Metabolism of 1‐, 3‐, and 6‐nitrobenzo[a]pyrene by intestinal microflora

Abstract

The compounds 1‐, 3‐, and 6‐nitrobenzo[a]pyrene (nitro‐BaP) are environmental pollutants and have been shown to be potent bacterial mutagens. The anaerobic metabolism of these isomeric nitro‐BaPs was investigated by the incubation of rat intestinal microflora with each isomer for 48 h. Aliquots were removed at several time intervals, extracted, fractionated by high‐pressure liquid chromatography (HPLQ, and the radioactivity determined. Metabolites were identified by comparison of their chromato‐graphic, ultraviolet‐visible absorption, and mass spectral properties with those of authentic standards. The order of the extent of nitroreduction for these isomers was 3‐nitro‐BaP> 6‐nitro‐BaP> 1‐nitro‐BaP. After 48 h of exposure, 84% of the added 3‐nitro‐BaP was present as 3‐amino‐BaP, 51% of the 6‐nitro‐BaP was metabolized to 6‐amino‐BaP, and 1‐nitro‐BaP was reduced to 1‐amino‐BaP (13%) and 1‐nitroso‐BaP (4%). The order of the extent of microbial nitroreduction for these nitro‐BaP isomers is different from the predictions based on electronic and steric hindrance effects. These results suggest that intestinal microflora nitroreductases exhibit a markedly high degree of substrate specificity toward nitro‐BaPs that affects the extent of nitroreduction.