Mutagenic activation of selected aminoazo compounds by rat liver: evidence for a cytochrome P-448 dependent reaction.

Mutagenicity of 3'-methyl-N,N-dimethyl-4-aminoazobenzene (3'-Me-DAB) and its N-demethylated products was examined using rat liver 9,000 g supernatant fraction (S-9) together with Salmonella typhimurium TA98 or TA100 as a tester strain. The expression of mutagenicity of 3'-Me-DAB required the presence of both microsomes and cytosol as sources of enzymes as well as NADPH as a cofactor. 3'-Me-DAB and its N-demethylated products showed no mutagenicity towards either strain when preincubated with S-9 from untreated rat livers. The involvement of a cytochrome P-450 species in the mutagenic activation was demonstrated with hepatic S-9 by using specific enzyme inducers and inhibitors. The treatment of rats with polychlorinated biphenyls or 3-methylcholanthrene resulted in a marked increase in the ability of S-9 to activate these compounds, whereas phenobarbital induction was not effective. All the mutagenic activities were considerably decreased by adding cytochrome c to the S-9 mixture, but the activation was insensitive to 1-(1-naphthyl)-2-thiourea and methimazole, high-affinity flavin-containing monooxygenase substrates. Carbon monoxide, metyrapone, and 2-diethylaminoethyl-2,2-diphenylvalerate hydrochloride, potent cytochrome P-450 inhibitors, had no inhibitory effect on the mutagenic activation. In contrast, 7,8-benzoflavone, a specific inhibitor of cytochrome P-448, considerably inhibited the reaction. These results suggest that cytochrome P-448 and a cytosol component are involved in the mutagenic activation of 3'-Me-DAB and its N-demethylated products.