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 undergo bioactivation that involves glutathione and cysteine S-conjugate formation. The cysteine S-conjugates thus formed may undergo cysteine conjugate beta-lyase-catalyzed biotransformation to form cytotoxic thiolates or thiiranes. 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-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 ester in the gas phase. Examination of these processes in the gas phase allowed direct observation of the formation of cysteine S-conjugate-derived thiolates and thiiranes, whose formation is inferred from condensed-phase results. The cysteine S-conjugates of these haloethenes exhibit distinctive patterns of mutagenicity that are thought to be correlated with the nature of the products formed by their cysteine conjugate beta-lyase-catalyzed biotransformation. In particular, S-(2-bromo-1,1,2-trifluoroethyl)-L-cysteine is mutagenic, whereas the chloro and fluoro analogues are not. It has been proposed that the mutagenicity of S-(2-bromo-1,1, 2-trifluoroethyl)-L-cysteine is correlated with the greater propensity of the bromine-containing cysteine S-conjugate to form a thiirane compared with those of the chlorine- or fluorine-containing conjugates. The ease of thiirane formation is consistent with the gas-phase results presented here, which show that the bromine-containing conjugate has a greater propensity to form a thiirane on anionic base-induced elimination than the chloro- or fluoro-substituted analogues. The blocked cysteine S-conjugates were deprotonated by gas-phase ion-molecule reactions with hydroxide, methoxide, and ethoxide ions and then allowed to decompose. The mechanisms for these decompositions are discussed as well as the insights into the bioactivation of these cysteine S-conjugates provided by the further decompositions of thiolate intermediates.