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

Proteome Damage Inflicted by Ionizing Radiation: Advancing a Theme in the Research of Miroslav Radman

Oxidative proteome damage has been implicated as a major contributor to cell death and aging. Protein damage and aging has been a particular theme of the recent research of Miroslav Radman. However, the study of how cellular proteins are damaged by oxidative processes is still in its infancy. Here we examine oxidative changes in the proteomes of four bacterial populations—wild type E. coli, two isolates from E. coli populations evolved for high levels of ionizing radiation (IR) resistance, and D. radiodurans—immediately following exposure to 3000 Gy of ionizing radiation. By a substantial margin, the most prominent intracellular oxidation events involve hydroxylation of methionine residues. Significant but much less frequent are carbonylation events on tyrosine and dioxidation events on tryptophan. A few proteins are exquisitely sensitive to targeted oxidation events, notably the active site of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in E. coli. Extensive experimental evolution of E. coli for IR resistance has decreased overall proteome sensitivity to oxidation but not to the level seen in D. radiodurans. Many observed oxidation events may reflect aspects of protein structure and/or exposure of protein surfaces to water. Proteins such as GAPDH and possibly Ef-Tu may have an evolved sensitivity to oxidation by H₂O₂.

Glutaredoxin: Discovery, redox defense and much more

Glutaredoxin, Grx, is a small protein containing an active site cysteine pair and was discovered in 1976 by Arne Holmgren. The Grx system, comprised of Grx, glutathione, glutathione reductase, and NADPH, was first described as an electron donor for Ribonucleotide Reductase but, from the first discovery in E.coli, the Grx family has impressively grown, particularly in the last two decades. Several isoforms have been described in different organisms (from bacteria to humans) and with different functions.

The unique characteristic of Grxs is their ability to catalyse glutathione-dependent redox regulation via glutathionylation, the conjugation of glutathione to a substrate, and its reverse reaction, deglutathionylation. Grxs have also recently been enrolled in iron sulphur cluster formation. These functions have been implied in various physiological and pathological conditions, from immune defense to neurodegeneration and cancer development thus making Grx a possible drug target.

This review aims to give an overview on Grxs, starting by a phylogenetic analysis of vertebrate Grxs, followed by an analysis of the mechanisms of action, the specific characteristics of the different human isoforms and a discussion on aspects related to human physiology and diseases.

Ionizing Radiation-induced Proteomic Oxidation in Escherichia coli

Recent work has begun to investigate the role of protein damage in cell death because of ionizing radiation (IR) exposure, but none have been performed on a proteome-wide basis, nor have they utilized MS (MS) to determine chemical identity of the amino acid side chain alteration. Here, we use Escherichia coli to perform the first MS analysis of IR-treated intact cells on a proteome scale. From quintuplicate IR-treated (1000 Gy) and untreated replicates, we successfully quantified 13,262 peptides mapping to 1938 unique proteins. Statistically significant, but low in magnitude (<2-fold), IR-induced changes in peptide abundance were observed in 12% of all peptides detected, although oxidative alterations were rare. Hydroxylation (+15.99 Da) was the most prevalent covalent adduct detected. In parallel with these studies on E. coli, identical experiments with the IR-resistant bacterium, Deinococcus radiodurans, revealed orders of magnitude less effect of IR on the proteome. In E. coli, the most significant target of IR by a wide margin was glyceraldehyde 3'-phosphate dehydrogenase (GAPDH), in which the thiol side chain of the catalytic Cys residue was oxidized to sulfonic acid. The same modification was detected in IR-treated human breast carcinoma cells. Sensitivity of GAPDH to reactive oxygen species (ROS) has been described previously in microbes and here, we present GAPDH as an immediate, primary target of IR-induced oxidation across all domains of life.

Protein S-glutathionylation reactions as a global inhibitor of cell metabolism for the desensitization of hydrogen peroxide signals

The pathogenesis of many human diseases has been attributed to the over production of reactive oxygen species (ROS), particularly superoxide (O₂^●-) and hydrogen peroxide (H₂O₂), by-products of metabolism that are generated by the premature reaction of electrons with molecular oxygen (O₂) before they reach complex IV of the respiratory chain. To date, there are 32 known ROS generators in mammalian cells, 16 of which reside inside mitochondria. Importantly, although these ROS are deleterious at high levels, controlled and temporary bursts in H₂O₂ production is beneficial to mammalian cells. Mammalian cells use sophisticated systems to take advantage of the second messaging properties of H₂O₂. This includes controlling its availability using antioxidant systems and negative feedback loops that inhibit the genesis of ROS at sites of production. At its core, ROS production depends on fuel metabolism. Therefore, desensitizing H₂O₂ signals would also require the temporary inhibition of fuel combustion and fluxes through metabolic pathways that promote ROS production. Additionally, this would also demand the diversion of fuels and nutrients into pathways that generate NADPH and other molecules required to maintain cellular redox buffering capacity. Therefore, fuel selection and metabolic flux plays an integral role in dictating the strength and duration of cellular redox signals. In the present review I provide an updated view on the function of protein S-glutathionylation, a ubiquitous redox sensitive modification involving the formation of a disulfide between the small molecular antioxidant glutathione and a cysteine residue, in the regulation of cellular metabolism on a global scale. To date, these concepts have mostly been reviewed at the level of mitochondrial bioenergetics in the contexts of health and disease. Careful examination of the literature revealed that glutathionylation is a temporary inhibitor of most metabolic pathways including glycolysis, the Krebs cycle, oxidative phosphorylation, amino acid metabolism, and fatty acid combustion, resulting in the diversion of fuels towards NADPH-producing pathways and the inhibition of ROS production. Armed with this information, I propose that protein S-glutathionylation reactions desensitize H₂O₂ signals emanating from catabolic pathways using a three-pronged regulatory mechanism; 1) inhibition of metabolic flux through pathways that promote ROS production, 2) diversion of metabolites towards pathways that support antioxidant defenses, and 3) direct inhibition of ROS-generating enzymes.

Competitive oxidation and ubiquitylation on the evolutionarily conserved cysteine confer tissue-specific stabilization of Insig-2

Insig-2 is an ER membrane protein negatively controlling lipid biosynthesis. Here, we find that Insig-2 is increased in the tissues, including liver, but unaltered in the muscle of gp78-deficient mice. In hepatocytes and undifferentiated C2C12 myoblasts, Insig-2 is ubiquitylated on Cys215 by gp78 and degraded. However, the C215 residue is oxidized by elevated reactive oxygen species (ROS) during C2C12 myoblasts differentiating into myotubes, preventing Insig-2 from ubiquitylation and degradation. The stabilized Insig-2 downregulates lipogenesis through inhibiting the SREBP pathway, helping to channel the carbon flux to ATP generation and protecting myotubes from lipid over-accumulation. Evolutionary analysis shows that the YECK (in which C represents Cys215 in human Insig-2) tetrapeptide sequence in Insig-2 is highly conserved in amniotes but not in aquatic amphibians and fishes, suggesting it may have been shaped by differential selection. Together, this study suggests that competitive oxidation-ubiquitylation on Cys215 of Insig-2 senses ROS and prevents muscle cells from lipid accumulation.

