S-thiolation of cytoplasmic cardiac creatine kinase in heart cells treated with diamide

Cardiovascular redox and ox stress proteomics

SIGNIFICANCE

Oxidative post-translational modifications (OPTMs) have been demonstrated as contributing to cardiovascular physiology and pathophysiology. These modifications have been identified using antibodies as well as advanced proteomic methods, and the functional importance of each is beginning to be understood using transgenic and gene deletion animal models. Given that OPTMs are involved in cardiovascular pathology, the use of these modifications as biomarkers and predictors of disease has significant therapeutic potential. Adequate understanding of the chemistry of the OPTMs is necessary to determine what may occur in vivo and which modifications would best serve as biomarkers.

RECENT ADVANCES

By using mass spectrometry, advanced labeling techniques, and antibody identification, OPTMs have become accessible to a larger proportion of the scientific community. Advancements in instrumentation, database search algorithms, and processing speed have allowed MS to fully expand on the proteome of OPTMs. In addition, the role of enzymatically reversible OPTMs has been further clarified in preclinical models.

CRITICAL ISSUES

The identification of OPTMs suffers from limitations in analytic detection based on the methodology, instrumentation, sample complexity, and bioinformatics. Currently, each type of OPTM requires a specific strategy for identification, and generalized approaches result in an incomplete assessment.

FUTURE DIRECTIONS

Novel types of highly sensitive MS instrumentation that allow for improved separation and detection of modified proteins and peptides have been crucial in the discovery of OPTMs and biomarkers. To further advance the identification of relevant OPTMs in advanced search algorithms, standardized methods for sample processing and depository of MS data will be required.

Prediction of reversibly oxidized protein cysteine thiols using protein structure properties

Protein cysteine thiols can be divided into four groups based on their reactivities: those that form permanent structural disulfide bonds, those that coordinate with metals, those that remain in the reduced state, and those that are susceptible to reversible oxidation. Physicochemical parameters of oxidation-susceptible protein thiols were organized into a database named the Balanced Oxidation Susceptible Cysteine Thiol Database (BALOSCTdb). BALOSCTdb contains 161 cysteine thiols that undergo reversible oxidation and 161 cysteine thiols that are not susceptible to oxidation. Each cysteine was represented by a set of 12 parameters, one of which was a label (1/0) to indicate whether its thiol moiety is susceptible to oxidation. A computer program (the C4.5 decision tree classifier re-implemented as the J48 classifier) segregated cysteines into oxidation-susceptible and oxidation-non-susceptible classes. The classifier selected three parameters critical for prediction of thiol oxidation susceptibility: (1) distance to the nearest cysteine sulfur atom, (2) solvent accessibility, and (3) pKa. The classifier was optimized to correctly predict 136 of the 161 cysteine thiols susceptible to oxidation. Leave-one-out cross-validation analysis showed that the percent of correctly classified cysteines was 80.1% and that 16.1% of the oxidation-susceptible cysteine thiols were incorrectly classified. The algorithm developed from these parameters, named the Cysteine Oxidation Prediction Algorithm (COPA), is presented here. COPA prediction of oxidation-susceptible sites can be utilized to locate protein cysteines susceptible to redox-mediated regulation and identify possible enzyme catalytic sites with reactive cysteine thiols.

Role of reversible, thioredoxin-sensitive oxidative protein modifications in cardiac myocytes

Reactive oxygen species (ROS) are important mediators of myocardial remodeling. However, the precise molecular mechanisms by which ROS exert their effects are incompletely understood. ROS induce oxidative posttranslational protein modifications that can regulate the function of structural, functional, and signaling proteins. For example, oxidative modification of free reactive thiols (S-thiolation) on the small G protein Ras increases Ras activity and thereby promotes ROS-dependent hypertrophic signaling in cardiac myocytes. By reducing thiols and restoring reversible thiol modifications, thioredoxin and glutaredoxin can act as regulators of ROS-mediated protein function. Understanding the regulation and functional relevance of oxidative protein modifications in myocardial remodeling may lead to new therapeutic strategies.

Taurine chloramine is more selective than hypochlorous acid at targeting critical cysteines and inactivating creatine kinase and glyceraldehyde-3-phosphate dehydrogenase

Hypochlorous acid (HOCl) and chloramines are produced by the neutrophil enzyme, myeloperoxidase. Both react readily with thiols, although chloramines differ from HOCl in discriminating between low molecular weight thiols on the basis of their pKa. Here, we have compared the reactivity of HOCl and taurine chloramine with thiol proteins by examining inactivation of creatine kinase (CK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). With both enzymes, loss of activity paralleled thiol loss. For CK both were complete at a 1:1 taurine chloramine:thiol mole ratio. For GAPDH each chloramine oxidized two thiols. Three times more HOCl than taurine chloramine was required for inactivation, indicating that HOCl is less thiol specific. Competition studies showed that thiols of CK were 4 times more reactive with taurine chloramine than thiols of GAPDH (rate constants of 1200 and 300 M-1s-1 respectively). These compare with 205 M-1s-1 for cysteine and are consistent with their lower pKa's. Both enzymes were equally susceptible to HOCl. GSH competed directly with the enzyme thiols for taurine chloramine and protected against oxidative inactivation. At lower GSH concentrations, mixed disulfides were formed. We propose that chloramines should preferentially attack proteins with low pKa thiols and this could be important in regulatory processes.

