Comparative mutagenicity of halomethanes and halonitromethanes in Salmonella TA100: structure-activity analysis and mutation spectra.

Halonitromethanes (HNMs) are a recently identified class of disinfection by-products (DPBs) in drinking water that are mutagenic in Salmonella and potent inducers of DNA strand breaks in mammalian cells. Here we compared the mutagenic potencies of the HNMs to those of their halomethane (HM) homologues by testing all nine HNMs and seven of the nine HMs (minus bromomethane and chloromethane) under the same conditions (the pre-incubation assay) in Salmonella TA100 +/- S9. We also determined the mutation spectra for several DBPs. In the presence of S9, all nine HNMs, but only three HMs, dibromomethane (DBM), dichloromethane (DCM), and bromochloromethane (BCM), were mutagenic. Only two DBPs of each class were mutagenic in the absence of S9. The HNMs were generally more potent mutagens than their HM homologues, and the brominated forms of both classes of DBPs were more mutagenic and cytotoxic than their chlorinated homologues. The HNMs were at least 10 times more cytotoxic than the HMs, and the cytotoxicity rankings in the presence of S9 were similar for the HNMs and the HMs. The addition of a nitro-group to BCM did not change the mutation spectra significantly, with both homologues inducing primarily (55-58%) GC --> AT transitions. The greater cytotoxic and mutagenic activities of the HNMs relative to the HMs are likely due to the greater intrinsic reactivity conferred by the nitro-group. Energy calculations predicted increased reactivity with increasing bromination and greater reactivity of the HNMs versus the HMs (Elumo values were approximately 20 kcal/mol lower for the HNMs compared to their HM homologues). Given that the HNMs also are potent genotoxins in mammalian cells [Environ. Sci. Technol. 38 (2004) 62] and are more mutagenic and 10x more cytotoxic in Salmonella than the HMs, whose levels are regulated in drinking water, further study of their occurrence and potential health effects is warranted.