Detection of S-glutathionylated proteins by glutathione S-transferase overlay

Profiling Glutathionylome in CD38-Mediated Epithelial-Mesenchymal Transition

Protein S-glutathionylation is an important posttranslational modification that regulates various cellular processes. However, changes in glutathionylome in epithelial-mesenchymal transition (EMT), a crucial cellular process for embryonic development, wound healing, and carcinoma progression and metastasis, have not been fully characterized. Our previous study revealed that CD38 overexpression decreased cellular nicotinamide adenine dinucleotide (NAD⁺) levels and caused cells to undergo EMT. In the present study, we engineered a cell system in which the glutathione synthetase (GS) mutant was expressed that catalyzed the formation of a glutathione analogue from azido-alanine to profile changes of glutathionylome in CD38-overexpressing cells. We identified 1298 glutathionylated proteins and revealed that proteins with changed glutathionylation levels involved in EMT associated pathways including epithelial adherens junction, actin cytoskeleton, and integrin signaling. Moreover, the glutathionylation level of 15-hydroxyprostaglandin dehydrogenase (15-PGDH) was increased in CD38-overexpressing cells. We further demonstrated that glutathionylation of Cys63 residue in 15-PGDH led to decreased enzymatic activity that could promote EMT by increasing prostaglandin E₂ (PGE₂). Taken together, these results indicate that the clickable glutathione is an effective probe for glutathionylome profiling, and glutathionylation of 15-PGDH on Cys63 inhibits its enzymatic activity to promote EMT.

S-glutathionylation, friend or foe in cardiovascular health and disease

Glutathione is a low molecular weight thiol that is present at high levels in the cell. The high levels of glutathione in the cell make it one of the most abundant antioxidants contributing to cellular redox homeostasis. As a general rule, throughout cardiovascular disease and progression there is an imbalance in redox homeostasis characterized by reactive oxygen species overproduction and glutathione underproduction. As research into these imbalances continues, glutathione concentrations are increasingly being observed to drive various physiological and pathological signaling responses. Interestingly in addition to acting directly as an antioxidant, glutathione is capable of post translational modifications (S-glutathionylation) of proteins through both chemical interactions and enzyme mediated events. This review will discuss both the chemical and enzyme-based S-glutathionylation of proteins involved in cardiovascular pathologies and angiogenesis.

A thioredoxin-dependent peroxiredoxin Q from Corynebacterium glutamicum plays an important role in defense against oxidative stress

Peroxiredoxin Q (PrxQ) that belonged to the cysteine-based peroxidases has long been identified in numerous bacteria, but the information on the physiological and biochemical functions of PrxQ remain largely lacking in Corynebacterium glutamicum. To better systematically understand PrxQ, we reported that PrxQ from model and important industrial organism C. glutamicum, encoded by the gene ncgl2403 annotated as a putative PrxQ, played important roles in adverse stress resistance. The lack of C. glutamicum prxQ gene resulted in enhanced cell sensitivity, increased ROS accumulation, and elevated protein carbonylation levels under adverse stress conditions. Accordingly, PrxQ-mediated resistance to adverse stresses mainly relied on the degradation of ROS. The physiological roles of PrxQ in resistance to adverse stresses were corroborated by its induced expression under adverse stresses, regulated directly by the stress-responsive ECF-sigma factor SigH. Through catalytical kinetic activity, heterodimer formation, and bacterial two-hybrid analysis, we proved that C. glutamicum PrxQ catalytically eliminated peroxides by exclusively receiving electrons from thioredoxin (Trx)/thioredoxin reductase (TrxR) system and had a broad range of oxidizing substrates, but a better efficiency for peroxynitrite and cumene hydroperoxide (CHP). Site-directed mutagenesis confirmed that the conserved Cys49 and Cys54 are the peroxide oxidation site and the resolving Cys residue, respectively. It was also discovered that C. glutamicum PrxQ mainly existed in monomer whether under its native state or functional state. Based on these results, a catalytic model of PrxQ is being proposed. Moreover, our result that C. glutamicum PrxQ can prevent the damaging effects of adverse stresses by acting as thioredoxin-dependent monomeric peroxidase could be further applied to improve the survival ability and robustness of the important bacterium during fermentation process.

