Bioactivation mechanisms of haloalkene cysteine S-conjugates modeled by gas-phase, ion-molecule reactions

Glutathione conjugate formation plays important roles in the detoxification and bioactivation of xenobiotics. A range of nephrotoxic haloalkenes under go bioactivation that involves glutathione and cysteine S-conjugate formati on. The cysteine S-conjugates thus formed may undergo cysteine conjugate be ta-lyase-catalyzed biotransformation to form cytotoxic thiolates or thiiran es. In the studies presented here, cysteine conjugate beta-lyase-catalyzed biotransformations were modeled by anion-induced elimination reactions of S -(2-bromo-1,1,2-trifluoroethyl)-N-acetyl-cysteine L-cysteine methyl ester, S-(2-chloro-1,1,2-trifluoroethyl)-N-acetyl-L-cysteine methyl ester, and S-( 2-fluoro-1,1,2-trifluoroethyl)-N-acetyl-L-cysteine methyl eater in the gas phase. Examination of these processes in the gas phase allowed direct obser vation of the formation of cysteine S-conjugate-derived thiolates and thiir anes, whose formation is inferred from condensed-phase results. The cystein e S-conjugates of these haloethenes exhibit distinctive patterns of mutagen icity that are thought to be correlated with the nature of the products for med by their cysteine conjugate beta-lyase-catalyzed biotransformation. In particular, S-(2-bromo-1,1,2-trifluoroethyl)-L-cysteine is mutagenic, where as the chloro and fluoro analogues are not. It has been proposed that the m utagenicity of S-(2-bromo-1,1,2-trifluoroethyl)-L-cysteine is correlated wi th the greater propensity of the bromine-containing cysteine S-conjugate to form a thiirane compared with those of the chlorine- or fluorine-containin g conjugates. The ease of thiirane formation is consistent with the gas-pha se results presented here, which show that the bromine-containing conjugate has a greater propensity to form a thiirane on anionic base-induced elimin ation than the chloro- or fluoro-substituted analogues. The blocked cystein e S-conjugates were deprotonated by gas-phase ion-molecule reactions with h ydroxide, methoxide, and ethoxide ions and then allowed to decompose. The m echanisms for these decompositions are discussed as well as the insights in to the bioactivation of these cysteine S-conjugates provided by the further decompositions of thiolate intermediates.