The increase in plasma BG activity after OP exposure is a much more sensitive biomarker of acute OP exposure than acetylcholinesterase (AChE) inhibition.

The angiotensin-converting enzyme inhibitor enalapril is hydrolysed to an active metabolite, enalaprilat, in the liver via carboxylesterase 1 (CES1). Previous studies show that variant rs71647871 in the CES1 gene affects the pharmacokinetics of enalapril on liver samples as well as healthy volunteers. This study included 286 Caucasian patients with arterial hypertension who received enalapril. The concentrations of enalapril and enalaprilat were determined before subsequent intake of the drug and 4 h after it with high-performance liquid chromatography (HPLC) and mass spectrometric detection. The study included genetic markers as follows: rs2244613, rs71647871 (c.428G>A, p.G143E) and three SNPs indicating the presence of a subtype CES1A1c (rs12149368, rs111604615 and rs201577108). Mean peak and trough enalaprilat concentrations, adjusted by clinical variables, were significantly lower in CES1 rs2244613 heterozygotes (by 16.6% and 19.6%) and in CC homozygotes (by 32.7% and 41.4%) vs. the AA genotype. In CES1A1c homozygotes, adjusted mean enalaprilat concentrations were 75% lower vs. heterozygotes and wild-type (WT) homozygotes. Pharmacogenetic markers of the CES1 gene may be a promising predictor for individualisation when prescribing enalapril.

The number of studies and reviews conducted for the Carboxylesterase gene is limited in comparison with other enzymes. Carboxylesterase (CES) gene or human carboxylesterases (hCES) is a multigene protein belonging to the alpha/beta-hydrolase family. Over the last decade, two major carboxylesterases (CES1 and CES2), located at 16q13-q22.1 on human chromosome 16 have been extensively studied as important mediators in the metabolism of a wide range of substrates. hCES1 is the most widely expressed enzyme in humans, and it is found in the liver. In this review, details regarding CES1 substrates include both inducers (e.g. Rifampicin) and inhibitors (e.g. Enalapril, Diltiazem, Simvastatin) and different types of hCES1 polymorphisms (nsSNPs) such as rs2244613 and rs71647871. along with their effects on various CES1 substrates were documented. Few instances where the presence of nsSNPs exerted a positive influence on certain substrates which are hydrolyzed via hCES1, such as anti-platelets like Clopidogrel when co-administered with other medications such as angiotensin-converting enzyme (ACE) inhibitors were also recorded. Remdesivir, an ester prodrug is widely used for the treatment of COVID-19, being a CES substrate, it is a potent inhibitor of CES2 and is hydrolyzed via CES1. The details provided in this review could give a clear-cut idea or information that could be used for further studies regarding the safety and efficacy of CES1 substrate.

The prodrug tenofovir alafenamide (TAF) is a first-line antiviral agent for the treatment of chronic hepatitis B infection. TAF activation involves multiple steps, and the first step is an ester hydrolysis reaction catalyzed by hydrolases. This study was to determine the contributions of carboxylesterase 1 (CES1) and cathepsin A (CatA) to TAF hydrolysis in the human liver. Our in vitro incubation studies showed that both CatA and CES1 catalyzed TAF hydrolysis in a pH-dependent manner. At their physiological pH environment, the activity of CatA (pH 5.2) was approximately 1,000-fold higher than that of CES1 (pH 7.2). Given that the hepatic protein expression of CatA was approximately 200-fold lower than that of CES1, the contribution of CatA to TAF hydrolysis in the human liver was estimated to be much greater than that of CES1, which is contrary to the previous perception that CES1 is the primary hepatic enzyme hydrolyzing TAF. The findings were further supported by a TAF incubation study with the CatA inhibitor telaprevir and the CES1 inhibitor bis-(p-nitrophenyl) phosphate. Moreover, an in vitro study revealed that the CES1 variant G143E (rs71647871) is a loss-of-function variant for CES1-mediated TAF hydrolysis. In summary, our results suggest that CatA may play a more important role in the hepatic activation of TAF than CES1. Additionally, TAF activation in the liver could be affected by CES1 genetic variation, but the magnitude of impact appears to be limited due to the major contribution of CatA to hepatic TAF activation. Significance Statement Contrary to the general perception that carboxylesterase 1 (CES1) is the major enzyme responsible for tenofovir alafenamide (TAF) hydrolysis in the human liver, the present study demonstrated that cathepsin A (CatA) may play a more significant role in TAF hepatic hydrolysis. Furthermore, the CES1 variant G143E (rs71647871) was found to be a loss-of-function variant for CES1-mediated TAF hydrolysis.

Remdesivir, an intravenous nucleotide prodrug, has been approved for treating COVID-19 in hospitalized adults and pediatric patients. Upon administration, remdesivir can be readily hydrolyzed to form its active form GS-441524, while the cleavage of the carboxylic ester into GS-704277 is the first step for remdesivir activation. This study aims to assign the key enzymes responsible for remdesivir hydrolysis in humans, as well as to investigate the kinetics of remdesivir hydrolysis in various enzyme sources. The results showed that remdesivir could be hydrolyzed to form GS-704277 in human plasma and the microsomes from human liver (HLMs), lung (HLuMs) and kidney (HKMs), while the hydrolytic rate of remdesivir in HLMs was the fastest. Chemical inhibition and reaction phenotyping assays suggested that human carboxylesterase 1 (hCES1A) played a predominant role in remdesivir hydrolysis, while cathepsin A (CTSA), acetylcholinesterase (AchE) and butyrylcholinesterase (BchE) contributed to a lesser extent. Enzymatic kinetic analyses demonstrated that remdesivir hydrolysis in hCES1A (SHUTCM) and HLMs showed similar kinetic plots and much closed K(m) values to each other. Meanwhile, GS-704277 formation rates were strongly correlated with the CES1A activities in HLM samples from different individual donors. Further investigation revealed that simvastatin (a therapeutic agent for adjuvant treating COVID-19) strongly inhibited remdesivir hydrolysis in both recombinant hCES1A and HLMs. Collectively, our findings reveal that hCES1A plays a predominant role in remdesivir hydrolysis in humans, which are very helpful for predicting inter-individual variability in response to remdesivir and for guiding the rational use of this anti-COVID-19 agent in clinical settings.

Human monocytic cells in blood have important roles in host defense and express the enzyme carboxylesterase 1 (CES1). This metabolic serine hydrolase plays a critical role in the metabolism of many molecules, including lipid mediators called prostaglandin glyceryl esters (PG-Gs), which are formed during cyclooxygenase-mediated oxygenation of the endocannabinoid 2-arachidonoylglycerol. Some PG-Gs have been shown to exhibit anti-inflammatory effects; however, they are unstable compounds, and their hydrolytic breakdown generates pro-inflammatory prostaglandins. We hypothesized that by blocking the ability of CES1 to hydrolyze PG-Gs in monocytes/macrophages, the beneficial effects of anti-inflammatory prostaglandin D(2)-glyceryl ester (PGD(2)-G) could be augmented. The goals of this study were to determine whether PGD(2)-G is catabolized by CES1, evaluate the degree to which this metabolism is blocked by small-molecule inhibitors, and assess the immunomodulatory effects of PGD(2)-G in macrophages. A human monocytic cell line (THP-1 cells) was pretreated with increasing concentrations of known small-molecule inhibitors that block CES1 activity [chlorpyrifos oxon (CPO), WWL229, or WWL113], followed by incubation with PGD(2)-G (10 M). Organic solvent extracts of the treated cells were analyzed by liquid chromatography with tandem mass spectrometry to assess levels of the hydrolysis product PGD(2). Further, THP-1 monocytes with normal CES1 expression (control cells) and "knocked-down" CES1 expression (CES1KD cells) were employed to confirm CES1's role in PGD(2)-G catabolism. We found that CES1 has a prominent role in PGD(2)-G hydrolysis in this cell line, accounting for about 50% of its hydrolytic metabolism, and that PGD(2)-G could be stabilized by the inclusion of CES1 inhibitors. The inhibitor potency followed the rank order: CPO > WWL113 > WWL229. THP-1 macrophages co-treated with WWL113 and PGD(2)-G prior to stimulation with lipopolysaccharide exhibited a more pronounced attenuation of pro-inflammatory cytokine levels (interleukin-6 and TNFalpha) than by PGD(2)-G treatment alone. In contrast, prostaglandin E(2)-glyceryl ester (PGE(2)-G) had opposite effects compared to those of PGD(2)-G, which appeared to be dependent on the hydrolysis of PGE(2)-G to PGE(2). These results suggest that the anti-inflammatory effects induced by PGD(2)-G can be further augmented by inactivating CES1 activity with specific small-molecule inhibitors, while pro-inflammatory effects of PGE(2)-G are attenuated. Furthermore, PGD(2)-G (and/or its downstream metabolites) was shown to activate the lipid-sensing receptor PPARgamma, resulting in altered "alternative macrophage activation" response to the Th2 cytokine interleukin-4. These findings suggest that inhibition of CES1 and other enzymes that regulate the levels of pro-resolving mediators such as PGD(2)-G in specific cellular niches might be a novel anti-inflammatory approach.

The present clinical trial investigated the impact of selected SNPs in CES1 on the metabolic activity of the enzyme. For this purpose, we used methylphenidate (MPH) as a pharmacological probe and the d-RA/d-MPH (metabolite/parent drug) ratios as a measure of enzymatic activity. This metabolic ratio (MR) was validated against the AUC ratios (AUCd -RA /AUCd -MPH ). CES1 SNPs from 120 volunteers were identified, and 12 SNPs fulfilling predefined inclusion criteria were analysed separately, comparing the effect of each genotype on the metabolic ratios. The SNP criteria were as follows: presence of Hardy-Weinberg equilibrium, a minor allele frequency >/= 0.01 and a clearly interpretable sequencing result in at least 30% of the individuals. Each participant ingested 10 mg of racemic methylphenidate, and blood samples were drawn prior to and 3 hours after drug administration. The SNP analysis confirmed the considerable impact of rs71647871 (G143E) in exon 4 on drug metabolism. In addition, three volunteers with markedly lower median MR, indicating decreased CES1 activity, harboured the same combination of three SNPs in intron 5. The median MR for these SNPs was 8.2 for the minor allele compared to 16.4 for the major alleles (P = 0.04). Hence, one of these or the combination of these SNPs could be of clinical significance considering that the median MR of the G143E group was 5.4. The precise genetic relationship of this finding is currently unknown, as is the clinical significance.

2-Arachidonoyl-glycerol (2-AG) is an endocannabinoid with anti-inflammatory properties. Blocking 2-AG hydrolysis to enhance CB2 signaling has proven effective in mouse models of inflammation. However, the expression of 2-AG lipases has never been thoroughly investigated in human leukocytes. Herein, we investigated the expression of seven 2-AG hydrolases by human blood leukocytes and alveolar macrophages (AMs) and found the following protein expression pattern: monoacylglycerol (MAG lipase; eosinophils, AMs, monocytes), carboxylesterase (CES1; monocytes, AMs), palmitoyl-protein thioesterase (PPT1; AMs), alpha/beta-hydrolase domain (ABHD6; mainly AMs), ABHD12 (all), ABHD16A (all), and LYPLA2 (lysophospholipase 2; monocytes, lymphocytes, AMs). We next found that all leukocytes could hydrolyze 2-AG and its metabolites derived from cyclooxygenase-2 (prostaglandin E2 -glycerol [PGE2 -G]) and the 15-lipoxygenase (15-hydroxy-eicosatetraenoyl-glycerol [15-HETE-G]). Neutrophils and eosinophils were consistently better at hydrolyzing 2-AG and its metabolites than monocytes and lymphocytes. Moreover, the efficacy of leukocytes to hydrolyze 2-AG and its metabolites was 2-AG >/= 15-HETE-G >> PGE2 -G for each leukocyte. Using the inhibitors methylarachidonoyl-fluorophosphonate (MAFP), 4-nitrophenyl-4-(dibenzo[d][1,3]dioxol-5-yl(hydroxy)methyl)piperidine-1-carboxyla te (JZL184), Palmostatin B, 4'-carbamoylbiphenyl-4-yl methyl(3-(pyridin-4-yl)benzyl)carbamate, N-methyl-N-[[3-(4-pyridinyl)phenyl]methyl]-4'-(aminocarbonyl)[1,1'-biphenyl]-4-yl ester carbamic acid (WWL70), 4'-[[[methyl[[3-(4-pyridinyl)phenyl]methyl]amino]carbonyl]oxy]-[1,1'-biphenyl]-4- carboxylic acid, ethyl ester (WWL113), tetrahydrolipstatin, and ML349, we could not pinpoint a specific hydrolase responsible for the hydrolysis of 2-AG, PGE2 -G, and 15-HETE-G by these leukocytes. Furthermore, JZL184, a selective MAG lipase inhibitor, blocked the hydrolysis of 2-AG, PGE2 -G, and 15-HETE-G by neutrophils and the hydrolysis of PGE2 -G and 15-HETE-G by lymphocytes, two cell types with limited/no MAG lipase. Using an activity-based protein profiling (ABPP) probe to label hydrolases in leukocytes, we found that they express many MAFP-sensitive hydrolases and an unknown JZL184-sensitive hydrolase of approximately 52 kDa. Altogether, our results indicate that human leukocytes are experts at hydrolyzing 2-AG and its metabolites via multiple lipases and probably via a yet-to-be characterized 52 kDa hydrolase. Blocking 2-AG hydrolysis in humans will likely abrogate the ability of human leukocytes to degrade 2-AG and its metabolites and increase their anti-inflammatory effects in vivo.

INTRODUCTION: Mammalian carboxylesterase enzymes are a highly conserved metabolic pathway involved in the metabolism of endogenous and exogenous compounds including many widely prescribed therapeutic agents. Recent advances in our understanding of genetic polymorphisms affecting enzyme activity have exposed potential therapeutic implications. Areas covered: The aims of this review are to provide an overview of carboxylesterase 1 (CES1) and carboxylesterase 2 (CES2) gene structure, to summarize the known polymorphism affecting substrate-drug metabolism, and to assess the potential therapeutic implications of genetic variations affecting enzyme function. Expert opinion: Genetic variability in carboxylesterase drug metabolism is a nascent area of research with only a handful of the thousands of SNPs investigated for their potential effects of enzyme activity or carboxylesterase-substrate disposition and therapeutics. It remains to be determined if the wide variability in enzyme activity can be explained by genetic variation, and used in personalized medicine to improve clinical outcomes.

The roots of Salvia miltiorrhiza ("Danshen") have been used in Chinese herbal medicine for centuries for a host of different conditions. While the exact nature of the active components of this material are unknown, large amounts of tanshinones are present in extracts derived from these samples. Recently, the tanshinones have been demonstrated to be potent human carboxylesterase (CE) inhibitors, with the ability to modulate the biological activity of esterified drugs. During the course of these studies, we also identified more active, irreversible inhibitors of these enzymes. We have purified, identified, and synthesized these molecules and confirmed them to be the anhydride derivatives of the tanshinones. These compounds are exceptionally potent inhibitors ( Ki < 1 nM) and can inactivate human CEs both in vitro and in cell culture systems and can modulate the metabolism of the esterified drug oseltamivir. Therefore, the coadministration of Danshen extracts with drugs that contain the ester chemotype should be minimized since, not only is transient inhibition of CEs observed with the tanshinones, but also prolonged irreversible inhibition arises via interaction with the anhydrides.