Nrf2-dysregulation correlates with reduced synthesis and low glutathione levels in experimental autoimmune encephalomyelitis

This study investigates the possible mechanism(s) underlying glutathione (GSH) deficiency in the mouse spinal cord during the course of myelin oligodendrocyte glycoprotein35-55 peptide-induced experimental autoimmune encephalomyelitis (EAE), a commonly used animal model of multiple sclerosis. Using the classical enzymatic recycling method and a newly developed immunodot assay, we first demonstrated that total GSH levels (i.e. free GSH plus all its adducts) are reduced in EAE, suggesting an impaired synthesis. The decline in the levels of this essential antioxidant tripeptide in EAE coincides temporally and in magnitude with a reduction in the amount of γ-glutamylcysteine ligase, the rate-limiting enzyme in GSH synthesis. Other enzymes involved in GSH biosynthesis, whose genes also contain antioxidant-response elements, including glutathione synthetase, cystine/glutamate antiporter, and γ-glutamyl transpeptidase (γ-GT) are diminished in EAE as well. Low levels of γ-glutamylcysteine ligase, glutathione synthetase, and γ-GT are the consequence of reduced mRNA expression, which correlates with diminished expression of the nuclear factor (erythroid-derived 2)-like 2 (Nrf2) in both the cytosol and nucleus. Interestingly, the low Nrf2 expression does not seem to be caused by increased degradation via Kelch-like ECH-associated protein 1-dependent or Kelch-like ECH-associated protein 1-independent mechanisms (such as glycogen synthetase kinase-3β activation), or by reduced levels of Nrf2 mRNA. This suggests that translation of this important transcription factor and/or other still unidentified post-translational processes are altered in EAE. These novel findings are central toward understanding how critical antioxidant and protective responses are lost in inflammatory demyelinating disorders.

The relevance of tissue thiol histochemistry to diagnostic hematopathology

Expression analyses suggest that alterations of the antioxidant state of some diffuse large B-cell lymphomas can assist prognosis; reversibly oxidized thiols may serve as a surrogate marker for identifying such cases. Little is known about the distribution of free thiols and reversibly oxidized thiols in human tissues. We developed a staining technique that enables visualization of tissue thiols in situ using bright field microscopy and validated it using gastrointestinal tissue specimens. We used our thiol staining technique to assess benign tonsillectomy and diffuse large B-cell lymphoma specimens. The gastrointestinal series revealed the presence of free thiols within epithelial cells and cells of the lamina propria. Staining for reversibly oxidized thiols was robust in gastric foveolar cells, intestinal goblet cells and the mucus they produce. Tonsillectomy specimens exhibited diffuse presence of free thiols. Staining for reversibly oxidized thiols was confined to germinal center macrophages and sinus histiocytes. Among the diffuse large B-cell lymphoma specimens, we observed strong staining for free thiols within malignant cells. By contrast to benign B-cells, the malignant cells demonstrated pronounced and diffuse staining for reversibly oxidized thiols. We demonstrated intrinsic differences between benign and malignant cells.

Cysteines under ROS attack in plants: a proteomics view

Plants generate reactive oxygen species (ROS) as part of their metabolism and in response to various external stress factors, potentially causing significant damage to biomolecules and cell structures. During the course of evolution, plants have adapted to ROS toxicity, and use ROS as signalling messengers that activate defence responses. Cysteine (Cys) residues in proteins are one of the most sensitive targets for ROS-mediated post-translational modifications, and they have become key residues for ROS signalling studies. The reactivity of Cys residues towards ROS, and their ability to react to different oxidation states, allow them to appear at the crossroads of highly dynamic oxidative events. As such, a redox-active cysteine can be present as S-glutathionylated (-SSG), disulfide bonded (S-S), sulfenylated (-SOH), sulfinylated (-SO2H), and sulfonylated (-SO3H). The sulfenic acid (-SOH) form has been considered as part of ROS-sensing pathways, as it leads to further modifications which affect protein structure and function. Redox proteomic studies are required to understand how and why cysteines undergo oxidative post-translational modifications and to identify the ROS-sensor proteins. Here, we update current knowledge of cysteine reactivity with ROS. Further, we give an overview of proteomic techniques that have been applied to identify different redox-modified cysteines in plants. There is a particular focus on the identification of sulfenylated proteins, which have the potential to be involved in plant signal transduction.