Role of oxidative modifications in atherosclerosis

This review focuses on the role of oxidative processes in atherosclerosis and its resultant cardiovascular events. There is now a consensus that atherosclerosis represents a state of heightened oxidative stress characterized by lipid and protein oxidation in the vascular wall. The oxidative modification hypothesis of atherosclerosis predicts that low-density lipoprotein (LDL) oxidation is an early event in atherosclerosis and that oxidized LDL contributes to atherogenesis. In support of this hypothesis, oxidized LDL can support foam cell formation in vitro, the lipid in human lesions is substantially oxidized, there is evidence for the presence of oxidized LDL in vivo, oxidized LDL has a number of potentially proatherogenic activities, and several structurally unrelated antioxidants inhibit atherosclerosis in animals. An emerging consensus also underscores the importance in vascular disease of oxidative events in addition to LDL oxidation. These include the production of reactive oxygen and nitrogen species by vascular cells, as well as oxidative modifications contributing to important clinical manifestations of coronary artery disease such as endothelial dysfunction and plaque disruption. Despite these abundant data however, fundamental problems remain with implicating oxidative modification as a (requisite) pathophysiologically important cause for atherosclerosis. These include the poor performance of antioxidant strategies in limiting either atherosclerosis or cardiovascular events from atherosclerosis, and observations in animals that suggest dissociation between atherosclerosis and lipoprotein oxidation. Indeed, it remains to be established that oxidative events are a cause rather than an injurious response to atherogenesis. In this context, inflammation needs to be considered as a primary process of atherosclerosis, and oxidative stress as a secondary event. To address this issue, we have proposed an "oxidative response to inflammation" model as a means of reconciling the response-to-injury and oxidative modification hypotheses of atherosclerosis.

Effects of age and caloric restriction on glutathione redox state in mice

The main purpose of this study was to determine whether the aging process in the mouse is associated with a pro-oxidizing shift in the redox state of glutathione and whether restriction of caloric intake, which results in the extension of life span, retards such a shift. Amounts of reduced and oxidized forms of glutathione (GSH and GSSG, respectively) and protein-glutathione mixed disulfides (protein-SSG) were measured in homogenates and mitochondria of liver, kidney, heart, brain, eye, and testis of 4, 10, 22, and 26 month old ad libitum-fed (AL) mice and 22 month old mice fed a diet containing 40% fewer calories than the AL group from the age of 4 months. The concentrations of GSH, GSSG, and protein-SSG vary greatly (approximately 10-, 30-, and 9-fold, respectively) from one tissue to another. During aging, the ratios of GSH:GSSG in mitochondria and tissue homogenates decreased, primarily due to elevations in GSSG content, while the protein-SSG content increased significantly. Glutathione redox potential in mitochondria became less negative, i.e., more pro-oxidizing, as the animal aged. Caloric restriction (CR) lowered the GSSG and protein-SSG content. Results suggest that the aging process in the mouse is associated with a gradual pro-oxidizing shift in the glutathione redox state and that CR attenuates this shift.

Glyceraldehyde phosphate dehydrogenase oxidation during cardiac ischemia and reperfusion

OBJECTIVES

Protein S-glutathiolation is a predicted mechanism by which protein thiol groups are oxidized during the oxidative stress of ischaemia and reperfusion. We measured protein S-thiolation during ischaemia and reperfusion and investigated the effect of this oxidative modification on the function of GAPDH.

METHODS

Glutathione was biotinylated (biotin-GSH) and used to probe for protein S-glutathiolation in isolated rat hearts using non-reducing Western blots and streptavidin-HRP. Streptavidin-agarose was used to purify S-glutathiolated proteins and these were identified using N-terminal sequencing and database searching.

RESULTS

Little protein S-glutathiolation occurred in control preparations, but this increased 15-fold during reperfusion. Protein S-glutathiolation was attenuated by the antioxidant mercaptopropionylglycine and was shown to occur only during the firstminutes of reperfusion. Affinity purification of the S-glutathiolated proteins showed 20 dominant S-glutathiolation substrates. A dominant S-thiolated protein was N-terminally sequenced (VKVGVNGFG) and HPLC peptide mapping gave additional sequence nearer the site of oxidation (TGVFTTMEKA). The first sequence was the N-terminus of GAPDH, and the second a peptide from the same protein starting at residue 96. GAPDH was immunopurified from aerobic, ischemic or reperfused hearts. Maleimidofluorescein labeling of purified GAPDH provided an index of its reduced thiol status. In the absence of DTT, ischemia induced a reduction in the number of free thiols on GAPDH that was reversed on reperfusion. When treated with DTT, the free thiol status of GAPDH could be increased in ischemic but not reperfused samples. Ischemia induced a reduction in GAPDH activity that was partially restored by reperfusion. DTT-treatment reactivated ischemic GAPDH, but had little effect on the activity from reperfused tissue. Mass spectra acquired from aerobic GAPDH preparations were relatively simple whereas spectra from ischemic or reperfused preparations were highly complex, possibly indicative of oxidation by multiple oxidants.

CONCLUSIONS

Many proteins, including GAPDH, are targets for S-glutathiolation during cardiac oxidative stress. GAPDH oxidation is associated with a loss in reduced cysteine status that correlates with the inactivation of this enzyme.

S-thiolation of HSP27 regulates its multimeric aggregate size independently of phosphorylation

HSP27 exists as large aggregates that breakdown after phosphorylation. We show rat cardiac HSP27 is S-thiolated during oxidant stress, and this modification, without phosphorylation, disaggregates multimeric HSP27. Biotinylated cysteine acts as a probe for thiolated proteins, which are detected using non-reducing Western blots probed with streptavidin-horseradish peroxidase. Controls show a low level of S-thiolation, which is increased 3.6-fold during post-ischemic reperfusion. S-thiolated proteins were purified using streptavidin-agarose, and Western immunoblotting showed HSP27 was present. We increased protein S-thiolation 10-fold with 10 microm H2O2 with or without a kinase inhibitor mixture (staurosporine, genistein, bisindolylmaleimide, SB203580, and PD98059). H2O2 alone induced the phosphorylation of HSP27 Ser-86 and Ser-45/Ser-59 of its homologue alphaB crystallin. However, kinase inhibition reduced phosphorylation of these sites below basal. Despite effective kinase inhibition, H2O2 still disaggregated HSP27, but not alphaB crystallin. This is consistent with the lack of an S-thiolation site on alphaB crystallin. Thus, we have demonstrated a novel mechanism of HSP27 multimeric size regulation. S-thiolation must occur at Cys-141, the only cysteine in rat HSP27.