Obesity often leads non-alcoholic fatty liver disease, insulin resistance and hyperlipidemia. Expression of carboxylesterase CES1 is positively correlated with increased lipid storage and plasma lipid concentration. Here we investigated structural and metabolic consequences of a single nucleotide polymorphism in CES1 gene that results in p.Gly143Glu amino acid substitution. We generated a humanized mouse model expressing CES1(WT) (control), CES1(G143E) and catalytically dead CES1(S221A) (negative control) in the liver in the absence of endogenous expression of the mouse orthologous gene. We show that the CES1(G143E) variant exhibits only 20% of the wild-type lipolytic activity. High-fat diet fed mice expressing CES1(G143E) had reduced liver and plasma triacylglycerol levels. The mechanism by which decreased CES1 activity exerts this hypolipidemic phenotype was determined to include decreased very-low density lipoprotein secretion, decreased expression of hepatic lipogenic genes and increased fatty acid oxidation as determined by increased plasma ketone bodies and hepatic mitochondrial electron transport chain protein abundance. We conclude that attenuation of human CES1 activity provides a beneficial effect on hepatic lipid metabolism. These studies also suggest that CES1 is a potential therapeutic target for non-alcoholic fatty liver disease management.

Several single nucleotide variations (SNVs) affect carboxylesterase 1 (CES1) activity, but the effects of genetic variants on CES1 gene expression have not been systematically investigated. Therefore, our aim was to investigate effects of genetic variants on CES1 gene expression in two independent whole blood sample cohorts of 192 (discovery) and 88 (replication) healthy volunteers and in a liver sample cohort of 177 patients. Furthermore, we investigated possible effects of the found variants on clopidogrel pharmacokinetics (n = 106) and pharmacodynamics (n = 46) in healthy volunteers, who had ingested a single 300 mg or 600 mg dose of clopidogrel. Using massively parallel sequencing, we discovered two CES1 SNVs, rs12443580 and rs8192935, to be strongly and independently associated with a 39% (p = 4.0 x 10(-13) ) and 31% (p = 2.5 x 10(-8) ) reduction in CES1 whole blood expression per copy of the minor allele. These findings were replicated in the replication cohort. However, these SNVs did not affect CES1 liver expression, or clopidogrel pharmacokinetics or pharmacodynamics. Conversely, the CES1 c.428G>A missense SNV (rs71647871) impaired the hydrolysis of clopidogrel, increased exposure to clopidogrel active metabolite and enhanced its antiplatelet effects. In conclusion, the rs12443580 and rs8192935 variants reduce CES1 expression in whole blood but not in the liver. These tissue-specific effects may result in substrate-dependent effects of the two SNVs on CES1-mediated drug metabolism.

The carboxylesterase 1 gene (CES1) encodes a hydrolase that metabolizes commonly used drugs. The CES1-related pseudogene, carboxylesterase 1 pseudogene 1 (CES1P1), has been implicated in gene exchange with CES1 and in the formation of hybrid genes including the carboxylesterase 1A2 gene (CES1A2). Hence, the CES1 region is complex. Using in silico PCR and alignment, we assessed the specificity of PCR-assisted procedures for genotyping CES1, CES1A2 and CES1P1 in studies identified in PubMed. We identified 33 such studies and excluded those that were not the first to use a procedure or lacked sequence information. After this 17 studies remained. Ten of these used haplotype-specific amplification, restriction enzyme treatment or amplicon sequencing, and included five that were predicted to lack specificity. All procedures for genotyping of single nucleotide polymorphisms in eight studies lacked specificity. One of these studies also used amplicon sequencing, thus being present in the group above. Some primers and their intended targets were mismatched. We provide experimental evidence that one of the procedures lacked specificity. Additionally, a complex pattern of segmental duplications in the CES1 region was revealed. In conclusion, many procedures for CES1, CES1A2 and CES1P1 genotyping appear to lack specificity. Knowledge about the segmental duplications may improve the typing of these genes.

Carboxylesterase 1 (CES1) is implicated in the metabolism of several commonly used drugs and other xenobiotics. The gene encoding this enzyme, CES1, is duplicated in some individuals. The original gene copy is called CES1A1. The duplicated version, CES1A2, is a hybrid of CES1A1 and the CES1-related pseudogene, CES1P1. Variants of CES1A2 with a weak and a strong promoter, respectively, have been reported. In addition, there are chimeric subtypes of CES1A1 that contain a segment of CES1P1. Collectively, this represents challenges to the genotyping of CES1 that previous procedures have had difficulties in solving, frequently leading to loss of specificity and inaccurate genotyping. Here, we report a novel and specific procedure that can selectively amplify CES1A1 and CES1A2 and accurately determine their variants. This procedure may be useful for personalization of treatments with drugs metabolized by CES1.

AIMS: This study investigated the influence of CES1 variations, including the single nucleotide polymorphism (SNP) rs71647871 (G143E) and variation in copy number, on the pharmacokinetics of a single oral dose of 10 mg methylphenidate.
METHODS: CES1 genotype was obtained from 200 healthy Danish Caucasian volunteers. Based on the genotype, 44 (19 males and 25 females) were invited to participate in an open, prospective trial involving six predefined genotypes: three groups with two, three and four CES1 copies, respectively; a group of carriers of the CES1 143E allele; a group of individuals homozygous for CES1A1c (CES1VAR); and a group having three CES1 copies, in which the duplication, CES1A2, had increased transcriptional activity. Plasma concentrations of methylphenidate and its primary metabolites were determined at scheduled time points.
RESULTS: Median AUC of d-methylphenidate was significantly larger in the group carrying the 143E allele (53.3 ng ml-1 h-1 , range 38.6-93.9) than in the control group (21.4 ng ml-1 h-1 , range 15.7-34.9) (P < 0.0001). Median AUC of d-methylphenidate was significantly larger in the group with four CES1 copies (34.5 ng ml-1 h-1 , range 21.3-62.8) than in the control group (P = 0.01) and the group with three CES1 copies (23.8 ng ml-1 h-1 , range 15.3-32.0, P = 0.03). There was no difference between the groups with two and three copies of CES1.
CONCLUSIONS: The 143E allele resulted in an increased AUC, suggesting a significantly decreased CES1 enzyme activity. Surprisingly, this was also the case in subjects with homozygous duplication of CES1, perhaps reflecting an undiscovered mutation affecting the activity of the enzyme.

Carboxylesterase 1 (CES1) is the predominant human hepatic hydrolase responsible for the metabolism of many clinically important medications. CES1 expression and activity vary markedly among individuals; and genetic variation is a major contributing factor to CES1 interindividual variability. In this study, we comprehensively examined the functions of CES1 nonsynonymous single nucleotide polymorphisms (nsSNPs) and haplotypes using transfected cell lines and individual human liver tissues. The 20 candidate variants include CES1 nsSNPs with a minor allele frequency >0.5% in a given population or located in close proximity to the CES1 active site. Five nsSNPs, including L40Ter (rs151291296), G142E (rs121912777), G147C (rs146456965), Y170D (rs148947808), and R171C (rs201065375), were loss-of-function variants for metabolizing the CES1 substrates clopidogrel, enalapril, and sacubitril. In addition, A158V (rs202121317), R199H (rs2307243), E220G (rs200707504), and T290M (rs202001817) decreased CES1 activity to a lesser extent in a substrate-dependent manner. Several nsSNPs, includingL40Ter (rs151291296), G147C (rs146456965), Y170D (rs148947808), and R171C (rs201065375), significantly reduced CES1 protein and/or mRNA expression levels in the transfected cells. Functions of the common nonsynonymous haplotypes D203E-A269S and S75N-D203E-A269S were evaluated using cells stably expressing the haplotypes and a large set of the human liver. Neither CES1 expression nor activity was affected by the two haplotypes. In summary, this study revealed several functional nsSNPs with impaired activity on the metabolism of CES1 substrate drugs. Clinical investigations are warranted to determine whether these nsSNPs can serve as biomarkers for the prediction of therapeutic outcomes of drugs metabolized by CES1.

Carboxylesterase 1 (CES1) hydrolyzes the prodrug clopidogrel to an inactive carboxylic acid metabolite. The effects of CES1 S75N (rs2307240,C>T) on clopidogrel response among 851 acute coronary syndrome patients who came from the north, central and south of China were studied. The occurrence ratios of each endpoint in the CC group were significantly higher than in the CT + TT group for cerebrovascular events (14% vs 4.8%, p < 0.001, OR = 0.31), acute myocardial infarction (15.1% vs 6.1%, p < 0.001, OR = 0.37) and unstable angina (62.8% vs 37.7%, p < 0.001, OR = 0.36). The results showed that there was a significant association between CES1 S75N (rs2307240) and the outcome of clopidogrel therapy. Moreover, the frequency of the T allele of rs2307240 in acute coronary syndrome patients (MAF = 0.22) was more than four times higher than that in the general public (MAF = 0.05).

The aim of this study was to identify demographic and genetic factors that significantly affect methylphenidate (MPH) pharmacokinetics (PK), and may help explain interindividual variability and further increase the safety of MPH. d-MPH plasma concentrations, demographic covariates, and carboxylesterase 1 (CES1) genotypes were gathered from 122 healthy adults and analyzed using nonlinear mixed effects modeling. The structural model that best described the data was a two-compartment disposition model with absorption transit compartments. Novel effects of rs115629050 and CES1 diplotypes, as well as previously reported effects of rs71647871 and body weight, were included in the final model. Assessment of the independent and combined effect of CES1 covariates identified several specific risk factors that may result in severely increased d-MPH plasma exposure.

OBJECTIVE: Most angiotensin-converting enzyme inhibitors (ACEIs) are prodrugs activated by carboxylesterase 1 (CES1). We investigated the prognostic importance of CES1 gene (CES1) copy number variation and the rs3815583 single-nucleotide polymorphism in CES1 among ACEI-treated patients with congestive heart failure (CHF). METHODS: Danish patients with chronic CHF enrolled in the previously reported Echocardiography and Heart Outcome Study were categorized according to their CES1 variants and followed up for up to 10 years. Risk for cardiovascular death and all-cause death was modeled by Cox proportional hazard analyses. RESULTS: A total of 491 ACEI-treated patients were included in the analyses. After a mean follow-up of 5.5 years, we found no difference in the risk for cardiovascular death and all-cause death between patients having three [hazard ratios (HRs) 1.06 (95% confidence interval (CI) 0.77-1.45) and 1.16 (95% CI 0.88-1.52)] or four [HRs 0.88 (95% CI 0.39-2.01) and 1.37 (95% CI 0.74-2.54)] CES1 copies and those with two copies, respectively. Similarly, no difference in the risk for cardiovascular and all-cause death was found for patients heterozygous [HRs 0.91 (95% CI 0.70-1.19) and 0.88 (95% CI 0.69-1.12)] or homozygous [HRs 0.58 (95% CI 0.30-1.15) and 0.82 (95% CI 0.48-1.39)] for the rs3815583 minor allele versus patients homozygous for the major allele. The active promoter of CES1A2 and the rs71647871 single-nucleotide polymorphism minor allele were detected at very low frequencies. CONCLUSION: This study did not support the use of CES1 copy number variation or rs3815583 as a predictor of fatal outcomes in ACEI-treated patients with CHF.

Genetic variations in drug-metabolizing enzymes have been reported to influence pharmacokinetics, drug dosage, and other aspects that affect therapeutic outcomes. Most particularly, non-synonymous single-nucleotide polymorphisms (nsSNPs) resulting in amino acid changes disrupt potential functional sites responsible for protein activity, structure, or stability, which can account for individual susceptibility to disease and drug response. Investigating the impact of nsSNPs at a protein's structural level is a key step in understanding the relationship between genetic variants and the resulting phenotypic changes. For this purpose, in silico structure-based approaches have proven their relevance in providing an atomic-level description of the underlying mechanisms. The present review focuses on nsSNPs in human carboxylesterase 1 (hCES1), an enzyme involved in drug metabolism. We highlight how prioritization of functional nsSNPs through computational prediction techniques in combination with structure-based approaches, namely molecular docking and molecular dynamics simulations, is a powerful tool in providing insight into the underlying molecular mechanisms of nsSNPs phenotypic effects at microscopic level. Examples of in silico studies of carboxylesterases (CESs) are discussed, ranging from exploring the effect of mutations on enzyme activity to predicting the metabolism of new hCES1 substrates as well as to guiding rational design of CES-selective inhibitors.

OBJECTIVE: CES1 encodes carboxylesterase-1, an important drug-metabolizing enzyme with high expression in the liver. Previous studies have reported a genomic translocation of the 5' region from the poorly expressed pseudogene CES1P1, to CES1, yielding the structural variant CES1VAR. The aim of this study was to characterize this translocation and its effect on CES1 expression in the human liver. MATERIALS AND
METHODS: Experiments were conducted in human liver tissues and cell culture (HepG2). The promoter and exon 1 of CES1 were sequenced by Sanger and Ion Torrent sequencing to identify gene translocations. The effects of CES1 5'UTRs on mRNA and protein expression were assessed by quantitative real-time PCR, allelic ratio mRNA analysis by primer extension (SNaPshot), quantitative targeted proteomics, and luciferase reporter gene assays.
RESULTS: Sequencing of CES1 identified two translocations: first, CES1VAR (17% minor allele frequency) comprising the 5'UTR, exon 1, and part of intron 1. A second shorter translocation, CES1SVAR, was observed excluding exon 1 and intron 1 regions (<0.01% minor allele frequency). CES1VAR is associated with 2.6-fold decreased CES1 mRNA and approximately 1.35-fold lower allelic mRNA. Luciferase reporter constructs showed that CES1VAR decreases luciferase activity 1.5-fold, whereas CES1SVAR slightly increases activity. CES1VAR was not associated with CES1 protein expression or metabolism of the CES1 substrates enalapril, clopidogrel, or methylphenidate in the liver. CONCLUSION: The frequent translocation variant CES1VAR reduces mRNA expression of CES1 in the liver by approximately 30%, but protein expression and metabolizing activity in the liver were not detectably altered - possibly because of variable CES1 expression masking small allelic effects. Whether drug therapies are affected by CES1VAR will require further in-vivo studies.

The oral anticoagulant prodrug dabigatran etexilate (DABE) is sequentially metabolized by intestinal carboxylesterase 2 (CES2) and hepatic carboxylesterase 1 (CES1) to form its active metabolite dabigatran (DAB). A recent genome-wide association study reported that the CES1 single nucleotide polymorphisms (SNPs) rs2244613 and rs8192935 were associated with lower DAB plasma concentrations in the Randomized Evaluation of Long-term Anticoagulation Therapy (RE-LY) study participants. In addition, gender differences in exposure to DAB were observed in clinical studies. The aim of this study was to examine the effect of CES1 genetic polymorphisms and gender on DABE activation using several in vitro approaches. The genotypes of the CES1 SNPs rs2244613, rs8192935, and the known loss-of-function CES1 variant rs71647871 (G143E), and the activation of DABE and its intermediate metabolites M1 and M2 were determined in 104 normal human liver samples. DABE, M1, and M2 activations were found to be impaired in human livers carrying the G143E variant. However, neither rs2244613 nor rs8192935 was associated with the activation in human livers. The incubation study of DABE with supernatant fractions (S9) prepared from the G143E-transfected cells showed that the G143E is a loss-of-function variant for DABE metabolism. Moreover, hepatic CES1 activity on M2 activation was significantly higher in female liver samples than male. Our data suggest that CES1 genetic variants and gender are important contributing factors to variability in DABE activation in humans. A personalized DABE treatment approach based on patient-specific CES1 genotypes and sex may have the potential to improve the efficacy and safety of DABE pharmacotherapy.

