Differential metabolism of acetanilide versus ethoxycoumarin and benzo[a]pyrene by two 3-methylcholanthrene-inducible forms of rat liver cytochrome P-450.
The present study compares the catalytic activities of two 3-methylcholanthrene (3-MC) inducible forms of cytochrome P-450. These isozymes (P-448HCB and P-448MC) were isolated from liver microsomes of rats treated with 3,4,5,3',4',5'-hexachlorobiphenyl (HCB) and 3-MC, respectively. Catalytic activities of the isozymes were compared in a reconstituted system and by antibody inhibition studies in microsomes. In a reconstituted system, P-448HCB had very little catalytic activity toward benzo[a]pyrene or ethoxycoumarin (substrates metabolized preferentially by P-448MC). In contrast, both isozymes had high turnover numbers for aniline and acetanilide. However, catalytic activities of the purified isozymes were affected dramatically by Emulgen 911, a nonionic detergent. Since nonionic detergents used in the purification of P-450 isozymes cannot be completely removed after purification, residual amounts of detergent probably affect turnover numbers in a reconstituted system. Therefore, specific antibodies to cytochromes P-448MC and P-448HCB were used to examine the contribution of these isozymes to microsomal metabolism. Antibody inhibition studies confirmed that the majority of benzo[a]pyrene and ethoxycoumarin metabolism in 3-MC-induced microsomes was catalyzed by cytochrome P-448MC. In contrast, P-448HCB accounted for the majority of the acetanilide hydroxylase activity in 3-MC- and HCB-induced microsomes. Neither isozyme contributed appreciably to metabolism of these substrates in control microsomes.
D000083:Acetanilides / Q000378:metabolism*; D000818:Animals; D001564:Benzo(a)pyrene; D001580:Benzopyrenes / Q000378:metabolism*; D002273:Carcinogens / Q000378:metabolism*; D003374:Coumarins / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D004790:Enzyme Induction; D008297:Male; D008748:Methylcholanthrene / Q000494:pharmacology*; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D011078:Polychlorinated Biphenyls / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013329:Structure-Activity Relationship
D000083:Acetanilides; D001580:Benzopyrenes; D002273:Carcinogens; D003374:Coumarins; C017299:7-ethoxycoumarin; D001564:Benzo(a)pyrene; D008748:Methylcholanthrene; D003577:Cytochrome P-450 Enzyme System; D011078:Polychlorinated Biphenyls; C027309:3,4,5,3',4',5'-hexachlorobiphenyl