Glutaredoxin 2 catalyzes the reversible oxidation and glutathionylation of mitochondrial membrane thiol proteins: implications for mitochondrial redox regulation and antioxidant DEFENSE

The redox poise of the mitochondrial glutathione pool is central in the response of mitochondria to oxidative damage and redox signaling, but the mechanisms are uncertain. One possibility is that the oxidation of glutathione (GSH) to glutathione disulfide (GSSG) and the consequent change in the GSH/GSSG ratio causes protein thiols to change their redox state, enabling protein function to respond reversibly to redox signals and oxidative damage. However, little is known about the interplay between the mitochondrial glutathione pool and protein thiols. Therefore we investigated how physiological GSH/GSSG ratios affected the redox state of mitochondrial membrane protein thiols. Exposure to oxidized GSH/GSSG ratios led to the reversible oxidation of reactive protein thiols by thiol-disulfide exchange, the extent of which was dependent on the GSH/GSSG ratio. There was an initial rapid phase of protein thiol oxidation, followed by gradual oxidation over 30 min. A large number of mitochondrial proteins contain reactive thiols and most of these formed intraprotein disulfides upon oxidation by GSSG; however, a small number formed persistent mixed disulfides with glutathione. Both protein disulfide formation and glutathionylation were catalyzed by the mitochondrial thiol transferase glutaredoxin 2 (Grx2), as were protein deglutathionylation and the reduction of protein disulfides by GSH. Complex I was the most prominent protein that was persistently glutathionylated by GSSG in the presence of Grx2. Maintenance of complex I with an oxidized GSH/GSSG ratio led to a dramatic loss of activity, suggesting that oxidation of the mitochondrial glutathione pool may contribute to the selective complex I inactivation seen in Parkinson's disease. Most significantly, Grx2 catalyzed reversible protein glutathionylation/deglutathionylation over a wide range of GSH/GSSG ratios, from the reduced levels accessible under redox signaling to oxidized ratios only found under severe oxidative stress. Our findings indicate that Grx2 plays a central role in the response of mitochondria to both redox signals and oxidative stress by facilitating the interplay between the mitochondrial glutathione pool and protein thiols.     

Minor role of oxidative stress during intermediate phase of acute pancreatitis in rats

Reactive oxygen species have been implicated in the pathogenesis of acute pancreatitis. Few studies have focused on the loss of endogenous antioxidants and molecular oxidative damage. Two acute pancreatitis models in rats; taurocholate (3% intraductal infusion) and cerulein (10 microg/kg/h), were used to study markers of oxidative stress: Glutathione, ascorbic acid, and their oxidized forms (glutathione disulfide and dehydroascorbic acid), malondialdehyde, and 4-hydroxynoneal in plasma and pancreas, as well as 7-hydro-8-oxo-2'-deoxyguanosine in pancreas. In both models, pancreatic glutathione depleted by 36-46% and pancreatic ascorbic acid depleted by 36-40% (p <.05). In the taurocholate model, plasma glutathione was depleted by 34% (p <.05), but there were no significant changes in plasma ascorbic acid or in plasma and pancreas dehydroascorbic acid, malondialdehyde, and 4-hydroxynoneal, and no significant changes in the pancreas glutathione disulfide/glutathione ratio. While pancreas glutathione disulfide/glutathione ratio increased in the cerulein model, there were no significant changes in plasma glutathione, plasma, or pancreas ascorbic acid, dehydroascorbic acid, 4-hydroxynoneal, and malondialdehyde, or in pancreas 7-hydro-8-oxo-2'-deoxyguanosine. Reactive oxygen species have a minor role in the intermediate stages of pancreatitis models.     