Protein-protein interactions of human glyoxalase II: findings of a reliable docking protocol

Glyoxalase II (GlxII) is an antioxidant glutathione-dependent enzyme, which catalyzes the hydrolysis of S-d-lactoylglutathione to form d-lactic acid and glutathione (GSH). The last product is the most important thiol reducing agent present in all eukaryotic cells that have mitochondria and chloroplasts. It is generally known that GSH plays a crucial role not only in the cellular redox state but also in various cellular processes. One of them is protein S-glutathionylation, a process that can occur through an oxidation reaction of proteins' thiol groups by GSH. Changes in protein S-glutathionylation have been associated with a range of human diseases such as diabetes, cardiovascular and pulmonary diseases, neurodegenerative diseases and cancer. Within a major project aimed at elucidating the role of GlxII in the mechanism of S-glutathionylation, a reliable computational protocol consisting of a protein-protein docking approach followed by atomistic Molecular Dynamics (MD) simulations was developed and it was applied to the prediction of molecular associations between human GlxII (in the presence and absence of GSH) and some proteins that are known to be S-glutathionylated in vitro, such as actin, malate dehydrogenase (MDH) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The computational results show a high propensity of GlxII to interact with actin and MDH through its active site and a high stability of the GlxII-protein systems when GSH is present. Moreover, close proximities of GSH with actin and MDH cysteine residues have been found, suggesting that GlxII could be able to perform protein S-glutathionylation by using the GSH molecule present in its catalytic site.

Consequences of Oxidative Stress on Plant Glycolytic and Respiratory Metabolism

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are present at low and controlled levels under normal conditions. These reactive molecules can increase to high levels under various biotic and abiotic conditions, resulting in perturbation of the cellular redox state that can ultimately lead to oxidative or nitrosative stress. In this review, we analyze the various effects that result from alterations of redox homeostasis on plant glycolytic pathway and tricarboxylic acid (TCA) cycle. Most documented modifications caused by ROS or RNS are due to the presence of redox-sensitive cysteine thiol groups in proteins. Redox modifications include Cys oxidation, disulfide bond formation, S-glutathionylation, S-nitrosylation, and S-sulfhydration. A growing number of proteomic surveys and biochemical studies document the occurrence of ROS- or RNS-mediated modification in enzymes of glycolysis and the TCA cycle. In a few cases, these modifications have been shown to affect enzyme activity, suggesting an operational regulatory mechanism in vivo. Further changes induced by oxidative stress conditions include the proposed redox-dependent modifications in the subcellular distribution of a putative redox sensor, NAD-glyceraldehyde-3P dehydrogenase and the micro-compartmentation of cytosolic glycolytic enzymes. Data from the literature indicate that oxidative stress may induce complex changes in metabolite pools in central carbon metabolism. This information is discussed in the context of our understanding of plant metabolic response to oxidative stress.

Targeting redox regulation to treat substance use disorder using N‐acetylcysteine

Substance use disorder (SUD) is a chronic relapsing disorder characterized by transitioning from acute drug reward to compulsive drug use. Despite the heavy personal and societal burden of SUDs, current treatments are limited and unsatisfactory. For this reason, a deeper understanding of the mechanisms underlying addiction is required. Altered redox status, primarily due to drug-induced increases in dopamine metabolism, is a unifying feature of abused substances. In recent years, knowledge of the effects of oxidative stress in the nervous system has evolved from strictly neurotoxic to include a more nuanced role in redox-sensitive signaling. More specifically, S-glutathionylation, a redox-sensitive post-translational modification, has been suggested to influence the response to drugs of abuse. In this review we will examine the evidence for redox-mediating drugs as therapeutic tools focusing on N-acetylcysteine as a treatment for cocaine addiction. We will conclude by suggesting future research directions that may further advance this field.

ROLE OF THIOLS IN OXIDATIVE STRESS

A well-regulated redox state is essential for normal physiological function and cellular metabolism. In most eukaryotic cells, protein cysteine thiols are most sensitive to fluctuations in the cellular redox state. Under normal physiological conditions, the cytosol has a highly reducing environment, which is due to high levels of reduced glutathione and complex system of redox enzymes that maintain glutathione in the reduced state. The reducing environment of the cytosol maintains most protein thiols in the reduced state; although some non-exposed cysteine could be present as disulfides. Upon physiological increase in cellular oxidants, such as due to growth factors, cytokines and thiol-disulfide exchange reactions, specific proteins could act as redox switches that regulate the conformation and activity of different proteins. This reversible post translational modification enables redox-sensitive dynamic changes in cell signaling and function. Physiological oxidative stress could lead to the formation of sulfenic acids, which are usually intermediate states of thiol oxidation that are converted to higher order oxidation states, intramolecular disulfides or mixed disulfides with glutathione. Such glutathiolation reactions have been found to regulate the function of several proteins involved in intracellular metabolism, signal transduction and cell structure. Excessive oxidative stress results in indiscriminate and irreversible oxidation of protein thiols, depletion of glutathione and cell death. Further elucidation of the relationship between changes in cell redox and thiol reactivity could provide a better understanding of how redox changes regulate cell function and how disruption of these relationships lead to tissue injury and dysfunction and the development of chronic diseases such as cancer and cardiovascular disease.

A possible S-glutathionylation of specific proteins by glyoxalase II: An in vitro and in silico study

Glyoxalase II, the second of 2 enzymes in the glyoxalase system, is a hydroxyacylglutathione hydrolase that catalyses the hydrolysis of S-d-lactoylglutathione to form d-lactic acid and glutathione, which is released from the active site. The tripeptide glutathione is the major sulfhydryl antioxidant and has been shown to control several functions, including S-glutathionylation of proteins. S-Glutathionylation is a way for the cells to store reduced glutathione during oxidative stress, or to protect protein thiol groups from irreversible oxidation, and few enzymes involved in protein S-glutathionylation have been found to date. In this work, the enzyme glyoxalase II and its substrate S-d-lactoylglutathione were incubated with malate dehydrogenase or with actin, resulting in a glutathionylation reaction. Glyoxalase II was also submitted to docking studies. Computational data presented a high propensity of the enzyme to interact with malate dehydrogenase or actin through its catalytic site and further in silico investigation showed a high folding stability of glyoxalase II toward its own reaction product glutathione both protonated and unprotonated. This study suggests that glyoxalase II, through a specific interaction of its catalytic site with target proteins, could be able to perform a rapid and specific protein S-glutathionylation using its natural substrate S-d-lactoylglutathione.

SIGNIFICANCE

This article reports for the first time a possible additional role of Glo2 that, after interacting with a target protein, is able to promote S-glutathionylation using its natural substrate SLG, a glutathione derived compound. In this perspective, Glo2 can play a new important regulatory role inS-glutathionylation, acquiring further significance in cellular post-translational modifications of proteins.

