S-glutathionylation of glyceraldehyde-3-phosphate dehydrogenase: role of thiol oxidation and catalysis by glutaredoxin

An increase in S-glutathionylated proteins in the Alzheimer's disease inferior parietal lobule, a proteomics approach

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by neurofibrillary tangles, senile plaques, and loss of synapses. Many studies support the notion that oxidative stress plays an important role in AD pathogenesis. Previous studies from our laboratory employed redox proteomics to identify oxidatively modified proteins in the AD inferior parietal lobule (IPL) and hippocampus. The proteins were consistent with biochemical or pathological alterations in AD and have been central to further investigations of the disease. The present study focused on the identification of specific targets of protein S-glutathionylation in AD and control IPL by using a redox proteomics approach. For AD IPL, we identified deoxyhemoglobin, alpha-crystallin B, glyceraldehyde phosphate dehydrogenase (GAPDH), and alpha-enolase as significantly S-glutathionylated relative to these brain proteins in control IPL. GAPDH and alpha-enolase were also shown to have reduced activity in the AD IPL. This study demonstrates that specific proteins are sensitive to S-glutathionylation, which most likely is due to their sensitivity to cysteine oxidation initiated by the increase in oxidative stress in the AD brain.

The thioredoxin-independent isoform of chloroplastic glyceraldehyde-3-phosphate dehydrogenase is selectively regulated by glutathionylation

In animal cells, many proteins have been shown to undergo glutathionylation under conditions of oxidative stress. By contrast, very little is known about this post-translational modification in plants. In the present work, we showed, using mass spectrometry, that the recombinant chloroplast A(4)-glyceraldehyde-3-phosphate dehydrogenase (A(4)-GAPDH) from Arabidopsis thaliana is glutathionylated with either oxidized glutathione or reduced glutathione and H(2)O(2). The formation of a mixed disulfide between glutathione and A(4)-GAPDH resulted in the inhibition of enzyme activity. A(4)-GAPDH was also inhibited by oxidants such as H(2)O(2). However, the effect of glutathionylation was reversed by reductants, whereas oxidation resulted in irreversible enzyme inactivation. On the other hand, the major isoform of photosynthetic GAPDH of higher plants (i.e. the A(n)B(n)-GAPDH isozyme in either A(2)B(2) or A(8)B(8) conformation) was sensitive to oxidants but did not seem to undergo glutathionylation significantly. GAPDH catalysis is based on Cys149 forming a covalent intermediate with the substrate 1,3-bisphosphoglycerate. In the presence of 1,3-bisphosphoglycerate, A(4)-GAPDH was fully protected from either oxidation or glutathionylation. Site-directed mutagenesis of Cys153, the only cysteine located in close proximity to the GAPDH active-site Cys149, did not affect enzyme inhibition by glutathionylation or oxidation. Catalytic Cys149 is thus suggested to be the target of both glutathionylation and thiol oxidation. Glutathionylation could be an important mechanism of regulation and protection of chloroplast A(4)-GAPDH from irreversible oxidation under stress.

Zinc center as redox switch--new function for an old motif

Oxidative stress affects a wide variety of different cellular processes. Now, an increasing number of proteins have been identified that use the presence of reactive oxygen species or alterations in the cellular thiol-disulfide state as regulators of their protein function. This review focuses on two members of this growing group of redox-regulated proteins that utilize a cysteine-containing zinc center as the redox switch: Hsp33, the first molecular chaperone, whose ability to protect cells against stress-induced protein unfolding depends on the presence of reactive oxygen species and RsrA, the first anti-sigma factor that uses a cysteine-containing zinc center to sense and respond to cellular disulfide stress.

Global methods to monitor the thiol-disulfide state of proteins in vivo

Cysteines play an important role in protein biochemistry. The unique chemical property and high reactivity of the free thiol group makes reduced cysteine a versatile component of catalytic centers and metal binding sites in many cytosolic proteins and oxidized cystine a stabilizing component in many secreted proteins. Moreover, cysteines readily react with reactive oxygen and nitrogen species to form reversible oxidative thiol modifications. As a result, these reversible thiol modifications have found a use as regulatory nano-switches in an increasing number of redox sensitive proteins. These redox-regulated proteins are able to adjust their activity quickly in response to changes in their redox environment. Over the past few years, a number of techniques have been developed that give insight into the global thiol-disulfide state of proteins in the cell. They have been successfully used to find substrates of thiol-disulfide oxidoreductases and to discover novel redoxregulated proteins. This review will provide an overview of the current techniques, focus on approaches to quantitatively describe the extent of thiol modification in vivo, and summarize their applications.