Chemical-proteomic strategies to investigate cysteine posttranslational modifications

The unique combination of nucleophilicity and redox-sensitivity that is characteristic of cysteine residues results in a variety of posttranslational modifications (PTMs), including oxidation, nitrosation, glutathionylation, prenylation, palmitoylation and Michael adducts with lipid-derived electrophiles (LDEs). These PTMs regulate the activity of diverse protein families by modulating the reactivity of cysteine nucleophiles within active sites of enzymes, and governing protein localization between soluble and membrane-bound forms. Many of these modifications are highly labile, sensitive to small changes in the environment, and dynamic, rendering it difficult to detect these modified species within a complex proteome. Several chemical-proteomic platforms have evolved to study these modifications and enable a better understanding of the diversity of proteins that are regulated by cysteine PTMs. These platforms include: (1) chemical probes to selectively tag PTM-modified cysteines; (2) differential labeling platforms that selectively reveal and tag PTM-modified cysteines; (3) lipid, isoprene and LDE derivatives containing bioorthogonal handles; and (4) cysteine-reactivity profiling to identify PTM-induced decreases in cysteine nucleophilicity. Here, we will provide an overview of these existing chemical-proteomic strategies and their effectiveness at identifying PTM-modified cysteine residues within native biological systems.

First proteomic study of S-glutathionylation in cyanobacteria

Glutathionylation, the reversible post-translational formation of a mixed disulfide between a cysteine residue and glutathione (GSH), is a crucial mechanism for signal transduction and regulation of protein function. Until now this reversible redox modification was studied mainly in eukaryotic cells. Here we report a large-scale proteomic analysis of glutathionylation in a photosynthetic prokaryote, the model cyanobacterium Synechocystis sp. PCC6803. Treatment of acellular extracts with N,N-biotinyl glutathione disulfide (BioGSSG) induced glutathionylation of numerous proteins, which were subsequently isolated by affinity chromatography on streptavidin columns and identified by nano LC-MS/MS analysis. Potential sites of glutathionylation were also determined for 125 proteins following tryptic cleavage, streptavidin-affinity purification, and mass spectrometry analysis. Taken together the two approaches allowed the identification of 383 glutathionylatable proteins that participate in a wide range of cellular processes and metabolic pathways such as carbon and nitrogen metabolisms, cell division, stress responses, and H2 production. In addition, the glutathionylation of two putative targets, namely, peroxiredoxin (Sll1621) involved in oxidative stress tolerance and 3-phosphoglycerate dehydrogenase (Sll1908) acting on amino acids metabolism, was confirmed by biochemical studies on the purified recombinant proteins. These results suggest that glutathionylation constitutes a major mechanism of global regulation of the cyanobacterial metabolism under oxidative stress conditions.

Proteomic identification and quantification of S-glutathionylation in mouse macrophages using resin-assisted enrichment and isobaric labeling

S-Glutathionylation (SSG) is an important regulatory posttranslational modification on protein cysteine (Cys) thiols, yet the role of specific cysteine residues as targets of modification is poorly understood. We report a novel quantitative mass spectrometry (MS)-based proteomic method for site-specific identification and quantification of S-glutathionylation across different conditions. Briefly, this approach consists of initial blocking of free thiols by alkylation, selective reduction of glutathionylated thiols, and covalent capture of reduced thiols using thiol affinity resins, followed by on-resin tryptic digestion and isobaric labeling with iTRAQ (isobaric tags for relative and absolute quantitation) for MS-based identification and quantification. The overall approach was initially validated by application to RAW 264.7 mouse macrophages treated with different doses of diamide to induce glutathionylation. A total of 1071 Cys sites from 690 proteins were identified in response to diamide treatment, with ~90% of the sites displaying >2-fold increases in SSG modification compared to controls. This approach was extended to identify potential SSG-modified Cys sites in response to H2O2, an endogenous oxidant produced by activated macrophages and many pathophysiological stimuli. The results revealed 364 Cys sites from 265 proteins that were sensitive to S-glutathionylation in response to H2O2 treatment, thus providing a database of proteins and Cys sites susceptible to this modification under oxidative stress. Functional analysis revealed that the most significantly enriched molecular function categories for proteins sensitive to SSG modifications were free radical scavenging and cell death/survival. Overall the results demonstrate that our approach is effective for site-specific identification and quantification of SSG-modified proteins. The analytical strategy also provides a unique approach to determining the major pathways and cellular processes most susceptible to S-glutathionylation under stress conditions.