Detection, quantitation, purification, and identification of cardiac proteins S-thiolated during ischemia and reperfusion

We have developed methods that allow detection, quantitation, purification, and identification of cardiac proteins S-thiolated during ischemia and reperfusion. Cysteine was biotinylated and loaded into isolated rat hearts. During oxidative stress, biotin-cysteine forms a disulfide bond with reactive protein cysteines, and these can be detected by probing Western blots with streptavidin-horseradish peroxidase. S-Thiolated proteins were purified using streptavidin-agarose. Thus, we demonstrated that reperfusion and diamide treatment increased S-thiolation of a number of cardiac proteins by 3- and 10-fold, respectively. Dithiothreitol treatment of homogenates fully abolished the signals detected. Fractionation studies indicated that the modified proteins are located within the cytosol, membrane, and myofilament/cytoskeletal compartments of the cardiac cells. This shows that biotin-cysteine gains rapid and efficient intracellular access and acts as a probe for reactive protein cysteines in all cellular locations. Using Western blotting of affinity-purified proteins we identified actin, glyceraldehyde-3-phosphate dehydrogenase, HSP27, protein-tyrosine phosphatase 1B, protein kinase Calpha, and the small G-protein ras as substrates for S-thiolation during reperfusion of the ischemic rat heart. MALDI-TOF mass fingerprint analysis of tryptic peptides independently confirmed actin and glyceraldehyde-3-phosphate dehydrogenase S-thiolation during reperfusion. This approach has also shown that triosephosphate isomerase, aconitate hydratase, M-protein, nucleoside diphosphate kinase B, and myoglobin are S-thiolated during post-ischemic reperfusion.

Development of a sensitive assay to detect reversibly oxidized protein cysteine sulfhydryl groups

Protein sulfhydryl groups can undergo reversible oxidation reactions in response to reactive oxygen and reactive nitrogen species. Sensitive detection of sulfhydryl group oxidation in specific proteins is required to further our understanding of protein redox changes in biological systems. In general, to detect reversible oxidation reactions the oxidized sulfur atom is reduced to a sulfhydryl group followed by a reaction with a quantifiable agent. Our aim was to develop a sensitive method to detect reversibly oxidized protein sulfhydryl groups in a Western blot format. Conjugation of methoxypolyethylene glycol-maleimide (MAL-PEG) to protein sulfhydryl groups was optimized. Once MAL-PEG forms a covalent bond with the protein, the MAL-PEG-protein conjugate can be detected as a band shift by western analysis. The efficiency of MAL-PEG conjugation to protein was determined with creatine kinase. MAL-PEG conjugated to approximately 100% of the available sulfhydryl groups on creatine kinase within 30 min. Band shift detection sensitivity was measured using the redox-regulated protein p53. MAL-PEG conjugation coupled to western analysis detected a minimum of 0.23 pmol of oxidized p53. The MAL-PEG conjugation method described in this communication can be used to assess the reversible sulfhydryl oxidation status of proteins for which antibodies suitable for western analysis are available.

Differential antioxidant enzyme activities and glutathione content between rat and rabbit conceptuses

Redox status regulates numerous cellular processes like transcription factor activation and binding, protein folding, and calcium sequestration. Because the most abundant reducing equivalent in the cell is glutathione (GSH), it could play a role for teratogens that cause oxidative stress and disrupt pathways involved in differentiation and proliferation. Investigation of the redox status of two species that have demonstrated differential sensitivity to teratogens represents a novel approach for determining the role of redox alteration in teratogenesis. Furthermore, examining specific regions of the embryo may also help to explain why certain tissues are uniquely sensitive, while others are resistant to oxidative insult. In the presented study, New Zealand White rabbit (GD 12) and Sprague Dawley rat embryos (GD 13) were removed from the uterus on days of similar development. Each embryo was dissected into three portions-the limbs, the head, and the trunk. Samples were placed in the appropriate buffers for the measurement of both direct and indirect redox status contributors-GSH, cysteine, thioredoxin, glutathione disulfide, protein-glutathione mixed disulfides, superoxide dismutase, glutathione peroxidase, and glutathione disulfide reductase. Species comparison of whole embryos indicated that the rabbit embryo possesses a higher redox potential (more oxidative) than the rat embryo. Findings, in general, show that the rabbit may be more sensitive to redox-altering teratogens because it is inherently more pro-oxidizing and may be more easily perturbed resulting in misregulation of cellular processes. Differences were most apparent in the limb as compared to the embryonic head and trunk, where the rabbit limb has a significantly more pro-oxidizing redox environment than the rat limb. Species comparisons like these may help in the understanding of how redox shifts affect cellular processes and would contribute to regulation of biochemical and molecular events that may be associated with mechanisms of teratogenesis. These may contribute to a more complete rationale for choosing a species for study and provide a better correlation with human developmental toxicants.

*NO, RSNO, ONOO-, NO+, *NOO, NOx--dynamic regulation of oxidant scavenging, nitric oxide stores, and cyclic GMP-independent cell signaling

Following its release from nitric oxide synthase, nitric oxide seldom perfuses the cytosol; rather this reactive mediator quickly interacts with available target molecules proximate to its site of release. Within the cell, virtually every component, low-molecular-weight oxidants and reductants, proteins, lipids, sugars, and nucleic acids can be modified by nitrogen oxides thus acting as potential targets for reactive nitrogen oxides. Adducts formed by nitrogen oxides often modulate the cellular activities of the target molecules, and these modified molecules may be differentially metabolized or localized. The formation of nitrogen oxide adducts can be a reversible process, and the reactive nitrogen species released may be specifically oxidized or reduced during the process. Recently, numerous studies have demonstrated that reversible nitration of cellular proteins acts to transduce molecular signals regulating such diverse processes as muscle contraction, neurotransmission, protein metabolism, and apoptosis. The vast numbers of molecules that undergo biologically relevant interactions with nitrogen oxides imply that the cellular concentration of nitrosated and nitrated species may effectively comprise a reserve or cellular store. Potentially, these nitroso reserves function as critical components of the overall redox status of the intracellular environs. Understanding the dynamic regulation of nitric oxide/nitrogen oxides release from these stores is likely to provide clues important in resolving the complex pathophysiology of poorly understood multifactorial disorders, including neurodegeneration, multiorgan failure, cardiomyopathy, and septic shock.

