Formation of mixed disulfides between glutathione and the cysteines of some proteins (glutathionylation) has been suggested as a mechanism through which protein functions can be regulated by the redox status. The aim of this study was to identify the proteins of T cell blasts that undergo glutathionylation under oxidative stress. To this purpose, we radiolabeled cellular glutathione with 35 S, exposed T cells to oxidants (diamide or hydrogen peroxide), and performed nonreducing, two-dimensional electrophoresis followed by detection of labeled proteins by phosphorimaging and their identification by mass spectrometry techniques. We detected several proteins previously not recognized to be glutathionylated, including cytoskeletal proteins (vimentin, myosin, tropomyosin, cofilin, profilin, and the already known actin), enzymes (enolase, aldolase, 6-phosphogluconolactonase, adenylate kinase, ubiquitinconjugating enzyme, phosphoglycerate kinase, triosephosphate isomerase, and pyrophosphatase), redox enzymes (peroxiredoxin 1, protein disulfide isomerase, and cytochrome c oxidase), cyclophilin, stress proteins (HSP70 and HSP60), nucleophosmin, transgelin, galectin, and fatty acid binding protein. Based on the presence of several protein isoforms in control cells, we suggest that enolase and cyclophilin are heavily glutathionylated under basal conditions. We studied the effect of glutathionylation on some of the enzymes identified in the present study and found that some of them (enolase and 6-phosphogluconolactonase) are inhibited by glutathionylation, whereas the enzymatic activity of cyclophilin (peptidylprolyl isomerase) is not. These findings suggest that protein glutathionylation might be a common mechanism for the global regulation of protein functions.

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To identify proteins undergoing glutathionylation (formation of protein-glutathione mixed disulfides) in human T cell blasts, we radiolabeled the glutathione pool with 35 S, exposed cells to the oxidant diamide, and analyzed cellular proteins by two-dimensional electrophoresis. One of the proteins undergoing glutathionylation was identified by molecular weight, isoelectric point, and immunoblotting as thioredoxin (Trx). Incubation of recombinant human Trx with glutathione disulfide or S-nitrosoglutathione led to the formation of glutathionylated Trx, identified by matrixassisted laser desorption ionization-time-of-flight mass spectrometry. The glutathionylation site was identified as Cys-72. Glutathionylation of rhTrx abolished its enzymatic activity as insulin disulfide reductase in the presence of NADPH and Trx reductase. Activity was, however, regained with sigmoidal kinetics, indicating a process of autoactivation due to the ability of Trx to deglutathionylate itself. These data suggest that the intracellular glutathione͞glutathione disulfide ratio, an indicator of the redox state of the cell, can regulate Trx functions reversibly through thiol-disulfide exchange reactions. T hioredoxin (Trx; ref. 1), a ubiquitous redox protein, is an essential cofactor electron donor for ribonucleotide reductase, but also has many other cellular functions, including regulation of transcription factors, apoptosis, and antioxidant activity and can act exogenously as a redox active growth factor (1, 2). The catalytic activity of Trx resides in its active site where the two redox active Cyss (Cys-31 and Cys-34 in human Trx) undergo reversible oxidation͞reduction. In addition to the conserved Cys residues in the active site, three additional structural Cys residues (Cys-61, Cys-68, and Cys-72) are present in the structure of human Trx.The present paper describes the experiments that led us to the conclusion that Trx can undergo glutathionylation in T cells exposed to oxidative stress. Protein glutathionylation, the formation of a disulfide between a Cys in a protein and the Cys in the tripeptide glutathione (GSH), is a modification that can be induced in cells by oxidative stress. Glutathionylation can occur by direct oxidation of a protein and GSH, by a thiol-disulfide exchange between a protein Cys and oxidized glutathione (GSSG), and also with the intermediacy of S-nitrosoglutathione (GSNO; refs. 3 and 4). Glutathionylation of proteins is reversible, as those proteins can be reduced by glutaredoxins (5, 6), and the process serves to regulate protein functions by the redox state of the cell (i.e., by the GSSG͞GSH ratio; refs. 7-9).To demonstrate the glutathionylation of Trx, we first labeled the intracellular GSH pool of the T-cell blasts with [ 35 S]Cys and analyzed the cell lysate by two dimensional electrophoresis under nonreducing conditions. A spot was found in the autoradiographic protein map with IP͞M r corresponding to those of Trx. In the second part of the study, we incubated recombinant human Trx in the presence or ...