Mutagenicity of nitric oxide and its inhibition by antioxidants

Nitric oxide (NO) is produced both by macrophages in vivo as a physiological response to infection and by a variety of cell types as an intercellular messenger. In addition, NO and nitrogen dioxide (NO₂) are significant components of many combustion processes. The ubiquitous exposure of humans to nitrogen oxides (NO_x), both endogenously and exogenously, may play a significant role in the carcinogenic process due to nitrosation of amines by NO_x. We report here that exposure to low concentrations of NO, alone or in combination with NO₂, results in significantly enhanced mutation in Salmonella typhimurium TA1535 using a modified Ames Salmonella reversion assay. The observed mutagenicity requires that the bacteria be actively dividing at the time of exposure to NO or NO₂, suggesting that the nitrogen oxides, or their reaction products, function as direct-acting mutagens and that the induced lesion is easily repairable by non-dividing cells. Exposure to NO resulted in a time- and dose-dependent increase in the number of revertants approximately proportional to the square of the NO concentration from 0 to 20 ppm. NO was a more effective mutagen relative to NO₂, however, the observed requirement for 0₂ suggests limited oxidation of NO (presumably to NO₂) is necessary. Numerous lipid- and aqueous-phase inhibitors of nitrosation, as well as a number of other general antioxidants and free-radical trapping agents, were examined for their effectiveness in blocking the mutagenic effects of NO. The mutagenic activity of NO was most effectively inhibited by β-carotene and tocopherols. BHT, dimethyl sulfoxide and mannitol also blocked the mutagenic effects of NO_x but appeared less effective than β-carotene or vitamin E, while ascorbate was ineffective as an inhibitor of mutation resulting from NO exposure.