Oseltamivir, an inactive anti-influenza virus prodrug, is activated (hydrolysed) in vivo by carboxylesterase 1 (CES1) to its active metabolite oseltamivir carboxylate. CES1 functions are significantly associated with certain CES1 genetic variants and some non-genetic factors. The purpose of this study was to investigate the effect of gender and several CES1 genetic polymorphisms on oseltamivir activation using a large set of individual human liver samples. CES1-mediated oseltamivir hydrolysis and CES1 genotypes, including the G143E (rs71647871), rs2244613, rs8192935, the -816A>C (rs3785161) and the CES1P1/CES1P1VAR, were determined in 104 individual human livers. The results showed that hepatic CES1 protein expression in females was 17.3% higher than that in males (p = 0.039), while oseltamivir activation rate in the livers from female donors was 27.8% higher than that from males (p = 0.076). As for CES1 genetic polymorphisms, neither CES1 protein expression nor CES1 activity on oseltamivir activation was significantly associated with the rs2244613, rs8192935, -816A>C or CES1P1/CES1P1VAR genotypes. However, oseltamivir hydrolysis in the livers with the genotype 143G/E was approximately 40% of that with the 143G/G genotype (0.7 +/- 0.2 versus 1.8 +/- 1.1 nmole/mg protein/min, p = 0.005). In summary, the results suggest that hepatic oseltamivir activation appears to be more efficient in females than that in males, and the activation can be impaired by functional CES1 variants, such as the G143E. However, clinical implication of CES1 gender differences and pharmacogenetics in oseltamivir pharmacotherapy warrants further investigations.

Sacubitril was recently approved by the Food and Drug Administration for use in combination with valsartan for the treatment of patients with heart failure with reduced ejection fraction. As a prodrug, sacubitril must be metabolized (hydrolyzed) to its active metabolite sacubitrilat (LBQ657) to exert its intended therapeutic effects. Thus, understanding the determinants of sacubitril activation will lead to the improvement of sacubitril pharmacotherapy. The objective of this study was to identify the enzyme(s) responsible for the activation of sacubitril, and determine the impact of genetic variation on sacubitril activation. First, an incubation study of sacubitril with human plasma and the S9 fractions of human liver, intestine, and kidney was conducted. Sacubitril was found to be activated by human liver S9 fractions only. Moreover, sacubitril activation was significantly inhibited by the carboxylesterase 1 (CES1) inhibitor bis-(p-nitrophenyl) phosphate in human liver S9. Further incubation studies with recombinant human CES1 and carboxylesterase 2 confirmed that sacubitril is a selective CES1 substrate. The in vitro study of cell lines transfected with wild-type CES1 and the CES1 variant G143E (rs71647871) demonstrated that G143E is a loss-of-function variant for sacubitril activation. Importantly, sacubitril activation was significantly impaired in human livers carrying the G143E variant. In conclusion, sacubitril is selectively activated by CES1 in human liver. The CES1 genetic variant G143E can significantly impair sacubitril activation. Therefore, CES1 genetic variants appear to be an important contributing factor to interindividual variability in sacubitril activation, and have the potential to serve as biomarkers to optimize sacubitril pharmacotherapy.

The aim of the study was to determine the effect of carboxylesterase 1 (CES1) genetic variation on the activation of angiotensin-converting enzyme inhibitor (ACEI) prodrugs. In vitro incubation study of human liver, intestine and kidney s9 fractions demonstrated that the ACEI prodrugs enalapril, ramipril, perindopril, moexipril and fosinopril are selectively activated by CES1 in the liver. The impact of CES1/CES1VAR and CES1P1/CES1P1VAR genotypes and diplotypes on CES1 expression and activity on enalapril activation was investigated in 102 normal human liver samples. Neither the genotypes nor the diplotypes affected hepatic CES1 expression and activity. Moreover, among several CES1 nonsynonymous variants studied in transfected cell lines, the G143E (rs71647871) was a loss-of-function variant for the activation of all ACEIs tested. The CES1 activity on enalapril activation in human livers with the 143G/E genotype was approximately one-third of that carrying the 143G/G. Thus, some functional CES1 genetic variants (for example, G143E) may impair ACEI activation, and consequently affect therapeutic outcomes of ACEI prodrugs.

Human Carboxylesterase 1 (hCES1) is the key liver microsomal enzyme responsible for detoxification and metabolism of a variety of clinical drugs. To analyse the role of the single N-linked glycan on the structure and activity of the enzyme, authentically glycosylated and aglycosylated hCES1, generated by mutating asparagine 79 to glutamine, were produced in human embryonic kidney cells. Purified enzymes were shown to be predominantly trimeric in solution by analytical ultracentrifugation. The purified aglycosylated enzyme was found to be more active than glycosylated hCES1 and analysis of enzyme kinetics revealed that both enzymes exhibit positive cooperativity. Crystal structures of hCES1 a catalytically inactive mutant (S221A) and the aglycosylated enzyme were determined in the absence of any ligand or substrate to high resolutions (1.86 A, 1.48 A and 2.01 A, respectively). Superposition of all three structures showed only minor conformational differences with a root mean square deviations of around 0.5 A over all Calpha positions. Comparison of the active sites of these un-liganded enzymes with the structures of hCES1-ligand complexes showed that side-chains of the catalytic triad were pre-disposed for substrate binding. Overall the results indicate that preventing N-glycosylation of hCES1 does not significantly affect the structure or activity of the enzyme.

Tenofovir alafenamide fumarate (TAF) is an oral phosphonoamidate prodrug of the HIV reverse transcriptase nucleotide inhibitor tenofovir (TFV). Previous studies suggested a principal role for the lysosomal serine protease cathepsin A (CatA) in the intracellular activation of TAF. Here we further investigated the role of CatA and other human hydrolases in the metabolism of TAF. Overexpression of CatA or liver carboxylesterase 1 (Ces1) in HEK293T cells increased intracellular TAF hydrolysis 2- and 5-fold, respectively. Knockdown of CatA expression with RNA interference (RNAi) in HeLa cells reduced intracellular TAF metabolism 5-fold. Additionally, the anti-HIV activity and the rate of CatA hydrolysis showed good correlation within a large set of TFV phosphonoamidate prodrugs. The covalent hepatitis C virus (HCV) protease inhibitors (PIs) telaprevir and boceprevir potently inhibited CatA-mediated TAF activation (50% inhibitory concentration [IC50] = 0.27 and 0.16 muM, respectively) in vitro and also reduced its anti-HIV activity in primary human CD4(+) T lymphocytes (21- and 3-fold, respectively) at pharmacologically relevant concentrations. In contrast, there was no inhibition of CatA or any significant effect on anti-HIV activity of TAF observed with cobicistat, noncovalent HIV and HCV PIs, or various prescribed inhibitors of host serine proteases. Collectively, these studies confirm that CatA plays a pivotal role in the intracellular metabolism of TAF, whereas the liver esterase Ces1 likely contributes to the hepatic activation of TAF. Moreover, this work demonstrates that a wide range of viral and host PIs, with the exception of telaprevir and boceprevir, do not interfere with the antiretroviral activity of TAF.

Carboxylesterase 1 (CES1) hydrolyzes the prodrug clopidogrel to an inactive carboxylic acid metabolite. We studied the pharmacokinetics and pharmacodynamics of 600 mg oral clopidogrel in healthy white volunteers, including 10 carriers and 12 noncarriers of CES1 c.428G>A (p.Gly143Glu, rs71647871) single nucleotide variation (SNV). Clopidogrel carboxylic acid to clopidogrel area under the plasma concentration-time curve from 0 hours to infinity (AUC0-infinity ) ratio was 53% less in CES1 c.428G>A carriers than in noncarriers (P = 0.009), indicating impaired hydrolysis of clopidogrel. Consequently, the AUC0-infinity of clopidogrel and its active metabolite were 123% (P = 0.004) and 67% (P = 0.009) larger in the c.428G>A carriers than in noncarriers. Consistent with these findings, the average inhibition of P2Y12 -mediated platelet aggregation 0-12 hours after clopidogrel intake was 19 percentage points higher in the c.428G>A carriers than in noncarriers (P = 0.036). In conclusion, the CES1 c.428G>A SNV increases clopidogrel active metabolite concentrations and antiplatelet effects by reducing clopidogrel hydrolysis to inactive metabolites.

AIM: The aim of the present study was to investigate the effects of the carboxylesterase 1 (CES1) c.428G > A (p.G143E, rs71647871) single nucleotide variation (SNV) on the pharmacokinetics of quinapril and enalapril in a prospective genotype panel study in healthy volunteers.
METHODS: In a fixed-order crossover study, 10 healthy volunteers with the CES1 c.428G/A genotype and 12 with the c.428G/G genotype ingested a single 10 mg dose of quinapril and enalapril with a washout period of at least 1 week. Plasma concentrations of quinapril and quinaprilat were measured for up to 24 h and those of enalapril and enalaprilat for up to 48 h. Their excretion into the urine was measured from 0 h to 12 h.
RESULTS: The area under the plasma concentration-time curve from 0 h to infinity (AUC0-infinity ) of active enalaprilat was 20% lower in subjects with the CES1 c.428G/A genotype than in those with the c.428G/G genotype (95% confidence interval of geometric mean ratio 0.64, 1.00; P = 0.049). The amount of enalaprilat excreted into the urine was 35% smaller in subjects with the CES1 c.428G/A genotype than in those with the c.428G/G genotype (P = 0.044). The CES1 genotype had no significant effect on the enalaprilat to enalapril AUC0-infinity ratio or on any other pharmacokinetic or pharmacodynamic parameters of enalapril or enalaprilat. The CES1 genotype had no significant effect on the pharmacokinetic or pharmacodynamic parameters of quinapril.
CONCLUSIONS: The CES1 c.428G > A SNV decreased enalaprilat concentrations, probably by reducing the hydrolysis of enalapril, but had no observable effect on the pharmacokinetics of quinapril.

Understanding the mechanistic basis of prodrug delivery and activation is critical for establishing species-specific prodrug sensitivities necessary for evaluating preclinical animal models and potential drug-drug interactions. Despite significant adoption of prodrug methodologies for enhanced pharmacokinetics, functional annotation of prodrug activating enzymes is laborious and often unaddressed. Activity-based protein profiling (ABPP) describes an emerging chemoproteomic approach to assay active site occupancy within a mechanistically similar enzyme class in native proteomes. The serine hydrolase enzyme family is broadly reactive with reporter-linked fluorophosphonates, which have shown to provide a mechanism-based covalent labeling strategy to assay the activation state and active site occupancy of cellular serine amidases, esterases, and thioesterases. Here we describe a modified ABPP approach using direct substrate competition to identify activating enzymes for an ethyl ester prodrug, the influenza neuraminidase inhibitor oseltamivir. Substrate-competitive ABPP analysis identified carboxylesterase 1 (CES1) as an oseltamivir-activating enzyme in intestinal cell homogenates. Saturating concentrations of oseltamivir lead to a four-fold reduction in the observed rate constant for CES1 inactivation by fluorophosphonates. WWL50, a reported carbamate inhibitor of mouse CES1, blocked oseltamivir hydrolysis activity in human cell homogenates, confirming CES1 is the primary prodrug activating enzyme for oseltamivir in human liver and intestinal cell lines. The related carbamate inhibitor WWL79 inhibited mouse but not human CES1, providing a series of probes for analyzing prodrug activation mechanisms in different preclinical models. Overall, we present a substrate-competitive activity-based profiling approach for broadly surveying candidate prodrug hydrolyzing enzymes and outline the kinetic parameters for activating enzyme discovery, ester prodrug design, and preclinical development of ester prodrugs.

The age-associated alteration in expression levels of carboxylesterases (CESs) can affect both intestinal and hepatic first-pass metabolism after oral administration of xenobiotic esters such as prodrugs. In this study, the age-related expression of CES isozymes and hydrolase activities were simultaneously investigated in liver, jejunum, and ileum from 8-, 46-, and 90-week-old rats. Rat liver expresses three major CES1 isozymes, Hydrolase A, Hydrolase B, and Hydrolase C, as well as one minor CES1 (Egasyn) and three minor CES2 isozymes (RL4, AY034877, and D50580). The mRNA and protein levels of major hepatic CES1 isozymes were decreased in an age-dependent manner, while those of minor CESs were maintained in all age groups. The hepatic hydrolase activity for temocapril was decreased in an age-dependent manner, accompanied by downregulation of Hydrolase B/C mRNA, while age-independent hydrolysis of propranolol derivatives was observed in rat liver, due to the contribution of Egasyn. Rat small intestine expresses one major CES2 (RL4) and four minor CESs (Hydrolase B, Hydrolase C, Egasyn, and AY034877). Interestingly, the expression of RL4 was age-dependently increased in both jejunum and ileum, while minor isozymes showed a constant expression across a wide age range. The up-regulation of RL4 expression with aging led to an increase of intestinal hydrolase activities for temocapril and propranolol derivatives. Consequently, age-dependent changes in the expression of CES isozymes affect the hydrolysis of xenobiotics in both rat liver and small intestine.

Esterases catalyze the hydrolysis of therapeutic drugs containing esters or amides in their structures. Human carboxylesterase (CES) and arylacetamide deacetylase (AADAC) are the major enzymes that catalyze the hydrolysis of drugs in the liver. Characterization of the enzyme(s) responsible for drug metabolism is required in drug development and to realize optimal drug therapy. Because multiple enzymes may show a metabolic potency for a given compound, inhibition studies using chemical inhibitors are useful tools to determine the contribution of each enzyme in human tissue preparations. The purpose of this study was to find specific inhibitors for human CES1, CES2, and AADAC. We screened 542 chemicals for the inhibition potency toward hydrolase activities of p-nitrophenyl acetate by recombinant CES1, CES2, and AADAC. We found that digitonin and telmisartan specifically inhibited CES1 and CES2 enzyme activity, respectively. Vinblastine potently inhibited both AADAC and CES2, but no specific inhibitor of AADAC was found. The inhibitory potency and specificity of these compounds were also evaluated by monitoring the effects on hydrolase activity of probe compounds of each enzyme (CES1: lidocaine, CES2: CPT-11, AADAC: phenacetin) in human liver microsomes. Telmisartan and vinblastine strongly inhibited the hydrolysis of CPT-11 and/or phenacetin, but digitonin did not strongly inhibit the hydrolysis of lidocaine, indicating that the inhibitory potency of digitonin was different between recombinant CES1 and liver microsomes. Although we could not find a specific inhibitor of AADAC, the combined use of telmisartan and vinblastine could predict the responsibility of human AADAC to drug hydrolysis.