Nitric oxide-cold stress signalling cross-talk, evolution of a novel regulatory mechanism

Plants enhance their cold stress tolerance by cold acclimation, a process which results in vast reprogramming of transcriptome, proteome and metabolome. Evidence suggests nitric oxide (NO) production during cold stress which regulates genes (especially the C-repeat binding factor (CBF) cold stress signalling pathway), diverse proteins including transcription factors (TFs) and phosphosphingolipids. About 59% (redox), 50% (defence/stress) and 30% (signalling) cold responsive proteins are modulated by NO-based post translational modifications (PTMs) namely S-nitrosylation, tyrosine nitration and S-glutathionylation, suggesting a cross-talk between NO and cold. Analysis of cold stress responsive deep proteome in apoplast, mitochondria, chloroplast and nucleus suggested continuation of this cross-talk in sub-cellular systems. Modulation of cold responsive proteins by these PTMs right from cytoskeletal elements in plasma membrane to TFs in nucleus suggests a novel regulation of cold stress signalling. NO-mediated altered protein transport in nucleus seems an important stress regulatory mechanism. This review addresses the NO and cold stress signalling cross-talk to present the overview of this novel regulatory mechanism.

Prediction of S-glutathionylated proteins progression in Alzheimer's transgenic mouse model using principle component analysis

To date, prediction of Alzheimer's disease (AD) is mainly based on clinical criteria because no well-established biochemical biomarkers for routine clinical diagnosis of AD currently exist. We developed an approach to aid in the early diagnosis of AD by using principal component analysis (PCA)-based spectral analysis of oxidized protein electrophoretic profiling. We found that the combination of capillary electrophoresis and PCA analysis of S-glutathionylation distribution characterization can be used in the sample classification and molecular weight (Mw) prediction. The comparison of leave-one-out AD versus non-AD gives the sensitivity of 100% and 93.33% in brain tissues and blood samples, respectively, while the specificity of 100% in brain and 90.0% in blood samples. Our findings demonstrate that PCA of S-glutathionylation electrophoretic profiling detects AD pathology features, and that the molecular weight based electrophoretic profiling of blood and brain S-glutathionylated proteins are sensitive to change, even at the early stage of the disease. Our results offer a previously unexplored diagnostic approach by using electrophoretic characteristics of oxidized proteins to serve as a predictor of AD progression and early stage screening.

A fluorometric method to quantify protein glutathionylation using glutathione derivatization with 2,3-naphthalenedicarboxaldehyde

This study reports the development of a new assay for the rapid determination of protein glutathionylation in tissues and cell lines using commercially available reagents and standard instrumentation. In this method cells are homogenized in the presence of N-ethylmaleimide to eliminate free thiols and the proteins are precipitated with acetone. Subsequently, the disulfide-bound glutathione is eluted from the protein by the addition of tris(2-carboxyethyl)phosphine and reacted with 2,3-napthalenedicarboxaldehyde to generate a highly fluorescent product. Lymphoblastoid cell lines were found to have glutathionylation levels in the range of 0.3-3 nmol/mg protein, which were significantly elevated after treatment of the cells with S-nitrosoglutathione. Mouse tissues including liver, kidney, lung, heart, brain, spleen, and testes were found to have glutathionylation levels between 1 and 2.5 nmol/mg protein and the levels tended to increase after treatment of mice with doxorubicin. In contrast, mouse skeletal muscle glutathionylation was significantly higher (4.2 ± 0.33 nmol/mg, p < 0.001) than in other tissues in untreated mice and decreased to 1.9 ± 0.15 nmol/mg after doxorubicin treatment. This new method allows rapid measurement of cellular glutathionylation in a high-throughput 96-well plate format.

Microtubule S-glutathionylation as a potential approach for antimitotic agents

Background

Microtubules have been one of the most effective targets for the development of anticancer agents. Cancer cells treated by these agents are characterized by cell arrest at G₂/M phase. Microtubule-targeting drugs are, therefore, referred to as antimitotic agents. However, the clinical application of the current antimitotic drugs is hampered by emerging drug resistance which is the major cause of cancer treatment failure. The clinical success of antimitotic drugs and emerging drug resistance has prompted a search for new antimitotic agents, especially those with novel mechanisms of action. The aim of this study was to determine whether microtubules can be S-glutathionylated in cancer cells and whether the glutathionylation will lead to microtubule dysfunction and cell growth inhibition. The study will determine whether microtubule S-glutathionylation can be a novel approach for antimitotic agents.

Methods

2-Acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylcarbonylamino)phenyl carbamoylsulfanyl]propionic acid (2-AAPA) was used as a tool to induce microtubule S-glutathionylation. UACC-62 cells, a human melanoma cell line, were used as a cancer cell model. A pull-down assay with glutathione S-transferase (GST)-agarose beads followed by Western blot analysis was employed to confirm microtubule S-glutathionylation. Immunofluorescence microscopy using a mouse monoclonal anti-α-tubulin-FITC was used to study the effect of the S-glutathionylation on microtubule function; mainly polymerization and depolymerization. Flow cytometry was employed to examine the effect of the S-glutathionylation on cell cycle distribution and apoptosis. Cell morphological change was followed through the use of a Zeiss AXIO Observer A1 microscope. Cancer cell growth inhibition by 2-AAPA was investigated with ten human cancer cell lines.