Destabilization and denaturation of cellular protein by glutathione depletion

This investigation tested the hypothesis that depletion of intracellular glutathione, in contrast to its oxidation, could lead to non-native oxidation of protein thiols, thereby trapping proteins in an unstable conformation. Chinese hamster cells were exposed to the α,β-unsaturated dicarboxylic acid diethylmaleate in order to produce rapid gluthathione (GSH) depletion without oxidation. Measurement of the fluorescence of oxidized 2′,7′-dichlorofluorescein diacetate indicated that reactive oxygen species accumulated in GSH depleted cells. Glutathione depletion was found to alter protein thiol/disulfide exchange ratios such that 17 to 23 nmol of protein SH/mg protein underwent oxidation. Differential scanning calorimetry (DSC) of glutathione depleted cells yielded a profile of specific heat capacity versus temperature that was characteristic of cells containing destabilized and denatured protein. In addition, cells depleted of glutathione exhibited a two-fold increase in NP-40 insoluble protein. These results indicate that depletion of intracellular glutathione caused oxidation of protein thiols, protein denaturation and aggregation and provide a mechanism to explain how GSH depletion can initiate stress responses.     

Thiol/disulfide redox equilibrium and kinetic behavior of chicken liver fatty acid synthase

Chicken liver fatty acid synthase is rapidly inactivated and cross-linked at pH 7.2 and 8.0 by incubation with low concentrations of common biological disulfides including glutathione disulfide, coenzyme A disulfide, and glutathione-coenzyme A-mixed disulfide. Glutathione disulfide inactivation of the enzyme is accompanied by the oxidation of a total of 4-5 enzyme thiols per monomer. Only one glutathione equivalent is incorporated per monomer as a protein-mixed disulfide, and its rate of incorporation is significantly slower than the rate of inactivation. The formation of protein-SS-protein disulfides results in significant cross-linking of enzyme subunits. The inactive enzyme is rapidly and completely reactivated, and the cross-linking is completely reversed by incubation of the enzyme with thiols (10-20 mM) including dithiothreitol, mercaptoethanol, and glutathione. In a glutathione redox buffer (GSH + GSSG), disulfide bond formation comes to equilibrium. The enzyme activity at equilibrium is dependent both on the ratio of glutathione to glutathione disulfide and on the total glutathione concentration. The equilibrium constant for the redox equilibration of fatty acid synthase in a glutathione redox buffer is 15 mM (Ered + GSSG in equilibrium Eox + 2GSH). The formation of at least one protein-protein disulfide per monomer dominates the redox properties of the enzyme while the formation of one protein-mixed disulfide with glutathione (Kmixed = 0.45) has little effect on activity. The oxidation equilibrium constant suggests that there would be no significant cycling between the reduced and the oxidized enzyme in response to likely physiological variations in the hepatic glutathione status. The possibility that changes in the concentration of cellular glutathione may act as a mechanism for metabolic control of other enzymes is discussed.     

Glutathione in plants: an integrated overview

Plants cannot survive without glutathione (γ-glutamylcysteinylglycine) or γ-glutamylcysteine-containing homologues. The reasons why this small molecule is indispensable are not fully understood, but it can be inferred that glutathione has functions in plant development that cannot be performed by other thiols or antioxidants. The known functions of glutathione include roles in biosynthetic pathways, detoxification, antioxidant biochemistry and redox homeostasis. Glutathione can interact in multiple ways with proteins through thiol-disulphide exchange and related processes. Its strategic position between oxidants such as reactive oxygen species and cellular reductants makes the glutathione system perfectly configured for signalling functions. Recent years have witnessed considerable progress in understanding glutathione synthesis, degradation and transport, particularly in relation to cellular redox homeostasis and related signalling under optimal and stress conditions. Here we outline the key recent advances and discuss how alterations in glutathione status, such as those observed during stress, may participate in signal transduction cascades. The discussion highlights some of the issues surrounding the regulation of glutathione contents, the control of glutathione redox potential, and how the functions of glutathione and other thiols are integrated to fine-tune photorespiratory and respiratory metabolism and to modulate phytohormone signalling pathways through appropriate modification of sensitive protein cysteine residues.     

Changes in protein and nonprotein thiol contents in bladder, kidney and liver of mice by the pesticide sodium-o-phenylphenol and their possible role in cellular toxicity.

