A series of aminobiphenyls and aminonaphthalenes were assayed for mutagenicity toward S. typhimurium, in the presence of rat liver 9000 g supernatant, to investigate the effects of positional isomerism and ortho-methyl substitution. Among the 3 possible aminobiphenyl isomers, only 4-aminobiphenyl was a potent mutagen, showing activity in S. typhimurium TA1538 and TA100, but not TA1535. 4-Amino-3-methylbiphenyl was a more potent mutagen in S. typhimurium TA1538 than was 4-aminobiphenyl, whereas both 3-amino-4-methylbiphenyl and 3-aminobiphenyl were inactive in this strain. 3-Amino-4-methylbiphenyl was mutagenic in S. typhimurium TA100, in contrast to 3-aminobiphenyl. These results demonstrate the enhancing effect of an ortho-methyl group on mutagenicity in the aminobiphenyls, which contrasts to the inhibitory effect of methyl substitution frequently observed in the 4-nitrobiphenyl system. Among the 2-aminonaphthalenes, 2-amino-3-methylnaphthalene was the most mutagenic compound in S. typhimurium TA1538, followed by 2-amino-1-methylnaphthalene and 2-aminonaphthalene. In S. typhimurium TA1535, however, 2-aminonaphthalene was a more potent mutagen than either of the ortho-methyl substituted derivatives. No significant mutagenic activity was observed for either 1-aminonaphthalene or 1-amino-2-methylnaphthalene in S. typhimurium TA1538 and TA1535. However, 1-aminonaphthalene was weakly mutagenic in S. typhimurium TA 100.
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