Thiol oxidation activates a novel redox-regulated coronary vasodilator mechanism involving inhibition of Ca2+ influx

This study examines the mechanism of relaxation of isolated endothelium-removed bovine coronary arteries (BCAs) to the thiol oxidant diamide. BCAs precontracted with KCl or the thromboxane A(2) receptor agonist U46619 showed a concentration-dependent reversible relaxation on exposure to 10 micromol/L to 1 mmol/L diamide. This relaxation was enhanced by an inhibitor of glutathione reductase, and it was not altered by severe hypoxia, the presence of inhibitors of soluble guanylate cyclase, K(+) channels, tyrosine kinases, or probes that modulate levels of superoxide. The relaxation was almost eliminated when BCAs were precontracted with a phorbol ester that causes a contraction that is largely independent of extracellular Ca(2+). The initial transient contraction elicited by 5-hydroxytryptamine in Ca(2+)-free solution was not altered by the presence of 1 mmol/L diamide; however, a subsequent tonic contraction on addition of CaCl(2) was inhibited by diamide. Diamide also inhibited contractions caused by the addition of CaCl(2) to Ca(2+)-free Krebs' buffer containing Bay K8644 (an L-type Ca(2+) channel opener) or KCl. Relaxation to diamide was attenuated by L-type Ca(2+) channel blockers (nifedipine and diltiazem). Thus, thiol oxidation elicited by diamide appears to activate a novel redox-regulated vasodilator mechanism that seems to inhibit extracellular Ca(2+) influx.

Regulation of protein function by S-glutathiolation in response to oxidative and nitrosative stress

Protein S-glutathiolation, the reversible covalent addition of glutathione to cysteine residues on target proteins, is emerging as a candidate mechanism by which both changes in the intracellular redox state and the generation of reactive oxygen and nitrogen species may be transduced into a functional response. This review will provide an introduction to the concepts of oxidative and nitrosative stress and outline the molecular mechanisms of protein regulation by oxidative and nitrosative thiol-group modifications. Special attention will be paid to recently published work supporting a role for S-glutathiolation in stress signalling pathways and in the adaptive cellular response to oxidative and nitrosative stress. Finally, novel insights into the molecular mechanisms of S-glutathiolation as well as methodological problems related to the interpretation of the biological relevance of this post-translational protein modification will be discussed.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Modification of creatine kinase by S-nitrosothiols: S-nitrosation vs. S-thiolation

Creatine kinase is reversibly inhibited by incubation with S-nitrosothiols. Loss of enzyme activity is associated with the depletion of 5,5'-dithiobis (2-nitrobenzoic acid)-accessible thiol groups, and is not due to nitric oxide release from RSNO. Full enzymatic activity and protein thiol content are restored by incubation of the S-nitrosothiol-modified protein with glutathione. S-nitroso-N-acetylpenicillamine, which contains a more sterically hindered S-nitroso group than S-nitrosoglutathione, predominantly modifies the protein thiol to an S-nitrosothiol via a transnitrosation reaction. In contrast, S-nitrosoglutathione modifies creatine kinase predominantly by S-thiolation. Both S-nitroso-N-acetylpenicillamine and S-nitrosoglutathione modify bovine serum albumin to an S-nitroso derivative. This indicates that S-thiolation and S-nitrosation are both relevant reactions for S-nitrosothiols, and the relative importance of these reactions in biological systems depends on both the environment of the protein thiol and on the chemical nature of the S-nitrosothiol.

Differential protein S-thiolation of glyceraldehyde-3-phosphate dehydrogenase isoenzymes influences sensitivity to oxidative stress

The irreversible oxidation of cysteine residues can be prevented by protein S-thiolation, in which protein -SH groups form mixed disulfides with low-molecular-weight thiols such as glutathione. We report here the identification of glyceraldehyde-3-phosphate dehydrogenase as the major target of protein S-thiolation following treatment with hydrogen peroxide in the yeast Saccharomyces cerevisiae. Our studies reveal that this process is tightly regulated, since, surprisingly, despite a high degree of sequence homology (98% similarity and 96% identity), the Tdh3 but not the Tdh2 isoenzyme was S-thiolated. The glyceraldehyde-3-phosphate dehydrogenase enzyme activity of both the Tdh2 and Tdh3 isoenzymes was decreased following exposure to H2O2, but only Tdh3 activity was restored within a 2-h recovery period. This indicates that the inhibition of the S-thiolated Tdh3 polypeptide was readily reversible. Moreover, mutants lacking TDH3 were sensitive to a challenge with a lethal dose of H2O2, indicating that the S-thiolated Tdh3 polypeptide is required for survival during conditions of oxidative stress. In contrast, a requirement for the nonthiolated Tdh2 polypeptide was found during exposure to continuous low levels of oxidants, conditions where the Tdh3 polypeptide would be S-thiolated and hence inactivated. We propose a model in which both enzymes are required during conditions of oxidative stress but play complementary roles depending on their ability to undergo S-thiolation.

