title: Human cytochrome P450-catalyzed conversion of the proestrogenic pesticide methoxychlor into an estrogen. Role of CYP2C19 and CYP1A2 in O-demethylation.
abstract: 1,1,1-Trichloro-2,2-bis(4-methoxyphenyl)ethane (methoxychlor) is a widely used pesticide that is pro-estrogenic. We have elucidated the human cytochrome P450 enzymes responsible for conversion of methoxychlor into its major metabolite, the mono-O-demethylated derivative (mono-OH-M) that is estrogenic. Incubation of methoxychlor with microsomes from insect cells overexpressing either CYP1A2, CYP2C18, or CYP2C19 yielded mono-OH-M with turnover numbers of 14.9, 15.5, and 39.1 nmol/min/nmol of P450, respectively. CYP2B6 and CYP2C9 were much less active. Incubations with purified CYP2C19 and CYP2C18 resulted in formation of mono-OH-M, and also the bis-demethylated metabolite. Co-incubation of liver microsomes with methoxychlor and various P450 isoform-selective inhibitors suggested involvement of several P450s in mono-O-demethylation, including CYP1A2, CYP2A6, CYP2C9, and CYP2C19. A role for CYP2C19, CYP1A2, and CYP2A6 was also indicated by multivariate regression analysis of the mono-O-demethylase activity in a panel of human liver microsomes characterized for isoform-specific catalytic activities (R2 = 0.96). Based on the totality of the evidence, CYP2C19 appears to be the major catalyst of methoxychlor mono-O-demethylation. However, in individuals lacking functional CYP2C19 (e.g. the "poor metabolizer" phenotype), CYP1A2 may play the predominant role. CYP2A6, CYP2C9, and CYP2B6 probably contribute to a lesser extent. Although CYP2C18 is an efficient methoxychlor demethylase, its expression in liver is reportedly low or absent, suggesting a negligible role for this enzyme in methoxychlor metabolism. Lengthy incubations of liver microsomes with methoxychlor produced other secondary and tertiary metabolites. Efficient conversion of methoxychlor to estrogenic mono-OH-M by liver microsomes suggests that methoxychlor has the potential to be estrogenic in humans, as observed in several animal species.
authors.affiliation: Department of Pharmacology and Molecular Toxicology, University of Massachusetts Medical Center, USA.
authors.forename: D M
authors.initials: DM
authors.lastname: Stresser
authors.forename: D
authors.initials: D
authors.lastname: Kupfer
mesh_terms: D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism; D065731:Cytochrome P-450 CYP2C19; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003640:Dealkylation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D045166:Estradiol Congeners / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D007306:Insecticides / Q000493:pharmacokinetics*; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008731:Methoxychlor / Q000493:pharmacokinetics*; D008862:Microsomes, Liver / Q000378:metabolism; D006899:Mixed Function Oxygenases / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D011994:Recombinant Proteins / Q000378:metabolism; D012044:Regression Analysis
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D045166:Estradiol Congeners; D007306:Insecticides; D007527:Isoenzymes; D011994:Recombinant Proteins; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; D019388:Cytochrome P-450 CYP1A2; D065731:Cytochrome P-450 CYP2C19; D008731:Methoxychlor