Dithiol Proteins as Guardians of the Intracellular Redox Milieu in Parasites: Old and New Drug Targets in Trypanosomes and Malaria-Causing Plasmodia

Parasitic diseases such as sleeping sickness, Chagas' heart disease, and malaria are major health problems in poverty-stricken areas. Antiparasitic drugs that are not only active but also affordable and readily available are urgently required. One approach to finding new drugs and rediscovering old ones is based on enzyme inhibitors that paralyze antioxidant systems in the pathogens. These antioxidant ensembles are essential to the parasites as they are attacked in the human host by strong oxidants such as peroxynitrite, hypochlorite, and H2O2. The pathogen-protecting system consists of some 20 thiol and dithiol proteins, which buffer the intraparasitic redox milieu at a potential of -250 mV. In trypanosomes and leishmania the network is centered around the unique dithiol trypanothione (N1,N8-bis(glutathionyl)spermidine). In contrast, malaria parasites have a more conservative dual antioxidative system based on glutathione and thioredoxin. Inhibitors of antioxidant enzymes such as trypanothione reductase are, indeed, parasiticidal but they can also delay or prevent resistance against a number of other antiparasitic drugs.

Protein thiol modifications visualized in vivo

Thiol-disulfide interconversions play a crucial role in the chemistry of biological systems. They participate in the major systems that control the cellular redox potential and prevent oxidative damage. In addition, thiol-disulfide exchange reactions serve as molecular switches in a growing number of redox-regulated proteins. We developed a differential thiol-trapping technique combined with two-dimensional gel analysis, which in combination with genetic studies, allowed us to obtain a snapshot of the in vivo thiol status of cellular proteins. We determined the redox potential of protein thiols in vivo, identified and dissected the in vivo substrate proteins of the major cellular thiol-disulfide oxidoreductases, and discovered proteins that undergo thiol modifications during oxidative stress. Under normal growth conditions most cytosolic proteins had reduced cysteines, confirming existing dogmas. Among the few partly oxidized cytosolic proteins that we detected were proteins that are known to form disulfide bond intermediates transiently during their catalytic cycle (e.g., dihydrolipoyl transacetylase and lipoamide dehydrogenase). Most proteins with highly oxidized thiols were periplasmic proteins and were found to be in vivo substrates of the disulfide-bond-forming protein DsbA. We discovered a substantial number of redox-sensitive cytoplasmic proteins, whose thiol groups were significantly oxidized in strains lacking thioredoxin A. These included detoxifying enzymes as well as many metabolic enzymes with active-site cysteines that were not known to be substrates for thioredoxin. H₂O₂-induced oxidative stress resulted in the specific oxidation of thiols of proteins involved in detoxification of H₂O₂ and of enzymes of cofactor and amino acid biosynthesis pathways such as thiolperoxidase, GTP-cyclohydrolase I, and the cobalamin-independent methionine synthase MetE. Remarkably, a number of these proteins were previously or are now shown to be redox regulated.

A differential thiol-trapping technique combined with two- dimensional gel analysis has been developed and used to visualize thiol-disulfide exchange reactions, which act as switches in redox-regulated proteins

Redox events in interleukin-1 signaling

There is increasing evidence that reactive oxygen species (ROS) are mediators in growth factor and cytokine signaling pathways. Mechanisms by which ROS can interfere with signaling cascades may include regulation of protein activities by the modification of essential cysteines. Modification can be performed chemically or enzyme-catalyzed. Enzymes catalyzing a reversible thiol modification within proteins are to be able to react with both, ROS and protein thiols. If hydroperoxides are involved, promising candidates are peroxiredoxins and glutathione peroxidases (GPx), especially the phospholipid hydroperoxide GPx. Interleukin-1, one of the key players in inflammatory response, stimulates the production of ROS itself, but its signaling cascade can also be influenced by ROS and by thiol modifying agents. Targets are located in early, intermediate, and late events in the signaling cascade. We here summarize what is known about the effects of thiol modifying agents, selenium and glutathione peroxidases, on the assembly of the IL-1 receptor signaling complex as an early event, on the activation of NF-kappa B as an intermediate event, and on the expression of cell adhesion molecules as a late event in IL-1 signaling.

