Enhanced mutagenicity of 2-nitropropane nitronate with respect to 2-nitropropane--possible involvement of free radical species.

The mutagenicities of the neutral and the anionic (nitronate) forms of 2-nitropropane (2-NP) were compared in S. typhimurium strains TA98, TA100 and TA102. The latter is a special strain sensitive to compounds producing oxidative damage to DNA at thymine-adenine base pair loci. Neutral 2-NP was not mutagenic in TA98, and produced significant mutagenic response in TA100 and TA102 only at levels of 55 mumoles/plate. In contrast, 2-NP nitronate was significantly mutagenic in TA98 at 14 mumoles/plate and, in strains TA100 and TA102, at approximately 4 mumoles/plate. Inclusion of S9 slightly increased the mutagenicity of 2-NP nitronate in TA102, but decreased it in TA100. The mutagenic response in TA102, but not in TA100, was inhibited by DMSO, a scavenger of hydroxyl radicals, in a dose-dependent manner. When 2-NP nitronate was incubated with thymidine and horseradish peroxidase-H2O2, the formation of 2-NP dimer (2,3-dimethyl-2,3-dinitrobutane), a condensation product of 2-NP free radicals, was observed. This was accompanied by 2-NP nitronate-dependent oxidation of thymidine to thymidine glycol, thymine glycol, 5-hydroxymethyldeoxyuridine and thymine, indicating that hydroxyl radicals and/or other reactive oxygen species capable of causing thymidine damage are also formed in this reaction. As a working hypothesis, we suggest that the genotoxicity of 2-NP nitronate may be due to the generation of DNA-damaging reactive forms of oxygen or 2-NP free radicals.