title: N4-hydroxylation of sulfamethoxazole by cytochrome P450 of the cytochrome P4502C subfamily and reduction of sulfamethoxazole hydroxylamine in human and rat hepatic microsomes.
abstract: The N4-hydroxylation of sulfamethoxazole (SMX) to its hydroxylamine (SMX-HA) metabolite is the first step in the formation of reactive metabolites responsible for mediating hypersensitivity reactions associated with this compound. In rat hepatic microsomes, the NADPH-dependent oxidation of SMX to SMX-HA was increased 3-fold by pretreatment of rats with phenobarbital. Other cytochrome P450 (CYP) inducers were ineffective. The constitutive and induced SMX N-hydroxylation activities were inhibited by tolbutamide, and induction of SMX-HA activity paralleled the induction of progesterone 21-hydroxylase activity, a marker for CYP2C6. SMX N-hydroxylation in phenobarbital-treated rat hepatic microsomes was inhibited 70% by anti-CYP2C6 antisera. Thus, the N4-hydroxylation of SMX by rat hepatic microsomes was mediated by members of the CYP2C subfamily, probably CYP2C6. In a panel of human microsomes, SMX-HA formation correlated with tolbutamide hydroxylase activity (r = 0.75; p = 0.01); CYP2C9 content (r = 0.79; p < 0.01) and was inhibited 70% by 500 microM tolbutamide and 90% by 100 microM sulfaphenazole. Recombinant CYP2C9 catalyzed the N-hydroxylation of SMX. SMX-HA formation in human hepatic microsomes was therefore mediated predominantly by CYP2C9. CYP-mediated reduction of SMX-HA to SMX was markedly induced in dexamethasone and phenobarbital-treated rat hepatic microsomes, and was attributed to CYP3A and CYP2B forms. In uninduced rat and human hepatic microsomes, SMX-HA reduction was mediated predominantly by an NADH-dependent microsomal hydroxylamine reductase under aerobic conditions. Under anaerobic conditions, troleandomycin at > or = 1 microM inhibited the reduction of SMX-HA in human hepatic microsomes by 45%, whereas sulfaphenazole had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Drug Metabolism, Merck Research Laboratories.
authors.forename: A E
authors.initials: AE
authors.lastname: Cribb
authors.forename: S P
authors.initials: SP
authors.lastname: Spielberg
authors.forename: G P
authors.initials: GP
authors.lastname: Griffin
mesh_terms: D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006801:Humans; D006900:Hydroxylation; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D010084:Oxidation-Reduction; D051381:Rats; D017207:Rats, Sprague-Dawley; D013420:Sulfamethoxazole / Q000031:analogs & derivatives* / Q000378:metabolism*
chemical_list: C059873:sulfamethoxazole hydroxylamine; D003577:Cytochrome P-450 Enzyme System; D013420:Sulfamethoxazole