Clopidogrel pharmacotherapy is associated with substantial interindividual variability in clinical response, which can translate into an increased risk of adverse outcomes. Clopidogrel, a recognized substrate of hepatic carboxylesterase 1 (CES1), undergoes extensive hydrolytic metabolism in the liver. Significant interindividual variability in the expression and activity of CES1 exists, which is attributed to both genetic and environmental factors. We determined whether CES1 inhibition and CES1 genetic polymorphisms would significantly influence the biotransformation of clopidogrel and alter the formation of the active metabolite. Coincubation of clopidogrel with the CES1 inhibitor bis(4-nitrophenyl) phosphate in human liver s9 fractions significantly increased the concentrations of clopidogrel, 2-oxo-clopidogrel, and clopidogrel active metabolite, while the concentrations of all formed carboxylate metabolites were significantly decreased. As anticipated, clopidogrel and 2-oxo-clopidogrel were efficiently hydrolyzed by the cell s9 fractions prepared from wild-type CES1 transfected cells. The enzymatic activity of the CES1 variants G143E and D260fs were completely impaired in terms of catalyzing the hydrolysis of clopidogrel and 2-oxo-clopidogrel. However, the natural variants G18V, S82L, and A269S failed to produce any significant effect on CES1-mediated hydrolysis of clopidogrel or 2-oxo-clopidogrel. In summary, deficient CES1 catalytic activity resulting from CES1 inhibition or CES1 genetic variation may be associated with higher plasma concentrations of clopidogrel-active metabolite, and hence may enhance antiplatelet activity. Additionally, CES1 genetic variants have the potential to serve as a biomarker to predict clopidogrel response and individualize clopidogrel dosing regimens in clinical practice.

The use of whole insect larvae as a source of recombinant proteins offers a more cost-effective method of producing large quantities of human proteins than conventional cell-culture approaches. Human carboxylesterase 1 has been produced in and isolated from whole Trichoplusia ni larvae. The recombinant protein was crystallized and its structure was solved to 2.2 resolution. The results indicate that the larvae-produced enzyme is essentially identical to that isolated from cultured Sf21 cells, supporting the use of this expression system to produce recombinant enzymes for crystallization studies.

Bioactivation of the antiviral agent oseltamivir to active oseltamivir carboxylate is catalyzed by carboxylesterase 1 (CES1). After the screening of 860 healthy Finnish volunteers for the CES1 c.428G>A (p.Gly143Glu, rs121912777) polymorphism, a pharmacokinetic study with 75 mg oseltamivir was carried out in c.428G>A carriers and noncarriers. Heterozygous c.428GA carriers (n = 9) had 18% larger values of oseltamivir area under the plasma concentration-time curve from 0 h to infinity (AUC(0-infinity)) (P = 0.025) and 23% smaller carboxylate-to-oseltamivir AUC(0-infinity) ratio (P = 0.006) than noncarriers (n = 12). This shows that the CES1 c.428G>A polymorphism impairs oseltamivir bioactivation in humans.

Organophosphorus (OP) nerve agents are potent suicide inhibitors of the essential neurotransmitter-regulating enzyme acetylcholinesterase. Due to their acute toxicity, there is significant interest in developing effective countermeasures to OP poisoning. Here we impart nerve agent hydrolysis activity into the human drug metabolism enzyme carboxylesterase 1. Using crystal structures of the target enzyme in complex with nerve agent as a guide, a pair of histidine and glutamic acid residues were designed proximal to the enzyme's native catalytic triad. The resultant variant protein demonstrated significantly increased rates of reactivation following exposure to sarin, soman, and cyclosarin. Importantly, the addition of these residues did not alter the high affinity binding of nerve agents to this protein. Thus, using two amino acid substitutions, a novel enzyme was created that efficiently converted a group of hemisubstrates, compounds that can start but not complete a reaction cycle, into bona fide substrates. Such approaches may lead to novel countermeasures for nerve agent poisoning.

Organophosphorus (OP) nerve agents are potent toxins that inhibit cholinesterases and produce a rapid and lethal cholinergic crisis. Development of protein-based therapeutics is being pursued with the goal of preventing nerve agent toxicity and protecting against the long-term side effects of these agents. The drug-metabolizing enzyme human carboxylesterase 1 (hCE1) is a candidate protein-based therapeutic because of its similarity in structure and function to the cholinesterase targets of nerve agent poisoning. However, the ability of wild-type hCE1 to process the G-type nerve agents sarin and cyclosarin has not been determined. We report the crystal structure of hCE1 in complex with the nerve agent cyclosarin. We further use stereoselective nerve agent analogs to establish that hCE1 exhibits a 1700- and 2900-fold preference for the P(R) enantiomers of analogs of soman and cyclosarin, respectively, and a 5-fold preference for the P(S) isomer of a sarin analog. Finally, we show that for enzyme inhibited by racemic mixtures of bona fide nerve agents, hCE1 spontaneously reactivates in the presence of sarin but not soman or cyclosarin. The addition of the neutral oxime 2,3-butanedione monoxime increases the rate of reactivation of hCE1 from sarin inhibition by more than 60-fold but has no effect on reactivation with the other agents examined. Taken together, these data demonstrate that hCE1 is only reactivated after inhibition with the more toxic P(S) isomer of sarin. These results provide important insights toward the long-term goal of designing novel forms of hCE1 to act as protein-based therapeutics for nerve agent detoxification.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT Association of UDP-glucuronosyltransferase 1A1 (UGT1A1) genetic polymorphisms *6 and *28 with reduced clearance of SN-38 and severe neutropenia in irinotecan therapy was demonstrated in Japanese cancer patients. * The detailed gene structure of CES1 has been characterized. * Possible functional SNPs in the promoter region have been reported. WHAT THIS STUDY ADDS * Association of functional CES1 gene number with AUC ratio [(SN-38 + SN-38G)/irinotecan], an in vivo index of CES activity, was observed in patients with irinotecan monotherapy. * No significant effects of major CES1 SNPs on irinotecan PK were detected. AIMS Human carboxylesterase 1 (CES1) hydrolyzes irinotecan to produce an active metabolite SN-38 in the liver. The human CES1 gene family consists of two functional genes, CES1A1 (1A1) and CES1A2 (1A2), which are located tail-to-tail on chromosome 16q13-q22.1 (CES1A2-1A1). The pseudogene CES1A3 (1A3) and a chimeric CES1A1 variant (var1A1) are also found as polymorphic isoforms of 1A2 and 1A1, respectively. In this study, roles of CES1 genotypes and major SNPs in irinotecan pharmacokinetics were investigated in Japanese cancer patients. METHODS CES1A diplotypes [combinations of haplotypes A (1A3-1A1), B (1A2-1A1), C (1A3-var1A1) and D (1A2-var1A1)] and the major SNPs (-75T>G and -30G>A in 1A1, and -816A>C in 1A2 and 1A3) were determined in 177 Japanese cancer patients. Associations of CES1 genotypes, number of functional CES1 genes (1A1, 1A2 and var1A1) and major SNPs, with the AUC ratio of (SN-38 + SN-38G)/irinotecan, a parameter of in vivo CES activity, were analyzed for 58 patients treated with irinotecan monotherapy. RESULTS The median AUC ratio of patients having three or four functional CES1 genes (diplotypes A/B, A/D or B/C, C/D, B/B and B/D; n= 35) was 1.24-fold of that in patients with two functional CES1 genes (diplotypes A/A, A/C and C/C; n= 23) [median (25th-75th percentiles): 0.31 (0.25-0.38) vs. 0.25 (0.20-0.32), P= 0.0134]. No significant effects of var1A1 and the major SNPs examined were observed. CONCLUSION This study suggests a gene-dose effect of functional CES1A genes on SN-38 formation in irinotecan-treated Japanese cancer patients.

Methylphenidate (MPH) is the most frequently prescribed drug in the treatment of attention deficit hyperactivity disorder (ADHD). Several pharmacogenetic studies suggested that catecholamine candidate genes influence individual MPH-responses, but these results are mostly contradictory. Genetic analyses of MPH metabolizing carboxylesterase 1 enzyme (CES1) have not been carried out, whereas, meta-analysis of CYP2D6 genetic variants has been already indicated significant pharmacogenetic differences in atomoxetine treatment. Here we present an association analysis of the CES1 Gly143Glu functional polymorphism in a Hungarian ADHD group (n = 173). The genotype frequencies were similar to that of the general population (5.8% vs 4.1% of Gly/Glu heterozygote). Pharmacogenetic analysis was conducted among 122 ADHD children treated with MPH. Neither the categorical analysis comparing 90 responders vs 32 non-responders, nor the dimensional analysis of Inattention and Hyperactivity-Impulsivity score reduction showed a significant main genotype effect. However, analyzing the daily dose, we observed an association with the rare 143Glu-variant: 5 patients in the responder group carrying the Glu-allele required lower doses of MPH for symptom reduction (0.410 +/- 0.127 vs 0.572 +/- 0.153 mg/kg, t(1,88) = 2.33, p = 0.022). This result warrants for further investigations of the CES1 gene in larger ADHD samples.

Oseltamivir phosphate is an ethyl ester prodrug widely used in the treatment and prevention of both Influenzavirus A and B infections. The conversion of oseltamivir to its active metabolite oseltamivir carboxylate is dependent on ester hydrolysis mediated by carboxylesterase 1 (CES1). We recently identified two functional CES1 variants p.Gly143Glu and p.Asp260fs in a research subject who displayed significant impairment in his ability to metabolize the selective CES1 substrate, methylphenidate. In vitro functional studies demonstrated that the presence of either of the two mutations can result in severe reductions in the catalytic efficiency of CES1 toward methylphenidate, which is required for hydrolysis and pharmacological deactivation. The aim of the present study was to investigate the function of these mutations on activating (hydrolyzing) oseltamivir to oseltamivir carboxylate using the cell lines expressing wild type (WT) and each mutant CES1. In vitro incubation studies demonstrated that the S9 fractions prepared from the cells transfected with WT CES1 and human liver tissues rapidly convert oseltamivir to oseltamivir carboxylate. However, the catalytic activity of the mutant hydrolases was dramatically hindered. The V(max) value of p.Gly143Glu was approximately 25% of that of WT enzyme, whereas the catalytic activity of p.Asp260fs was negligible. These results suggest that the therapeutic efficacy of oseltamivir could be compromised in treated patients expressing either functional CES1 mutation. Furthermore, the potential for increased adverse effects or toxicity as a result of exposure to high concentrations of the nonhydrolyzed prodrug should be considered.

The human carboxylesterase 1 (CES1) gene encodes for the enzyme carboxylesterase 1, a serine esterase governing both metabolic deactivation and activation of numerous therapeutic agents. During the course of a study of the pharmacokinetics of the methyl ester racemic psychostimulant methylphenidate, profoundly elevated methylphenidate plasma concentrations, unprecedented distortions in isomer disposition, and increases in hemodynamic measures were observed in a subject of European descent. These observations led to a focused study of the subject's CES1 gene. DNA sequencing detected two coding region single-nucleotide mutations located in exons 4 and 6. The mutation in exon 4 is located in codon 143 and leads to a nonconservative substitution, p.Gly143Glu. A deletion in exon 6 at codon 260 results in a frameshift mutation, p.Asp260fs, altering residues 260-299 before truncating at a premature stop codon. The minor allele frequency of p.Gly143Glu was determined to be 3.7%, 4.3%, 2.0%, and 0% in white, black, Hispanic, and Asian populations, respectively. Of 925 individual DNA samples examined, none carried the p.Asp260fs, indicating it is an extremely rare mutation. In vitro functional studies demonstrated the catalytic functions of both p.Gly143Glu and p.Asp260fs are substantially impaired, resulting in a complete loss of hydrolytic activity toward methylphenidate. When a more sensitive esterase substrate, p-nitrophenyl acetate was utilized, only 21.4% and 0.6% catalytic efficiency (V(max)/K(m)) were determined in p.Gly143Glu and p.Asp260fs, respectively, compared to the wild-type enzyme. These findings indicate that specific CES1 gene variants can lead to clinically significant alterations in pharmacokinetics and drug response of carboxylesterase 1 substrates.

The organophosphorus nerve agents sarin, soman, tabun, and VX exert their toxic effects by inhibiting the action of human acetylcholinesterase, a member of the serine hydrolase superfamily of enzymes. The current treatments for nerve agent exposure must be administered quickly to be effective, and they often do not eliminate long-term toxic side effects associated with organophosphate poisoning. Thus, there is significant need for effective prophylactic methods to protect at-risk personnel from nerve agent exposure, and protein-based approaches have emerged as promising candidates. We present the 2.7 A resolution crystal structures of the serine hydrolase human carboxylesterase 1 (hCE1), a broad-spectrum drug metabolism enzyme, in covalent acyl-enzyme intermediate complexes with the chemical weapons soman and tabun. The structures reveal that hCE1 binds stereoselectively to these nerve agents; for example, hCE1 appears to react preferentially with the 10(4)-fold more lethal PS stereoisomer of soman relative to the PR form. In addition, structural features of the hCE1 active site indicate that the enzyme may be resistant to dead-end organophosphate aging reactions that permanently inactivate other serine hydrolases. Taken together, these data provide important structural details toward the goal of engineering hCE1 into an organophosphate hydrolase and protein-based therapeutic for nerve agent exposure.

Human carboxylesterase 1 (hCE1) is a drug and endobiotic-processing serine hydrolase that exhibits relatively broad substrate specificity. It has been implicated in a variety of endogenous cholesterol metabolism pathways including the following apparently disparate reactions: cholesterol ester hydrolysis (CEH), fatty acyl Coenzyme A hydrolysis (FACoAH), acyl-Coenzyme A:cholesterol acyltransfer (ACAT), and fatty acyl ethyl ester synthesis (FAEES). The structural basis for the ability of hCE1 to perform these catalytic actions involving large substrates and products has remained unclear. Here we present four crystal structures of the hCE1 glycoprotein in complexes with the following endogenous substrates or substrate analogues: Coenzyme A, the fatty acid palmitate, and the bile acids cholate and taurocholate. While the active site of hCE1 was known to be promiscuous and capable of interacting with a variety of chemically distinct ligands, these structures reveal that the enzyme contains two additional ligand-binding sites and that each site also exhibits relatively non-specific ligand-binding properties. Using this multisite promiscuity, hCE1 appears structurally capable of assembling several catalytic events depending, apparently, on the physiological state of the cellular environment. These results expand our understanding of enzyme promiscuity and indicate that, in the case of hCE1, multiple non-specific sites are employed to perform distinct catalytic actions.

Carboxylesterases hydrolyze a large array of endogenous and exogenous ester-containing compounds, including pyrethroid insecticides. Herein, we report the specific activities and kinetic parameters of human carboxylesterase (hCE)-1 and hCE-2 using authentic pyrethroids and pyrethroid-like, fluorescent surrogates. Both hCE-1 and hCE-2 hydrolyzed type I and II pyrethroids with strong stereoselectivity. For example, the trans-isomers of permethrin and cypermethrin were hydrolyzed much faster than corresponding cis-counterparts by both enzymes. Kinetic values of hCE-1 and hCE-2 were determined using cypermethrin and 11 stereoisomers of the pyrethroid-like, fluorescent surrogates. K(m) values for the authentic pyrethroids and fluorescent surrogates were in general lower than those for other ester-containing substrates of hCEs. The pyrethroid-like, fluorescent surrogates were hydrolyzed at rates similar to the authentic pyrethroids by both enzymes, suggesting the potential of these compounds as tools for high throughput screening of esterases that hydrolyze pyrethroids.

Human carboxylesterase 1 (hCE1) exhibits broad substrate specificity and is involved in xenobiotic processing and endobiotic metabolism. We present and analyze crystal structures of hCE1 in complexes with the cholesterol-lowering drug mevastatin, the breast cancer drug tamoxifen, the fatty acyl ethyl ester (FAEE) analogue ethyl acetate, and the novel hCE1 inhibitor benzil. We find that mevastatin does not appear to be a substrate for hCE1, and instead acts as a partially non-competitive inhibitor of the enzyme. Similarly, we show that tamoxifen is a low micromolar, partially non-competitive inhibitor of hCE1. Further, we describe the structural basis for the inhibition of hCE1 by the nanomolar-affinity dione benzil, which acts by forming both covalent and non-covalent complexes with the enzyme. Our results provide detailed insights into the catalytic and non-catalytic processing of small molecules by hCE1, and suggest that the efficacy of clinical drugs may be modulated by targeted hCE1 inhibitors.