Mixed results with mixed disulfides

A period of research with Helmut Sies in the 1980s is recalled. Our experiments aimed at an in-depth understanding of metabolic changes due to oxidative challenges under near-physiological conditions, i.e. perfused organs. A major focus were alterations of the glutathione and the NADPH/NADP(+) system by different kinds of oxidants, in particular formation of glutathione mixed disulfides with proteins. To analyze mixed disulfides, a test was adapted which is widely used until today. The observations in perfused rat livers let us believe that glutathione-6-phosphate dehydrogenase (G6PDH), i.a. might be activated by glutathionylation. Although we did not succeed to verify this hypothesis for the special case of G6PDH, the regulation of enzyme/protein activities by glutathionylation today is an accepted posttranslational mechanism in redox biology in general. Our early experimental approaches are discussed in the context of present knowledge.

Allicin Induces Thiol Stress in Bacteria through S-Allylmercapto Modification of Protein Cysteines

Allicin (diallyl thiosulfinate) from garlic is a highly potent natural antimicrobial substance. It inhibits growth of a variety of microorganisms, among them antibiotic-resistant strains. However, the precise mode of action of allicin is unknown. Here, we show that growth inhibition of Escherichia coli during allicin exposure coincides with a depletion of the glutathione pool and S-allylmercapto modification of proteins, resulting in overall decreased total sulfhydryl levels. This is accompanied by the induction of the oxidative and heat stress response. We identified and quantified the allicin-induced modification S-allylmercaptocysteine for a set of cytoplasmic proteins by using a combination of label-free mass spectrometry and differential isotope-coded affinity tag labeling of reduced and oxidized thiol residues. Activity of isocitrate lyase AceA, an S-allylmercapto-modified candidate protein, is largely inhibited by allicin treatment in vivo. Allicin-induced protein modifications trigger protein aggregation, which largely stabilizes RpoH and thereby induces the heat stress response. At sublethal concentrations, the heat stress response is crucial to overcome allicin stress. Our results indicate that the mode of action of allicin is a combination of a decrease of glutathione levels, unfolding stress, and inactivation of crucial metabolic enzymes through S-allylmercapto modification of cysteines.

Disulfide-Linked Dinitroxides for Monitoring Cellular Thiol Redox Status through Electron Paramagnetic Resonance Spectroscopy

Intracellular thiol-disulfide redox balance is crucial to cell health, and may be a key determinant of a cancer's response to chemotherapy and radiation therapy. The ability to assess intracellular thiol-disulfide balance may thus be useful not only in predicting responsiveness of cancers to therapy, but in assessing predisposition to disease. Assays of thiols in biology have relied on colorimetry or fluorimetry, both of which require UV-visible photons, which do not penetrate the body. Low-frequency electron paramagnetic resonance imaging (EPRI) is an emerging magnetic imaging technique that uses radio waves, which penetrate the body well. Therefore, in combination with tailored imaging agents, EPRI affords the opportunity to image physiology within the body. In this study, we have prepared water-soluble and membrane-permeant disulfide-linked dinitroxides, at natural isotopic abundance, and with D,(15)N-substitution. Thiols such as glutathione cleave the disulfides, with simple bimolecular kinetics, to yield the monomeric nitroxide species, with distinctive changes in the EPR spectrum. Using the D,(15)N-substituted disulfide-dinitroxide and EPR spectroscopy, we have obtained quantitative estimates of accessible intracellular thiol in cultured human lymphocytes. Our estimates are in good agreement with published measurements. This suggests that in vivo EPRI of thiol-disulfide balance is feasible. Finally, we discuss the constraints on the design of probe molecules that would be useful for in vivo EPRI of thiol redox status.

Biological chemistry and functionality of protein sulfenic acids and related thiol modifications

Selective modification of proteins at cysteine residues by reactive oxygen, nitrogen or sulfur species formed under physiological and pathological states is emerging as a critical regulator of protein activity impacting cellular function. This review focuses primarily on protein sulfenylation (-SOH), a metastable reversible modification connecting reduced cysteine thiols to many products of cysteine oxidation. An overview is first provided on the chemistry principles underlining synthesis, stability and reactivity of sulfenic acids in model compounds and proteins, followed by a brief description of analytical methods currently employed to characterize these oxidative species. The following chapters present a selection of redox-regulated proteins for which the -SOH formation was experimentally confirmed and linked to protein function. These chapters are organized based on the participation of these proteins in the regulation of signaling, metabolism and epigenetics. The last chapter discusses the therapeutic implications of altered redox microenvironment and protein oxidation in disease.

Development of a Clickable Probe for Profiling of Protein Glutathionylation in the Central Cellular Metabolism of E. coli and Drosophila

Protein glutathionylation is an important post-translational modification that regulates many cellular processes, including energy metabolism, signal transduction, and protein homeostasis. Global profiling of glutathionylated proteins (denoted as glutathionylome) is crucial for understanding redox-regulated signal transduction. Here, we developed a novel method based on click reaction and proteomics to enrich and identify the glutathionylated peptides in Escherichia coli and Drosophila lysates, in which 937 and 1,930 potential glutathionylated peptides were identified, respectively. Bioinformatics analysis showed that the cysteine residue next to negatively charged amino acid residues has a higher frequency of glutathionylation. Importantly, we found that most proteins associated with metabolic pathways were glutathionylated and that the glutathionylation sites of metabolic enzymes were highly conserved among different species. Our results indicate that the glutathione analog is a useful tool to characterize protein glutathionylation, and glutathionylation of metabolic enzymes, which play important roles in regulating cellular metabolism, is conserved.

Protective role of bacillithiol in superoxide stress and Fe-S metabolism in Bacillus subtilis

Glutathione (GSH) serves as the prime thiol in most organisms as its depletion increases antibiotic and metal toxicity, impairs oxidative stress responses, and affects Fe and Fe–S cluster metabolism. Many gram-positive bacteria lack GSH, but instead produce other structurally unrelated yet functionally equivalent thiols. Among those, bacillithiol (BSH) has been recently identified in several low G+C gram-positive bacteria. In this work, we have explored the link between BSH and Fe–S metabolism in Bacillus subtilis. We have identified that B. subtilis lacking BSH is more sensitive to oxidative stress (paraquat), and metal toxicity (Cu(I) and Cd(II)), but not H₂O₂. Furthermore, a slow growth phenotype of BSH null strain in minimal medium was observed, which could be recovered upon the addition of selected amino acids (Leu/Ile and Glu/Gln), supplementation of iron, or chemical complementation with BSH disulfide (BSSB) to the growth medium. Interestingly, Fe–S cluster containing isopropylmalate isomerase (LeuCD) and glutamate synthase (GOGAT) showed decreased activities in BSH null strain. Deficiency of BSH also resulted in decreased levels of intracellular Fe accompanied by increased levels of manganese and altered expression levels of Fe–S cluster biosynthetic SUF components. Together, this study is the first to establish a link between BSH and Fe–S metabolism in B. subtilis.

