Evidence for cytochrome P450 2A6 and 3A4 as major catalysts for N'-nitrosonornicotine α-hydroxylation by human liver microsomes

The tobacco specific carcinogen N'-nitrosonornicotine (NNN), is believed to be a causative agent for esophageal cancer in smokers. NNN requires metabolic activation to exert its carcinogenic potential. Metabolism occurs through cytochrome P450 (P450) catalyzed 2'- and 5'-hydroxylation, which generates unstable metabolites that decompose to 4-hydroxy-1-(3-pyridyl)-1-butanone ('keto alcohol') and 4-hydroxy-4-(3-pyridyl)butanal, respectively. The latter cyclyzes to 5-(3-pyridyl)-2-hydroxytetrahydrofuran ('lactol'). 2'-Hydroxylation of NNN is believed to be the pathway critical for esophogeal NNN carcinogenesis in the rat. The ability of human liver microsomes and expressed human P450s to metabolize [5-³H]NNN to keto alcohol and lactol was determined by reverse phase HPLC with radioflow detection. At low NNN concentrations, 11 human liver microsomes metabolized NNN primarily by 5'-hydroxylation to lactol. This reaction was strongly correlated (r = 0.92) with coumarin 7-hydroxylation, suggesting that NNN 5'-hydroxylation is catalyzed mainly by P450 2A6. 2'-Hydroxylation of NNN by human liver microsomes correlated with 6β-hydroxylation of testosterone, a P450 3A4-specific activity (r = 0.94). The relative rates of 2'- and 5'-hydroxylation by human P450s 2A6, 2E1, 2D6 and 3A4 expressed in Sf9 cells by the baculovirus-insect cell expression system, and human P450 3A4 produced by stable expression in Chinese hamster ovary cells, were determined. Human P450 2A6 metabolized 1 μM NNN exclusively by 5'-hydroxylation. The rate of lactol formation was 317 pmol/min per nmol P450. Human P450s 2E1 and 2D6 also metabolized NNN only to lactol, but at much lower rates, 0.4 and 0.8 pmol/min per nmol of P450 respectively. In contrast, the metabolism of NNN by expressed human P450 3A4 was specific for keto alcohol formation. The K(m) for 5'-hydroxylation by baculovirus-expressed P450 2A6. was 2.1 μM, and k(cat) was 953 pmol/min per nmol of P450. The K(m) for lactol formation by human liver microsomes containing high levels of P450 2A6 was 5 μM. Human liver microsomes exhibited a K(m) of 312 μM for keto alcohol formation. Coumarin, 8-methoxypsoralen (P450 2A6 inhibitors), and anti-2A6 monoclonal antibody were strong inhibitors of NNN-derived lactol formation in human liver microsomes. Troleandomycin, an inhibitor of P450 3A4, effectively inhibited the metabolism of NNN to keto alcohol by human liver microsomes. These results are consistent with P450 2A6 mediated 5'-hydroxylation and P450 3A4 mediated 2'-hydroxylation of NNN in human liver microsomes.