Inhibitor Report for: Leptophos

The neurotoxic potential of trichlorfon, diazinon, phosmet, dichlorvos, phosphamidon and coumaphos was evaluated for their ability to inhibit brain neurotoxic esterase (NTE) activity in adult hens. Leptophos was used as a reference neurotoxic agent. All compounds were administered at high single oral doses and the NTE and acetylcholinesterase (AchE) activities were measured at 24 h and 6 w later. With the exception of leptophos, all compounds produced severe cholinergic signs associated with > 80% inhibition of brain AchE at 24 h. On the other hand, brain NTE activity was 86% inhibited by leptophos and to lesser extents by trichlorfon (76%), phosphamidon (74%), coumaphos (70%) and dichlorvos (70%). However, none of the latter compounds produced clinical delayed neurotoxicity as was observed with leptophos. It was concluded that trichlorfon, phosphamidon, coumaphos and dichlorvos are potentially neurotoxic because of their ability to inhibit brain NTE activity, but the extent of inhibition required for development of clinical delayed neurotoxicity (> 80%) is not likely to occur with any of these compounds due to their severe cholinergic activity.

Ten day old chick sympathetic ganglia cultured in a microslide assembly were treated with a selected group of organophosphate pesticides to evaluate their cytotoxicity ranges, and the usefulness of such a model for screening pesticides. Examination by phase contrast and light microscopy for chemically-induced morphological alteration of nerve fibers, glial cells and neurons provided the criteria for quantitation and assessment of the toxic effects. Concentrations that produced half-maximal effects ranged from 1 x 10(-6)M (severely toxic) for methylparathian, diazinon, paraoxon, mevinphos, diisopropylfluorophosphate, tri-o-tolyl phosphate and its mixed isomers to a 1 x 10(-3)M (intermediate) for malathion, leptophos, coumaphos, mono- and dicrotophos. Some or no effects were evident at 1 x 10(2-)M for O'ethyl-O-p-nitrophenyl phenyl phosphonothioate, tri-m-tolylphosphate, chlorpyriphos and triphenyl phosphate. In all instances, nerve fibers were more sensitive than neurons or glial cells to insecticides. All cellular growth was inhibited at 1 x 10(-2)M (except triphenyl phosphate). Below 1 x 10(-7)M, no inhibitory effects were evident. The secondary abnormalities included decreased cellular migration, diffuse cellular growth pattern, increased vacuolization, nerve fiber swelling and cellular degeneration. The cytotoxic effects of these chemicals do not appear to be related to in vivo toxicity or cholinesterase inhibition potential.

The neurotoxic potential of trichlorfon, diazinon, phosmet, dichlorvos, phosphamidon and coumaphos was evaluated for their ability to inhibit brain neurotoxic esterase (NTE) activity in adult hens. Leptophos was used as a reference neurotoxic agent. All compounds were administered at high single oral doses and the NTE and acetylcholinesterase (AchE) activities were measured at 24 h and 6 w later. With the exception of leptophos, all compounds produced severe cholinergic signs associated with > 80% inhibition of brain AchE at 24 h. On the other hand, brain NTE activity was 86% inhibited by leptophos and to lesser extents by trichlorfon (76%), phosphamidon (74%), coumaphos (70%) and dichlorvos (70%). However, none of the latter compounds produced clinical delayed neurotoxicity as was observed with leptophos. It was concluded that trichlorfon, phosphamidon, coumaphos and dichlorvos are potentially neurotoxic because of their ability to inhibit brain NTE activity, but the extent of inhibition required for development of clinical delayed neurotoxicity (> 80%) is not likely to occur with any of these compounds due to their severe cholinergic activity.

The ability of bromine and rat liver microsomes (RLM) to convert organophosphorus (OP) protoxicants to esterase inhibitors was determined by measuring acetylcholinesterase (AChE) and neuropathy target esterase (NTE) inhibition. Species specific differences in susceptibility to esterase inhibition were determined by comparing the extent of esterase inhibition observed in human neuroblastoma cells and hen, bovine, and rodent brain homogenates. OP protoxicants examined included tri-o-tolyl phosphate (TOTP), O-ethyl O-p-nitrophenyl phenylphosphonothioate (EPN), leptophos, fenitrothion, fenthion, and malathion. Bromine activation resulted in greater AChE inhibition than that produced by RLM activation for equivalent concentrations of fenitrothion, malathion, and EPN. For EPN and leptophos, bromine activation resulted in greater inhibition of NTE than RLM. Only preincubation with RLM activated TOTP; resultant inhibition of AChE was less in hen brain (13 +/- 3%) than in neuroblastoma cells (73 +/- 1%) at 10(-6) M. In contrast, 10(-6) M RLM-activated TOTP produced more inhibition of hen brain NTE (89 +/- 6%) than NTE of human neuroblastoma cells (72 +/- 7%). Human neuroblastoma cells and brain homogenates from hens, the accepted animal model for study of OP-induced neurotoxicity, were relatively similar in sensitivity to esterase inhibition. Homogenates from hens were more sensitive to NTE inhibition induced by phenyl saligenin phosphate (PSP), an active congener of TOTP, than were homogenates from less susceptible species (mouse, rat, bovine). AChE of hen brain homogenates was also more sensitive than homogenates from other species to malaoxon, the active form of malathion.