Protein quality control under oxidative stress conditions

Accumulation of reactive oxygen and chlorine species (RO/CS) is generally regarded to be a toxic and highly undesirable event, which serves as contributing factor in aging and many age-related diseases. However, it is also put to excellent use during host defense, when high levels of RO/CS are produced to kill invading microorganisms and regulate bacterial colonization. Biochemical and cell biological studies of how bacteria and other microorganisms deal with RO/CS have now provided important new insights into the physiological consequences of oxidative stress, the major targets that need protection, and the cellular strategies employed by organisms to mitigate the damage. This review examines the redox-regulated mechanisms by which cells maintain a functional proteome during oxidative stress. We will discuss the well-characterized redox-regulated chaperone Hsp33, and we will review recent discoveries demonstrating that oxidative stress-specific activation of chaperone function is a much more widespread phenomenon than previously anticipated. New members of this group include the cytosolic ATPase Get3 in yeast, the Escherichia coli protein RidA, and the mammalian protein α2-macroglobulin. We will conclude our review with recent evidence showing that inorganic polyphosphate (polyP), whose accumulation significantly increases bacterial oxidative stress resistance, works by a protein-like chaperone mechanism. Understanding the relationship between oxidative and proteotoxic stresses will improve our understanding of both host-microbe interactions and how mammalian cells combat the damaging side effects of uncontrolled RO/CS production, a hallmark of inflammation.

Analytical approaches to the diagnosis and treatment of aging and aging-related disease: redox status and proteomics

Basal levels of oxidants are indispensible for redox signaling to produce adaptive cellular responses such as vitagenes linked to cell survival; however, at higher levels, they are detrimental to cells, contributing to aging and to the pathogenesis of numerous age-related diseases. Aging is a complex systemic process and the major gap in aging research reminds the insufficient knowledge about pathways shifting from normal "healthy" aging to disease-associated pathological aging. The major complication of normal "healthy" aging is in fact the increasing risk of age-related diseases such as cardiovascular diseases, diabetes mellitus, and neurodegenerative pathologies that can adversely affect the quality of life in general, with enhanced incidences of comorbidities and mortality. In this context, global "omics" approaches may help to dissect and fully study the cellular and molecular mechanisms of aging and age-associated processes. The proteome, being more close to the phenotype than the transcriptome and more stable than the metabolome, represents the most promising "omics" field in aging research. In the present study, we exploit recent advances in the redox biology of aging and discuss the potential of proteomics approaches as innovative tools for monitoring at the proteome level the extent of protein oxidative insult and related modifications with the identification of targeted proteins.

Evidence for thiol/disulfide exchange reactions between tubulin and glyceraldehyde-3-phosphate dehydrogenase

While thiol redox reactions are a common mechanism to regulate protein structure and function, protein disulfide bond formation is a marker of oxidative stress that has been linked to neurodegeneration. Both tubulin and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) contain multiple cysteines that have been identified as targets for oxidation to disulfides, S-nitrosation and S-glutathionylation. We show that GAPDH is one of three prominent brain microtubule-associated proteins (MAPs), in addition to MAP-2 and tau, with reactive cysteines. We detected a threefold to fourfold increase in tubulin cysteine oxidation by hydrogen peroxide in the presence of rabbit muscle GAPDH by 5-iodoacetamidofluorescein labeling and by Western blot detection of higher molecular weight inter-chain tubulin disulfides. In thiol/disulfide exchange experiments, tubulin restored ∼50% of oxidized GAPDH cysteines and the equilibrium favored reduced GAPDH. Further, we report that oxidized GAPDH is repaired by the thioredoxin reductase system (TRS). Restoration of GAPDH activity after reduction by both tubulin and the TRS was time-dependent suggesting conformational changes near the active site cysteine149. The addition of brain MAPs to oxidized tubulin reduced tubulin disulfides and labeling of MAP-2 and of GAPDH decreased. Because the extent of tubulin repair of oxidized GAPDH was dependent on buffer strength, we conclude that electrostatics influence thiol/disulfide exchange between the two proteins. The novel interactions presented herein may protect GAPDH from inhibition under oxidative stress conditions.

Evolutionary development of redox regulation in chloroplasts

SIGNIFICANCE

The post-translational modification of thiol groups stands out as a key strategy that cells employ for metabolic regulation and adaptation to changing environmental conditions. Nowhere is this more evident than in chloroplasts-the O2-evolving photosynthetic organelles of plant cells that are fitted with multiple redox systems, including the thioredoxin (Trx) family of oxidoreductases functional in the reversible modification of regulatory thiols of proteins in all types of cells. The best understood member of this family in chloroplasts is the ferredoxin-linked thioredoxin system (FTS) by which proteins are modified via light-dependent disulfide/dithiol (S-S/2SH) transitions.

RECENT ADVANCES

Discovered in the reductive activation of enzymes of the Calvin-Benson cycle in illuminated chloroplast preparations, recent studies have extended the role of the FTS far beyond its original boundaries to include a spectrum of cellular processes. Together with the NADP-linked thioredoxin reductase C-type (NTRC) and glutathione/glutaredoxin systems, the FTS also plays a central role in the response of chloroplasts to different types of stress.

CRITICAL ISSUES

The comparisons of redox regulatory networks functional in chloroplasts of land plants with those of cyanobacteria-prokaryotes considered to be the ancestors of chloroplasts-and different types of algae summarized in this review have provided new insight into the evolutionary development of redox regulation, starting with the simplest O2-evolving organisms.

FUTURE DIRECTIONS

The evolutionary appearance, mode of action, and specificity of the redox regulatory systems functional in chloroplasts, as well as the types of redox modification operating under diverse environmental conditions stand out as areas for future study.

Oxidant sensing by reversible disulfide bond formation

Maintenance of the cellular redox balance is crucial for cell survival. An increase in reactive oxygen, nitrogen, or chlorine species can lead to oxidative stress conditions, potentially damaging DNA, lipids, and proteins. Proteins are very sensitive to oxidative modifications, particularly methionine and cysteine residues. The reversibility of some of these oxidative protein modifications makes them ideally suited to take on regulatory roles in protein function. This is especially true for disulfide bond formation, which has the potential to mediate extensive yet fully reversible structural and functional changes, rapidly adjusting the protein's activity to the prevailing oxidant levels.

Glutaredoxins in thiol/disulfide exchange

SIGNIFICANCE

Glutaredoxins (Grxs) are small oxidoreductases of the thioredoxin family of proteins regulating the thiol redox state of several proteins. Thereby, Grxs are key elements in redox signaling.