Oxidative stress in malaria parasite-infected erythrocytes: host–parasite interactions

Experimenta naturae, like the glucose-6-phosphate dehydrogenase deficiency, indicate that malaria parasites are highly susceptible to alterations in the redox equilibrium. This offers a great potential for the development of urgently required novel chemotherapeutic strategies. However, the relationship between the redox status of malarial parasites and that of their host is complex. In this review article we summarise the presently available knowledge on sources and detoxification pathways of reactive oxygen species in malaria parasite-infected red cells, on clinical aspects of redox metabolism and redox-related mechanisms of drug action as well as future prospects for drug development. As delineated below, alterations in redox status contribute to disease manifestation including sequestration, cerebral pathology, anaemia, respiratory distress, and placental malaria. Studying haemoglobinopathies, like thalassemias and sickle cell disease, and other red cell defects that provide protection against malaria allows insights into this fine balance of redox interactions. The host immune response to malaria involves phagocytosis as well as the production of nitric oxide and oxygen radicals that form part of the host defence system and also contribute to the pathology of the disease. Haemoglobin degradation by the malarial parasite produces the redox active by-products, free haem and H(2)O(2), conferring oxidative insult on the host cell. However, the parasite also supplies antioxidant moieties to the host and possesses an efficient enzymatic antioxidant defence system including glutathione- and thioredoxin-dependent proteins. Mechanistic and structural work on these enzymes might provide a basis for targeting the parasite. Indeed, a number of currently used drugs, especially the endoperoxide antimalarials, appear to act by increasing oxidant stress, and novel drugs such as peroxidic compounds and anthroquinones are being developed.

Different types of glutathionylation of hemoglobin can exist in intact erythrocytes

Glutathionylation of hemoglobin (Hb) was studied by incubation of intact human erythrocytes with 1 mM tert-butylhydroperoxide (tBHP). Electrophoresis of the membranes showed a time dependent increase of membrane-bound Hb alpha chain until 10 min, and immunoblotting study showed that membrane-bound Hb alpha chain reacted with anti-glutathione antibody only after 10 min. Concomitant with the Hb alpha chain, membrane associated actin, spectrin, and glyceraldehyde 3-phosphate dehydrogenase reacted with the antibody. Cytosolic Hb of the control erythrocytes reacted with anti-glutathione antibody. Together with our previous paper, the present study indicates that at least three different types of glutathionylation of Hb can exist in erythrocytes. The first type is a mixed disulfide bond between reduced glutathione (GSH) and normal Hb. The second type is a disulfide bond between the cysteine 93 of metHb beta chain and oxidized glutathione (GSSG), and the third type is a disulfide bond between the other cysteine residues of metHb alpha chain and/or metHb beta chain and GSSG.

Human mitochondrial glutaredoxin reduces S-glutathionylated proteins with high affinity accepting electrons from either glutathione or thioredoxin reductase

Glutaredoxins catalyze glutathione-dependent thiol disulfide oxidoreductions via a GSH-binding site and active cysteines. Recently a second human glutaredoxin (Grx2), which is targeted to either mitochondria or the nucleus, was cloned. Grx2 contains the active site sequence CSYC, which is different from the conserved CPYC motif present in the cytosolic Grx1. Here we have compared the activity of Grx2 and Grx1 using glutathionylated substrates and active site mutants. The kinetic studies showed that Grx2 catalyzes the reduction of glutathionylated substrates with a lower rate but higher affinity compared with Grx1, resulting in almost identical catalytic efficiencies (k(cat)/K(m)). Permutation of the active site motifs of Grx1 and Grx2 revealed that the CSYC sequence of Grx2 is a prerequisite for its high affinity toward glutathionylated proteins, which comes at the price of lower k(cat). Furthermore Grx2 was a substrate for NADPH and thioredoxin reductase, which efficiently reduced both the active site disulfide and the GSH-glutaredoxin intermediate formed in the reduction of glutathionylated substrates. Using this novel electron donor pathway, Grx2 reduced low molecular weight disulfides such as CoA but with particular high efficiency glutathionylated substrates including GSSG. These results suggest an important role for Grx2 in protection and recovery from oxidative stress.