Egasyn-beta-glucuronidase complex is located at the luminal site of liver microsomal endoplasmic reticulum. When organophosphorus insecticides (OP) are incorporated into the liver microsomes, they become tightly bound to egasyn, a carboxylesterase isozyme, and subsequently, beta-glucuronidase (BG) is dissociated and released into blood. Consequently, the increase in plasma BG activity becomes a good biomarker of OP exposure. Thus, the single administration of EPN (O-ethyl O-p-nitrophenylphenylphosphonothioate), acephate and chlorpyrifos increased plasma BG activity in approximately 100-fold the control level in rats. The increase in plasma BG activity after OP exposure is a much more sensitive biomarker of acute OP exposure than acetylcholinesterase (AChE) inhibition.

Imidapril is an angiotensin-converting enzyme inhibitor that is widely used in treating hypertension, although the responses vary among individuals. We investigated whether a single nucleotide polymorphism at position -816 of the carboxylesterase 1 (CES1) gene, which activates imidapril in the liver, is involved in the responsiveness to imidapril medication. A total of 105 Japanese hypertensives with systolic/diastolic blood pressures (SBP/DBP) of 140/90 mmHg or higher were prescribed 5-10 mg/day of imidapril. At baseline, blood pressure levels were not different between patients with and those without the -816C allele (AA vs. AC+ CC groups). After 8 weeks of treatment, we classified the responders and non-responders based on the decline in their blood pressures, and found that the responder rate was significantly higher in the AC+CC group than in the AA group (p=0.0331). Also, the reduction in SBP was significantly greater in the AC+CC group than in the AA group (24.7+/-11.8 vs. 17.6+/-16.8 mmHg, p=0.0184). Furthermore, an in vitro reporter assay revealed that the -816C construct had significantly higher promoter activity (p<0.0001). These findings suggest that the A(-816)C polymorphism affects the transcriptional activity, and that this may account for the responsiveness to imidapril.

Capecitabine is an oral prodrug of 5-fluorouracil that is indicated for the treatment of breast and colorectal cancers. A three-step in vivo-targeted activation process requiring carboxylesterases, cytidine deaminase, and thymidine phosphorylase converts capecitabine to 5-fluorouracil. Carboxylesterases hydrolyze capecitabine's carbamate side chain to form 5'-deoxy-5-fluorocytidine (5'-DFCR). This study examines the steady-state kinetics of recombinant human carboxylesterase isozymes carboxylesterase (CES) 1A1, CES2, and CES3 for hydrolysis of capecitabine with a liquid chromatography/mass spectroscopy assay. Additionally, a spectrophotometric screening assay was utilized to identify drugs that may inhibit carboxylesterase activation of capecitabine. CES1A1 and CES2 hydrolyze capecitabine to a similar extent, with catalytic efficiencies of 14.7 and 12.9 min(-1) mM(-1), respectively. Little catalytic activity is detected for CES3 with capecitabine. Northern blot analysis indicates that relative expression in intestinal tissue is CES2 > CES1A1 > CES3. Hence, intestinal activation of capecitabine may contribute to its efficacy in colon cancer and toxic diarrhea associated with the agent. Loperamide is a strong inhibitor of CES2, with a K(i) of 1.5 muM, but it only weakly inhibits CES1A1 (IC(50) = 0.44 mM). Inhibition of CES2 in the gastrointestinal tract by loperamide may reduce local formation of 5'-DFCR. Both CES1A1 and CES2 are responsible for the activation of capecitabine, whereas CES3 plays little role in 5'-DFCR formation.

Recent scientific advances have revealed the identity of several enzymes involved in the synthesis, storage and catabolism of intracellular neutral lipid storage droplets. An enzyme that hydrolyzes stored triacylglycerol (TG), triacylglycerol hydrolase (TGH), was purified from porcine, human and murine liver microsomes. In rodents, TGH is highly expressed in liver as well as heart, kidney, small intestine and adipose tissues, while in humans TGH is mainly expressed in the liver, adipose and small intestine. TGH localizes to the endoplasmic reticulum and lipid droplets. The TGH genes are located within a cluster of carboxylesterase genes on human and mouse chromosomes 16 and 8, respectively. TGH hydrolyzes stored TG, and in the liver, the lipolytic products are made available for VLDL-TG synthesis. Inhibition of TGH activity also inhibits TG and apolipoprotein B secretion by primary hepatocytes. A role for TGH in basal TG lipolysis in adipocytes has been proposed. TGH expression and activity is both developmentally and hormonally regulated. A model for the function of TGH is presented and discussed with respect to tissue specific functions.

Human carboxylesterases 1 and 2 (CES1 and CES2) catalyze the hydrolysis of many exogenous compounds. Alterations in carboxylesterase sequences could lead to variability in both the inactivation of drugs and the activation of prodrugs. We resequenced CES1 and CES2 in multiple populations (n = 120) to identify single-nucleotide polymorphisms and confirmed the novel SNPs in healthy European and African individuals (n = 190). Sixteen SNPs were found in CES1 (1 per 300 bp) and 11 in CES2 (1 per 630 bp) in at least one population. Allele frequencies and estimated haplotype frequencies varied significantly between African and European populations. No association between SNPs in CES1 or CES2 was found with respect to RNA expression in normal colonic mucosa; however, an intronic SNP (IVS10-88) in CES2 was associated with reduced CES2 mRNA expression in colorectal tumors. Functional analysis of the novel polymorphisms described in this study is now warranted to identify putative roles in drug metabolism.

Human chromosome 16 features one of the highest levels of segmentally duplicated sequence among the human autosomes. We report here the 78,884,754 base pairs of finished chromosome 16 sequence, representing over 99.9% of its euchromatin. Manual annotation revealed 880 protein-coding genes confirmed by 1,670 aligned transcripts, 19 transfer RNA genes, 341 pseudogenes and three RNA pseudogenes. These genes include metallothionein, cadherin and iroquois gene families, as well as the disease genes for polycystic kidney disease and acute myelomonocytic leukaemia. Several large-scale structural polymorphisms spanning hundreds of kilobase pairs were identified and result in gene content differences among humans. Whereas the segmental duplications of chromosome 16 are enriched in the relatively gene-poor pericentromere of the p arm, some are involved in recent gene duplication and conversion events that are likely to have had an impact on the evolution of primates and human disease susceptibility.

As a base for human transcriptome and functional genomics, we created the "full-length long Japan" (FLJ) collection of sequenced human cDNAs. We determined the entire sequence of 21,243 selected clones and found that 14,490 cDNAs (10,897 clusters) were unique to the FLJ collection. About half of them (5,416) seemed to be protein-coding. Of those, 1,999 clusters had not been predicted by computational methods. The distribution of GC content of nonpredicted cDNAs had a peak at approximately 58% compared with a peak at approximately 42%for predicted cDNAs. Thus, there seems to be a slight bias against GC-rich transcripts in current gene prediction procedures. The rest of the cDNAs unique to the FLJ collection (5,481) contained no obvious open reading frames (ORFs) and thus are candidate noncoding RNAs. About one-fourth of them (1,378) showed a clear pattern of splicing. The distribution of GC content of noncoding cDNAs was narrow and had a peak at approximately 42%, relatively low compared with that of protein-coding cDNAs.

Human carboxylesterase 1 (hCE1) is a broad-spectrum bioscavenger that plays important roles in narcotic metabolism, clinical prodrug activation, and the processing of fatty acid and cholesterol derivatives. We determined the 2.4 A crystal structure of hCE1 in complex with tacrine, the first drug approved for treating Alzheimer's disease, and compare this structure to the Torpedo californica acetylcholinesterase (AcChE)-tacrine complex. Tacrine binds in multiple orientations within the catalytic gorge of hCE1, while it stacks in the smaller AcChE active site between aromatic side chains. Our results show that hCE1's promiscuous action on distinct substrates is enhanced by its ability to interact with ligands in multiple orientations at once. Further, we use our structure to identify tacrine derivatives that act as low-micromolar inhibitors of hCE1 and may provide new avenues for treating narcotic abuse and cholesterol-related diseases.

We present the first crystal structures of a human protein bound to analogs of cocaine and heroin. Human carboxylesterase 1 (hCE1) is a broad-spectrum bioscavenger that catalyzes the hydrolysis of heroin and cocaine, and the detoxification of organophosphate chemical weapons, such as sarin, soman and tabun. Crystal structures of the hCE1 glycoprotein in complex with the cocaine analog homatropine and the heroin analog naloxone provide explicit details about narcotic metabolism in humans. The hCE1 active site contains both specific and promiscuous compartments, which enable the enzyme to act on structurally distinct chemicals. A selective surface ligand-binding site regulates the trimer-hexamer equilibrium of hCE1 and allows each hCE1 monomer to bind two narcotic molecules simultaneously. The bioscavenger properties of hCE1 can likely be used to treat both narcotic overdose and chemical weapon exposure.

Triacylglycerol hydrolase is a microsomal enzyme that hydrolyzes stored cytoplasmic triacylglycerol in the liver and participates in the lipolysis/re-esterification cycle during the assembly of very-low-density lipoproteins. The structure-activity relationship of the enzyme was investigated by site-directed mutagenesis and heterologous expression. Expression of human TGH in Escherichia coli yields a protein without enzymatic activity, which suggests that posttranslational processing is necessary for the catalytic activity. Expression in baculovirus-infected Sf-9 cells resulted in correct processing of the N-terminal signal sequence and yielded a catalytically active enzyme. A putative catalytic triad consisting of a nucleophilic serine (S221), glutamic acid (E354), and histidine (H468) was identified. Site-directed mutagenesis of the residues (S221A, E354A, and H468A) yielded a catalytically inactive enzyme. CD spectra of purified mutant proteins were very similar to that of the wild-type enzyme, which suggests that the mutations did not affect folding. Human TGH was glycosylated in the insect cells. Mutagenesis of the putative N-glycosylation site (N79A) yielded an active nonglycosylated enzyme. Deletion of the putative C-terminal endoplasmic reticulum retrieval signal (HIEL) did not result in secretion of the mutant protein. A model of human TGH structure suggested a lipase alpha/beta hydrolase fold with a buried active site and two disulfide bridges (C87-C116 and C274-C285).

Triacylglycerol hydrolase mobilizes stored triacylglycerol some of which is used for very-low-density lipoprotein assembly in the liver. A full-length cDNA coding for a human triacylglycerol hydrolase (hTGH) was isolated from a human liver cDNA library. The cDNA has an open reading frame of 576 amino acids with a cleavable 18-amino-acid signal sequence. The deduced amino acid sequence shows that the protein belongs to the carboxylesterase family. The hTGH was highly expressed in Escherichia coli as a 6xHis-tagged fusion protein, with the tag at the N-terminus in place of the signal peptide. However, the expressed protein was insoluble and inactive. Expression was confirmed by immunoblotting and N-terminal amino acid sequencing of the purified protein. Expression of hTGH with its native signal sequence and a C-terminal 6xHis-tag in Sf9 cells using the baculovirus expression system yielded active enzyme. N-terminal amino acid sequencing of the purified expressed protein showed correct processing of the signal peptide. The enzyme also undergoes glycosylation within the endoplasmic reticulum lumen. The results suggest that hTGH expressed in insect cells is properly folded. Therefore, baculovirus expression of hTGH and facile purification of the His-tagged enzyme will allow detailed characterization of the structure/activity relationship.

The sensitive technique of RT-PCR was used to identify cholesteryl ester hydrolase (CEH) expressed in human macrophages. This enzyme is thought to regulate the availability of intracellular free cholesterol for efflux. The expected 667-bp product was obtained starting with RNA from human peripheral blood and THP-1 monocytes and macrophages. The cDNA for human macrophage CEH was then cloned by PCR-based screening of a lambda-gt11 cDNA library. The full-length cDNA was sequenced and found to exhibit 76% homology (at the nucleotide and conceptually translated protein level) to hepatic CEH, an enzyme shown to be involved in hepatic cholesterol homeostasis and regulated by cholesterol at the transcription level via sterol response elements in the proximal promoter. Identification of the conserved catalytic triad (Ser(221, His(468), and Glu354)) and the SEDCLY motif places human macrophage CEH in the family of carboxylesterases. A greater than 20-fold increase in CEH activity was observed when COS-1 and COS-7 cells were transiently transfected with an eukaryotic expression vector, pcDNA3.1/V5/His-TOPO, containing the cDNA for human macrophage CEH. Using this full-length cDNA as a probe, a 2.2-kb transcript was identified by Northern blot analysis of total RNA from human peripheral blood and THP -1 macrophages. Overexpression of human macrophage CEH resulted in an impairment of upregulation of low-density lipoprotein (LDL) receptor mRNA in Chinese hamster ovary (CHO-K1) cells grown in cholesterol-deficient environment. These data identify the human macrophage CEH, demonstrate its expression in human peripheral blood macrophage and human macrophage cell line, THP-1, and suggest its role in the intracellular cholesteryl ester metabolism.

Serious intoxications and incidences due to misuse of organophosphorus insecticides (OP) have been reported for over three decades. In this meeting I am talking about the following three topics. (1) Epidemiological studies on the use of OP in Japan, Taiwan and Thailand. The National Research Institute of Police Science (NRIPS) in Japan has published the annual report of the epidemiological studies on the criminal and suicide events. The numbers of the incidence of pesticides are approximately 22%-30% of the total incidences, during 1991 and 1996. The incidence of pesticides has been gradually reduced for past six years. This seems to be influenced by the decrease of the incidences of paraquat having extremely high toxicity. According to the epidemiological data in the National Poison Center in Taiwan, unlike the prevalence of drug poisoning exposures in most western countries, pesticides poisoning exposures are numerous in Taiwan. The number of pesticide intoxication cases is 6,872 out of 23,436 of total case numbers of various intoxications during the 1985 and 1997. The fatality rate is much higher in Taiwan than in most western countries, with 5.65% of all poisoning exposures resulted in death. Paraquot is the leading cause of death in Taiwan. The ratio of death to total OP intoxication cases is 11.5% including accidental exposure and suicide. According to the report of the Division of Epidemiology, Ministry of Public Health, Thailand during the 1990-1995, the overall morbidity due to poisoning was 30 per 100,000 population with 300 deaths per year. The morbidity rate of reported pesticide poisoning in the whole country was 10 per 100,000 population. (2) Sarin victims in Tokyo. On March 20, 1995, a terrorist attack using sarin (isopropylmethylphosphonofluoridate) occurred on the Tokyo subway. Many people inhaled the sarin gas and collapsed in the train. Eventually, 12 people died and over 5,000 were injured. The results of the forensic studies showed that methylphosphonic acid, the hydrolysis product of sarin, bound to AChE in the cerebellums of the victims was separated and identified using GCMS. (3) A sensitive and rapid biomarker of OP-poisoning. Beta-glucuronidase (beta-G) is loosely bound to egasyn which is one of carboxylesterase isozymes in the liver microsomes. The oxon formed from OP in the liver microsomes is covalently bound to egasyn and causes the cleavage of the egasyn-glucuronidase complex, leading to the release of beta-G into the blood. In fact, plasma beta-G activities is a much more sensitive and rapid biomarker than the blood cholinesterase inhibition. In conclusion, OPs cause serious toxicity in both acute and chronic exposures. The epidemiological data in Taiwan and Thailand show that paraquat intoxication cause the most serious irreversible damage in the patients. In 1995, terrorists attack using sarin which is one of the most toxic OP, on the Tokyo subway, and 12 people died. This is an example of the acute intoxication of OP and some people have been still in hospitalized. Recently, the present authors have reported that the plasma beta-G activity is a novel biomarker of OP intoxication. The plasma beta-G is rapidly and significantly increased after OP exposure, and this is a much more sensitive to OP intoxication than ChE inhibition.

