Conjugation of haloalkanes by bacterial and mammalian glutathione transferases: Mono- and dihalomethanes

A primary route of metabolism of dihalomethanes occurs via glutathione (GSH ) transferase-catalyzed conjugation. Mammalian theta class GSH transferases and a group of bacterial dichloromethane dehalogenases are able to catalyz e the hydrolytic dehalogenation of dihalomethanes via GSH conjugation and s ubsequent formation of HCHO. Dihalomethanes have been shown to induce rever tants in Salmonella typhimurium TA 1535 expressing theta class GSH transfer ases. Two mammalian theta class GSH transferases (rat GST 5-5 and human GST T1) and the bacterial dehalogenase DM11 were compared in the in vitro conj ugation of CH3Cl and using in vitro assays (HCHO formation) and the S. typh imurium mutagenesis assay with the dihalomethanes CH2Cl2, CH2Br2, CH2BrCl, CH2ICl, CH2I2, and CH2ClF. GSTs 5-5 and TI had similar characteristics and exhibited first-order rather than Michaelis-Menten kinetics for HCHO format ion over the range of dihalomethane concentrations tested. In contrast, the DM11 enzyme displayed typical hyperbolic Michaelis-Menten kinetics for all of the compounds tested. A similar pattern was observed for the conjugatio n of CH3Cl The reversion tests with S. typhimurium expressing DM11 or GST 5 -5 showed a concentration-dependent increase in revertants for most of the dihalomethanes, and DM11 produced revertants at dihalomethane concentration s lower than GST 5-5. Collectively, the results indicate that rates of conv ersion of dihalomethanes to HCHO are not correlated with mutagenicity and t hat GSH conjugates are genotoxic. The results are compared with the conjuga tion and genotoxicity of haloethanes in the preceding paper in this issue [ Wheeler, J. B., Stourman, N. V., Armstrong, R. N., and Guengerich, F. P. (2 001) Chem. Res. Toxicol. 14, 1107-1117]. The halide order appears most impo rtant in the dihalomethane conjugation reactions catalyzed by GST 5-5 and l ess so in GST T1 and DM11, probably due to changes in the rate-limiting ste ps.