Acute treatment of mice with Na-o-phenylphenol or phenylbenzoquinone, an electrophilic metabolite of o-phenylphenol, resulted in differential depletion of contents of protein and nonprotein thiols in bladder, kidney and liver. Maximum decrease in the levels of protein and nonprotein reduced thiols was observed in bladder (by both agents) and was followed by kidney (by both agents) and liver (phenylbenzoquinone only). The reason for this differential changes in reduced thiol contents remains to be understood. The content of protein and nonprotein disulfides was higher in bladder of mice treated with Na-o-phenylphenol compared to that observed in untreated mice bladder. Phenyl 2,5'-p-benzoquinone mediated in vivo depletion of nonprotein and protein thiols suggests that Na-o-phenylphenol treatment may decrease in vivo thiols via the formation of phenylbenzoquinone. Increased disulfide formation is considered to represent an index of oxidative stress produced by chemical. Increases in the level of protein and nonprotein disulfides in bladder suggest as observed in this study that administration of Na-o-phenylphenol to mice produced oxidative stress in bladder. Products of redox cycling of xenobiotics are known to cause cellular toxicity via altering the homeostasis of thiol status. Therefore, it is concluded that decreases in protein thiol contents either via alkylation and/or oxidation of sulfhydryl groups of proteins and increases in disulfide contents presumably by products of redox cycling of Na-o-phenylphenol may play a role in Na-o-phenylphenol-induced cellular toxicity.     

Redox regulation of beta-actin during integrin-mediated cell adhesion

Redox sensitivity of actin toward an exogenous oxidative stress has recently been reported. We report here the first evidence of in vivo actin redox regulation by a physiological source of reactive oxygen species, specifically those species generated by integrin receptors during cell adhesion. Actin oxidation takes place via the formation of a mixed disulfide between cysteine 374 and glutathione; this modification is essential for spreading and for cytoskeleton organization. Impairment of actin glutathionylation, either through GSH depletion or expression of the C374A redox-insensitive mutant, greatly affects cell spreading and the formation of stress fibers, leading to inhibition of the disassembly of the actinomyosin complex. These data suggest that actin glutathionylation is essential for cell spreading and cytoskeleton organization and that it plays a key role in disassembly of actinomyosin complex during cell adhesion.     

Reversible glutathionylation of complex I increases mitochondrial superoxide formation

Increased production of reactive oxygen species (ROS) by mitochondria is involved in oxidative damage to the organelle and in committing cells to apoptosis or senescence, but the mechanisms of this increase are unknown. Here we show that ROS production by mitochondrial complex I increases in response to oxidation of the mitochondrial glutathione pool. This correlates with thiols on the 51- and 75-kDa subunits of complex I forming mixed disulfides with glutathione. Glutathionylation of complex I increases superoxide production by the complex, and when the mixed disulfides are reduced, superoxide production returns to basal levels. Within intact mitochondria oxidation of the glutathione pool to glutathione disulfide also leads to glutathionylation of complex I, which correlates with increased superoxide formation. In this case, most of this superoxide is converted to hydrogen peroxide, which can then diffuse into the cytoplasm. This mechanism of reversible mitochondrial ROS production suggests how mitochondria might regulate redox signaling and shows how oxidation of the mitochondrial glutathione pool could contribute to the pathological changes that occur to mitochondria during oxidative stress.     

In vivo targets of S-thiolation in Chlamydomonas reinhardtii

Glutathionylation is the major form of S-thiolation in cells. This reversible redox post-translational modification consists of the formation of a mixed disulfide between a free thiol on a protein and a molecule of glutathione. This recently described modification, which is considered to occur under oxidative stress, can protect cysteine residues from irreversible oxidation, and alter positively or negatively the activity of diverse proteins. This modification and its targets have been mainly studied in non-photosynthetic organisms so far. We report here the first proteomic approach performed in vivo on photosynthetically competent cells, using the eukaryotic unicellular green alga Chlamydomonas reinhardtii with radiolabeled [(35)S]cysteine to label the glutathione pool and diamide as oxidant. This method allowed the identification of 25 targets, mainly chloroplastic, involved in various metabolic processes. Several targets are related to photosynthesis, such as the Calvin cycle enzymes phosphoglycerate kinase and ribose-5-phosphate isomerase. A number of targets, such as chaperones and peroxiredoxins, are related to stress responses. The glutathionylation of HSP70B, chloroplastic 2-Cys peroxiredoxin and isocitrate lyase was confirmed in vitro on purified proteins and the targeted residues were identified.     