Actin S-glutathionylation: evidence against a thiol-disulphide exchange mechanism

Many proteins, including actin, are targets for S-glutathionylation, the reversible formation of mixed disulphides between protein cysteinyl thiol groups and glutathione (GSH) that can be induced in cells by oxidative stress. Proposed mechanisms of protein S-glutathionylation follow mainly two distinct pathways. One route involves the initial oxidative modification of a reduced protein thiol to an activated protein, which may then react with GSH to the mixed disulphide. The second route involves the oxidative modification of GSH to an activated form such as glutathione disulphide (GSSG), which may then react with a reduced protein thiol, yielding the corresponding protein mixed disulphide. We show here that physiological levels of GSSG induce a little extent of actin S-glutathionylation. Instead, actin with the exposed cysteine thiol activated by diamide or 5,5'-dithiobis(2-nitrobenzoic acid) reacts with physiological levels of GSH, incorporating about 0.7 mol GSH/mol protein. Differently, an extremely high concentration of GSSG induces an increased level of S-glutathionylation that causes a 50% inhibition in actin polymerization not reversed by dithiotreitol. In mammalian cells, GSH is present in millimolar concentrations and is in about 100-fold excess over GSSG. The high concentration of GSSG required for obtaining a significant actin S-glutathionylation as well as attendant irreversible changes in protein functions make unlikely that actin may be S-glutathionylated by a thiol-disulphide exchange mechanism within the cell.

Cooperative interaction between ascorbate and glutathione during mitochondrial impairment in mesencephalic cultures

A decrease in total glutathione, and aberrant mitochondrial bioenergetics have been implicated in the pathogenesis of Parkinson's disease. Our previous work exemplified the importance of glutathione (GSH) in the protection of mesencephalic neurons exposed to malonate, a reversible inhibitor of mitochondrial succinate dehydrogenase/complex II. Additionally, reactive oxygen species (ROS) generation was an early, contributing event in malonate toxicity. Protection by ascorbate was found to correlate with a stimulated increase in protein-glutathione mixed disulfide (Pr-SSG) levels. The present study further examined ascorbate-glutathione interactions during mitochondrial impairment. Depletion of GSH in mesencephalic cells with buthionine sulfoximine potentiated both the malonate-induced toxicity and generation of ROS as monitored by dichlorofluorescein diacetate (DCF) fluorescence. Ascorbate completely ameliorated the increase in DCF fluorescence and toxicity in normal and GSH-depleted cultures, suggesting that protection by ascorbate was due in part to upstream removal of free radicals. Ascorbate stimulated Pr-SSG formation during mitochondrial impairment in normal and GSH-depleted cultures to a similar extent when expressed as a proportion of total GSH incorporated into mixed disulfides. Malonate increased the efflux of GSH and GSSG over time in cultures treated for 4, 6 or 8 h. The addition of ascorbate to malonate-treated cells prevented the efflux of GSH, attenuated the efflux of GSSG and regulated the intracellular GSSG/GSH ratio. Maintenance of GSSG/GSH with ascorbate plus malonate was accompanied by a stimulation of Pr-SSG formation. These findings indicate that ascorbate contributes to the maintenance of GSSG/GSH status during oxidative stress through scavenging of radical species, attenuation of GSH efflux and redistribution of GSSG to the formation of mixed disulfides. It is speculated that these events are linked by glutaredoxin, an enzyme shown to contain both dehydroascorbate reductase as well as glutathione thioltransferase activities.

The sugar-metabolic enzymes aldolase and triose-phosphate isomerase are targets of glutathionylation in Arabidopsis thaliana: detection using biotinylated glutathione

GSH has multiple actions in physiological responses of plants, but the molecular mechanisms are not fully understood. GSH plays an important role in functional alteration of proteins by reversible covalent incorporation (glutathionylation) in vertebrate cells. To investigate the function of glutathionylation in plant cells, we examined glutathionylated proteins in the suspension-cultured cells of Arabidopsis using biotinylated GSH. Biotinylated GSH was incorporated into about 20 proteins. Two of these proteins were identified as the key enzymes for sugar metabolism, triose-phosphate isomerase (TPI) and putative plastidic aldolase. Recombinant TPI was inactivated by GSSG, and it was reactivated by GSH. The physiological roles of glutathionylation of TPI and aldolase in sugar metabolism are discussed.

