Activation of bis-electrophiles to mutagenic conjugates by human O6-alkylguanine-DNA alkyltransferase.

O(6)-Alkylguanine DNA-alkyl transferase (AGT) has been shown to conjugate both 1,2-dibromoethane and dibromomethane, yielding AGT inactivation, DNA-AGT cross-linking, and enhanced mutagenicity. A variety of related chemicals were examined to determine if similar phenomena occur. Among the compounds examined in these systems (histidine operon reversion in Escherichia coli and Salmonella typhimurium tester strains), a strong halide order was generally observed, with increasing activities in the order I > Br >> Cl. At least one Br atom appeared to be required for human AGT-dependent mutations, and compounds with only Cl did not inhibit AGT and were not activated to genotoxins. Of a series of haloforms tested (CHX(3), X = Br or Cl), all were without effect. Among a series of alpha,omega-disubstituted dihaloalkanes (Br or I), the inactivation of AGT increased with methylene chain length (at least up to n = 5) but the most mutagenic activity (AGT-dependent) was seen with n = 1-3. The effects with n = 1 or 2 were expected from previous results; the mutagenic effect with n = 3 and the reduction with n > 3 may represent a balance between AGT reaction, stability, and reactivity, in the absence of anchimeric assistance. A strong AGT-dependent mutation was observed for 1,3-butadiene diepoxide. We conclude that numerous bis-electrophiles show AGT-dependent activation to mutagenic conjugates. Haloforms and dichloroalkanes are therefore not an issue, but bromohaloalkanes and 1,3-butadiene diepoxide are potential problems. These observations are of relevance in considering toxicity and risks of some chemicals used in industrial applications.