Protein thiol oxidation and formation of S-glutathionylated cyclophilin A in cells exposed to chloramines and hypochlorous acid

Neutrophil oxidants, including the myeloperoxidase products, HOCl and chloramines, have been linked to endothelial dysfunction in inflammatory diseases such as atherosclerosis. As they react preferentially with sulfur centers, thiol proteins are likely to be cellular targets. Our objectives were to establish whether there is selective protein oxidation in vascular endothelial cells treated with HOCl or chloramines, and to identify sensitive proteins. Cells were treated with HOCl, glycine chloramine and monochloramine, reversibly oxidized cysteines were labeled and separated by 1D or 2D SDS-PAGE, and proteins were characterized by mass spectrometry. Selective protein oxidation was observed, with chloramines and HOCl causing more changes than H(2)O(2). Cyclophilin A was one of the most sensitive targets, particularly with glycine chloramine. Cyclophilin A was also oxidized in Jurkat T cells where its identity was confirmed using a knockout cell line. The product was a mixed disulfide with glutathione, with glutathionylation at Cys-161. Glyceraldehyde-3-phosphate dehydrogenase, peroxiredoxins and cofilin were also highly sensitive to HOCl/chloramines. Cyclophilins are becoming recognized as redox regulatory proteins, and glutathionylation is an important mechanism for redox regulation. Cells lacking Cyclophilin A showed more glutathionylation of other proteins than wild-type cells, suggesting that cyclophilin-regulated deglutathionylation could contribute to redox changes in HOCl/chloramine-exposed cells.     

Regulation of protein tyrosine phosphatase 1B in intact cells by S-nitrosothiols

Protein tyrosine phosphatases (PTPases) contain an active site cysteine which when oxidized leads to loss of phosphatase activity and accumulation of phosphoproteins. For example, oxidants produced following EGF stimulation inhibit PTP1B and enhance EGF receptor phosphorylation. Because NO-derived species also modify reactive thiols, we postulated that NO would reversibly inhibit PTP1B. In our studies we exposed A431 or Jurkat cells to NO donors and measured PTP1B activity or used 3-maleimidylpropionylbiocytin (MPB) to measure thiol redox status. Nitrosothiols led to a rapid inhibition of PTP1B through a mechanism that was greatly enhanced by addition of cysteine to the medium. Analysis of thiol oxidation status using immunoprecipitated PTP1B showed modification consistent with loss of activity. Both enzyme inhibition and modification were reversible in intact cells or after addition of DTT to cell lysates. While DTT reversed oxidation, ascorbate did not, suggesting that formation of a mixed disulfide (possibly glutathionylation) rather than S-nitrosylation accounts for PTP1B inhibition. Importantly, PTP1B inhibition by nitrosothiols led to EGF receptor phosphorylation even in the absence of exogenously added EGF. These findings suggest an important role for NO in modulating signaling pathways since inhibition of PTPases could potentially enhance or prolong activity of phosphoproteins.     

Oxidation and S-nitrosylation of cysteines in human cytosolic and mitochondrial glutaredoxins: effects on structure and activity

Glutathione (GSH) is the major intracellular thiol present in 1-10-mm concentrations in human cells. However, the redox potential of the 2GSH/GSSG (glutathione disulfide) couple in cells varies in association with proliferation, differentiation, or apoptosis from -260 mV to -200 or -170 mV. Hydrogen peroxide is transiently produced as second messenger in receptor-mediated growth factor signaling. To understand oxidation mechanisms by GSSG or nitric oxide-related nitrosylation we studied effects on glutaredoxins (Grx), which catalyze GSH-dependent thiol-disulfide redox reactions, particularly reversible glutathionylation of protein sulfhydryl groups. Human Grx1 and Grx2 contain Cys-Pro-Tyr-Cys and Cys-Ser-Tyr-Cys active sites and have three and two additional structural Cys residues, respectively. We analyzed the redox state and disulfide pairing of Cys residues upon GSSG oxidation and S-nitrosylation. Cytosolic/nuclear Grx1 was partly inactivated by both S-nitrosylation and oxidation. Inhibition by nitrosylation was reversible under anaerobic conditions; aerobically it was stronger and irreversible, indicating inactivation by nitration. Oxidation of Grx1 induced a complex pattern of disulfide-bonded dimers and oligomers formed between Cys-8 and either Cys-79 or Cys-83. In addition, an intramolecular disulfide between Cys-79 and Cys-83 was identified, predicted to have a profound effect on the three-dimensional structure. In contrast, mitochondrial Grx2 retains activity upon oxidation, did not form disulfide-bonded dimers or oligomers, and could not be S-nitrosylated. The dimeric iron sulfur cluster-coordinating inactive form of Grx2 dissociated upon nitrosylation, leading to activation of the protein. The striking differences between Grx1 and Grx2 reflect their diverse regulatory functions in vivo and also adaptation to different subcellular localization.     