Delayed neuropathy and inhibition of soluble neuropathy target esterase (NTE) and acetylcholinesterase (AChE) activities in different regions of brain and spinal cord of adult hens were studied after the intravenous administration of leptophos (30 mg/kg), tri-o-cresyl phosphate (TOCP 40 mg/kg) or dipterex (200 mg/kg). The level of NTE activity varied according to the regions of the central nervous system (CNS) of the control (normal) hen, being higher in the cerebrum (74.1 micromol of phenyl valerate hydrolyzed/10 minutes/mg protein) and in the cerebellum (68.7), and lower in the spinal cord (44.5 in cervical, 55.6 in thoracic and 50.0 in lumbar cord). Hens given leptophos and TOCP demonstrated delayed neuropathy with obvious inhibition of NTE, but the times of onset and the degrees of peak inhibition of NTE activity were different: 6-24 hours after dosing and 73-82% of normal activity for leptophos, and 24-48 hours and 45-80% for TOCP, respectively. Furthermore, the average inhibition of NTE during 6-48 hours after dosing, (called here 'period average inhibition') was also significantly different between the leptophos group (63-73%) and TOCP group (40-64%). Hens given dipterex did not demonstrate delayed neuropathy, and had the least peak inhibition and period average inhibition of NTE activity among the 3 groups. Ratios of NTE inhibition/AChE inhibition were higher in the leptophos group (0.91-1.24) and TOCP group (1.13-2.45) than in the dipterex group (0.25-0.79). These results indicate that the distribution of NTE in the soluble fraction of membrane proteins is different in different regions of the CNS, and that the degree of peak inhibition of NTE activity and the time of onset of peak inhibition induced by organophosphorus compounds (OPs) also differ for different OPs. Thus, practical and useful NTE measurements should identify the peak inhibition and period inhibition in several nervous tissue regions.

The effects of various insecticides on the mycelial growth, sporulation and conidial germination of Metarhizium anisopliae var. anisopliae isolate E9 were studied in the laboratory. Chlorpyrifos was the most toxic organophosphate to mycelial growth and sporulation at all concentrations. Temephos, malathion and leptophos were highly toxic to sporulation while malathion was the most inhibitory to germination. The carbamates, carbofuran, methomyl and oxamyl were moderately toxic to mycelial growth and sporulation while oxamyl had an adverse effect on germination. The pyrethroids (pyrethrin, permethrin and resmethrin) and the insect growth regulators (diflubenzuron and methoprene) were not inhibitory to the various developmental stages of isolate E9. The chlorinated hydrocarbons (chlordane, lindane and toxaphene) were more deleterious than all other insecticide groups tested. Among the fungicides, benomyl and maneb produced the greatest inhibition.

A simple and efficient cleanup method for gas chromatographic determination of 23 organophosphorus pesticides in crops including onion is described. The sample was extracted with acetone. The extract was purified with coagulating solution, which contained ammonium chloride and phosphoric acid, and then filtered by suction. The filtrate was diluted with NaCl solution and reextracted with benzene. The organic layer was evaporated and injected into a gas chromatograph equipped with a flame photometric detector (FPD) and fused silica capillary columns (0.53 mm id) coated with silicone equivalent to OV-1701, OV-1, and SE-52. Onion extract, which contained FPD interferences, was cleaned up on a disposable silica cartridge. Recoveries of most organophosphorus pesticides from spiked crops: mandarin orange, tomato, spinach, sweet pepper, broccoli, lettuce, and onion at levels of 0.02-0.28 ppm, exceeded 80%, but the water-soluble pesticides dichlorvos and dimethoate gave poor recoveries in all crops; the nonpolar pesticides disulfoton, chlorpyrifos, fenthion, prothiophos, and leptophos were not recovered quantitatively in spinach, sweet pepper, broccoli, and lettuce. IBP, edifenphos, phosmet, and pyridaphenthion were not recovered from onion because of adsorption to the silica cartridge. The detection limits ranged from 1.25 to 17.5 ppb on a crop basis.

The toxic and teratogenic effects of 4 organophosphorus compounds (phenyl saliginen cyclic phosphate (PSCP), leptophos-oxon (LPTO), tri-o-tolyl phosphate (TOTP), and paraoxon (PXN] were investigated in the embryos of 3 species of frogs. Developmental abnormalities were observed in surviving embryos of each of the 3 species following exposure to PSCP at concentrations as low as 500 ppb for 24 h. LPTO, while being toxic to gray treefrog embryos at concentrations as low as 2.2 ppm, did not induce developmental abnormalities. TOTP and PXN were neither toxic nor teratogenic at concentrations of 10 ppm and 100 ppm respectively.

Ten day old chick sympathetic ganglia cultured in a microslide assembly were treated with a selected group of organophosphate pesticides to evaluate their cytotoxicity ranges, and the usefulness of such a model for screening pesticides. Examination by phase contrast and light microscopy for chemically-induced morphological alteration of nerve fibers, glial cells and neurons provided the criteria for quantitation and assessment of the toxic effects. Concentrations that produced half-maximal effects ranged from 1 x 10(-6)M (severely toxic) for methylparathian, diazinon, paraoxon, mevinphos, diisopropylfluorophosphate, tri-o-tolyl phosphate and its mixed isomers to a 1 x 10(-3)M (intermediate) for malathion, leptophos, coumaphos, mono- and dicrotophos. Some or no effects were evident at 1 x 10(2-)M for O'ethyl-O-p-nitrophenyl phenyl phosphonothioate, tri-m-tolylphosphate, chlorpyriphos and triphenyl phosphate. In all instances, nerve fibers were more sensitive than neurons or glial cells to insecticides. All cellular growth was inhibited at 1 x 10(-2)M (except triphenyl phosphate). Below 1 x 10(-7)M, no inhibitory effects were evident. The secondary abnormalities included decreased cellular migration, diffuse cellular growth pattern, increased vacuolization, nerve fiber swelling and cellular degeneration. The cytotoxic effects of these chemicals do not appear to be related to in vivo toxicity or cholinesterase inhibition potential.