S-nitrosylation and S-glutathiolation of protein sulfhydryls by S-nitroso glutathione

The modification of reactive protein sulfhydryls by S-nitrosoglutathione and other NO donors has been studied by gel isoelectric focusing. S-nitrosylated, unmodified, and S-glutathiolated protein forms are differentiated by this method. With specific antibodies for the protein of interest, both S-nitrosylation and S-glutathiolation of the protein were analyzed in mixtures obtained as soluble tissue or cell extracts. The effect of S-nitrosoglutathione (GSNO) on purified phosphorylase b, on carbonic anhydrase III in an extract from rat liver, and on H-ras expressed in Escherichia coli was examined. When fresh GSNO reacted with pure phosphorylase b, only S-nitrosylated forms of the protein were observed. Likewise the NO donors, amyl nitrite, spermine NONOate, and diethylamine NONOate, all generated S-nitrosylated phosphorylase b. When crude mixtures of proteins from rat liver (containing carbonic anhydrase III) or from E. coli (containing an overexpressed form of H-ras) were exposed to fresh GSNO, both the S-nitrosylated and the S-glutathiolated forms of the proteins were observed. It is suggested that reactive intermediates from the breakdown of GSNO are responsible for the observed S-glutathiolation. These experiments show that both S-nitrosylated and S-glutathiolated forms of proteins may be generated by the addition of GSNO to mixtures containing proteins with reactive sulfhydryls. These protein modifications may exhibit metabolic consequences independent of the release of nitric oxide.

Regulation of ubiquitin-conjugating enzymes by glutathione following oxidative stress

Upon oxidative stress cells show an increase in the oxidized glutathione (GSSG) to reduced glutathione (GSH) ratio with a concomitant decrease in activity of the ubiquitinylation pathway. Because most of the enzymes involved in the attachment of ubiquitin to substrate proteins contain active site sulfhydryls that might be covalently modified (thiolated) upon enhancement of GSSG levels (glutathiolation), it appeared plausible that glutathiolation might alter ubiquitinylation rates upon cellular oxidative stress. This hypothesis was explored using intact retina and retinal pigment epithelial (RPE) cell models. Exposure of intact bovine retina and RPE cells to H2O2 (0.1-1.7 micromol/mg) resulted in a dose-dependent increase in the GSSG:GSH ratio and coincident dose-dependent reductions in the levels of endogenous ubiquitin-activating enzyme (E1)-ubiquitin thiol esters and endogenous protein-ubiquitin conjugates and in the ability to form de novo retinal protein-125I-labeled ubiquitin conjugates. Oxidant-induced decrements in ubiquitin conjugates were associated with 60-80% reductions in E1 and ubiquitin-conjugating enzyme (E2) activities as measured by formation of ubiquitin thiol esters. When GSH levels in RPE cells recovered to preoxidation levels following H2O2 removal, endogenous E1 activity and protein-ubiquitin conjugates were restored. Evidence that S thiolation of E1 and E2 enzymes is the biochemical link between cellular redox state and E1/E2 activities includes: (i) 5-fold increases in levels of immunoprecipitable, dithiothreitol-labile 35S-E1 adducts in metabolically labeled, H2O2-treated, RPE cells; (ii) diminished formation of E1- and E2-125I-labeled ubiquitin thiol esters, oligomerization of E225K, and coincident reductions in protein-125I-labeled ubiquitin conjugates in supernatants from nonstressed retinas upon addition of levels of GSSG equivalent to levels measured in oxidatively stressed retinas; and (iii) partial restoration of E1 and E2 activities and levels of protein-125I-labeled ubiquitin conjugates in supernatants from H2O2-treated retinas when GSSG:GSH ratios were restored to preoxidation levels by the addition of physiological levels of GSH. These data suggest that the cellular redox status modulates protein ubiquitinylation via reversible S thiolation of E1 and E2 enzymes, presumably by glutathione.

Decreased sulfhydryl groups in the reperfused myocardial tissue of a rat model of myocardial infarction

The aim of this study was to determine whether myocardial injury resulting from temporary ischemia followed by reperfusion can be measured by assaying sulfhydryl groups in the affected tissue of a rat model of myocardial infarction. We studied 3 groups: a control group (n = 6), which underwent surgery without left coronary artery (LCA) ligation; group NoR (n = 9), in which the LCA was ligated for 3 h; and group I + R (n = 7), in which 30 min LCA ligation was followed by 3 h reperfusion. The sulfhydryl group content of myocardial tissue was assayed by measuring the fluorescence produced by incubating heart sections with N-(7-dimethylamino-4-methyl-3-coumarinyl) maleimide (DACM), which binds sulfhydryl groups. The fluorescence intensity (FI) of normal and infarcted myocardium was quantified by our computerized system of microscopic fluorophotometry. Indices such as sulfhydryl group content, the size of the low-FI area [% AREA(lower FI)] and the relative decrease in FI [%FI(decrease)]) in the infarct zone were calculated. Both %AREA(lower FI) and %FI(decrease) were significantly higher in the infarcted zone of animals in NoR and I + R groups than in control animals. Both indices were higher in infarct tissue from animals in the I + R group than in the NoR group. These changes suggest that sulfhydryl group content is significantly reduced in tissue that has been subjected to ischemia-reperfusion. Microscopic fluorophotometry, as defined by DACM staining of myocardial tissue, may help to delineate areas of myocardial reperfusion injury.