Glutathione--functions and metabolism in the malarial parasite Plasmodium falciparum

When present as a trophozoite in human erythrocytes, the malarial parasite Plasmodium falciparum exhibits an intense glutathione metabolism. Glutathione plays a role not only in antioxidative defense and in maintaining the reducing environment of the cytosol. Many of the known glutathione-dependent processes are directly related to the specific lifestyle of the parasite. Reduced glutathione (GSH) supports rapid cell growth by providing electrons for deoxyribonucleotide synthesis and it takes part in detoxifying heme, a product of hemoglobin digestion. Free radicals generated in the parasite can be scavenged in reaction sequences involving the thiyl radical GS* as well as the thiolate GS-. As a substrate of glutathione S-transferase, glutathione is conjugated to non-degradable compounds including antimalarial drugs. Furthermore, it is the coenzyme of the glyoxalase system which detoxifies methylglyoxal, a byproduct of the intense glycolysis taking place in the trophozoite. Proteins involved in GSH-dependent processes include glutathione reductase, glutaredoxins, glyoxalase I and II, glutathione S-transferases, and thioredoxins. These proteins, as well as the ATP-dependent enzymes of glutathione synthesis, are studied as factors in the pathophysiology of malaria but also as potential drug targets. Methylene blue, an inhibitor of the structurally known P. falciparum glutathione reductase, appears to be a promising antimalarial medication when given in combination with chloroquine.

Selenium-dependent enzymes in endothelial cell function

Glutathione peroxidases and thioredoxin reductases are the main selenoproteins expressed by endothelial cells. These enzymes reduce hydroperoxides, their role in endothelial cell physiology, however, by far exceeds prevention of oxidative damage. Reactive oxygen and nitrogen species, especially superoxide, hydroperoxides, and nitric oxide, are crucial signaling molecules in endothelial cells. Their production is regulated by vascular NAD(P)H oxidases and the endothelial nitric oxide synthase. Their metabolism and physiological functions are coordinated by glutathione peroxidases and the thioredoxin/thioredoxin reductase system. Endothelial selenoproteins are involved in the regulation of the vascular tone by maintaining the superoxide anion/nitric oxide balance, of cell adhesion by controlling cell adhesion molecule expression, of apoptosis via inhibition/activation of apoptosis signal-regulating kinase-1, and of eicosanoid production by controlling the activity of cyclooxygenases and lipoxygenases. Accordingly, they regulate inflammatory processes and atherogenesis. The underlying mechanisms are various and differ between individual selenoproteins. Scavenging of hydroperoxides not only prevents oxidative damage, but also interferes with signaling cascades and enzymes involved. Modulation of proteins by hydroperoxide-driven thiol/disulfide exchange is a novel mechanism that needs to be further investigated. A better understanding of the complex interplay of selenoproteins in regulating endothelial cell functions will help to develop a rationale for an improvement of health by an optimum selenium supply.

20 S proteasome from Saccharomyces cerevisiae is responsive to redox modifications and is S-glutathionylated

The 20 S proteasome core purified from Saccharomyces cerevisiae is inhibited by reduced glutathione (GSH), cysteine (Cys), or the GSH precursor gamma-glutamylcysteine. Chymotrypsin-like activity was more affected by GSH than trypsin-like activity, whereas the peptidylglutamyl-hydrolyzing activity (caspase-like) was not inhibited by GSH. Cys-sulfenic acid formation in the 20 S core was demonstrated by spectral characterization of the Cys-S(O)-4-nitrobenzo-2-oxa-1,3-diazole adduct, indicating that 20 S proteasome Cys residues might react with reduced sulfhydryls (GSH, Cys, and gamma-glutamylcysteine) through the oxidized Cys-sulfenic acid form. S-Glutahionylation of the 20 S core was demonstrated in vitro by GSH-biotin incorporation and by decreased alkylation with monobromobimane. Compounds such as N-ethylmaleimide (-S-sulfhydril H alkylating), dimedone (-SO sulfenic acid H reactant), or 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (either -SH or -SOH reactant) highly inhibited proteasomal chymotrypsin-like activity. In vivo experiments revealed that 20 S proteasome extracted from H(2)O(2)-treated cells showed decreased chymotrypsin-like activity accompanied by S-glutathionylation as demonstrated by GSH release from the 20 S core after reduction with NaBH(4). Moreover, cells pretreated with H(2)O(2) showed decreased reductive capacity assessed by determination of the GSH/oxidized glutathione ratio and increased protein carbonyl levels. The present results indicate that at the physiological level the yeast 20 S proteasome is regulated by its sulfhydryl content, thereby coupling intracellular redox signaling to proteasome-mediated proteolysis.