Inhibitor Report for: Fenthion-oxon

A simultaneous analytical method for the organophosphorus insecticide fenthion and its five metabolites (fenthion oxon, fenthion oxon sulfoxide, fenthion oxon sulfone, fenthion sulfoxide, and fenthion sulfone) was developed based on ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). Five matrices (brown rice, chili pepper, orange, potato, and soybean) were selected to validate the method. The target compounds were analyzed using positive electrospray ionization in the multiple reaction monitoring mode. For the best sensitivity in regard to the detector response, water and methanol containing formic acid (0.1%) were selected as the mobile phase. The optimum extraction efficiency was obtained through a citrate-buffered QuEChERS (quick, easy, cheap, effective, rugged, and safe) method. Recovery tests were carried out at three spiking levels (n = 3). At all fortification levels, the accuracy and precision results were between 70% and 120% with a relative standard deviation of >=15%. The limit of quantitation was 0.01 mg/kg, and the correlation coefficients (r(2)) of the matrix-matched calibration curves were >0.99. Significant signal suppression in the detector responses were observed for all matrices, suggesting that a compensation method, such as matrix-matched calibration, is required to provide accurate quantitative results. The applicability of the presented method was confirmed for the simultaneous analysis of fenthion and its metabolites in various crops.

Although fenthion (FEN) is widely used as a broad spectrum insecticide on various crops in many countries, very scant data are available on its biotransformation in humans. In this study the in vitro human hepatic FEN biotransformation was characterized, identifying the relative contributions of cytochrome P450 (CYPs) and/or flavin-containing monooxygenase (FMOs) by using single c-DNA expressed human enzymes, human liver microsomes and cytosol and CYP/FMO-specific inhibitors. Two major metabolites, FEN-sulfoxide and FEN-oxon (FOX), are formed by some CYPs although at very different levels, depending on the relative CYP hepatic content. Formation of further oxidation products and the reduction of FEN-sulfoxide back to FEN by the cytosolic aldehyde oxidase enzyme were ruled out. Comparing intrinsic clearance values, FOX formation seemed to be favored and at low FEN concentrations CYP2B6 and 1A2 are mainly involved in its formation. At higher levels, a more widespread CYP involvement was evident, as in the case of FEN-sulfoxide, although a higher efficiency of CYP2C family was suggested. Hepatic FMOs were able to catalyze only sulfoxide formation, but at low FEN concentrations hepatic FEN sulfoxidation is predominantly P450-driven. Indeed, the contribution of the hepatic isoforms FMO(3) and FMO(5) was generally negligible, although at high FEN concentrations FMO's showed activities comparable to the active CYPs, accounting for up to 30% of total sulfoxidation. Recombinant FMO(1) showed the highest efficiency with respect to CYPs and the other FMOs, but it is not expressed in the adult human liver. This suggests that FMO(1)-catalysed sulfoxidation may represent the major extra-hepatic pathway of FEN biotransformation.

The in vivo metabolism of fenthion, an organophosphorus pesticide, and its sulfoxide (fenthion sulfoxide) was examined in goldfish (Carassius auratus). When goldfish were administered fenthion i.p. at a dose of 100 mg/kg, two metabolites were isolated from the tank water. They were identified as fenthion sulfoxide and fenthion oxon, in which > P = S of fenthion is transformed to > P = O, by comparing their mass and UV spectra, and their behavior in HPLC and TLC, with those of authentic standards. However, fenthion sulfone was not detected as a metabolite. The amounts of fenthion, fenthion sulfoxide and fenthion oxon excreted within 4 days were 2.7, 3.4 and 2.5%, of the initial dose of fenthion, respectively. Unchanged fenthion was detected in the body of the fish to the extent of 42-50% of the dose after 10 days, but fenthion sulfoxide and fenthion oxon showed very low concentrations. When fenthion sulfoxide was administered to the fish, about 70% of the dose was excreted unchanged into the tank water within 24 h, but little of the reduced compound, fenthion, was found. In contrast, fenthion was detected at 2.1% of dose in the body of goldfish as a metabolite of fenthion sulfoxide. The fact that fenthion is metabolized to the toxic oxon form in fish presumably has environmental and health implication for its use as a pesticide.