Metabolism of carcinogenic heterocyclic and aromatic amines by recombinant human cytochrome P450 enzymes

The N-hydroxylation of carcinogenic arylamines represents an initial stepin their metabolic activation. Animal studies have shown that this reactionis catalyzed by the cytochrome P450 (P450) enzymes P450 1A1 and P450 1A2.In this study, utilizing enzymes expressed in Escherichia coli (andpurified) or in human B-lymphoblastoid cells, the catalytic activities ofrecombinant human P450 1A1, P450 1A2, and P450 3A4 for N- hydroxylation ofseveral carcinogenic arylamines were determined. P450 1A2 from bothexpression systems catalyzed the N-hydroxylation of 4- aminobiphenyl andthe heterocyclic amines, 2-amino-3-methylimidazo[4,5- f/quinoline (IQ),2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), and2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Rates were similar,with values of 1.1-7.8 nmol/min/nmol P450. In contrast, P450 1A1 catalyzedN-hydroxylation of only PhIP, and no activity was observed with P450 3A4.Further kinetic analysis with purified P450 1A2 showed similar Km and Vmaxvalues for N-hydroxylation of the arylamines. Furafylline and fluvoxamine,inhibitors of P450 1A2 activity in human liver microsomes, were found to beinhibitory of the recombinant P450 1A2 N-hydroxylation activity. Resultsfrom this study are supportive of a major role for human P450 1A2 in themetabolic activation of arylamines.