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.