Results

Our investigation demonstrated that cell morphology was changed and microtubules were S-glutathionylated in the presence of 2-AAPA in UACC-62 cells. Accordingly, microtubules were found depolymerized and cells were arrested at G₂/M phase. The affected cells were found to undergo apoptosis. Cancer growth inhibition experiments demonstrated that the concentrations of 2-AAPA required to produce the effects on microtubules were compatible to the concentrations producing cancer cell growth inhibition.

Conclusions

The data from this investigation confirms that microtubule S-glutathionylation leads to microtubule dysfunction and cell growth inhibition and can be a novel approach for developing antimitotic agents.

Redox regulation in photosynthetic organisms: focus on glutathionylation

SIGNIFICANCE

In photosynthetic organisms, besides the well-established disulfide/dithiol exchange reactions specifically controlled by thioredoxins (TRXs), protein S-glutathionylation is emerging as an alternative redox modification occurring under stress conditions. This modification, consisting of the formation of a mixed disulfide between glutathione and a protein cysteine residue, can not only protect specific cysteines from irreversible oxidation but also modulate protein activities and appears to be specifically controlled by small disulfide oxidoreductases of the TRX superfamily named glutaredoxins (GRXs).

RECENT STUDIES

In recent times, several studies allowed significant progress in this area, mostly due to the identification of several plant proteins undergoing S-glutathionylation and to the characterization of the molecular mechanisms and the proteins involved in the control of this modification.

CRITICAL ISSUES

This article provides a global overview of protein glutathionylation in photosynthetic organisms with particular emphasis on the mechanisms of protein glutathionylation and deglutathionylation and a focus on the role of GRXs. Then, we describe the methods employed for identification of glutathionylated proteins in photosynthetic organisms and review the targets and the possible physiological functions of protein glutathionylation.

FUTURE DIRECTIONS

In order to establish the importance of protein S-glutathionylation in photosynthetic organisms, future studies should be aimed at delineating more accurately the molecular mechanisms of glutathionylation and deglutathionylation reactions, at identifying proteins undergoing S-glutathionylation in vivo under diverse conditions, and at investigating the importance of redoxins, GRX, and TRX, in the control of this redox modification in vivo.

S-Glutathionyl quantification in the attomole range using glutaredoxin-3-catalyzed cysteine derivatization and capillary gel electrophoresis with laser-induced fluorescence detection

S-glutathionylation (Pr-SSG) is a specific post-translational modification of cysteine residues by the addition of glutathione. S-Glutathionylated proteins induced by oxidative or nitrosative stress play an essential role in understanding the pathogenesis of the aging and age-related disorder, such as Alzheimer's disease (AD). The purpose of this research is to develop a novel and ultrasensitive method to accurately and rapidly quantify the Pr-SSG by using capillary gel electrophoresis with laser-induced fluorescence detection (CGE-LIF). The derivatization method is based on the specific reduction of protein-bound S-glutathionylation with glutaredoxin (Grx) and labeling with thiol-reactive fluorescent dye (Dylight 488 maleimide). The experiments were performed by coupling the derivatization method with CGE-LIF to study electrophoretic profiling in in vitro oxidative stress model-S-glutathionylated bovine serum albumin (BSA-SSG), oxidant-induced human colon adenocarcinoma (HT-29) cells, brain tissues, and whole blood samples from an AD transgenic (Tg) mouse model. The results showed almost an eightfold increase in S-glutathionyl abundance when subjecting HT-29 cells in an oxidant environment, resulting in Pr-SSG at 232 ± 10.64 (average ±SD; n=3) nmol/mg. In the AD-Tg mouse model, an initial quantitative measurement demonstrated the extent of protein S-glutathionylation in three brain regions (hippocampus, cerebellum, and cerebrum), ranging from 1 to 10 nmol/mg. Additionally, we described our developed method to potentially serve as a highly desirable diagnostic tool for monitoring S-glutathionylated protein profile in minuscule amount of whole blood. The whole blood samples for S-glutathionyl expression of 5-month-old AD-Tg mice are quantified as 16.3 μmol/L (=7.2 nmol/mg protein). Altogether, this is a fast, easy, and accurate method, reaching the lowest limit of Pr-SSG detection at 1.8 attomole (amol) level, reported to date.

