Metabolism of l-nitrobenzo[a]pyrene by rat liver microsomes to potent mutagenic metabolites

1-,3- and 6-Nitrobenzo[a]pyrene (nitro-BaP), which are prototypes of nitro polycyclic aromatic hydrocarbons (nitro-PAHs) derived from a carcinogenic parent PAH, benzo[a]pyrene, are environmental contaminants and potent bacterial mutagens. In this study, the aerobic and hypoxic metabolism of 1-nitro-BaP by rat liver microsomes was studied. Aerobic metabolism of 1-nitro-BaP yielded 1-nitro-BaP trans-7,8- and 9,10-dihydrodiol, while metabolism under hypoxic conditions yielded 1-amino-BaP. The metabolites formed from aerobic metabolism of 1-nitro-BaP and 1-nitro-BaP trans-9,10-dihydrodiol by liver microsomes of untreated rats and rats pretreated with 3-methylcholanthrene and phenobarbital were quantified. Comparison of these results with those obtained with BaP and BaP trans-9,10-dihydrodiol indicates that nitro substitution at the 1-position of BaP markedly affects the regioselectivity of the P-450-containing enzymes. 1-Nitro-BaP and the three metabolites were potent mutagens in Salmonella typhimurium TA98, both in the absence and in the presence of an exogenous metabolic activation system (S9). The direct and S9-mediated mutagenicities of 1-nitro-BaP and the two dihydrodiols were decreased in the nitroreductase-deficient strain TA98NR, while TA98/1,8-DNP6, an O-acetylase-deficient strain, was less sensitive to the two dihydrodiols, both with and without S9, and 1-nitro-BaP with S9. 1-Amino-BaP was equally mutagenic in all three tester strains. These observations indicate that: the metabolism of 1-nitro-BaP involves several pathways leading to mutagenic activation; the major activation pathways of 1-nitro-BaP involve nitroreduction; nitroreduction followed by O-acetylation is the major activation pathway of 1-nitro-BaP trans-7,8- and 9,10-dihydrodiol; and 1-amino-BaP is a potent direct-acting mutagen.