Lysosomal beta-glucuronidase shows a dual localization in mouse liver, where a significant fraction is retained in the endoplasmic reticulum (ER) by interaction with an ER-resident carboxyl esterase called egasyn. This interaction of mouse egasyn (mEg) with murine beta-glucuronidase (mGUSB) involves binding of the C-terminal 8 residues of the mGUSB to the carboxylesterase active site of the mEg. We isolated the recombinant human homologue of the mouse egasyn cDNA and found that it too binds human beta-glucuronidase (hGUSB). However, the binding appears not to involve the active site of the human egasyn (hEg) and does not involve the C-terminal 18 amino acids of hGUSB. The full-length cDNA encoding hEg was isolated from a human liver cDNA library using full-length mEg cDNA as a probe. The 1941-bp cDNA differs by only a few bases from two previously reported cDNAs for human liver carboxylesterase, allowing the anti-human carboxylesterase antiserum to be used for immunoprecipitation of human egasyn. The cDNA expressed bis-p-nitrophenyl phosphate (BPNP)-inhibitable esterase activity in COS cells. When expressed in COS cells, it is localized to the ER. The intracellular hEg coimmunoprecipitated with full-length hGUSB and with a truncated hGUSB missing the C-terminal 18-amino-acid residue when extracts of COS cells expressing both proteins were treated with anti-hGUSB antibody. It did not coimmunoprecipitate with mGUSB from extracts of coexpressing COS cells. Unlike mEg, hEg was not released from the hEg-GUSB complex with BPNP. Thus, hEg resembles mEg in that it binds hGUSB. However, it differs from mEg in that (i) it does not appear to use the esterase active site for binding since treatment with BPNP did not release hEg from hGUSB and (ii) it does not use the C terminus of GUSB for binding, since a C-terminal truncated hGUSB (the C-terminal 18 amino acids are removed) bound as well as nontruncated hGUSB. Evidence is presented that an internal segment of 51 amino acids between 228 and 279 residues contributes to binding of hGUSB by hEg.

The DNA sequence encoding a novel human brain carboxylesterase (CES) has been determined. The protein is predicted to have 567 amino acids, including conserved motifs, such as GESAGG, GXXXXEFG, and GDHGD which comprise a catalytic triad, and the endoplasmic reticulum retention motif (HXEL-COOH) observed in CES families. Their gene products exhibited hydrolase activity towards temocapril, p-nitrophenyl-acetate and long-chain acyl-CoA. Since the molecular masses of these gene products are similar to those that exist in capillary endothelial cells of the human brain [Yamamda et al. (1994) Brain Res. 658, 163-167], these CES isozymes may function as a blood-brain barrier to protect the central nervous system from ester or amide compounds.

The expression and androgen regulation of egasyn, the endoplasmic reticulum-targeting protein of beta-D-glucuronidase, was examined in the mouse-epididymis. The proximal (caput) and distal (corpus & cauda) epididymal tissue extracts were prepared by homogenization and sonication in buffered Triton X-100 solution, and high speed centrifugation. The supernatant when resolved by 2D-PAGE under non-denaturing conditions and stained for esterase activity showed that the distal (but not proximal) epididymis of the normal mouse contain several specific forms of esterases. These forms include a series of four variants (pI 5.2-5.75) with high mobility (HM) and esterase activity, and three faintly staining variants (beginning at pI 6.0) with low mobility (LM). Several lines of evidence indicate that the specific esterases seen in the corpus/cauda epididymidis are egasyn-esterases. Firstly, these molecular forms were not seen in the distal epididymal extracts from the egasyn-deficient mouse. Secondly, the HM forms can be immunoprecipitated with anti-egasyn antibody, suggesting the presence of free egasyn. Finally, the LM forms disappeared after heat treatment (56 degrees C for 8 min), a condition known to dissociate egasyn:beta-D-glucuronidase complex. This result indicates that a small amount of egasyn is complexed with beta-D-glucuronidase. Immunoblotting (Western blot) studies (using anti-egasyn antibody) following resolution of egasyn released from the egasyn:beta-D-glucuronidase complex revealed a single band of an apparent molecular weight 64 kDa in the distal (but not proximal) epididymis, indicating that the mouse epididymal egasyn is identical or very similar to the liver egasyn. Castration of mice lead to the appearance of free and complexed egasyn forms in the proximal epididymis. Testosterone supplementation to the castrated mice resulted in the disappearance of the induced egasyn forms from the caput epididymidis. Taken together, these results indicate that the expression of egasyn in the epididymis is region-specific and is differentially regulated by androgens.

Hepatic retinyl ester hydrolase (REH) activity was isolated from porcine and human liver and characterized, and some of its properties were compared with those of other retinyl-ester-splitting enzymes. Sequence analysis revealed that the REH proteins are structurally similar to non-specific carboxylesterases and distinct from bile salt-activated lipases and cholesterol esterases. Pig REH, a 64-kDa protein, hydrolyzed retinyl palmitate at a rate of 595 nmol x h(-1) x mg(-1) protein in the presence of 100 mM Chaps with an apparent Km value for retinyl palmitate of 27.5 microM. The pH optimum was 7.0-9.2. Its human counterpart has a molecular mass of 65 kDa and a pH optimum near 6.5. In the presence of Chaps, pig REH activity was stimulated up to 1.7-fold by various non-ionic detergents. The ranking order of retinyl palmitate cleavage initiated by the stimuli was n-dodecylglucoside > octanoyl-N-methylglucamide > n-octyglucoside > n-dodecylmaltoside > Triton X-114 > Triton X-100. Porcine REH was effectively inhibited by alpha-tocopherol and bis-(4-nitrophenyl) phosphate [(Np)2P]. The structural, immunological and catalytic features, pH dependence, and the effect of (Np)2P on enzyme activity of pig REH are similar to those reported for the non-specific carboxylesterase ES-4. However, ES-4 differed from REH in molecular mass and the requirement of Chaps or Chaps-like detergents as cofactor. Judging from these results, pig REH may be a non-specific carboxylesterase isoform.

A human liver carboxylesterase (hCE-2) that catalyzes the hydrolysis of the benzoyl group of cocaine and the acetyl groups of 4-methylumbelliferyl acetate, heroin, and 6-monoacetylmorphine was purified from human liver. The purified enzyme exhibited a single band on SDS-polyacrylamide gel electrophoresis with a subunit mass of approximately 60 kDa. The native enzyme was monomeric. The isoelectric point of hCE-2 was approximately 4.9. Treatment with endoglycosidase H caused an increase in electrophoretic mobility indicating that the liver carboxylesterase was a glycoprotein of the high mannose type. The complete cDNA nucleotide sequence was determined. The authenticity of the cDNA was confirmed by a perfect sequence match of 78 amino acids derived from the hCE-2 purified from human liver. The mature 533-amino acid enzyme encoded by this cDNA shared highest sequence identity with the rabbit liver carboxylesterase form 2 (73%) and the hamster liver carboxylesterase AT51p (67%). Carboxylesterases with high sequence identity to hCE-2 have not been reported in mouse and rat liver. hCE-2 exhibited different drug ester substrate specificity from the human liver carboxylesterase called hCE-1, which hydrolyzes the methyl ester of cocaine. hCE-2 had higher catalytic efficiencies for hydrolysis of 4-methylumbelliferyl acetate, heroin, and 6-monoacetylmorphine and greater inhibition by eserine than hCE-1. hCE-2 may play an important role in the degradation of cocaine and heroin in human tissues.

The expression and androgen regulation of beta-glucuronidase molecular forms were examined in mouse epididymis, liver, and kidney. Two-dimensional polyacrylamide gel electrophoresis performed under nondenaturing conditions showed that, compared to liver and kidney, which contain four microsomal (M1-M4) and a major lysosomal (L) form of beta-glucuronidase, the epididymis revealed regional differences and tissue specificity in the expression of the various molecular forms of the enzyme. Only the lysosomal form (pI 5.4-6.1) is present in the caput epididymidis while the corpus/cauda contains the lysosomal form, the free X form (pI 5.9-6.3) and the four microsomal forms (X form complexed with egasyn). Mutant mice that lack egasyn have no microsomal forms in the distal epididymis. In epididymal fluid, the lysosomal form is found throughout the epididymis, whereas the X form appears only in the corpus/cauda epididymidis. Sodium dodecyl Sulfate (SDS)-gel electrophoresis and western blot analysis of immunoprecipitated beta-glucuronidase revealed only one band corresponding to the L form (apparent molecular weight 74 kDa) in the caput epididymidis and two bands in the corpus/cauda (apparent molecular weights 73 and 75 kDa), corresponding to L and X forms, respectively. Castration of mice led to the suppression of the regional differences in the appearance of X and M forms in the epididymis. Testosterone supplementation to castrated mice restored the characteristic electrophoretic pattern of beta-glucuronidase in the caput epididymidis. In the liver and kidney, castration has no effect on the expression of the molecular forms, whereas androgen treatment induced the X form in the kidney. Histochemical localization of beta-glucuronidase confirmed the region specificity seen in the epididymis and in addition revealed cell specificity in the expression of beta-glucuronidase. These results indicate that beta-glucuronidase shows tissue specificity and, in the case of the epididymis, region and cell specificity. In addition, the enzyme in the different tissues responds differentially to androgens.

Bile salt stimulated cholesterol esterase is predominantly synthesized in the pancreas. However, this enzyme is also synthesized by the liver and was found to be present in plasma. The physiologic role of the systemic cholesterol esterase has not been clearly defined. In the current study, the human hepatoma cell line HepG2 was used as a model to determine the role of cholesterol esterase on hepatic uptake of high density lipoprotein (HDL)-associated cholesteryl esters. The results showed that hepatic uptake of the cholesteryl esters analog [3H]cholesteryl ether on reconstituted HDL was inhibited by anti-cholesterol esterase antibodies. The HDL-associated cholesteryl ester transported to HepG2 cells was also increased 2-fold in the presence of taurocholate, an activator of the cholesterol esterase. These results suggest that liver-derived cholesterol esterase may play an important role in cellular uptake of cholesteryl esters from HDL. This hypothesis was supported by demonstrating the ability of exogenously added cholesterol esterase to further enhance hepatic uptake of HDL-associated cholesteryl esters. The results of the current study also showed that cholesterol esterase increased free-to-esterified cholesterol ratio in the lipoprotein. Thus, alteration of HDL structure and composition contributes to the cholesterol esterase-induced cellular uptake of HDL-associated cholesteryl esters. On the basis of these observations, we propose that liver-derived cholesterol esterase may play an important role in lipoprotein metabolism.

The involvement of carboxylesterase, acetylcholinesterase, butyrylcholinesterase and cholesterol esterase in pharmacology and toxicology are well recognized. However, there are few papers concerning the comparative studies of these serine hydrolases in terms of molecular level. Recently, we have studied various aspects of carboxylesterases using cDNAs of carboxylesterase isozymes purified from 9 animal species and human liver microsomes, and found that there is high homology of the N-terminal amino acid sequences of the isozymes tested. On the other hand, we compared the amino acid sequences at the active site of the individual esterases and found that the sequences of all esterases tested are strictly conserved. These results strongly suggest that the esterases involved are classified into the serine hydrolase super family.

Multiple forms of carboxylesterase have been identified in rat liver, and five carboxylesterases (designated hydrolases A, B, C, S, and egasyn) have been cloned. Hydrolases A, B, C, and egasyn all have a C-terminal consensus sequence (HXEL) for retaining proteins in the endoplasmic reticulum, and these carboxylesterases are found in rat liver microsomes. In contrast, hydrolase S lacks this C-terminal consensus sequence and is presumed to be secreted. In order to test this hypothesis, a polyclonal antibody was raised against recombinant hydrolase S from cDNA-directed expression in Escherichia coli. In addition to hydrolases A, B, and C (57-59 kDa), this antibody recognized a 67-kDa protein in rat liver microsomes and a 71-kDa protein in rat serum. The 71-kDa protein detected in rat serum was also detected in the extracellular medium from primary cultures of rat hepatocytes. Non-denaturing gel electrophoresis with staining for esterase activity showed that a serum carboxylesterase comigrated with the 71-kDa protein. Immunoprecipitation of the 71-kDa enzyme from rat serum decreased esterase activity toward 1-naphthylacetate and para-nitrophenylacetate. The 71-kDa protein immunoprecipitated from rat serum had an N-terminal amino acid sequence identical to that predicted from the cDNA encoding hydrolase S, providing further evidence that hydrolase S is synthesized in and secreted by the liver. The levels of the 67-kDa protein in rat liver microsomes and the levels of the 71-kDa protein in rat serum were co-regulated. Deglycosylation of microsomes and serum converted the 67- and 71-kDa proteins to a 58-kDa peptide, which matches the molecular mass calculated from the cDNA for hydrolase S. These results suggest that the 67-kDa protein in liver microsomes is a precursor form of hydrolase S that undergoes further glycosylation before being secreted into serum. In rats, liver appears to be the only source of hydrolase S because no mRNA encoding hydrolase S could be detected in several extrahepatic tissues. Serum carboxylesterases have been found to play an important role in lipid metabolism and detoxication of organophosphates, therefore, the secretion of hydrolase S and the modulation of its expression by xenobiotics may have physiological as well as toxicological significance.

beta-Glucuronidase is retained within the endoplasmic reticulum (ER) via complex formation with esterase-22 (egasyn), which in turn has a COOH-terminal HTEL ER retention sequence. To identify the regions of glucuronidase that interact with egasyn, complex formation was assayed in COS cells cotransfected with egasyn cDNA and with either deletion constructs of glucuronidase or with constructs containing specific glucuronidase propeptide sequences appended to the carboxyl terminus of a rat secretory protein alpha 1-acid glycoprotein. The region of glucuronidase essential for complex formation is a linear octamer sequence at the COOH terminus of the propeptide. A portion of this octamer is similar to a sequence near the reactive site of serpins. This and associated data indicate that an interaction related to that between serine proteinases and their serpin inhibitors retains beta-glucuronidase within the ER. Further, attachment of this octamer sequence provides an alternative method of targeting proteins to the ER lumen of any cell that contains egasyn. These and related results demonstrate that complex formation with esterases/proteinases within the ER is important in the subcellular targeting and/or processing of certain proteins.