S-glutathionylation: from molecular mechanisms to health outcomes

Redox homeostasis governs a number of critical cellular processes. In turn, imbalances in pathways that control oxidative and reductive conditions have been linked to a number of human disease pathologies, particularly those associated with aging. Reduced glutathione is the most prevalent biological thiol and plays a crucial role in maintaining a reduced intracellular environment. Exposure to reactive oxygen or nitrogen species is causatively linked to the disease pathologies associated with redox imbalance. In particular, reactive oxygen species can differentially oxidize certain cysteine residues in target proteins and the reversible process of S-glutathionylation may mitigate or mediate the damage. This post-translational modification adds a tripeptide and a net negative charge that can lead to distinct structural and functional changes in the target protein. Because it is reversible, S-glutathionylation has the potential to act as a biological switch and to be integral in a number of critical oxidative signaling events. The present review provides a comprehensive account of how the S-glutathionylation cycle influences protein structure/function and cellular regulatory events, and how these may impact on human diseases. By understanding the components of this cycle, there should be opportunities to intervene in stress- and aging-related pathologies, perhaps through prevention and diagnostic and therapeutic platforms.

Unveiling the roles of the glutathione redox system in vivo by analyzing genetically modified mice

Redox status affects various cellular activities, such as proliferation, differentiation, and death. Recent studies suggest pivotal roles of reactive oxygen species not only in pathogenesis under oxidative insult but also in intracellular signal transduction. Glutathione is present in several millimolar concentrations in the cytoplasm and has multiple roles in the regulation of cellular homeostasis. Two enzymes, γ-glutamylcysteine synthetase and glutathione synthetase, constitute the de novo synthesis machinery, while glutathione reductase is involved in the recycling of oxidized glutathione. Multidrug resistant proteins and some other transporters are responsible for exporting oxidized glutathione, glutathione conjugates, and S-nitrosoglutathione. In addition to antioxidation, glutathione is more positively involved in cellular activity via its sulfhydryl moiety of a molecule. Animals in which genes responsible for glutathione metabolism are genetically modified can be used as beneficial and reliable models to elucidate roles of glutathione in vivo. This review article overviews recent progress in works related to genetically modified rodents and advances in the elucidation of glutathione-mediated reactions.

In situ analysis of protein S-glutathionylation in lung tissue using glutaredoxin-1-catalyzed cysteine derivatization

Protein S-glutathionylation (PSSG) is a posttranslational modification that involves the conjugation of the small antioxidant molecule glutathione to cysteine residues and is emerging as a critical mechanism of redox-based signaling. PSSG levels increase under conditions of oxidative stress and are controlled by glutaredoxins (Grx) that, under physiological conditions, preferentially deglutathionylate cysteines and restore sulfhydryls. Both the occurrence and distribution of PSSG in tissues is unknown because of the labile nature of this oxidative event and the lack of specific reagents. The goal of this study was to establish and validate a protocol that enables detection of PSSG in situ, using the property of Grx to deglutathionylate cysteines. Using Grx1-catalyzed cysteine derivatization, we evaluated PSSG content in mice subjected to various models of lung injury and fibrosis. In control mice, PSSG was detectable primarily in the airway epithelium and alveolar macrophages. Exposure of mice to NO(2) resulted in enhanced PSSG levels in parenchymal regions, while exposure to O(2) resulted in minor detectable changes. Finally, bleomycin exposure resulted in marked increases in PSSG reactivity both in the bronchial epithelium as well as in parenchymal regions. Taken together, these findings demonstrate that Grx1-based cysteine derivatization is a powerful technique to specifically detect patterns of PSSG expression in lungs, and will enable investigations into regional changes in PSSG content in a variety of diseases.

Detection of protein glutathionylation

Recent studies indicate that protein glutathionylation is an important regulatory mechanism. The develop-ment of redox proteomics techniques to identify proteins undergoing glutathionylation has a key role in defining the importance of this post-translational modification, although the available methods are not yet comparable to those for the study of other modifications like phosphorylation. We describe here methods that have been successfully employed to identify in vitro glutathionylated proteins.

