The role of cytochrome P4503A1/2 in the sex-specific sulfoxidation of the hexachlorobutadiene metabolite, N-acetyl-S- (pentachlorobutadienyl) -L-cysteine in rats
Werner, M.; Birner, G.; Dekant, W.
Drug Metabolism and Disposition the Biological Fate of Chemicals 23(8): 861-868
1995
ISSN/ISBN: 0090-9556 PMID: 7493554 Document Number: 440054
Hexachlorobuta-1,3-diene (HCBD) is a selective nephrotoxin and a potent nephrocarcinogen in rodents. Its toxicity and carcinogenicity is based on a multistep bioactivation pathway. Glutathione conjugation seems to be the only bioactivation pathway for HCBD leading to reactive intermediates, which are thought to be responsible for the observed nephrotoxic effects. Recent in vivo studies revealed a novel urinary metabolite in male, but not female, rats after administration of (14C)HCBD. This metabolite was identified as (E)-N-acetyl-S-(1,2,3,4,4-pentachlorobutadienyl)-L-cysteine-sulfoxide (N-Ac-PCBC-SO). The objective of this study was to elucidate the enzyme(s) involved in this particular oxidation and to find an explanation for the sex differences in the formation of N-AcPCBC-SO. Both cytochrome P450 and flavin-containing monooxygenases (FMO) may catalyze the oxidation of N-Ac-PCBC-SO. The use of various competitive and allosteric inhibitors of cytochrome P450 and FMO (i.e. metyrapone, N-benzylimidazole, thiobenzamide, CO, n-octylamine, and heat inactivation studies) showed that the sulfoxidation of (E)-N-acetyl-S-(1,2,3,4,4-pentachlorobutadienyl)-L-cysteine is catalyzed by cytochrome P450 enzymes. In microsomes from male rats pretreated with pyridine, phenobarbital, and dexamethasone, an increase in the rates of sulfoxide formation was only seen in microsomes from dexamethasone- and phenobarbital-induced animals. Moreover, troleandomycin, a selective chemical inhibitor for enzymes of the cytochrome P4503A family, inhibited sulfoxide formation by gt 80%. Correlation of sulfoxide formation with testosterone 6-beta-hydroxylation, a marker of cytochrome P4503A1/2 in the rat, underlined the finding that cytochrome P4503A is the predominant cytochrome P450 responsible for this particular oxidation. Cytochromes P4502B1/2B2, however, have been shown to be minor contributors to the sulfoxidation reaction in phenobarbital-induced liver microsomes, as indicated by the use of an inhibitory antibody for rat cytochromes P4502B1/2 and by performing kinetic studies in the presence of troleandomycin. Beside the beta-lyase-mediated bioactivation of haloalkenes, the formation of chlorovinyl sulfoxides may represent an additional pathway leading to reactive intermediates, which could bind to macromolecules as Michael-acceptor substrates without further activation.