The influence of methyl substitution on the mutagenicity of nitronaphthalenes and nitrobiphenyls

A series of nitrobiphenyls, nitronaphthalenes, and their methyl-substituted derivatives were assayed for mutagenicity toward S. typhimurium TA98 and TA100. In assays conducted in the absence of rat liver S9 fraction, substitution of a methyl group ortho to the nitro group decreased mutagenicity (3-methyl-4-nitrobiphenyl, 2-methyl-1-nitronaphthalene, and 3-methyl-2-nitronaphthalene). The mutagenicity of 4-nitrobiphenyl was also inhibited by methyl substitution at the 2'-position (2'-methyl-4-nitrobiphenyl), and at both the 3- and 2'-positions (3,2'-dimethyl-4-nitrobiphenyl). In assays conducted in the presence of rat liver S9 fraction, inhibition of mutagenicity by methyl substitution was demonstrated for 2-methyl-1-nitronaphthalene, 3-methyl-2-nitronaphthalene and 3,2'-dimethyl-4-nitrobiphenyl. Thus, methyl substitution of nitrobiphenyls and nitronaphthalenes generally decreased mutagenicity, when assays were conducted in the absence of rat liver S9 fraction. However, in the presence of rat liver S9 fraction, the inhibitory effect of methyl substitution on mutagenicity was less pronounced. These results contrast to the usual enhancing effect of ortho-methyl substitution of the corresponding aromatic amines and their N-oxidized derivatives (hydroxylamines and C-nitroso compounds).