Mechanisms of synergism in the mutagenicity of cadmium and N-methyl-N-nitrosourea in Salmonella typhimurium: the effect of pH.

Cadmium enhances the mutagenic effect of N-methyl-N-nitrosourea (MNU) in a synergistic manner in Salmonella typhimurium. In the range of doses that yield synergistic effects, it is by itself highly cytotoxic and only weakly mutagenic. A decrease in pH from 7 to 6 markedly decreases cadmium toxicity, causing a 4-fold increase in the surviving fraction. The same shift in pH markedly increases the dose-dependent mutagenesis of MNU, whether MNU is acting alone or in combination with cadmium and increases the synergism at lower doses of cadmium. Thus, the synergism appears to depend on the mutagenicity of MNU and not on the cytotoxicity of Cd. The combined mutagenic effect of Cd and MNU is comparable in tester strain TA1535 and TA100, which contains the error-prone (SOS) repair-enhancing pKM101 plasmid. Thus the synergistic effect of cadmium is not enhanced by the induction of SOS processing. The hydrolysis of MNU is higher at pH 7 than at pH 6 but cadmium has no effect on the rate of MNU hydrolysis at either pH, and therefore does not influence the concentration of the active electrophile of MNU. Cadmium does not enhance the mutagenesis of ethylnitrosourea, the ethylating analogue of MNU, indicating that the synergism is specific for methylated DNA lesions. These data suggest that cadmium acts either directly by modifying the nature or indirectly by inhibiting the repair of methylation damage.