RECENT ADVANCES

Redox signaling via protein thiols depends on reversible oxidative modifications induced mainly by reactive oxygen/nitrogen species and glutathione (GSH) in form of its oxidized disulfide or S-nitroso-glutathione. Grxs contribute to redox signaling by the catalysis of glutathionylation, de-glutathionylation, as well as reduction of disulfide bridges via two distinct enzymatic mechanisms. The dithiol mechanism utilizes both active site cysteines to reduce disulfides, whereas the monothiol mechanism utilizes only the N-terminal active site cysteine for the reduction of GSH mixed disulfides. The sphere of action of Grxs continues to grow with the recent identification of novel targets.

CRITICAL ISSUES

Because of limited methodological tools, the identification of new substrates for oxidoreductases in general is one of the biggest challenges in this research area.

FUTURE DIRECTIONS

With this review, we provide a condensed summary of the current knowledge of thiol/disulfide exchange reactions catalyzed by Grxs regarding the mechanistic, structural, and functional aspects. The latter will be of high importance for future research directions, gaining novel insights into redox signaling in general, and the role of Grxs in particular.

Alzheimer's disease: redox dysregulation as a common denominator for diverse pathogenic mechanisms

Alzheimer's disease (AD) is the most common cause of dementia and a progressive neurodegeneration that appears to result from multiple pathogenic mechanisms (including protein misfolding/aggregation, involved in both amyloid β-dependent senile plaques and tau-dependent neurofibrillary tangles), metabolic and mitochondrial dysfunction, excitoxicity, calcium handling impairment, glial cell dysfunction, neuroinflammation, and oxidative stress. Oxidative stress, which could be secondary to several of the other pathophysiological mechanisms, appears to be a major determinant of the pathogenesis and progression of AD. The identification of oxidized proteins common for mild cognitive impairment and AD suggests that key oxidation pathways are triggered early and are involved in the initial progression of the neurodegenerative process. Abundant data support that oxidative stress, also considered as a main factor for aging, the major risk factor for AD, can be a common key element capable of articulating the divergent nature of the proposed pathogenic factors. Pathogenic mechanisms influence each other at different levels. Evidence suggests that it will be difficult to define a single-target therapy resulting in the arrest of progression or the improvement of AD deterioration. Since oxidative stress is present from early stages of disease, it appears as one of the main targets to be included in a clinical trial. Exploring the articulation of AD pathogenic mechanisms by oxidative stress will provide clues for better understanding the pathogenesis and progression of this dementing disorder and for the development of effective therapies to treat this disease.

Using quantitative redox proteomics to dissect the yeast redoxome

To understand and eventually predict the effects of changing redox conditions and oxidant levels on the physiology of an organism, it is essential to gain knowledge about its redoxome: the proteins whose activities are controlled by the oxidation status of their cysteine thiols. Here, we applied the quantitative redox proteomic method OxICAT to Saccharomyces cerevisiae and determined the in vivo thiol oxidation status of almost 300 different yeast proteins distributed among various cellular compartments. We found that a substantial number of cytosolic and mitochondrial proteins are partially oxidized during exponential growth. Our results suggest that prevailing redox conditions constantly control central cellular pathways by fine-tuning oxidation status and hence activity of these proteins. Treatment with sublethal H(2)O(2) concentrations caused a subset of 41 proteins to undergo substantial thiol modifications, thereby affecting a variety of different cellular pathways, many of which are directly or indirectly involved in increasing oxidative stress resistance. Classification of the identified protein thiols according to their steady-state oxidation levels and sensitivity to peroxide treatment revealed that redox sensitivity of protein thiols does not predict peroxide sensitivity. Our studies provide experimental evidence that the ability of protein thiols to react to changing peroxide levels is likely governed by both thermodynamic and kinetic parameters, making predicting thiol modifications challenging and de novo identification of peroxide sensitive protein thiols indispensable.

The disulfide proteome and other reactive cysteine proteomes: analysis and functional significance

Ten years ago, proteomics techniques designed for large-scale investigations of redox-sensitive proteins started to emerge. The proteomes, defined as sets of proteins containing reactive cysteines that undergo oxidative post-translational modifications, have had a particular impact on research concerning the redox regulation of cellular processes. These proteomes, which are hereafter termed "disulfide proteomes," have been studied in nearly all kingdoms of life, including animals, plants, fungi, and bacteria. Disulfide proteomics has been applied to the identification of proteins modified by reactive oxygen and nitrogen species under stress conditions. Other studies involving disulfide proteomics have addressed the functions of thioredoxins and glutaredoxins. Hence, there is a steadily growing number of proteins containing reactive cysteines, which are probable targets for redox regulation. The disulfide proteomes have provided evidence that entire pathways, such as glycolysis, the tricarboxylic acid cycle, and the Calvin-Benson cycle, are controlled by mechanisms involving changes in the cysteine redox state of each enzyme implicated. Synthesis and degradation of proteins are processes highly represented in disulfide proteomes and additional biochemical data have established some mechanisms for their redox regulation. Thus, combined with biochemistry and genetics, disulfide proteomics has a significant potential to contribute to new discoveries on redox regulation and signaling.

Engineering the robustness of Clostridium acetobutylicum by introducing glutathione biosynthetic capability

To improve the aero- and solvent tolerance of the solvent-producing Clostridium acetobutylicum, glutathione biosynthetic capability was introduced into C. acetobutylicum DSM1731 by cloning and over-expressing the gshAB genes from Escherichia coli. Strain DSM1731(pITAB) produces glutathione, and shows a significantly improved survival upon aeration and butanol challenge, as compared with the control. In addition, strain DSM1731(pITAB) exhibited an improved butanol tolerance and an increased butanol production capability, as compared with the recombinant strains with only gshA or gshB gene. These results illustrated that introducing glutathione biosynthetic pathway, which is redundant for the metabolism of C. acetobutylicum, can increase the robustness of the host to achieve a better solvent production.

DJ-1 loss by glutaredoxin but not glutathione depletion triggers Daxx translocation and cell death

Environmental and genetic causes are implicated in the etiopathogenesis of Parkinson's disease (PD), a neurodegenerative movement disorder. DJ-1, a putative gene recessively linked to early onset PD, functions as an antioxidant, transcriptional co-activator, and molecular chaperone. We examined DJ-1 status following global perturbation of protein thiol homeostasis by depleting cellular antioxidant glutathione or downregulating glutaredoxin 1, a thiol disulfide oxidoreductase, wherein both paradigms generate oxidative stress. While these perturbations did not affect expression of DJ-1 mRNA, downregulation of glutaredoxin 1 but not glutathione depletion caused loss of DJ-1 protein, translocation of Daxx (a death-associated protein) from nucleus, and cell death. Overexpression of wild-type DJ-1, but not the cysteine mutants, prevented Daxx translocation and cytotoxicity. Protease inhibitors prevented constitutive DJ-1 loss. Residual DJ-1 was present in reduced state, indicating that DJ-1 when oxidized was degraded through proteolysis. Thus, loss of DJ-1 occurring through its oxidative modification and subsequent proteolysis mediated through dysregulation of thiol disulfide oxidoreductase may contribute to pathogenesis of sporadic PD, thus providing a link between environmental challenges and constitutive levels of this vital protein.

