Glutathione S-Transferase Conjugation of Organophosphorus Pesticides Yields S-Phospho-, S-Aryl-, and S-Alkylglutathione Derivatives

Abstract

Pesticide detoxification is a central feature of selective toxicity and safety evaluation. Two of the principal enzymes involved are GSH S-transferases (GSTs) and cytochrome P450s acting alone and together. More than 100 pesticides are organophosphorus (OP) compounds, but with few exceptions, their GSH conjugates have not been directly observed in vitro or in vivo. The major insecticides chlorpyrifos (CP) and diazinon are of particular interest as multifunctional substrates with diverse metabolites, while ClP(S)(OEt) 2 and the cotton defoliant tribufos are possible precursors of phosphorylated GSH conjugates. Formation of GSH conjugates by GST with GSH was studied in vitro with and without metabolic activation by human liver microsomes or P450 3A4 with NADPH. Metabolites were analyzed by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS). Five GSH conjugates were identified from CP and chlorpyrifos oxon (CPO), i.e., GSCP and GSCPO in which the 6-chloro substituent of CP and CPO, respectively, is displaced by GSH; S-(3,5,6-trichloropyridin-2-yl)glutathione; S-(3,5-dichloro-6-hydroxypyridin-2-yl)glutathione; and S-ethylglutathione. GST of a human liver microsomal preparation but not P450 3A4 with GSH metabolized CP to GSCP. With GST and GSH, diazinon and diazoxon gave S-(2-isopropyl-4-methylpyrimidin-6-yl)glutathione and ClP(S)(OEt) 2 yielded GSP(S)(OEt) 2. With microsomes, NADPH, GST, and GSH tribufos gave GSP(O)(SBu) 2. The liver of intraperitoneally treated mice contained GSCP from CP, GSP(S)(OEt) 2 from ClP(S)(OEt) 2, and GSP(O)(SBu) 2 from tribufos. GSP(S)(OEt) 2 and GSP(O)(SBu) 2 are the first S-phosphoglutathione metabolites observed in vitro and in vivo directly by LC-ESI-MS. Nine other OP pesticides gave only O-dealkylation in the GST/GSH system. GST-catalyzed metabolism joins P450s and hydrolases as important contributors to OP detoxification.

Phytotransformation of Phorate by Brassica juncea (Indian Mustard)

“…The metabolism of many xenobiotic compounds including herbicides proceeds by glutathione conjugation reactions, but its involvement in the metabolism of organophosphorus insecticides has not been demonstrated yet. Some of the organophosphates have been reported to be detoxified by glutathione conjugation in mammals (Fujioka and Casida 2007). In this study, the enhancement in the activity of GST on phorate exposure suggests that plants have a similar mechanism for its detoxification.…”

Review of the Toxicology of Chlorpyrifos With an Emphasis on Human Exposure and Neurodevelopment

“…In the kidney the cysteine moiety may be acetylated to form the mercapturic acid or may lose alanine resulting in a metabolite with a free sulfhydryl group (Dekant, 2001). Glutathione conjugates of chlorpyrifos were detected in the livers of mice treated intraperitoneally with chlorpyrifos, (Fujioka and Casida, 2007), suggesting that this could be a significant pathway for metabolism of chlorpyrifos (see later discussion on human metabolism of chlorpyrifos).…”

Polymorphisms of pesticide-metabolizing genes in children living in intensive farming communities

“…GSTs, in addition to detoxifying reactive intermediates, also play a role by dealkylating methyl or ethyl groups (e.g., methyl parathion and parathion, respectively) and dearylating OPs (e.g., diazinon) (Fujioka and Casida, 2007). As null genotype for these genes may confer higher susceptibility to disease and disease outcome from chemical exposures (Rossini et al, 2002), the determination of GSTM1 and GSTT1 genotype polymorphism may be helpful as an individual marker of susceptibility in OP-exposed population (Singh et al, 2011).…”