The S-thiolating activity of membrane gammaglutamyltransferase: formation of cysteinyl-glycine mixed disulfides with cellular proteins and in the cell microenvironment.

Previous studies have documented that activity of the plasma membrane enzyme gamma-glutamyltransferase (GGT) is accompanied by prooxidant processes, with production of reactive oxygen species and oxidation of cellular protein thiols. The present work was aimed to verify the occurrence and extent of S-thiolation mediated by GGT and characterize the molecular species involved in mixed disulfide formation. Experiments show that the cysteinyl-glycine (CG) originating from cellular GGT-mediated glutathione (GSH) metabolism can efficiently thiolate cellular proteins, as well as proteins present in the extracellular environment. With cells presenting high levels of GGT expression, basal levels of CG-containing protein mixed disulfides are detectable, in cellular proteins, as well as in proteins of the culture medium. Stimulation of GGT activity in these cells by administration of substrates results in an increase of CG mixed disulfide formation and a concomitant decrease of GSH-containing disulfides, likely due to GGT-dependent removal of GSH from the system. The findings reported suggest that binding of CG ("protein S-cysteylglycylation") may represent an as yet unrecognized function of membrane GGT, likely playing a regulatory role(s) in the cell and its surroundings.