Molecular Mechanisms of Glutaredoxin Enzymes: Versatile Hubs for Thiol–Disulfide Exchange between Protein Thiols and Glutathione

The tripeptide glutathione (GSH) and its oxidized form glutathione disulfide (GSSG) constitute a key redox couple in cells. In particular, they partner protein thiols in reversible thiol–disulfide exchange reactions that act as switches in cell signaling and redox homeostasis. Disruption of these processes may impair cellular redox signal transduction and induce redox misbalances that are linked directly to aging processes and to a range of pathological conditions including cancer, cardiovascular diseases and neurological disorders. Glutaredoxins are a class of GSH-dependent oxidoreductase enzymes that specifically catalyze reversible thiol–disulfide exchange reactions between protein thiols and the abundant thiol pool GSSG/GSH. They protect protein thiols from irreversible oxidation, regulate their activities under a variety of cellular conditions and are key players in cell signaling and redox homeostasis. On the other hand, they may also function as metal-binding proteins with a possible role in the cellular homeostasis and metabolism of essential metals copper and iron. However, the molecular basis and underlying mechanisms of glutaredoxin action remain elusive in many situations. This review focuses specifically on these aspects in the context of recent developments that illuminate some of these uncertainties.     

Glutathionylation of trypanosomal thiol redox proteins

Trypanosomatids, the causative agents of several tropical diseases, lack glutathione reductase and thioredoxin reductase but have a trypanothione reductase instead. The main low molecular weight thiols are trypanothione (N(1),N(8)-bis-(glutathionyl)spermidine) and glutathionyl-spermidine, but the parasites also contain free glutathione. To elucidate whether trypanosomes employ S-thiolation for regulatory or protection purposes, six recombinant parasite thiol redox proteins were studied by ESI-MS and MALDI-TOF-MS for their ability to form mixed disulfides with glutathione or glutathionylspermidine. Trypanosoma brucei mono-Cys-glutaredoxin 1 is specifically thiolated at Cys(181). Thiolation of this residue induced formation of an intramolecular disulfide bridge with the putative active site Cys(104). This contrasts with mono-Cys-glutaredoxins from other sources that have been reported to be glutathionylated at the active site cysteine. Both disulfide forms of the T. brucei protein were reduced by tryparedoxin and trypanothione, whereas glutathione cleaved only the protein disulfide. In the glutathione peroxidase-type tryparedoxin peroxidase III of T. brucei, either Cys(47) or Cys(95) became glutathionylated but not both residues in the same protein molecule. T. brucei thioredoxin contains a third cysteine (Cys(68)) in addition to the redox active dithiol/disulfide. Treatment of the reduced protein with GSSG caused glutathionylation of Cys(68), which did not affect its capacity to catalyze reduction of insulin disulfide. Reduced T. brucei tryparedoxin possesses only the redox active Cys(32)-Cys(35) couple, which upon reaction with GSSG formed a disulfide. Also glyoxalase II and Trypanosoma cruzi trypanothione reductase were not sensitive to thiolation at physiological GSSG concentrations.     

