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

Glutathione (GSH) transferases are generally involved in the detoxication o f xenobiotic chemicals. However, conjugation can also activate compounds an d result in DNA modification. Activation of 1,2-dihaloethanes (BrCH2CH2Br, BrCH2CH2Cl, and ClCH2CH2Cl) was investigated using two mammalian theta clas s GSH transferases (rat GST 5-5 and human GST T1) and a bacterial dichlorom ethane dehalogenase (DM11). Although the literature suggests that the bacte rial dehalogenase does not catalyze reactions with CH3Cl, ClCH2CH2Cl, or CH 3CHCl2, we found a higher enzyme efficiency for DM11 than for the mammalian GSH transferases in conjugating CH3Cl, CH3CH2Cl, and CH3CH2Br. Enzymatic r ates of activation of 1,2-dihaloethanes were determined in vitro by measuri ng S,S-ethylene-bis-GSH, the major product trapped by nonenzymatic reaction with the substrate GSH. Salmonella typhimurium TA 1535 systems expressing each of these GSH transferases were used to determine mutagenicity. Rates o f formation of S,S-ethylene-bis-GSH by the GSH transferases correlated with the mutagenicity determined in the reversion assays for the three 1,2-diha loethanes, consistent with the view that half-mustards are the mutagenic pr oducts of the GSH transferase reactions. Half-mustards [S-(2-haloethyl)GSH] containing either F, Cl, or Br (as the leaving group) were tested for thei r abilities to induce revertants in S. typhimurium, and rates of hydrolysis were also determined. GSH transferases do not appear to be involved in the breakdown of the half-mustard intermediates. A halide order (Br > Cl) was observed for both GSH transferase-catalyzed mutagenicity and S,S-ethylene-b is-GSH formation from 1,2-dihaloethanes, with the single exception (both as says) of BrCH2CH2Cl reaction with DM11, which was unexpectedly high. The la ck of substrate saturation seen for conjugation of dihalomethanes with GSTs 5-5 and T1 was also observed with the mono- and 1,2-dihaloethanes [Wheeler , J. B., Stourman, N. V., Thier, R., Dommermuth, A., Vuilleumier, S., Rose, J. A., Armstrong, R. N., and Guengerich, F. P. (2001) Chem. Res. Toxicol. 14, 1118-1127], indicative of an inherent difference in the catalytic mecha nisms of the bacterial and mammalian GSH transferases.