Factors influencing the mutagenic activity of the colon carcinogen 1,2-dimethylhydrazine in Salmonella typhimurium strain TA 1535 in vitro

The colon carcinogen 1,2-dimethylhydrazine (SMDH), a non-mutagenin the standard Ames assay, has been shown in previous experimentsto become weakly mutagenic in Salmonella TA1535 in vitro, whenspecific test conditions were used. The present studies wereperformed to determine more precisely the nature of metabolicfactors and experimental conditions for optimal mutagenesisof SDMH in the same strain of Salmonella. First, it was confirmedthat both the presence of rat liver S9 fractions (25 µl/mlincubation mixture) and prolonged pre-incubation periods inliquid medium of at least 120 min were necessary to elicit SDMHmutagenesis. In contrast to results obtained with dimethylnitrosamine,which served as a model compound for the activation throughoxidative, cytochrome P-450- and NADPH-dependent enzymatic processes,the activation of SDMH to mutagenic factors was not dependenton the presence of NADPH: in fact, NADPH strongly reduced theSDMH-induced mutation yields. It was also observed that growthof the indicator bacteria is an important prerequisite for mutationinduction by SDMH. Aminoacetonitrile and disulfiram, two inhibitorsof SDMH metabolism and carcinogenicity in mammals, also stronglyinhibited SDMH mutagenesis in the present in vitro assay. Itcan, therefore, be concluded that (i) the right test protocolis of crucial importance for the detection of SDMH as a bacterialmutagen, and (ii) that activation pathways in vitro are (partially)different from presumed in vivo metabolism and activation.