COMPARATIVE METABOLISM OF THE TOBACCO-RELATED CARCINOGENS BENZO[A]PYRENE, 4-(METHYLNITROSAMINO)-1-(3-PYRIDYL)-1-BUTANONE, 4-(METHYLNITROSAMINO)-1-(3-PYRIDYL)-1-BUTANOL AND N'-NITROSONORNICOTINE IN HUMAN HEPATIC MICROSOMES

We compared the metabolism in human hepatic microsomes of three tobacc o smoke carcinogens believed to be involved in the induction of cancer in humans: benzo[a]pyrene aP),4-(methylnitrosamino)-1-(3-pyridyl)-1-b utanone (NNK), and N'-nitrosonomicotine (NNN). The metabolism of 4-(me thylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), a major metabolite of NNK, was also investigated. Although the metabolism of some of these compounds by human enzymes or tissue preparations has been previously examined in some studies, they have never been compared in the same hu man hepatic samples. Moreover, there have been no previous reports of NNAL metabolism by human tissues or enzymes. The tritium-labeled carci nogens (3 mu M) were incubated with 10 different human hepatic microso mal preparations and cofactors for 10-20 min, and the products were an alyzed by radioflow HPLC. NNN was the best substrate for oxidative met abolism, with the 5'-hydroxylation pathway being the predominant one o bserved (mean +/- SD = 31 +/- 17 pmol/min/mg protein). alpha-Hydroxyla tion of NNK by the methylene and methyl hydroxylation metabolic activa tion pathways was the next fastest reaction, with rates of 3.1 +/- 1.9 and 3.3 +/- 1.1 pmol/min/mg protein, respectively. Metabolism of BaP resulted in the formation of dihydrodiols and phenols; trans-7,8-dihyd ro-7,8-dihydroxy-BaP, its major proximate carcinogen, was formed at a rate of 1.1 +/- 0.61 pmol/min/mg protein. cu-Hydroxylation of NNAL pro ceeded at a rate of 0.53 +/- 0.26 pmol/min/mg protein. The results of this study demonstrate that human hepatic microsomes metabolize all of these tobacco carcinogens resulting in a substantial stream of electr ophilic intermediates capable of binding to DNA, The relative rates of oxidative metabolism to electrophiles or their precursors were NNN > NNK > Bap > NNAL. Correlation studies indicated involvement of cytochr ome P4502A6 in the 8'-hydroxylation of NNN and cytochrome P4503A4 in t he cu-methylene hydroxylation and pyridine-N-oxidation of NNK and NNAL . The results of this study provide the first data on the comparative metabolism of these important carcinogens in human hepatic microsomes.