An enzyme with acyl coenzyme A:cholesterol acyltransferase (ACAT) activity was isolated from porcine liver, and sequences derived from trypsinized peptides indicated homology to liver carboxylesterase. By use of degenerate primers, human cDNA clones were identified, which were identical to human liver carboxylesterase. Expression of the full-length cDNA in Chinese hamster ovary (CHO) cells led to an approximately threefold increase in cellular ACAT activity. This was accompanied by an approximately 20-fold increase of cellular cholesteryl ester content. By light and electron microscopy, recombinant CHO cells contained numerous lipid droplets that were not present in control CHO cells. Expression of an antisense cDNA in HepG2 cells reduced cellular ACAT activity by 35% compared with control. To further investigate the role of the enzyme in cellular cholesterol homeostasis, regulation of the mRNA was investigated in 7-day cultured human mononuclear phagocytes (MNPs). When these cells were incubated in lipoprotein-deficient serum for 18 hours, the mRNA for ACAT/carboxylesterase was almost not detectable on Northern blots, whereas after incubation with acetylated low-density lipoproteins, a strong hybridization signal was obtained. This is evidence that the mRNA of ACAT/carboxylesterase is induced by cholesterol loading. It is concluded from the data presented that ACAT/carboxylesterase is relevant for cellular cholesterol esterification in vivo. The regulation in MNPs indicates that the enzyme is also involved in foam cell formation during early atherogenesis.

The psychomotor stimulant cocaine is inactivated primarily by hydrolysis to benzoylecgonine, the major urinary metabolite of the drug. A non-specific carboxylesterase was purified from human liver that catalyzes the hydrolysis of the methyl ester group of cocaine to form benzoylecgonine. In the presence of ethanol, the enzyme also catalyzes the transesterification of cocaine producing the pharmacologically active metabolite cocaethylene (benzoylecgonine ethyl ester). The carboxylesterase obeys simple Michaelis-Menten kinetics with Km values of 116 microM for cocaine and 43 mM for ethanol. The enzymatic activity suggests that it may play an important role in regulating the detoxication of cocaine and in the formation of the active metabolite cocaethylene. Additionally, the enzyme catalyzes the formation of ethyloleate from oleic acid and ethanol. The carboxylesterase was purified from autopsy liver by gel filtration, chromatofocusing, ion-exchange, and hydrophobic interaction chromatography to purity by SDS-PAGE and agarose gel isoelectric focusing. The subunit molecular weight was determined to be 59,000 and the native molecular weight was estimated to be 170,000 from a calibrated gel filtration column, suggesting that the active enzyme is a trimer. The isoelectric point was approximately 5.8. Digestion of carbohydrate residues on the protein with an acetylglucosaminidase plus binding to several lectins indicates that the enzyme is glycosylated. The esterase was cleaved with two proteases, and the amino acid sequences from fourteen peptides were used to search GenBank. Two identical matches were found corresponding to carboxylesterase cDNAs from human liver and lung.

Commercially available Wistar rats are genetically heterogeneous with respect to acid beta-D-glucuronidase (EC 3.2.1.31) in liver tissue: Of 43 rats studied, 27 animals exhibited only approximately 40% catalytic activity of the remaining group. Analysis by subcellular and density gradient fractionation, and polyacrylamide gel electrophoresis techniques showed that livers with high activity exhibit a dual enzyme distribution in lysosomes (app. 76%) and endoplasmic reticulum (named "microsomal enzyme form"; app. 24%), whereas those with low activity not only lack the microsomal enzyme form - presumably due to the absence of egasyn - but also display reduced lysosomal enzyme activity.

A cDNA, designated hCE, encoding the entire sequence of a carboxylesterase, was isolated from a human liver lambda gt11 library. The hCE-deduced protein sequence contained 568 amino acids, including an 18 amino acid signal peptide sequence, and had a calculated molecular mass of the mature protein of 60,609 Da. A second cDNA, designated hCEv, was isolated from the same lambda gt11 library and contained a 3-bp deletion resulting in the loss of the final amino acid in the signal peptide sequence (Ala-1) and a second 3-bp deletion leading to an in-frame loss of Gln345. Expression of mRNA corresponding to both hCE and hCEv was detected in eight adult human liver samples, with individual levels varying 5-fold (hCE) and 12-fold (hCEv). A single immunoreactive protein was detected in 13 adult human liver samples when probed with antibody directed against a rat carboxylesterase. Based on allele-specific oligonucleotide hybridizations, we believe that the hCE and hCEv cDNAs represent two distinct members of the carboxylesterase family. The carboxylesterase genes were localized to human chromosome 16 using a somatic cell hybrid mapping strategy. Baculovirus expression of hCE in Sf9 cells produced a protein with an estimated molecular mass of 59,000 Da. This enzyme was able to hydrolyze aromatic and aliphatic esters but possessed no catalytic activity toward amides or a fatty acyl CoA ester. Baculovirus-mediated expression of the hCEv cDNA yielded a second protein of 56,000 Da resulting from inefficient N-glycosylation of the hCEv protein. Although the substrate specificity for the hCEv protein was identical to that of expressed hCE for any given substrate, the specific activity for the hCE protein was always higher than that for the hCEv protein. Tunicamycin inhibition studies provided the first evidence that N-glycosylation of these luminal enzymes is essential for maximal catalytic activity.

A cDNA encoding human liver carboxylesterase and its gene were isolated. Nucleotide sequence analyses of the cDNA revealed that the predicted enzyme protein consists of 567 amino acids, including 18 amino acids of a putative signal peptide. Comparison of the deduced amino acid sequences of this enzyme with those of seven other carboxylesterases in various mammalian species, together with experimental data from several other laboratories, showed that these enzymes can be classified into three groups depending on the sequences at their carboxyl terminals and the presence or absence of one exon. A human carboxylesterase gene was found to span approximately 30 kb and to have 14 small exons. Alignments of this gene with those of human cholinesterase and rat cholesterol esterase indicated insertional sites at some introns and homologous amino acid sequences around them, although these genes have different numbers of exons. Thus the results supported the conclusion that these esterases evolved from a common ancestral gene.

Egasyn is localized within the lumen of the endoplasmic reticulum (ER) where it complexes with and thus causes sequestration of a considerable portion of beta-glucuronidase. Egasyn has an HTEL sequence at its carboxyl terminus rather than the KDEL sequence that serves as a retention signal for many ER lumenal proteins. To determine whether the HTEL sequence acts as an ER retention signal and/or functions in complex formation, HTEL-deleted egasyn was expressed in mammalian cell lines. The majority of HTEL-deleted egasyn was secreted, while wild type egasyn was retained in the ER. Furthermore, the egasyn HTEL sequence, when added to the carboxyl termini of two secretory proteins, mouse esterase, Es-N, and rat alpha 1-acid glycoprotein (AGP), caused retention of both proteins within the ER, demonstrating that the HTEL sequence is both necessary and sufficient for retention of egasyn and, by extension, the egasyn-glucuronidase complex within the ER. Other carboxyl terminal tetrapeptides including HIEL and HVEL, naturally occurring in other ER lumenal proteins, were also sufficient for ER retention of AGP, while HTEHT and HTEHK were inefficient in ER retention. The HTEL sequence, in contrast, is not required for egasyn-glucuronidase complex formation. Further, the complex is apparently unstable outside the ER since it was not visible in the medium of cells transfected with egasyn lacking the HTEL sequence despite abundant secretion of this egasyn. These results show that it is possible to localize proteins within the lumen of the ER if they form complexes with ER lumenal proteins containing an intrinsic ER retention sequence.

A human liver lambda gt11 library was screened with antibodies raised to a purified rat liver carboxylesterase, and several clones were isolated and sequenced. The longest cDNA contained an open reading frame of 507 amino acids that represented 92% of the sequence of a mature carboxylesterase protein. This sequence possessed many structural features that are highly conserved among rabbit and rat liver carboxylesterase proteins, including Ser, His, and Asp residues that comprise the active site, two pairs of Cys residues that may participate in disulfide bond formation, and one Asn-Xxx-Thr site for N-linked carbohydrate addition. When the clone was used to probe human liver genomic DNA that had been digested with various restriction enzymes, many hybridizing bands of differing intensities were observed. The results suggest that the carboxylesterases exist as several isoenzymes in humans, and that they are encoded by multiple genes.

We identified a 60-kDa diisopropylfluorophosphate-(DFP) reactive protein in human bronchoalveolar lavage fluid, at a yield of 50-100 pmol/lavage. The protein is associated with the cell-free lavage fluid sediment, which consists mainly of surfactant. [3H]DFP labeling is inhibited by heating to 56 degrees C, 2 mM phenylmethylsulfonylfluoride and 1 mM bis(4-nitrophenyl)-phosphate. An identical 60-kDa [3H]DFP-reactive protein is present in the insoluble fraction of alveolar macrophage-conditioned culture medium and in total membrane preparations of alveolar macrophages. The [3H]DFP-labeled protein was purified approximately 30-fold from lavage fluid sediment by size-exclusion (Sephacryl S-200) and ion-exchange (Mono-Q) chromatography. Cyanogen bromide treatment of the partially purified protein produced a major labeled peptide of 14 kDa with an NH2-terminal sequence 90% identical to a region of form 1 rabbit liver microsomal carboxylesterase. Esterase activity in unlabeled starting material, detected using p-nitrophenyl valerate as substrate, copurified with the [3H]DFP-labeled enzyme. Degenerate oligonucleotide primers were designed based on the partial amino acid sequence and on a highly conserved region of known liver carboxylesterase sequences. Polymerase chain reaction using these primers and reverse-transcribed human alveolar macrophage mRNA yielded a 354-base pair product which was then used to screen a human alveolar macrophage cDNA library. A complete esterase sequence was obtained from two incomplete, overlapping clones, and is virtually identical to human liver carboxylesterase partial sequences. Northern blot analysis demonstrated a single approximately 1.7-kilobase transcript in human monocytes and alveolar macrophages, with much higher levels in the latter. These data indicate that human alveolar macrophages both contain and release a serine esterase that is apparently identical to liver microsomal carboxylesterase. Its enzymatic profile suggests it is a major component of alveolar macrophage-nonspecific esterase activity. We hypothesize that it acts as a detoxication enzyme in the lung.

Mouse egasyn cDNA was inserted into expression vector pCDpoly and transfected into mammalian cell lines. Transfected human HepG2 cells, monkey COS-1 cells, and mouse L cells expressed egasyn-esterase catalytic activity. Within COS-1 cells, egasyn was localized to the endoplasmic reticulum. Although individual cells produced large amounts of egasyn, no secretion was observed. No beta-glucuronidase-egasyn complexes were formed in transfected HepG2 or COS-1 cells. However, these complexes were readily detected in transfected L cells. Although the signal for retention of egasyn in the endoplasmic reticulum appears to be species independent, the signal for association with beta-glucuronidase is species restricted.

A human liver carboxylesterase (CE)-encoding cDNA has been cloned using synthetic oligodeoxyribonucleotides (oligos) based on the known amino acid (aa) sequences of rabbit and rat liver CEs. The oligos hybridize specifically to DNA encoding liver CEs. The longest cDNA obtained from screening several cDNA libraries encodes about 80% of the protein and translates into an aa sequence which has a high degree of similarity with the sequences of liver CEs from other species. On hybridization to mRNA isolated from human liver, the cDNA gave a single band of about 2.0 kb consistent with its encoding a protein of less than 68 kDa. DNA obtained from a number of human livers and probed with the CE cDNA gave identical hybridization patterns. These patterns were moderately complex by comparison with published data.

To investigate the potential role of the COOH-terminal peptides in retaining a family of soluble carboxylesterases in the lumen of the endoplasmic reticulum, the pI 6.1 esterase cDNA has been cloned into the pKCR3 vector for transient expression in COS cells. The plasmid-encoded product appeared to be identical to the authentic enzyme: it was active on alpha-naphthyl acetate and behaved as a homotrimer of noncovalently bound 60-kDa subunits which contain a single, endo-beta-N-acetylglucosaminidase H-sensitive oligosaccharide chain. This enzyme was retained in the transgenic COS cells. In contrast, a mutated form ending in HVER-COOH was secreted, indicating that the natural terminus HVEL-COOH contains topogenic information, with the ultimate Leu residue as an essential part. Variants of pI 6.1 esterase ending in HIEL-COOH, or HTEL-COOH were retained in cells to the same extent as the wild-type protein. Therefore, the sequences HIEL and HTEL present at the COOH termini of several liver esterases (rabbit forms 1 and 2, human esterase, mouse egasyn, and rat pI 6.4 esterase) most likely have a function in their localization in the endoplasmic reticulum. Finally, an HDEL-COOH variant of pI 6.1 esterase was also normally retained, demonstrating that this signal can be correctly decoded by the sorting machinery of mammalian cells. Cell retention signals of the type HXEL-COOH appear to be common in higher eukaryotes and tolerate considerable variation at the antepenultimate X residue.

Liver microsomal beta-glucuronidase is stabilized within microsomal vesicles by complexation with the accessory protein, named egasyn. In this study, we showed that egasyn is identical to one of the carboxylesterase isozymes and organophosphorus and carbamate insecticides, acetanilide which is a specific substrate of egasyn and halothane caused a rapid dissociation of the egasyn-microsomal beta-glucuronidase complex when administered in vivo or when added in vitro to isolated hepatocytes. The dissociation was relatively specific to organophosphates, carbamates, but not pyrethroids. Dissociation of the egasyn-beta-glucuronidase complex in vivo by organophosphates was followed by massive and rapid secretion of microsomal beta-glucuronidase into plasma. From these results, we concluded that release of liver microsomal beta-glucuronidase is the most rapid and sensitive marker to organophosphorus or carbamate insecticide-induced intoxication.

Human monocyte/macrophage serine esterase (HMSE), commonly known as acid esterase or alpha-naphthylacetate esterase, comprises a group of five enzyme variants that can be distinguished by their isoelectric points from esterase variants of the other normal human blood cell populations. A cDNA for one of the monocytic enzyme variants (HMSE1) was cloned from a U-937 lambda gt11 cDNA library by screening with an oligonucleotide mixture designed according to amino acid sequence data of the purified enzyme. The cDNA contains 1,727 bp with an open reading frame of 1,512 bp coding for a protein of 503 amino acid residues. HMSE1 cDNA represents the first cloned monocyte/macrophage-specific serine esterase and its sequence shows up to 77% homology to other known serine esterases of different species. The amino acid composition of the putative active site of HMSE1 as deduced from the nucleotide sequence corresponds with the active sites of other serine esterases but not with the active sites of serine proteases. Hybridization of the cDNA with RNA of separated normal blood cell populations and hematopoietic cell lines shows restricted expression within the monocyte/macrophage lineage.

The post-translational processing of beta-glucuronidase in BW5147 mouse lymphoma cells is slow relative to other newly synthesized lysosomal enzymes. To characterize this slow maturation the acid hydrolase was immunoprecipitated from cells pulse-labeled with [2-3H]mannose. Radiolabeled beta-glucuronidase migrated as the precursor form of the enzyme for up to 4 h of chase, whereas another acid hydrolase, beta-galactosidase, was processed completely to its mature form within this same time period. Both beta-glucuronidase and beta-galactosidase obtained high levels of mannose 6-phosphate (Man 6-P) within 60 min of their biosynthesis. The Man 6-P content of beta-galactosidase declined rapidly during a subsequent chase while that of beta-glucuronidase remained high during the first 4 h of chase and then slowly declined. 3H-Labeled phosphorylated high mannose-type oligosaccharides isolated from beta-glucuronidase after 1 h of chase were composed primarily of species with one or two phosphodiester groups, but oligosaccharides with one and two phosphomonoesters became the predominant phosphorylated species with longer chase times. The phosphorylated oligosaccharides attached to other newly synthesized acid hydrolases, on the other hand, contained primarily phosphodiester species at all chase times. When BW5147 cells were pulsed with [3H]mannose and chased in the presence of monensin to disrupt transport, the number of phosphorylated oligosaccharides recovered from beta-glucuronidase was comparable to the quantity recovered from the enzyme produced by non-drug-treated cells. The number of phosphorylated units recovered from all other newly synthesized acid hydrolases, however, was greater in the presence of the ionophore than in its absence. Nondenaturing gel electrophoresis studies indicated that beta-glucuronidase existed in two forms at steady state within BW5147 cells and, as such, was similar to liver beta-glucuronidase in which a large percentage of the enzyme was present as a complex bound to egasyn. These data suggest that newly synthesized beta-glucuronidase produced by BW5147 cells complexes with an egasyn-like protein within the endoplasmic reticulum. This interaction retards the enzyme's migration through the secretory apparatus but does not prevent its access to Golgi-associated processing enzymes.