Formation of protein-glutathione mixed disulfides in the developing rat conceptus following diamide treatment in vitro

Protein-glutathione mixed disulfide (protein-S-SG) formation was investigated in developing rat conceptuses during early organogenesis (gestational day 10, GD 10) using the whole embryo culture system. Low levels of protein-S-SG (25.0 +/- 6.6 pmoles resolved GSH/conceptus) were found in conceptuses under normal culture conditions. Incubation of the conceptuses with 75-500 microM diamide (a thiol oxidant) resulted in rapid increases in protein-S-SG (to 2- to 16-fold that of control values) in a dose-dependent manner during 30 min of the culture period. Approximately 20% of the observed cytosolic glutathione (GSH) depletion following diamide (500 microM) could be accounted for as mixed disulfides of protein sulfhydryls, when determined in whole conceptual tissues after 15 min. The most extensive S-thiolation of protein sulfhydryls by GSH was observed in visceral yolk sac (VYS) when compared to embryo proper and ectoplacental cone. This result indicates that the most abundant, sensitive, or accessible protein sulfhydryls were found in the VYS. Inhibition of glutathione disulfide reductase activity by pretreatment of the conceptuses with 25 microM BCNU for 2 hr potentiated protein-S-SG formation elicited by 75 microM diamide. Reincubation of the conceptuses in fresh media, following the 15-min treatment with 500 microM diamide, reversed both the GSH depletion and the protein-S-SG formation in conceptal tissues. The reduction of the protein-S-SG was dependent on adequate intracellular GSH levels and was inhibited when GSH was rapidly depleted by subsequent addition of N-ethylmaleimide (NEM, 100 microM). Under the same experimental conditions, addition of 1 mM dithiothreitol (DTT) did not significantly enhance the GSH restoration rate nor the protein-S-SG reduction rate. The results also indicated that low levels of intracellular cysteine do not play an important role in the reduction of protein-S-SG. Protein-S-SG formation may be important for cellular regulation and in mediating the embryotoxicity elicited by diamide or other oxidative stresses.

Carbonic anhydrase III. Oxidative modification in vivo and loss of phosphatase activity during aging

Oxidative modification of DNA, lipids, and proteins occurs as a consequence of reaction with free radicals and activated oxygen. Oxidative modification of total cellular proteins has been described under many pathologic and experimental conditions, but no specific proteins have been identified as in vivo targets for oxidative modification. Utilizing an immunochemical method for detection of oxidatively modified proteins, we identified a protein in rat liver that was highly oxidized. It was purified to homogeneity and identified as carbonic anhydrase, isozyme III. Its characteristics match those previously described for a protein that was lost during aging of the rat, senescence marker protein-1. Carbonic anhydrase III was purified from rats aged 2, 10, and 18 months, and the proteins were characterized. All three preparations were highly oxidatively modified as assessed by their carbonyl content. The enzyme has three known catalytic activities, and the specific activities for carbon dioxide hydration and for ester hydrolysis decreased during aging by approximately 30%. However, the third activity, that of a phosphatase, was virtually lost during aging. While the physiologic role of carbonic anhydrase III is unknown, we suggest that it functions in an oxidizing environment, which leads to its own oxidative modification.

Irreversible inactivation of protein kinase C by a peptide-substrate analog

Protein kinase C (PKC) is a phospholipid-dependent isozyme family that plays a pivotal role in mammalian signal-transduction pathways that mediate cell growth and differentiation and pathological developments, such as the acquisition of drug resistance by cancer cells. Several peptide-substrate analogs have been shown to reversibly inhibit PKC with high potency and selectivity, but peptide-substrate analogs that antagonize PKC by forming a covalent complex with the enzyme have not been reported. The development of active site-directed irreversible inactivators of PKC could provide new insights into the catalytic mechanism and might ultimately lead to the design of novel therapeutics targeted at PKC. In this report, we show that the peptide-substrate analog Arg-Lys-Arg-Cys-Leu-Arg-Arg-Leu (RKRCLRRL) irreversibly inactivates PKC in a dithiothreitol-sensitive manner. The inactivation mechanism most consistent with our results is the formation of a covalent linkage between the inhibitor-peptide and the enzyme at its active-site. Limited proteolysis of PKC produces a catalytic-domain fragment that is independent of the phospholipid cofactor. RKRCLRRL antagonized the histone kinase activity of PKC and its catalytic-domain fragment with similar efficacies, achieving > 50% inactivation at an RKRCLRRL concentration of 10 microM. In contrast, RKRCLRRL analogs with single amino acid substitutions at Cys were non-inhibitory. The inactivated complex of the catalytic-domain fragment and RKRCLRRL was stable upon dilution, and the inactivation of PKC and the catalytic-domain fragment by RKRCLRRL was quenched by dithiothreitol, providing evidence that the enzyme and the synthetic peptide may be covalently linked in an inactivated complex by a disulfide bond. Substrates and substrate analogs protected the catalytic-domain fragment against inactivation by RKRCLRRL, providing evidence that inactivation entailed binding of RKRCLRRL at the active-site of the enzyme. S-Thiolation is the formation of mixed disulfides between proteins and low molecular weight thiols. PKC is thought to have a highly reactive Cys residue in its active-site, and Cys residues that are flanked by basic residues, as is the case in RKRCLRRL, display enhanced reactivity. Our results support an inactivation mechanism that entails S-thiolation of the active-site of PKC by RKRCLRRL. This is the first report of irreversible inactivation of PKC by an active site-directed peptide.

Proposal for experimental studies to evaluate sodium hypochlorite dialysate in retroviral treatment

Sodium hypochlorite (NaOCl) is widely used to inactivate retroviruses topically and on environmental surfaces. This proposal establishes the thesis that sodium hypochlorite and its related oxygen free radicals can be administered in minute quantities in vivo to achieve a reduction in retroviral titer within the infected individual. Published reports of animal studies and accidental sodium hypochlorite infusion in much greater concentrations have indicated that the protein depletion and oxidation of sulfhydryl compounds is reversible and possibly preventable by administration of disulfide reducing agents. Various methods of infusion can include the ex vivo retroviral inactivation of plasma utilizing extracorporeal circulation through a continuous centrifugal plasma separator. The utilization of infusion of low-concentration sodium hypochlorite dialysate for retroviral inactivation merits immediate experimental study. Chlorinated tap-water and table salt ingestion must also be among the environmental factors studied for correlation to HIV infection.