Methods for analysis of protein glutathionylation and their application to photosynthetic organisms

Protein S-glutathionylation, the reversible formation of a mixed-disulfide between glutathione and protein thiols, is involved in protection of protein cysteines from irreversible oxidation, but also in protein redox regulation. Recent studies have implicated S-glutathionylation as a cellular response to oxidative/nitrosative stress, likely playing an important role in signaling. Considering the potential importance of glutathionylation, a number of methods have been developed for identifying proteins undergoing glutathionylation. These methods, ranging from analysis of purified proteins in vitro to large-scale proteomic analyses in vivo, allowed identification of nearly 200 targets in mammals. By contrast, the number of known glutathionylated proteins is more limited in photosynthetic organisms, although they are severely exposed to oxidative stress. The aim of this review is to detail the methods available for identification and analysis of glutathionylated proteins in vivo and in vitro. The advantages and drawbacks of each technique will be discussed as well as their application to photosynthetic organisms. Furthermore, an overview of known glutathionylated proteins in photosynthetic organisms is provided and the physiological importance of this post-translational modification is discussed.

Radical-free biology of oxidative stress

Free radical-induced macromolecular damage has been studied extensively as a mechanism of oxidative stress, but large-scale intervention trials with free radical scavenging antioxidant supplements show little benefit in humans. The present review summarizes data supporting a complementary hypothesis for oxidative stress in disease that can occur without free radicals. This hypothesis, which is termed the "redox hypothesis," is that oxidative stress occurs as a consequence of disruption of thiol redox circuits, which normally function in cell signaling and physiological regulation. The redox states of thiol systems are sensitive to two-electron oxidants and controlled by the thioredoxins (Trx), glutathione (GSH), and cysteine (Cys). Trx and GSH systems are maintained under stable, but nonequilibrium conditions, due to a continuous oxidation of cell thiols at a rate of about 0.5% of the total thiol pool per minute. Redox-sensitive thiols are critical for signal transduction (e.g., H-Ras, PTP-1B), transcription factor binding to DNA (e.g., Nrf-2, nuclear factor-kappaB), receptor activation (e.g., alphaIIbbeta3 integrin in platelet activation), and other processes. Nonradical oxidants, including peroxides, aldehydes, quinones, and epoxides, are generated enzymatically from both endogenous and exogenous precursors and do not require free radicals as intermediates to oxidize or modify these thiols. Because of the nonequilibrium conditions in the thiol pathways, aberrant generation of nonradical oxidants at rates comparable to normal oxidation may be sufficient to disrupt function. Considerable opportunity exists to elucidate specific thiol control pathways and develop interventional strategies to restore normal redox control and protect against oxidative stress in aging and age-related disease.

Redox-based regulation of signal transduction: principles, pitfalls, and promises

Oxidants are produced as a by-product of aerobic metabolism, and organisms ranging from prokaryotes to mammals have evolved with an elaborate and redundant complement of antioxidant defenses to confer protection against oxidative insults. Compelling data now exist demonstrating that oxidants are used in physiological settings as signaling molecules with important regulatory functions controlling cell division, migration, contraction, and mediator production. These physiological functions are carried out in an exquisitely regulated and compartmentalized manner by mild oxidants, through subtle oxidative events that involve targeted amino acids in proteins. The precise understanding of the physiological relevance of redox signal transduction has been hampered by the lack of specificity of reagents and the need for chemical derivatization to visualize reversible oxidations. In addition, it is difficult to measure these subtle oxidation events in vivo. This article reviews some of the recent findings that illuminate the significance of redox signaling and exciting future perspectives. We also attempt to highlight some of the current pitfalls and the approaches needed to advance this important area of biochemical and biomedical research.

Molecular mechanisms and potential clinical significance of S-glutathionylation

Protein S-glutathionylation, the reversible binding of glutathione to protein thiols (PSH), is involved in protein redox regulation, storage of glutathione, and protection of PSH from irreversible oxidation. S-Glutathionylated protein (PSSG) can result from thiol/disulfide exchange between PSH and GSSG or PSSG; direct interaction between partially oxidized PSH and GSH; reactions between PSH and S-nitrosothiols, oxidized forms of GSH, or glutathione thiyl radical. Indeed, thiol/disulfide exchange is an unlikely intracellular mechanism for S-glutathionylation, because of the redox potential of most Cys residues and the GSSG export by most cells as a protective mechanism against oxidative stress. S-Glutathionylation can be reversed, following restoration of a reducing GSH/GSSG ratio, in an enzyme-dependent or -independent manner. Currently, definite evidence of protein S-glutathionylation has been clearly demonstrated in few human diseases. In aging human lenses, protein S-glutathionylation increases; during cataractogenesis, some of lens proteins, including alpha- and beta-crystallins, form both mixed disulfides and disulfide-cross-linked aggregates, which increase with cataract severity. The correlation of lens nuclear color and opalescence intensity with protein S-glutathionylation indicates that protein-thiol mixed disulfides may play an important role in cataractogenesis and development of brunescence in human lenses. Recently, specific PSSG have been identified in the inferior parietal lobule in Alzheimer's disease. However, much investigation is needed to clarify the actual involvement of protein S-glutathionylation in many human diseases.