Charge-based analysis of antibodies with engineered cysteines

THIOMABs are antibodies with an engineered unpaired cysteine residue on each heavy chain that can be used as intermediates to generate antibody-drug conjugates. Multiple charge variant peaks were observed during cation-exchange chromatography (CEX) and imaged capillary isoelectric focusing (cIEF) analysis of several different THIOMABs. This charge heterogeneity was due to cysteinylation and/or glutathionylation at the engineered and unpaired cysteines through disulfide bonds formed during the cell culture process. Cysteine treatment followed by analysis using CEX, LC/MS and electrophoresis demonstrates that cysteine is a mild reductant that can remove glutathione and cysteine bound to the engineered cysteines without disrupting the inter- or intra-chain disulfide bonds of antibodies. We further demonstrated that using a cysteine/cystine redox pair (rather than cysteine alone) can not only effectively remove glutathione at the engineered cysteines, but also generate homogeneously cysteinylated species, which resulted in one main peak in both CEX-HPLC and imaged cIEF assays for antibodies with engineered and unpaired cysteines.     

Glutathione as an Antioxidant and Regulatory Molecule in Plants Under Abiotic Stress Conditions

The glutathione (GSH)/glutathione disulfide (GSSG) redox couple is involved in several physiologic processes in plants under both optimal and stress conditions. It participates in the maintenance of redox homeostasis in the cells. The redox state of the GSH/GSSG couple is defined by its reducing capacity and the half-cell reduction potential, and differs in the various organs, tissues, cells, and compartments, changing during the growth and development of the plants. When characterizing this redox couple, the synthesis, degradation, oxidation, and transport of GSH and its conjugation with the sulfhydryl groups of other compounds should be considered. Under optimal growth conditions, the high GSH/GSSG ratio results in a reducing environment in the cells which maintains the appropriate structure and activity of protein molecules because of the inhibition of the formation of intermolecular disulfide bridges. In response to abiotic stresses, the GSH/GSSG ratio decreases due to the oxidation of GSH during the detoxification of reactive oxygen species (ROS) and changes in its metabolism. The lower GSH/GSSG ratio activates various defense mechanisms through a redox signalling pathway, which includes several oxidants, antioxidants, and stress hormones. In addition, GSH may control gene expression and the activity of proteins through glutathionylation and thiol-disulfide conversion. This review discusses the size and redox state of the GSH pool, including their regulation, their role in redox signalling and defense processes, and the changes caused by abiotic stress.     

Regulation of protein function by glutathionylation

The main function of reduced glutathione (GSH) is to protect from oxidative stress as a reactive oxygen scavenger. However, in the context of redox regulation, the ratio between GSH and its oxidized form (GSSG) determines the redox state of redox-sensitive cysteines in some proteins and, thus, acts as a signaling system. While GSH/GSSG can catalyze oxido-reduction of intra- and inter-chain disulfides by thiol-disulfide exchange, this review focuses on the formation of mixed disulfides between glutathione and proteins, also known as glutathionylation. The review discusses the regulatory role of this post-translational modification and the role of protein disulfide oxidoreductases (thioredoxin/thioredoxin reductase, glutaredoxin, protein disulfide isomerase) in the reversibility of this process.     

Protein glutathionylation in health and disease

Background

It is now recognized that protein cysteines exist not only as free thiols or intramolecular disulfides, that help maintain the 3D structure of proteins, but can also undergo different types of oxidation, one of which is glutathionylation, or the formation of mixed disulfides with glutathione (GSH).

Scope of the review

We will discuss how proteins can undergo glutathionylation and how this can affect the protein characteristics/function. Glutathionylation is reversible and de-glutathionylation can be catalysed by protein thiol–disulfide oxidoreductases. Genetic modification of the expression of these enzymes, particularly glutaredoxin, using overexpression, knockout mice or siRNA, is becoming an important tool to study the role of protein glutathionylation. While in the past this post-translational modification was mainly known in the context of oxidative stress, measurement of glutathionylated proteins in patients is pointing out a potential importance if this modification in pathogenesis and could identify new biomarkers. We also wanted to point out the main findings in the role of glutathionylation in diseases and drug action.

Major conclusions

We identify two major open problems in the field, namely the complexity of the mechanisms responsible for glutathionylation and de-glutathionylation, as well as what makes a protein susceptible to glutathionylation.

General significance

This review underlines the peculiarities of this post-translational modification and their biological role. This article is part of a Special Issue entitled Cellular functions of glutathione.