Tat protein of human immunodeficiency virus type 1 represses expression of manganese superoxide dismutase in HeLa cells

Using a HeLa cell line stably transfected with the tat gene from human immunodeficiency virus type 1, we have found that the expression of the regulatory Tat protein suppresses the expression of cellular Mn-containing superoxide dismutase (Mn-SOD). This enzyme is one of the cell's primary defenses against oxygen-derived free radicals and is vital for maintaining a healthy balance between oxidants and antioxidants. The parental HeLa cells expressed nearly equivalent amounts of Cu,Zn- and Mn-SOD isozymes. Those cells expressing the Tat protein, however, contained 52% less Mn-SOD activity than parental cells, whereas that of the Cu,Zn enzyme was essentially unchanged. The steady-state levels of Mn-SOD-specific RNAs were also lower in the HeLa-tat cell line than in the parental line. No difference was seen in the steady-state levels of Cu,Zn-SOD-specific RNAs. In addition to the decreased Mn-SOD-activity, HeLa-tat cell showed evidence of increased oxidative stress. Carbonyl proteins were markedly higher, and total cellular sulfhydryl content decreased in cell extracts at a faster rate, probably reflecting ongoing lipid peroxidation. HeLa and HeLa-tat extracts were incubated with radiolabeled Mn-SOD transcripts, and the reaction products were subjected to UV crosslinking, digestion with ribonuclease A, and electrophoretic analysis. The results suggest a direct interaction between Tat protein and Mn-SOD gene transcripts.

Reversible inactivation of recombinant rat liver guanidinoacetate methyltransferase by glutathione disulfide

Recombinant rat liver guanidinoacetate methyltransferase is inactivated by glutathione disulfide (GSSG) following pseudo-first-order kinetics. A second-order rate constant of 20.8 M-1 min-1 is obtained at pH 7.5 and 30 degrees C. The inactivation is fully reversed by glutathione (GSH) in a pseudo-first-order fashion with a second-order rate constant of 11.1 M-1 min-1. The rate of inactivation is not affected by S-adenosylmethionine or guanidinoacetate, but complete protection against inactivation is observed in the presence of sinefungin plus guanidinoacetate. At equilibrium in the buffers containing various concentrations of GSH and GSSG, the enzyme shows activities that are dependent on the ratio but not on the total concentration of GSH and GSSG. A hyperbolic relationship is obtained between enzyme activity and [GSH]/[GSSG] ratio. The inactivation by GSSG is associated with the disappearance of approximately 1 mol of sulfhydryl group per mole of enzyme. These results indicate that inactivation of guanidinoacetate methyltransferase by GSSG is the consequence of the formation of a mixed disulfide between a protein thiol and glutathione. The equilibrium constant for the redox reaction, E-SH + GSSG in equilibrium with E-SSG + GSH, obtained from the equilibrium data (1.69) is in good agreement with the value determined as the ratio of second-order rate constants for reactivation and inactivation (1.87). The cysteine residue engaged in the mixed disulfide with glutathione is identified as Cys-15 by peptide analysis after consecutive treatment of the GSSG-inactivated enzyme with N-ethylmaleimide, 2-mercaptoethanol, and [14C]iodoacetate. The GSSG-inactivated enzyme binds S-adenosyl-methionine but not guanidinoacetate in the presence and absence of sinefungin. Native guanidinoacetate methyltransferase binds guanidinoacetate in the presence of sinefungin. The low overall redox equilibrium constant of 1.7-1.9 found for the reaction between guanidinoacetate methyltransferase and GSSG suggests that the activity of the enzyme is not amenable to modulation by the change in intracellular [GSH]/[GSSG] ratio.

Reduction (dethiolation) of protein mixed-disulfides; distribution and specificity of dethiolating enzymes and N,N'-bis(2-chlorethyl)-N-nitrosourea inhibition of an NADPH-dependent cardiac dethiolase

The S-thiolated proteins phosphorylase b (Phb) and carbonic anhydrase III (CAIII) were prepared with [3H]glutathione in a reaction initiated with diamide. These substrates were used to measure the rate of reduction (dethiolation) of protein mixed-disulfides by enzymes with properties similar to those of thioredoxin and glutaredoxin. This enzyme activity is termed a dethiolase since the identities of the enzymes are still unknown. The dethiolation of either S-[3H]glutathiolated Phb or S-[3H]glutathiolated CAIII was employed in tissue assays and for study of two partially purified dethiolases from cardiac tissue. NADPH-dependent dethiolase activity was most abundant except in rat liver and muscle. Total dethiolase activity was approximately 10-fold higher in neutrophils, 3T3-L1 cells, and Escherichia coli than in other sources. Rat skeletal muscle had 3- to 4-fold higher dethiolase activity than rat heart or liver. These data indicate that protein dethiolase activity is ubiquitous and that normal expression of the two dethiolase activities varies considerably. A partially purified cardiac NADPH-dependent dethiolase acted on Phb approximately 1.5 times faster than CAIII, and a glutathione (GSH)-dependent dethiolase acted on Phb 3 times faster than CAIII. The Km for glutathione for the GSH-dependent dethiolase was 15 microM with Phb as substrate and 10 microM with CAIII. Thus, the GSH-dependent dethiolase is probably not affected by normal changes in the cardiac glutathione content (normally approximately 3 mM). Partially purified cardiac NADPH-dependent dethiolase was inactivated by BCNU (N,N'-bis(2-chloroethyl)-N-nitrosourea) and the GSH-dependent dethiolase was unaffected under similar conditions. In a soluble extract from bovine heart, 200 microM BCNU inhibited NADPH-dependent dethiolase by more than 60% but did not affect GSH-dependent activity. These results demonstrate that BCNU is a selective inhibitor of the NADPH-dependent dethiolase.

