Metabolism of N-nitrosobenzylmethylamine by human cytochrome P-450 enzymes

N-Nitrosobenzylmethy/amine (NBzMA) is a potent esophageal carcinogen in rod ents, and has been found as a dietary contaminant in certain areas of China where esophageal cancer is endemic. To determine which cytochrome P-450 en zymes in humans are primarily responsible for NBzMA metabolism, microsomes from lymphoblastoid cell lines expressing a panel of human cytochrome P-450 s (CYP1A1, CYP7A2, CYP2A6, CYP2B6, CYP2D6, CYP2E1, CYP2C9, CYP2C19, and CYP 3A4) and a panel of 10 different human liver microsomal preparations were e xamined for their abilities to metabolize [H-3] NBzMA. In addition, the abi lity of human liver microsomes to form various NBzMA metabolites was correl ated with the abilities of these preparations to metabolize coumarin, ethox y-resorufin, chlorzoxazone, 7-ethoxy-4-trifiuoromethylcoumarin, S-mephenyto in, and nifedipine. NBzMA metabolites were quantitated by reversed-phase hi gh-performance liquid chromatography (HPLC) coupled with flow-through radio activity detection. Major metabolites included benzaldehyde, benzyl alcohol , benzoic acid, and several uncharacterized radioactive peaks. Of the repre sentative P-450 activities, only CYP2E1 and CYP2A6 catalyzed substantial me tabolism of NBzMA. Compared to CYP2E1, CYP2A6 metabolized NBzMA more readil y. NBzMA acted as a potent inhibitor of coumarin 7-hydroxylation in CYP2A6 microsomes. Human liver microsomes metabolized NBzMA readily. NBzMA metabol ite formation was most highly correlated with coumarin 7-hydroxylase activi ty, a marker of CYP2A6 activity. 8-Methoxypsoralen substantially inhibited NBzMA metabolism in human hepatic microsomes. When the effects of the poten t isothiocyanates PEITC and PHITC were analyzed on microsomes from cell lin es expressing CYP2E1 and CYP2A6, it was found that PEITC inhibited both enz ymes, PHITC was the more effective inhibitor of CYP2E1, and PHITC was an in effective inhibitor of CYP2A6. Collectively, these data indicate that CYP2A 6 and, to a lesser degree, CYP2E1 are important P-450 enzymes in the activa tion of NBzMA in human systems.