Enzymes involved in the metabolism of the carcinogen 2-nitroanisole: evidence for its oxidative detoxication by human cytochromes P450.

2-Nitroanisole (2-NA) is an important industrial pollutant and a potent carcinogen for rodents. Determining the capability of humans to metabolize 2-NA and understanding which human cytochrome P450 (P450) enzymes are involved in its activation and/or detoxification are important to assess an individual's susceptibility to this environmental carcinogen. We compared the ability of hepatic microsomal samples from different species including human to metabolize 2-NA. Comparison between experimental animals and human P450 enzymes is essential for the extrapolation of animal carcinogenicity data to assess human health risk. Human hepatic microsomes generated a pattern of 2-NA metabolites, reproducing that formed by hepatic microsomes of rats and rabbits. An O-demethylated metabolite of 2-NA (2-nitrophenol) and two ring-oxidized derivatives of this metabolite (2,6-dihydroxynitrobenzene and 2,X-dihydroxynitrobenzene) were produced. No nitroreductive metabolism leading to the formation of o-anisidine was evident with hepatic microsomes of any species. Likewise, no DNA binding of 2-NA metabolite(s) measured with either tritium-labeled 2-NA or the (32)P-postlabeling technique was detectable in microsomes. Therefore, hepatic microsomal P450 enzymes participate in the detoxication reactions of this environmental carcinogen. Using hepatic microsomes of rabbits pretreated with specific P450 inducers, microsomes from Baculovirus transfected insect cells expressing recombinant human P450 enzymes, purified P450 enzymes, and selective P450 inhibitors, we found that human recombinant P450 2E1, 1A1, and 2B6, as well as orthologous rodent P450 enzymes, are the most efficient enzymes metabolizing 2-NA. The role of specific P450 enzymes in the metabolism of 2-NA in human hepatic microsomes was investigated by correlating specific P450-dependent reactions with the levels of 2-NA metabolites formed by the same microsomes and by examining the effects of specific inhibitors of P450 enzymes on 2-NA metabolism. On the basis of these studies, we attribute most of the 2-NA oxidation metabolism in human microsomes to P450 2E1. These results, the first report on the metabolism of 2-NA by human P450 enzymes, clearly demonstrate that P450 2E1 is the major human enzyme oxidizing this carcinogen in human liver.