title: Contribution of carboxylesterase and cytochrome P450 to the bioactivation and detoxification of isocarbophos and its enantiomers in human liver microsomes.
abstract: Organophosphorus pesticides are the most widely used pesticides in modern agricultural systems to ensure good harvests. Isocarbophos (ICP), with a potent acetylcholinesterase inhibitory effect is widely utilized to control a variety of leaf-eating and soil insects. However, the characteristics of the bioactivation and detoxification of ICP in humans remain unclear. In this study, the oxidative metabolism, esterase hydrolysis, and chiral inversion of ICP in human liver microsomes (HLMs) were investigated with the aid of a stereoselective LC/MS/MS method. The depletion of ICP in HLMs was faster in the absence of carboxylesterase inhibitor (BNPP) than in the presence of NADPH and BNPP, with t1/2 of 5.2 and 90 min, respectively. Carboxylesterase was found to be responsible for the hydrolysis of ICP, the major metabolic pathway. CYP3A4, CYP1A2, CYP2D6, CYP2C9, and CYP2C19 were all involved in the secondary metabolism pathway of desulfuration of ICP. Flavin-containing monooxygenase (FMO) did not contribute to the clearance of ICP. The hydrolysis and desulfuration of (±)ICP, (+)ICP, and (-)ICP in HLMs follow Michaelis-Menten kinetics. Individual enantiomers of ICP and its oxidative desulfuration metabolite isocarbophos oxon (ICPO) were found to be inhibitors of acetylcholinesterases at different extents. For example, (±)ICPO is more potent than ICP (IC50 0.031μM vs. 192μM), whereas (+)ICPO is more potent than (-)ICPO (IC50 0.017μM vs. 1.55μM). Given the finding of rapid hydrolysis of ICP and low abundance of oxidative metabolites presence in human liver, the current study highlights that human liver has a greater capacity for detoxification of ICP.
authors.affiliation: Key Laboratory of Drug Metabolism and Pharmacokinetics, Beijing Institute of Pharmacology and Toxicology, Beijing 100850, China State Key Laboratory of Toxicology and Medical Countermeasures, Beijing 100850, China.
authors.forename: Xiao-Mei
authors.initials: XM
authors.lastname: Zhuang
authors.affiliation: Key Laboratory of Drug Metabolism and Pharmacokinetics, Beijing Institute of Pharmacology and Toxicology, Beijing 100850, China State Key Laboratory of Toxicology and Medical Countermeasures, Beijing 100850, China.
authors.forename: Xia
authors.initials: X
authors.lastname: Wei
authors.affiliation: Key Laboratory of Drug Metabolism and Pharmacokinetics, Beijing Institute of Pharmacology and Toxicology, Beijing 100850, China State Key Laboratory of Toxicology and Medical Countermeasures, Beijing 100850, China.
authors.forename: Yan
authors.initials: Y
authors.lastname: Tan
authors.affiliation: Key Laboratory of Drug Metabolism and Pharmacokinetics, Beijing Institute of Pharmacology and Toxicology, Beijing 100850, China State Key Laboratory of Toxicology and Medical Countermeasures, Beijing 100850, China.
authors.forename: Wei-Bin
authors.initials: WB
authors.lastname: Xiao
authors.affiliation: Key Laboratory of Drug Metabolism and Pharmacokinetics, Beijing Institute of Pharmacology and Toxicology, Beijing 100850, China State Key Laboratory of Toxicology and Medical Countermeasures, Beijing 100850, China.
authors.forename: Hai-Ying
authors.initials: HY
authors.lastname: Yang
authors.affiliation: Key Laboratory of Drug Metabolism and Pharmacokinetics, Beijing Institute of Pharmacology and Toxicology, Beijing 100850, China State Key Laboratory of Toxicology and Medical Countermeasures, Beijing 100850, China.
authors.forename: Jian-Wei
authors.initials: JW
authors.lastname: Xie
authors.affiliation: Millennium Pharmaceuticals, Inc., Cambridge, Massachusetts 02139.
authors.forename: Chuang
authors.initials: C
authors.lastname: Lu
authors.affiliation: Key Laboratory of Drug Metabolism and Pharmacokinetics, Beijing Institute of Pharmacology and Toxicology, Beijing 100850, China State Key Laboratory of Toxicology and Medical Countermeasures, Beijing 100850, China amms_hli@126.com.
authors.forename: Hua
authors.initials: H
authors.lastname: Li
mesh_terms: D000110:Acetylcholinesterase / Q000737:chemistry; D000818:Animals; D043182:Carboxylesterase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002800:Cholinesterase Inhibitors / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics* / Q000494:pharmacology; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004593:Electrophorus; D004789:Enzyme Activation; D006801:Humans; D008658:Inactivation, Metabolic; D007306:Insecticides / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics* / Q000494:pharmacology; D008294:Malathion / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics / Q000494:pharmacology; D008862:Microsomes, Liver / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism; D015394:Molecular Structure; D011994:Recombinant Proteins / Q000737:chemistry; D013237:Stereoisomerism
chemical_list: D002800:Cholinesterase Inhibitors; D007306:Insecticides; D011994:Recombinant Proteins; C549713:isocarbophos; D003577:Cytochrome P-450 Enzyme System; D043182:Carboxylesterase; D000110:Acetylcholinesterase; D008294:Malathion
keywords: enantiomers; enzyme kinetics; isocarbophos; isocarbophos oxon; toxicity