A significant portion of murine hepatocyte beta-glucuronidase is maintained within the endoplasmic reticulum (ER) by complex formation with the esterase active site of the protein egasyn. The carboxyl-terminal propeptide of the precursor form of glucuronidase appears important in localization of glucuronidase to the ER since a naturally occurring mutation in it is associated with decreased levels of ER glucuronidase. A sequence similarity was noted between the carboxyl-terminal propeptide and portions of the conserved sequences of the reactive site region of members of the serpin (serine proteinase inhibitor) superfamily. Also, previous studies had shown that a synthetic peptide, corresponding to the propeptide region, was a specific and potent inhibitor of the esterase activity of purified egasyn. Taken together, these results suggest that (a) the egasyn-glucuronidase system may use a novel mechanism related to that of serine proteinases and their inhibitors in complex formation and in subsequent localization of glucuronidase within the ER and that (b) a possible function of ER glucuronidase is to modulate the esterase activity of egasyn.

The proenzyme form of beta-glucuronidase is compartmentalized in large quantities within the endoplasmic reticulum by binding to the esterase, egasyn. Also, the propeptide of the proenzyme form of beta-glucuronidase is likely located at the carboxyl terminus. We have, therefore, tested if this carboxyl-terminal peptide is important in binding to egasyn. A polyclonal antibody to a 30-mer synthetic peptide, corresponding to the carboxyl-terminal 30 amino acids of pro-beta-glucuronidase, provided evidence that egasyn binds to the carboxyl terminus of beta-glucuronidase. This antibody interacted with proenzyme beta-glucuronidase-egasyn complexes in which one, two, or three egasyn molecules were bound to the beta-glucuronidase tetramer, but not with those complexes (M4) which contained four egasyn molecules. We interpret these results as indicating that all available carboxyl termini of the beta-glucuronidase proenzyme tetramer are shielded by egasyn in the M4 complexes. The same antibody did not recognize the mature lysosomal form of beta-glucuronidase, indicating that only the proenzyme form of microsomal beta-glucuronidase contains the original carboxyl terminus. Also, the synthetic 30-mer was found to be a specific and potent inhibitor (50% inhibition at 1.3 microM) of the esterase activity of purified egasyn but exhibited little inhibitory activity toward other purified esterases including a rat trifluoroacetylated esterase or egasyn esterase from another species. Together, these data describe a potent interaction of the exposed carboxyl terminus of precursor glucuronidase with the esterase catalytic site of egasyn, which in turn results in the specific localization of glucuronidase within the lumen of the endoplasmic reticulum.

It was demonstrated that the inbred strain EHBR had the C phenotype of esterase-3 judging from the absence of liver microsomal beta-glucuronidase and the pattern of esterase activities of liver homogenates after analytical isoelectric focusing. In addition, in the strain EHBR, liver microsomal hydrolase activities of acetanilide and isocarboxazid which are hydrolyzed well by esterase-3 were lower than in outbred Sprague-Dawley rat and inbred LEW rat having the D phenotype of esterase-3. These results suggest that the phenotype difference of esterase-3 is possible to cause the strain differences of liver microsomal carboxylesterase activities.

Three differently modified forms of beta-glucuronidase are known to exist: a microsomal enzyme form (M) existing in tissues where egasyn, a second microsomal protein, is present; and an acidic (La; complex-type oligosaccharide) and a basic (Lb; non-complex type oligosaccharide) lysosomal form which occur in all mouse tissues. Lb predominates in tissues containing microsomal beta-glucuronidase, La in those lacking it. In pulse-labelling experiments using mouse strain C57BL/6 liver containing egasyn (Eg+/Eg+) and microsomal enzyme, about half of the newly synthesized beta-glucuronidase was processed to the microsomal enzyme form, which was evidently further processed to Lb, and about half directly to La. In contrast, in liver of the congenic line C57BL/6.YBR Es-1b Eg0 that lacks egasyn (Eg0/Eg0) and microsomal enzyme, most of the labelled beta-glucuronidase was processed to La, and only a minor portion to Lb. Newly synthesized enzyme appeared first in microsomal, then in light and heavy lysosomal fractions of Eg+/Eg+ liver. In Eg0/Eg0 liver, no labelled enzyme was measurable in the microsomes, but it appeared rapidly in both types of lysosomes. Taken together these findings indicate that the microsomal enzyme form serves as a precursor of Lb, and that La is synthesized independently. The apparent half-life of La is only two-thirds that of Lb; this fact accounts for the reduced beta-glucuronidase activity in Eg0/Eg0 liver, which contains La as the predominant form.

Mouse liver beta-glucuronidase is stabilized within microsomal vesicles by complexation with the accessory protein egasyn. The location of the beta-glucuronidase-egasyn complex and free egasyn within microsomal vesicles was investigated. Surprisingly, it was found that neither the complex nor free egasyn are intrinsic membrane components. Rather, both are either free within the vesicle lumen or only weakly bound to the inside of the vesicle membrane. This conclusion was derived from release studies using low concentrations of Triton X-100 or controlled sonication. Both the intact complex and free egasyn were released in parallel with lumenal proteins, not with intrinsic membrane components. Also, beta-glucuronidase was protected from digestion by proteinase K by the membrane of microsomal vesicles. The hydrophilic nature of both the complex and free egasyn was confirmed by phase separation experiments with the detergent Triton X-114. Egasyn is one of an unusual group of esterases that, despite being located within the lumen or only weakly bound to the lumenal surface of the endoplasmic reticulum, do not enter the secretory pathway.

Organophosphorous compounds, which are potent inhibitors of egasyn-esterase activity, caused a rapid dissociation of the high molecular weight egasyn-microsomal beta-glucuronidase complex when administered in vivo or when added in vitro to microsomal suspensions. The dissociation was relatively specific to phosphodiester inhibitors of the esterase active site. Also, the egasyn-esterase active site was inaccessible to substrates and to inhibitors when egasyn was complexed to beta-glucuronidase. Dissociation of the egasyn-microsomal beta-glucuronidase complex in vivo by organophosphorous compounds was followed by massive and rapid secretion of microsomal beta-glucuronidase, but not egasyn, into plasma. These experiments implicate the egasyn-esterase active site in attachment of microsomal beta-glucuronidase to egasyn by a novel mechanism that, in turn, compartmentalizes beta-glucuronidase within the endoplasmic reticulum.

Recent experiments have demonstrated that egasyn not only sequesters beta-glucuronidase in microsomes by forming high molecular weight complexes with beta-glucuronidase, but also has carboxyl esterase activity. We have found several new phenotypes of egasyn-esterase after electrophoresis and isoelectric focusing of liver homogenates and purified egasyn of inbred and wild mouse strains. Several phenotypes corresponded in relative mobility and relative isoelectric point among inbred strains to that recently reported for esterase-22 by Eisenhardt and von Deimling [(1982). Comp. Biochem. Physiol. 73B:719]. This genetic evidence, plus a wide variety of comparative biochemical and physiological data, indicates that egasyn is identical to esterase-22. Both parental types of egasyn isozymes are expressed in heterozygous F1 progeny, suggesting that alterations in the egasyn structural gene are responsible for the altered isoelectric points. Also, egasyn is a monomer since no new esterase bands appear in F1 progeny. The variants in isoelectric point of egasyn map at or near the egasyn (Eg) gene within the esterases of cluster 1 near Es-9 on chromosome 8.

The accumulation of the relatively large amounts of beta-glucuronidase in microsomal fractions of normal mice depends on formation of complexes with the protein egasyn. Unexpectedly, it was found that the egasyn gene also affects the processing of beta-glucuronidase, which is segregated to lysosomes. In egasyn-positive mice lysosomal beta-glucuronidase from liver has a mean pI of 5.9 with a minor proportion at pI 5.4, whereas in egasyn-negative mice the proportion of the two lysosomal forms is reversed. Combined experiments measuring susceptibility to neuraminidase and to endoglycosidase H and specific binding to Ricinus communis lectin-agarose columns showed that the alterations in isoelectric point were associated with a decrease in complex oligosaccharides of lysosomal beta-glucuronidase in egasyn-positive mice. Since this alteration occurs not only in a congenic strain carrying the Eg0 gene but also in several other inbred strains that are homozygous for this gene, it is considered to be a genuine effect of the Eg gene rather than other genes that might regulate oligosaccharide processing. Also, the alteration is likely to be a result of direct physical interaction of the egasyn protein and lysosomal beta-glucuronidase, since a second lysosomal enzyme, beta-galactosidase, which does not form complexes with egasyn, is unaffected. The results suggest a model in which egasyn not only causes accumulation of beta-glucuronidase in the microsomal compartment but also acts upon the precursor to lysosomal beta-glucuronidase to alter its interaction with trans-Golgi-apparatus processing enzymes.

The glycoprotein egasyn complexes with and stabilizes precursor beta-glucuronidase in microsomes of several mouse organs. Several observations indicate egasyn is, in addition, an esterase. Liver homogenates of egasyn-positive strains have specific electrophoretically separable esterases which are absent in egasyn-negative mice. These esterases react with anti-egasyn serum. A specific esterase was likewise complexed with immunopurified microsomal beta-glucuronidase. The esterases were, like egasyn and microsomal beta-glucuronidase, concentrated in the microsomal subcellular fraction. Egasyn which is not bound to beta-glucuronidase, which represents 80-90% of total liver egasyn, is not complexed with other liver proteins. Egasyn, therefore, specifically stabilizes beta-glucuronidase in microsomes. The esterase activity is inhibited by bis-p-nitrophenyl phosphate indicating it is a carboxyl esterase. Several possible functions of egasyn-esterase activity are discussed.

A binding protein with apparent specificity for beta-glucuronidase has been partially purified from a Triton X-100 extract of rat liver microsomes by affinity chromatography on glucuronidase-Sepharose 2B. It appears that once removed from the membrane, this binding protein self-aggregates to form large macromolecular complexes. With the use of polyacrylamide gel electrophoretic and sucrose density gradient ultracentrifugation assays to monitor the conversion of glucuronidase tetramer to a very high molecular weight complex, it was shown that the binding activity is heatlabile and protease-sensitive. However, binding activity is not influenced by salts, carbohydrates, other proteins or glycoproteins, or by extensive periodate oxidation of beta-glucuronidase, nor does binding occur with any other protein tested. The binding protein does not discriminate against any form of beta-glucuronidase from any rat organ tested. However, the binding protein does show organ localization, being present in the liver and kidney but not the spleen. The possible relationship of this binding protein to egasyn, a membrane protein which stabilizes beta-glucuronidase in mouse liver endoplasmic reticulum, is discussed.

Mouse beta-glucuronidase has a dual intracellular localization, being present in both endoplasmic reticulum and lysosomes of several tissues. Previous studies demonstrated that the protein egasyn is complexed with microsomal but not lysosomal glucuronidase and that a mutant lacking egasyn is deficient in microsomal, but not lysosomal, glucuronidase. By means of a recently developed radioimmunoassay for egasyn, the relationship between microsomal glucuronidase levels and egasyn levels has been examined in various adult tissues, during postnatal development in liver, and after androgen induction of glucuronidase in kidney. The results indicate that the relative availability of egasyn determines the balance between glucuronidase incorporation into membranes and that into lysosomes.

Previous studies have suggested that the binding of mouse flucuronidase to endoplasmic reticulum membrane is stabilized by the membrane protein egasyn. Using a radioimmunoassay for egasyn, we have now examined the inheritance of egasyn levels in mice. Mice of the inbred strain C57BL/6J, which have normal levels of microsomal glucuronidase, contained 56 +/- 10 mug egasyn per gram of liver. Mice of the inbred strain YBR, which carry the Eg0 mutation resulting in the absence of microsomal glucuronidase, did not contain detectable levels of egasyn. The F1 progeny of these two strains contained intermediate levels of egasyn, 25 +/- 4 mug egasyn per gram of liver. Progeny from the backcross of these F1 animals to YBR were distributed equally into two discrete phenotypic classes. One class lacked both egasyn and microsomal glucuronidase, while the other class contained 25 +/- 3 mug egasyn per gram of liver and contained normal levels of microsomal glucuronidase. Thus egasyn levels are determined by the Eg locus and show additive inheritance. These results suggest that the Eg gene codes for egasyn and that it is the inability to produce egasyn that results in a deficiency of microsomal glucuronidase in the Eg0 mutant.

Glucuronidase present in lysosomes of mouse liver occurs as the free tetramer, whereas glucuronidase present in endoplasmic reticulum occurs in macromolecular complexes containing one to four molecules of the protein egasyn. Earlier genetic and biochemical studies suggest that these complexes, or M forms, function to stabilize the membrane binding of glucoronidase. The detergent Triton X-100 extracts glucuronidase-egasyn complexes intact and they dissociate in the presence of the detergent deoxycholate or upon heating. We have now purfied egasyn by releasing it from antiglucuronidase immunoprecipitates of M forms under relatively mild conditions, such as treatment with deoxycholate or heating at 50 degrees. Isolated egasyn is a glycoprotein of molecular weight about 64,000 and is not unusually hydrophobic in amino acid composition. Monospecific antibody to egasyn was raised. This antibody showed no cross-reactivity with purified beta-glucuronidase and antibody to glucuronidase failed to react with purified egasyn; however, both antibodies bound to egasyn-glucuronidase complexes. A procedure for the radioimmunoassay of egasyn was developed utilizing egasyn labeled with iodine 125. Most of the antigenic sites of egasyn in homogenates of normal liver are masked after extraction with Triton X-100 and only become immunoreactive after exposure to deoxycholate. After unmasking, mouse liver proved to contain about 56 mug of egasyn/g, nearly all of which is localized to the microsomal fraction. Of this total only about 10% was complexed with glucuronidase, suggesting theat the bulk of the egasyn present may be complexed with other proteins. Mice of the inbred strain YBR, which carry the EgO mutation resulting in the absence of microsomal glucuronidase, lacked immunoreactive egasyn, suggesting that the primary defect in this strain lies in the unavailabililty of agasyn to form complexes. There is now considerable evidence in support of the concept that the microsomal forms of glucuronidase exist in membranes complexed with egasyn and that formation of these complexes is required for maintenance of glucuronidase in membranes. Egasyn may represent one of a class of membrane anchor proteins that each stabilize the membrane binding of a charcteristic set of proteins.

Two polypeptide chains are present in murine beta-glucuronidase precipitated with a specific anti-beta-glucuronidase antibody F(ab)2 fragment. One is the catalytic subunit of beta-glucuronidase and the other has the properties predicted for the hypothetical beta-gluronidase membrane anchor protein. The new protein, named egasyn, is associated with microsomal, but no lysosomal beta-glucuronidase. It is released from the microsomal beta-glucuronidase complex by heat treatment. The YBR strain of mice carrying the Eg degrees mutation does not form an egasyn-beta-glucuronidase complex and is unable to retain beta-glucuronidase on microsomal membranes.