Conjugation of Haloalkanes by Bacterial and Mammalian Glutathione Transferases:  Mono- and Dihalomethanes

Abstract

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 catalyze the hydrolytic dehalogenation of dihalomethanes via GSH conjugation and subsequent formation of HCHO. Dihalomethanes have been shown to induce revertants in Salmonella typhimurium TA 1535 expressing theta class GSH transferases. 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 conjugation of CH(3)Cl and using in vitro assays (HCHO formation) and the S. typhimurium mutagenesis assay with the dihalomethanes CH(2)Cl(2), CH(2)Br(2), CH(2)BrCl, CH(2)ICl, CH(2)I(2), and CH(2)ClF. GSTs 5-5 and T1 had similar characteristics and exhibited first-order rather than Michaelis-Menten kinetics for HCHO formation 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 conjugation of CH(3)Cl. 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 concentrations lower than GST 5-5. Collectively, the results indicate that rates of conversion of dihalomethanes to HCHO are not correlated with mutagenicity and that GSH conjugates are genotoxic. The results are compared with the conjugation and genotoxicity of haloethanes in the preceding paper in this issue [Wheeler, J. B., Stourman, N. V., Armstrong, R. N., and Guengerich, F. P. (2001) Chem. Res. Toxicol. 14, 1107-1117]. The halide order appears most important in the dihalomethane conjugation reactions catalyzed by GST 5-5 and less so in GST T1 and DM11, probably due to changes in the rate-limiting steps.

Glutathione S Transferases: Biochemistry, Polymorphism and Role in Colorectal Carcinogenesis

“…However, in some instances the glutathione conjugate is more reactive than the parent compound such as short-chain alkyl halides that contain two functional groups and 1,2-dihaloethanes, where the glutathione conjugate however, rearranges to form an episulfonium intermediate that is responsible for modifying DNA [24]. Also conjugation of GSH with the solvent dichloromethane results in the formation of the highly unstable Schloromethylglutathione, containing an electrophilic center capable of modifying DNA [24,25].…”

Characterization of a Mutagenic DNA Adduct Formed from 1,2-Dibromoethane by O6-Alkylguanine-DNA Alkyltransferase

“…Of these, S- [2-(N 7 -guanyl)ethyl]glutathione accounts for up to 95% of total DNA adducts (24) and its formation produces G:C to A:T transition mutations (25). GST was shown to enhance DBE genotoxicity in Salmonella typhimurium strains TA100 and TA1535, where mutations at specific guanines are needed to produce reversions (22,26,27). The extent to which the various pathways for activation of DBE (activation by microsomes, by GST or by AGT) contribute to its genotoxicity are not yet clear.…”

Bird Glutathione S-transferases: Endogenous and Exogenous Toxic Insults Read more at http://nexusacademicpublishers.com/table_contents_detail/4/872/html#CUM6EltbRFTzsEeL.99

“…The conjugation of GSH with the solvent dichloromethane facilitate formation of the highly unstable Schloromethlglutathione which capable to modify DNA (Guengerich et al, 2003;Wheeler et al, 2001).…”