N-Acetylcysteine infusion does not affect glucose disposal during prolonged moderate-intensity exercise in humans

There is evidence that reactive oxygen species (ROS) signalling is required for normal increases in glucose uptake during contraction of isolated mouse skeletal muscle, and that AMP-activated protein kinase (AMPK) is involved. The aim of this study was to determine whether ROS signalling is involved in the regulation of glucose disposal and AMPK activation during moderate-intensity exercise in humans. Nine healthy males completed 80 min of cycle ergometry at 62 +/- 1% of peak oxygen consumption ( V(O(2)peak).A 6,6-(2)H-glucose tracer was infused at rest and during exercise, and in a double-blind randomised cross-over design, N-acetylcysteine (NAC) or saline (CON) was co-infused. NAC was infused at 125 mg kg(1) h(1) for 15 min and then at 25 mg kg(1) h(1) for 20 min before and throughout exercise. NAC infusion elevated plasma NAC and cysteine, and muscle NAC and cysteine concentrations during exercise. Although neither NAC infusion nor exercise significantly affected muscle reduced or oxidised glutathione (GSH or GSSG) concentration (P > 0.05), S-glutathionylation (an indicator of oxidative stress) of a protein band of approximately 270 kDa was increased approximately 3-fold with contraction and this increase was prevented by NAC infusion. Despite this, exercised-induced increases in tracer determined glucose disposal, plasma lactate, plasma non-esterified fatty acids (NEFAs), and decreases in plasma insulin were not affected by NAC infusion. In addition, skeletal muscle AMPKalpha and acetyl-CoA carboxylase-beta (ACCbeta) phosphorylation increased during exercise by approximately 3- and approximately 6-fold (P < 0.05), respectively, and this was not affected by NAC infusion. Unlike findings in mouse muscle ex vivo, NAC does not attenuate skeletal muscle glucose disposal or AMPK activation during moderate-intensity exercise in humans.

Thioredoxins function as deglutathionylase enzymes in the yeast Saccharomyces cerevisiae

Background

Protein-SH groups are amongst the most easily oxidized residues in proteins, but irreversible oxidation can be prevented by protein glutathionylation, in which protein-SH groups form mixed disulphides with glutathione. Glutaredoxins and thioredoxins are key oxidoreductases which have been implicated in regulating glutathionylation/deglutathionylation in diverse organisms. Glutaredoxins have been proposed to be the predominant deglutathionylase enzymes in many plant and mammalian species, whereas, thioredoxins have generally been thought to be relatively inefficient in deglutathionylation.

Results

We show here that the levels of glutathionylated proteins in yeast are regulated in parallel with the growth cycle, and are maximal during stationary phase growth. This increase in glutathionylation is not a response to increased reactive oxygen species generated from the shift to respiratory metabolism, but appears to be a general response to starvation conditions. Our data indicate that glutathionylation levels are constitutively high in all growth phases in thioredoxin mutants and are unaffected in glutaredoxin mutants. We have confirmed that thioredoxins, but not glutaredoxins, catalyse deglutathionylation of model glutathionylated substrates using purified thioredoxin and glutaredoxin proteins. Furthermore, we show that the deglutathionylase activity of thioredoxins is required to reduce the high levels of glutathionylation in stationary phase cells, which occurs as cells exit stationary phase and resume vegetative growth.

Conclusions

There is increasing evidence that the thioredoxin and glutathione redox systems have overlapping functions and these present data indicate that the thioredoxin system plays a key role in regulating the modification of proteins by the glutathione system.

Use of dimedone-based chemical probes for sulfenic acid detection methods to visualize and identify labeled proteins

Reversible thiol modification is a major component of the modulation of cell-signaling pathways by reactive oxygen species. Hydrogen peroxide, peroxynitrite, or lipid hydroperoxides are all able to oxidize cysteines to form cysteine sulfenic acids; this reactive intermediate can be directly reduced to thiol by cellular reductants such as thioredoxin or further participate in disulfide bond formation with glutathione or cysteine residues in the same or another protein. To identify the direct protein targets of cysteine modification and the conditions under which they are oxidized, a series of dimedone-based reagents linked to affinity or fluorescent tags have been developed that specifically alkylate and trap cysteine sulfenic acids. In this chapter, we provide detailed methods using one of our biotin-tagged reagents, DCP-Bio1, to identify and monitor proteins that are oxidized in vitro and in vivo. Using streptavidin-linked agarose beads, this biotin-linked reagent can be used to affinity capture labeled proteins. Stringent washing of the beads prior to elution minimizes the contamination of the enriched material with unlabeled proteins through coimmunoprecipitation or nonspecific binding. In particular, we suggest including DTT in one of the washes to remove proteins covalently linked to biotinylated proteins through a disulfide bond, except in cases where these linked proteins are of interest. We also provide methods for targeted approaches monitoring cysteine oxidation in individual proteins, global approaches to follow total cysteine oxidation in the cell, and guidelines for proteomic analyses to identify novel proteins with redox sensitive cysteines.

Proteome changes during meat aging in tough and tender beef suggest the importance of apoptosis and protein solubility for beef aging and tenderization

Within a population of Charolais young bulls, two extreme groups of longissimus thoracis muscle samples, classified according to Warner-Bratzler shear force (WBSF) of 55 degrees C grilled meat, were analyzed by 2D-electrophoresis. Muscle analyses were performed on 4 bulls of the "tender" group (WBSF=27.7+/-4.8 N) and 4 bulls of the "tough" group (WBSF=41.2+/-6.1 N), at 3 post-mortem times: D0, samples taken within 10 min post-mortem; D5 and D21, samples kept at 4 degrees C under vacuum during 5 and 21 days. Proteins of muscle samples were separated in two fractions based on protein solubility in Tris buffer: "soluble" and "insoluble". Proteins of both fractions were separated by 2D-electrophoresis. Evolution of spots during the 3 post-mortem times was analyzed by hierarchical classification (HCA). Three clusters of proteins presenting similar evolution profiles provided accurate classification of post-mortem times and showed the translocation of some chaperone proteins and glycolytic enzymes from the soluble fraction to the insoluble fraction between D0 and D5. Cellular structure dismantlement and proteolysis was observed at D21. Effect of group ("tender" vs "tough") on spot intensities was tested by ANOVA. At D0, higher quantity of proteins of the inner and outer membrane of mitochondria was found in the tender group suggesting a more extensive degradation of mitochondria that may be related to the apoptotic process.

