DNA-damaging effects of genotoxins in mixture: Nonadditive effects of aflatoxin B-1 and N-acetylaminofluorene on their mutagenicity in Salmonella typhimurium

Most animal genotoxicity studies have used exposures to single chemicals; h umans, however, are potentially exposed to mixtures of genotoxins. Cancer a nd developmental toxicity risks associated with genotoxins in mixture are g enerally estimated by assuming additivity of the components. Two or more ge notoxins acting sequentially or simultaneously may present a greater or les ser hazard than that predicted by simple addition of their potencies. Previ ously, we studied the effect of one genotoxin on the binding of a second ge notoxin to DNA in an in vitro system and demonstrated that consecutive bind ing of the two toxins was not additive. In the present study, the effect of one genotoxin on the mutagenicity of another was evaluated for two well-kn own genotoxins using the Salmonella assay. Pretreatment of frameshift strai ns TA98 and TA1538 with AFB(1)-8,9-epoxide (17.3 ng/plate) enhanced the mut agenicity induced by subsequent exposure to N-acetoxy-acetylaminofluorene ( N-AcO-AAF) similar to 2-3 times above theoretical values for additivity. Pr etreatment of base-substitution strain TA100 with N-AcO-AAF (0.1 mu g/plate ) inhibited the mutagenicity following subsequent exposure to AFB(1)-8,9-ep oxide by 3 times below the theoretical additive value. Concentration-respon se relationships for these enhancing or inhibitory effects were demonstrate d using increasing concentrations of the first genotoxin during pretreatmen t. These results demonstrate effects, other than additive, of sequential ex posures to two genotoxins on the induction of mutations in a bacterial syst em.