Bacterial Nitroreductase

Nitroreductase similar to azoreductase is exclusively of bacterial origin in the lumen of the intestine. The enzyme is required for the mutagenic activity of nitrocompounds (64). Nitroreductase generates reactive nitroso and N-hydroxyintermediates in the course of converting aromatic amines. 1-nitropyrene is formed by the reaction of nitrogen oxides with the combustion product pyrene. The presence of 1-nitropyrene in diesel exhaust makes exposure to this compound a real risk. 1-nitropyrene is mutagenic in bacterial test systems, and carcinogenic when administered to the rat. When 1-nitropyrene was fed to conventional rats 5% to 6% of the dose was detected in the feces as 1-aminopyrene (69). When the same feeding experiment was performed with germfree rats no 1-aminopyrene was detected in the feces. The reduction of 1-nitropyrene to 1-aminopyrene is a carcinogen activation process, and the results cited above indicate that the intestinal microflora are important in the activation of 1-nitropyrene.

Mixed bacterial fecal specimens obtained from humans have been shown to reduce 6-nitrochrysene to 6-aminochrysene, a compound that causes cancer in mice (70). The intermediate nitrosopolychic aromatic hydrocarbons generated in the conversion of the nitro groups to an amine are highly reactive compounds that can alter DNA.

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