Measuring the poise of thiol/disulfide couples in vivo

The reduction potentials (E(h)) for the redox couples GSH/GSSG and cysteine/cystine (Cys/CySS) in plasma are useful indicators of systemic oxidative stress and other medically relevant physiological states. This article describes a sensitive method for determining plasma levels of GSH, GSSG, Cys, and CySS used to calculate the in vivo E(h) values. The method uses iodoacetate to alkylate free thiols, derivatization with dansyl chloride to fluorescently tag amino groups, and HPLC and fluorescence to separate, detect, and quantify the molecules. Benefits of the method, such as sensitivity and dynamic range, are described, as are caveats, such as the importance of preventing red blood cell hemolysis and limitations in quantification of GSSG. General principles of redox chemistry and previous studies showing that the compounds are more oxidized than predicted from their standard reduction potentials are reviewed. The calculated in vivo E(h) is a convenient and informative way of summarizing the redox environment of plasma and is also useful for studies of cerebrospinal fluid, lymph, bronchoalveolar lavage fluid, human biopsies, and a broad range of in vitro cell culture conditions.

Novel roles for GAPDH in cell death and carcinogenesis

Growing evidence points to the fact that glucose metabolism has a central role in carcinogenesis. Among the enzymes controlling this energy production pathway, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is of particular interest. Initially identified as a glycolytic enzyme and considered as a housekeeping gene, this enzyme is actually tightly regulated and is involved in numerous cellular functions. Particularly intriguing are recent reports describing GAPDH as a regulator of cell death. However, its role in cell death is unclear; whereas some studies point toward a proapoptotic function, others describe a protective role and suggest its participation in tumor progression. In this study, we highlight recent findings and discuss potential mechanisms through which cells regulate GAPDH to fulfill its diverse functions to influence cell fate.

Thiol-based redox switches in eukaryotic proteins

For many years, oxidative thiol modifications in cytosolic proteins were largely disregarded as in vitro artifacts, and considered unlikely to play significant roles within the reducing environment of the cell. Recent developments in in vivo thiol trapping technology combined with mass spectrometric analysis have now provided convincing evidence that thiol-based redox switches are used as molecular tools in many proteins to regulate their activity in response to reactive oxygen and nitrogen species. Reversible oxidative thiol modifications have been found to modulate the function of proteins involved in many different pathways, starting from gene transcription, translation and protein folding, to metabolism, signal transduction, and ultimately apoptosis. This review will focus on three well-characterized eukaryotic proteins that use thiol-based redox switches to influence gene transcription, metabolism, and signal transduction. The transcription factor Yap1p is a good illustration of how oxidative modifications affect the function of a protein without changing its activity. We use glyeraldehyde-3-phosphate dehydrogenase to demonstrate how thiol modification of an active site cysteine re-routes metabolic pathways and converts a metabolic enzyme into a pro-apoptotic factor. Finally, we introduce the redox-sensitive protein tyrosine phosphatase PTP1B to illustrate that reversibility is one of the fundamental aspects of redox-regulation.

Oxidation of cysteine 645 of cobalamin-independent methionine synthase causes a methionine limitation in Escherichia coli

Cobalamin-independent methionine synthase (MetE) catalyzes the final step in Escherichia coli methionine biosynthesis but is inactivated under oxidative conditions, triggering a methionine deficiency. This study demonstrates that the mutation of MetE cysteine 645 to alanine completely eliminates the methionine auxotrophy imposed by diamide treatment, suggesting that modulation of MetE activity via cysteine 645 oxidation has significant physiological consequences for oxidatively stressed cells.

Glutathionylation of the pro-apoptotic protein p53 in Alzheimer's disease brain: implications for AD pathogenesis

Alzheimer's disease (AD) is an age-related neurodegenerative disorder. The exact mechanism for the AD pathogenesis is not clearly understood. However, a number of hypotheses have been proposed to explain the pathogenesis of AD. One the hypotheses is the oxidative stress hypothesis that is supported by a number of studies which reported an increase in the levels of reactive oxygen/reactive nitrogen species and their products with a concomitant decrease in the levels of antioxidant enzymes in AD brain. In the present study, we measured in AD brain the expression levels of different forms (monomer, dimer and tetramer) of the pro-apoptotic protein, p53, and observed greater levels of p53 monomer and dimer in AD brain compared to control. In addition, we also showed the selective glutathionylation of monomeric and dimeric form of p53 in AD brain. We propose that glutathionylation of p53 may prevent the formation of tetramer, an aggregate form required for effective action of p53, and may be involved in oxidative stress conditions and neurodegeneration observed in this dementing disorder.

Protein S-glutathionylation: a regulatory device from bacteria to humans

S-Glutathionylation is the specific post-translational modification of protein cysteine residues by the addition of the tripeptide glutathione, the most abundant and important low-molecular-mass thiol within most cell types. Protein S-glutathionylation is promoted by oxidative or nitrosative stress but also occurs in unstressed cells. It can serve to regulate a variety of cellular processes by modulating protein function and to prevent irreversible oxidation of protein thiols. Recent findings support an essential role for S-glutathionylation in the control of cell-signalling pathways associated with viral infections and with tumour necrosis factor-(-induced apoptosis. Glyceraldehyde-3-phosphate dehydrogenase has recently been implicated in the regulation of endothelin-1 synthesis by a novel, S-glutathionylation-based mechanism involving messenger RNA stability. Moreover, recent studies have identified S-glutathionylation as a redox signalling mechanism in plants.

Thiol chemistry and specificity in redox signaling

Exposure of cells to sublethal oxidative stress results in the modulation of various signaling pathways. Oxidants can activate and inactivate transcription factors, membrane channels, and metabolic enzymes, and regulate calcium-dependent and phosphorylation signaling pathways. Oxidation and reduction of thiol proteins are thought to be the major mechanisms by which reactive oxidants integrate into cellular signal transduction pathways. This review focuses on mechanisms for sensing and transmitting redox signals, from the perspective of their chemical reactivity with specific oxidants. We discuss substrate preferences for different oxidants and how the kinetics of these reactions determines how each oxidant will react in a cell. This kinetic approach helps to identify initial oxidant-sensitive targets and elucidate mechanisms involved in transmission of redox signals. It indicates that only those proteins with very high reactivity, such as peroxiredoxins, are likely to be direct targets for hydrogen peroxide. Other more modestly reactive thiol proteins such as protein tyrosine phosphatases are more likely to become oxidized by an indirect mechanism. The review also examines oxidative changes observed during receptor-mediated signaling, the strengths and limitations of detection methods for reactive oxidant production, and the evidence for hydrogen peroxide acting as the second messenger. We discuss areas where observations in cell systems can be rationalized with the reactivity of specific oxidants and where further work is needed to understand the mechanisms involved.