Human p53 is inhibited by glutathionylation of cysteines present in the proximal DNA-binding domain during oxidative stress

The cellular mechanisms that modulate the redox state of p53 tumor suppressor remain unclear, although its DNA binding function is known to be strongly inhibited by oxidative and nitrosative stresses. We show that human p53 is subjected to a new and reversible posttranslational modification, namely, S-glutathionylation in stressed states, including DNA damage. First, a rapid and direct incorporation of biotinylated GSH or GSSG into the purified recombinant p53 protein was observed. The modified p53 had a significantly weakened ability to bind its consensus DNA sequence. Reciprocal immunoprecipitations and a GST overlay assay showed that p53 in tumor cells was marginally glutathionylated; however, the level of modification increased greatly after oxidant and DNA-damaging treatments. GSH modification coexisted with the serine phophorylations in activated p53, and the thiol-conjugated protein was present in nuclei. When tumor cells treated with camptothecin or cisplatin were subsequently exposed to glutathione-enhancing agents, p53 underwent dethiolation accompanied by detectable increases in the level of p21waf1 expression, relative to the DNA-damaging drugs alone. Mass spectrometry of GSH-modified p53 protein identified cysteines 124, 141, and 182, all present in the proximal DNA-binding domain, as the sites of glutathionylation. Biotinylated maleimide also reacted rapidly with Cys141, implying that this is the most reactive cysteine on the p53 surface. The glutathionylatable cysteines were found to exist in a negatively charged microenvironment in cellular p53. Molecular modeling studies located Cys124 and -141 at the dimer interface of p53 and showed glutathionylation of either residue would inhibit p53-DNA association and also interfere with protein dimerization. These results show for the first time that shielding of reactive cysteines contributes to a negative regulation for human p53 and imply that such an inactivation of the transcription factor may represent an acute defensive response with significant consequences for oncogenesis.

Serum levels of S-glutathionylated proteins as a risk-marker for arteriosclerosis obliterans

BACKGROUND

Oxidative stress plays a role in the development of chronic peripheral arterial disease (PAD) because under these conditions redox regulation is impaired, inducing the S-glutathionylation of proteins. A method of estimating the levels of S-glutathionylated proteins has been developed using biotinylated glutathione S-transferase, which allows the study of their crucial role in the oxidative stress-related progression of PAD.

METHODS AND RESULTS

The serum levels of S-glutathionylated proteins were examined in 41 patients with arteriosclerosis obliterans (ASO) and 38 age-matched non-ASO patients using biotinylated glutathione S-transferase. The levels were higher in the patients with ASO, even early on, and positively correlated with the ankle/brachial index. In vitro, the levels of S-glutathionylated proteins were reduced in the presence of glutathione and glutaredoxin.

CONCLUSIONS

Serum levels of S-glutathionylated proteins are a sensitive risk-marker for ASO at an early stage.

Detecting oxidative post-translational modifications in proteins

Oxidative stress induces various post-translational modifications (PTM); some are reversible in vivo via enzymatic catalysis. The present paper reviews specific procedures for the detection of oxidative PTM in proteins, most of them including electrophoresis. Main topics are carbonylated and glutathionylated proteins as well as modification of selected amino acids (Cys, Tyr, Met, Trp, Lys).

In situ detection of S-glutathionylated proteins following glutaredoxin-1 catalyzed cysteine derivatization

S-glutathionylation is rapidly emerging as an important post-translational modification, responsible for transducing oxidant signals. However, few approaches are available that allow visualization of glutathione mixed disulfides in intact cells. We describe here a glutaredoxin1-dependent cysteine derivatization and labeling approach, in order to visualize S-glutathionylation patterns in situ. Using this new method, marked S-glutathionylation was observed in epithelial cells, which was predominant at membrane ruffles. As expected, the labeling intensity was further enhanced in response to bolus oxidant treatments, or in cells overexpressing Nox1 plus its coactivators. In addition, manipulation of endogenous levels of glutaredoxin-1 via RNAi, or overexpression resulted in altered sensitivity to H2O2 induced formation of glutathione mixed disulfides. Overall, the derivatization approach described here preferentially detects S-glutathionylation and provides an important means to visualize this post-translational modification in sub-cellular compartments and to investigate its relation to normal physiology as well as pathology.