EVIDENCE FOR METABOLIC-ACTIVATION OF N'-NITROSONORNICOTINE AND N-NITROSOBENZYL METHYLAMINE BY A RAT NASAL COUMARIN HYDROXYLASE

Kinetic parameters were determined for the hydroxylation of N'-nitroso nornicotine (NNN), N-nitrosobenzylmethylamine (NBzMA), coumarin, and e thoxycoumarin catalyzed by rat nasal mucosa microsomes. NNN is a tobac co-specific nitrosamine that, in rats, causes tumors in the nasal cavi ty and esophagus, whereas NBzMA induces tumors in rat esophagus. Both nitrosamines require Lu-hydroxylation to exert their carcinogenic effe cts. NNN, NBzMA, coumarin, and ethoxycoumarin were all extensively hyd roxylated by rat nasal mucosa microsomes. The K-M values for the hydro xylation of each substrate were low, ranging between 2 and 5 mu M. 2'- and 5'-Hydroxylation of NNN were catalyzed to a similar extent. NBzMA was metabolized predominantly to benzaldehyde, the product of debenzy lation, or methylene hydroxylation. 4-(Methylnitrosamino)-1-(3-pyridyl )-1-butanone (NNK), NNN, and NBzMA were inhibitors of coumarin and eth oxycoumarin hydroxylation. NNN hydroxylation by nasal mucosa microsome s was inhibited by coumarin, ethoxycoumarin, NNK, and NBzMA but not by N-nitrosodimethylamine. 8-Methoxypsoralen, a potent inhibitor of P450 2A6- and 2a5-dependent coumarin hydroxylation in human and mouse live r microsomes, also significantly inhibited NNN activation. The results of this study suggest that the four substrates examined are hydroxyla ted by closely related P450 enzymes in rat nasal mucosa and that a cou marin hydroxylase metabolizes both NNN and NBzMA.