Quantifying changes in the thiol redox proteome upon oxidative stress in vivo

Antimicrobial levels of reactive oxygen species (ROS) are produced by the mammalian host defense to kill invading bacteria and limit bacterial colonization. One main in vivo target of ROS is the thiol group of proteins. We have developed a quantitative thiol trapping technique termed OxICAT to identify physiologically important target proteins of hydrogen peroxide (H(2)O(2)) and hypochlorite (NaOCl) stress in vivo. OxICAT allows the precise quantification of oxidative thiol modifications in hundreds of different proteins in a single experiment. It also identifies the affected proteins and defines their redox-sensitive cysteine(s). Using this technique, we identified a group of Escherichia coli proteins with significantly (30-90%) oxidatively modified thiol groups, which appear to be specifically sensitive to either H(2)O(2) or NaOCl stress. These results indicate that individual oxidants target distinct proteins in vivo. Conditionally essential E. coli genes encode one-third of redox-sensitive proteins, a finding that might explain the bacteriostatic effect of oxidative stress treatment. We identified a select group of redox-regulated proteins, which protect E. coli against oxidative stress conditions. These experiments illustrate that OxICAT, which can be used in a variety of different cell types and organisms, is a powerful tool to identify, quantify, and monitor oxidative thiol modifications in vivo.

S-glutathionylation: indicator of cell stress and regulator of the unfolded protein response

The specific posttranslational modification of protein cysteine residues by the addition of the tripeptide glutathione is termed S-glutathionylation. This process is promoted by oxidative and nitrosative stress but also occurs in unstressed cells. Altered levels of S-glutathionylation in some proteins have been associated with numerous pathologies, many of which have been linked to redox stress in the endoplasmic reticulum (ER). Proper protein folding is dependent upon controlled redox conditions within the ER, and it seems that ER conditions can in turn affect rates of S-glutathionylation. This article seeks to bring together the ways through which these processes are interrelated and considers the implications of these interrelationships upon therapeutic approaches to disease.

Biochemical characterization of glutaredoxins from Chlamydomonas reinhardtii reveals the unique properties of a chloroplastic CGFS-type glutaredoxin

Glutaredoxins (GRXs) are small ubiquitous disulfide oxidoreductases known to use GSH as electron donor. In photosynthetic organisms, little is known about the biochemical properties of GRXs despite the existence of approximately 30 different isoforms in higher plants. We report here the biochemical characterization of Chlamydomonas GRX1 and GRX3, the major cytosolic and chloroplastic isoforms, respectively. Glutaredoxins are classified on the basis of the amino acid sequence of the active site. GRX1 is a typical CPYC-type GRX, which is reduced by GSH and exhibits disulfide reductase, dehydroascorbate reductase, and deglutathionylation activities. In contrast, GRX3 exhibits unique properties. This chloroplastic CGFS-type GRX is not reduced by GSH and has an atypically low redox potential (-323 +/- 4 mV at pH 7.9). Remarkably, GRX3 can be reduced in the light by photoreduced ferredoxin and ferredoxin-thioredoxin reductase. Both GRXs proved to be very efficient catalysts of A(4)-glyceraldehyde-3-phosphate dehydrogenase deglutathionylation, whereas cytosolic and chloroplastic thioredoxins were inefficient. Glutathionylated A(4)-glyceraldehyde-3-phosphate dehydrogenase is the first physiological substrate identified for a CGFS-type GRX.

The inhibition of glyceraldehyde-3-phosphate dehydrogenase by nitroxyl (HNO)

Nitroxyl (HNO) has received recent and significant interest due to its novel and potentially important pharmacology. However, the chemical/biochemical mechanism(s) responsible for its biological activity remain to be established. Some of the most important biological targets for HNO are thiols and thiol proteins. Consistent with this, it was recently reported that HNO inhibits the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a protein with a catalytically important cysteine thiol at its active site. Interestingly, it was reported that intracellular GAPDH inhibition occurred without significantly altering the cellular thiol redox status of glutathione. Herein, the nature of this reaction specificity was examined. HNO is found to irreversibly inhibit GAPDH in a manner that can be protected against by one of its substrates, glyceraldehyde-3-phosphate (G-3-P). These results are consistent with the idea that HNO has the ability to react with and oxidize a variety of intracellular thiols and the ease or facility of cellular re-reduction of the thiol targets can determine the target specificity.

S-cysteinylation is a general mechanism for thiol protection of Bacillus subtilis proteins after oxidative stress

S-Thiolation is crucial for protection and regulation of thiol-containing proteins during oxidative stress and is frequently achieved by the formation of mixed disulfides with glutathione. However, many Gram-positive bacteria including Bacillus subtilis lack the low molecular weight (LMW) thiol glutathione. Here we provide evidence that S-thiolation by the LMW thiol cysteine represents a general mechanism in B. subtilis. In vivo labeling of proteins with [(35)S]cysteine and nonreducing two-dimensional PAGE analyses revealed that a large subset of proteins previously identified as having redox-sensitive thiols are modified by cysteine in response to treatment with the thiol-specific oxidant diamide. By means of multidimensional shotgun proteomics, the sites of S-cysteinylation for six proteins could be identified, three of which are known to be S-glutathionylated in other organisms.

Glutathione supplementation potentiates hypoxic apoptosis by S-glutathionylation of p65-NFkappaB

In murine embryonic fibroblasts, N-acetyl-L-cysteine (NAC), a GSH generating agent, enhances hypoxic apoptosis by blocking the NFkappaB survival pathway (Qanungo, S., Wang, M., and Nieminen, A. L. (2004) J. Biol. Chem. 279, 50455-50464). Here, we examined sulfhydryl modifications of the p65 subunit of NFkappaB that are responsible for NFkappaB inactivation. In MIA PaCa-2 pancreatic cancer cells, hypoxia increased p65-NFkappaB DNA binding and NFkappaB transactivation by 2.6- and 2.8-fold, respectively. NAC blocked these events without having an effect on p65-NFkappaB protein levels and p65-NFkappaB nuclear translocation during hypoxia. Pharmacological inhibition of the NFkappaB pathway also induced hypoxic apoptosis, indicating that the NFkappaB signaling pathway is a major protective mechanism against hypoxic apoptosis. In cell lysates after hypoxia and treatment with N-ethylmaleimide (thiol alkylating agent), dithiothreitol (disulfide reducing agent) was not able to increase binding of p65-NFkappaB to DNA, suggesting that most sulfhydryls in p65-NFkappaB protein were in reduced and activated forms after hypoxia, thereby being blocked by N-ethylmaleimide. In contrast, with hypoxic cells that were also treated with NAC, dithiothreitol increased p65-NFkappaB DNA binding. Glutaredoxin (GRx), which specifically catalyzes reduction of protein-SSG mixed disulfides, reversed inhibition of p65-NFkappaB DNA binding in extracts from cells treated with hypoxia plus NAC and restored NFkappaB activity. This finding indicated that p65-NFkappaB-SSG was formed in situ under hypoxia plus NAC conditions. In cells, knock-down of endogenous GRx1, which also promotes protein glutathionylation under hypoxic radical generating conditions, prevented NAC-induced NFkappaB inactivation and hypoxic apoptosis. The results indicate that GRx-dependent S-glutathionylation of p65-NFkappaB is most likely responsible for NAC-mediated NFkappaB inactivation and enhanced hypoxic apoptosis.