Mutagenicity and toxicity of the DNA alkylation carcinogens 1,2-dimethylhydrazine and azoxymethane in Escherichia coli and Salmonella typhimurium.

DNA alkylating agents such as 1,2-dimethylhydrazine (SDMH) and azoxymethane (AOM) are potent carcinogens and are widely used to induce colon tumors in experimental animals. However, standard bacterial mutagenesis assays have failed to detect the mutagenic effects of these chemicals. Using derivatives of a set of Escherichia coli test strains developed by Cupples and Miller (Proc. Natl. Acad. Sci. USA, 86, 5345, 1989), we hve demonstrated that under two conditions, SDMH and AOM induced point mutations by several-fold in a dose-dependent manner: (i) of six possible base substitutions, they only induced GC-->AT transitions; and (ii) the cells must be deficient in O6-methylguanine (O6MeG) DNA methyltransferase (MTase) activity. SDMH and AOM up to 200 microg/ml were unable to induce His+ revertants in a Salmonella Ames test strain TA1535 (GC-->AT); however, in the absence of mammalian S9 extract, His+ revertants increased up to 55-fold upon treatment of an isogenic Salmonella strain deficient in MTase activity. These results indicate that SDMH and AOM are indeed bacterial mutagens and that lesions induced by them are the target of DNA repair MTases, which probably include mutagenic and carcinogenic lesions such as O6MeG. Furthermore, variable responses of bacterial species to SDMH- and AOM-induced mutagenicity suggests a difference either in the metabolism of potential mutagens or in the repair of specific lesions. Since O6MeG is not only a mutagenic lesion but also a lethal lesion if left unrepaired, we compared the mutagenicity and toxicity of SDMH and AOM with an SN-type methylating carcinogen. N-methyl-N'-nitro-N-nitrosoguanidine, and conclude that SDMH and AOM are weak bacterial mutagens.