Metabolism of 2-acetylaminofluorene by two 3-methylcholanthrene-inducible forms of rat liver cytochrome P-450.

The present study examines the contribution of two major 3-methylcholanthrene (3-MC)-inducible forms of rat liver cytochrome P-450 (P-448MC and P-448HCB) to the metabolism of 2-acetylaminofluorene (AAF). In a reconstituted enzyme system, purified rat liver P-448MC metabolized AAF at a 10-fold greater rate than P-448HCB. The major metabolites produced by cytochrome P-448MC were 3-hydroxy (OH) (30%), 5-OH (24%), 7-OH (22%), and 9-OH (10%). N-OH-AAF (3%) was a minor metabolite. In contrast, P-448HCB catalyzed the N-hydroxylation of AAF preferentially (15% of total metabolites). The other primary metabolites formed by this isozyme were 7-OH-AAF (30%), 5-OH-AAF (29%), and 9-OH-AAF (25%). Cytochrome P-448HCB catalyzed the formation of less 3-OH-AAF (7%) than did P-448MC (30%). Since cytochrome P-448HCB is immunochemically related to P-448MC, specific antisera to both isozymes were made by immunoabsorption with the appropriate antigen bound covalently to Sepharose. Anti-P-448MC inhibited AAF metabolism approximately 43% in microsomes from 3-MC-induced male rats and 30% in microsomes from rats treated with 3,4,5,3',4',5'-hexachlorobiphenyl (HCB), another 3-MC-type inducer. Anti-P-448HCB inhibited total metabolism of AAF by only 22 and 38% in microsomes from 3-MC- and HCB-induced rats. However, anti-P-448HCB inhibited N-hydroxylation by 60% in both 3-MC- and HCB-induced microsomes. Anti-P-448MC did not inhibit N-hydroxylation. Neither antibody affected AAF metabolism in control microsomes. These data suggest that, in rat liver, two 3-MC-inducible izozymes of cytochrome P-450 metabolize AAF; however, N-hydroxylation is catalyzed primarily by one of these isozymes, cytochrome P-448HCB.