Theoretical evaluation of two plausible routes for bioactivation of S-(1,1-difluoro-2,2-dihaloethyl)-L-cysteine conjugates: thiirane vs thionoacyl fluoride pathway.

The selective nephrotoxicity of halogenated alkenes has been attributed to a glutathione (GSH) S-conjugate pathway involving enzymatic hydrolysis to the cysteine S-conjugate and beta-lyase bioactivation to thiolates, which are presumed to give rise to the ultimate mutagenic or cytotoxic reactive species. Studies have shown that the brominated S-(2,2-dihalo-1,1-difluoroethyl)-L-cysteine conjugates are mutagenic in the Ames test, whereas the nonbrominated analogues are nonmutagenic. While careful experimentation has contributed much to current understanding, the ultimate reactive species responsible for the differing mutagenic effects remain unknown. Computational methods were applied to the investigation of two proposed metabolic pathways leading from the thiolate to either a thiirane or thionoacyl fluoride intermediate, both electrophilic species presumed capable of binding to proteins or DNA. Studied were six F-, Cl-, and Br-substituted 2,2-dihalo-1,1-difluoroethane-1-thiolates (2,2-dihalo-DFETs). Pathway preference was determined for each thiolate by comparison of reaction energy profiles and activation energies. At all but the lowest level of ab initio theory, a thionoacyl fluoride pathway was predicted for 2,2-difluoro-DFET, while a thiirane pathway was energetically preferred for the brominated 2,2-dihalo-DFETs. These results offer a clear mechanism-based rationale for distinguishing 2,2-difluoro-DFET from the brominated 2,2-dihalo-DFETs, while the results are less clear for the 2,2-dichloro and 2-chloro-2-fluoro-DFETs, which at the highest level of ab initio treatment had a relatively small energy preference (2.4 kcal/mol) for the thiirane pathway. The predicted clear preference for a thiirane pathway for the brominated 2,2-dihalo-DFETs is not consistent with a recently proposed pathway involving alpha-thiolactone formation through a thionoacyl fluoride intermediate [Finkelstein, M. B., et al. (1995) J. Am. Chem. Soc. 117, 9590-9591], but is supported by results of a recent study providing experimental evidence for thiirane formation from the brominated 2,2-dihalo-DFETs [Finkelstein, M. B., et al. (1996) Chem. Res. Toxicol. 9, 227-231].