Phosphorylase and creatine kinase modification by thiol-disulfide exchange and by xanthine oxidase-initiated S-thiolation

The reaction of glycogen phosphorylase b and creatine kinase with glutathione disulfide, cystine, and cystamine was compared by direct analysis on electrofocusing gels. This method was useful for individual proteins or for mixtures of the proteins. Millimolar concentrations of glutathione disulfide were required for both proteins and the rate of modification of each protein was similar. The reaction of glutathione disulfide with creatine kinase was inhibited by reduced glutathione (GSH), but the effect on the reaction with phosphorylase was minimal. Cystine and cystamine were required in micromolar amounts to effectively form the disulfide adducts. Both proteins were modified by cystine but cystamine reacted only with phosphorylase. Cystamine (10 microM) was an effective inhibitor of the reaction of phosphorylase b with 2 mM glutathione disulfide. S-thiolation of creatine kinase inactivated the enzyme and a direct assay of the enzyme activity could be used to quantitate S-thiolation of this protein by each of the disulfides. The effect of each disulfide on enzyme activity confirmed the results obtained by gel electrofocusing. Glutathione disulfide and cystine both inactivated the enzyme while cystamine had no effect on the activity. S-thiolation of phosphorylase had no observable effect on any activity parameter, but it effectively prevented binding of phosphorylase to high-molecular-weight glycogen, probably at the glycogen storage site of phosphorylase. The rate of S-thiolation of a mixture of phosphorylase and creatine kinase by thiol-disulfide exchange with glutathione disulfide was compared to the rate of S-thiolation of these proteins by a xanthine oxidase-initiated process (presumably due to protein sulfhydryl activation by reactive oxygen species). The xanthine oxidase-initiated mechanism was somewhat faster than thiol-disulfide exchange with both proteins. It was shown that GSH inhibited S-thiolation of creatine kinase by this mechanism as well as by thiol-disulfide exchange. It is suggested that both mechanisms may play a role in protein S-thiolation in vivo. For proteins that are typified by creatine kinase, the concentration of GSH in the cells may determine whether the S-thiolated form of the protein accumulates. For proteins typified by phosphorylase b, the accumulation of S-thiolated forms may be more independent of GSH.

The mechanisms of reduction of protein mixed disulfides (dethiolation) in cardiac tissue

Dethiolation of proteins (reduction of protein mixed disulfides) by NADPH-dependent and glutathione (GSH)-dependent enzymes, and by nonenzymatic reaction with GSH, was studied by electrofocusing methodology with glycogen phosphorylase b and creatine kinase as substrates. Phosphorylase b was not rapidly dethiolated by reduced glutathione alone, but a cardiac extract catalyzed rapid dethiolation by both an NADPH-dependent and a GSH-dependent process. In contrast, creatine kinase was actively dethiolated by GSH. This GSH-dependent dethiolation was not enhanced by a soluble extract of bovine heart. Creatine kinase was also not dethiolated by an NADPH-dependent process. Partial purification of the phosphorylase dethiolases showed that the NADPH-dependent dethiolase had both a high-molecular-weight and a low-molecular-weight component The properties of these components were similar to those of thioredoxin and thioredoxin reductase. These two components were sensitive to inhibition by phenylarsine oxide and inhibition was reversed by addition of a dithiol. In contrast, GSH-dependent dethiolation required a single component of low molecular weight. This process was less sensitive to phenylarsine oxide inhibition. These studies show that two cytosolic proteins, phosphorylase b and creatine kinase, were dethiolated by different mechanisms. Phosphorylase b was dethiolated by both NADPH-dependent and GSH-dependent enzymes found in a soluble extract of bovine heart. In contrast, creatine kinase was rapidly dethiolated nonenzymatically by GSH alone.

Reduction of protein mixed disulfides (dethiolation) by Escherichia coli thioredoxin: a study with glycogen phosphorylase b and creatine kinase

The role of thioredoxin in the reduction of protein mixed disulfides (dethiolation) was studied by electrofocusing methodology with glycogen phosphorylase b and creatine kinase as substrates for the reaction. Glycogen phosphorylase b was effectively dethiolated by Escherichia coli thioredoxin with dithiothreitol as the reductant, while creatine kinase could not be dethiolated by this mechanism. The rate of dethiolation of phosphorylase b was dependent on the concentration of thioredoxin up to a maximum at 20 microM when the concentration of phosphorylase b was 4 microM in monomer. Rat heart contained a thioredoxin reductase activity that could use added E. coli thioredoxin to dethiolate phosphorylase b and the same concentration of thioredoxin as above was required. This activity was not expressed with creatine kinase as the substrate. Cardiac tissue was shown to have a similar endogenous dethiolating activity. These results suggest that thioredoxin may play an important role in dethiolating specific proteins that might become S-thiolated during oxidative stress of cardiac tissue.

Specific S-thiolation of a 30-kDa cytosolic protein from rat liver under oxidative stress

Thin-gel isoelectric focusing (IEF) is a simple and sensitive method of quantifying S-thiolation of individual proteins (protein mixed-disulfide formation). IEF of rat liver cytosol identified one major protein (pI 7.0) which underwent S-thiolation with glutathione disulfide to produce two acidic bands with pIs 6.4 and 6.1. The S-thiolated forms of the protein were purified by preparative isoelectric focusing. An apparent molecular mass of 30 kDa was determined by SDS/polyacrylamide gel electrophoresis. The 30-kDa protein amounted to 7 +/- 2% of the total cytosolic protein on IEF. The most abundant soluble protein of freshly isolated hepatocytes, with an identical isoelectric point to the liver 30-kDa protein, was modified in a similar manner in response to oxidative stress induced by model compounds. Addition of 50 microM tert-butyl hydroperoxide, 50 microM diamide [1,1-azobis(N,N'-dimethylformamide)] or 20 microM menadione (2-methyl-1,4-naphthoquinone) initiated the S-thiolation within less than 2 min in the hepatocytes. These compounds, at the concentrations employed, did not result in cell death. Menadione produced slowly progressive S-thiolation of the protein, while tert-butyl hydroperoxide or diamide produced rapid S-thiolation that decreased quickly after 2 min.

S-thiolation of creatine kinase and glycogen phosphorylase b initiated by partially reduced oxygen species

S-thiolation of cardiac creatine kinase and skeletal muscle glycogen phosphorylase b was initiated by reduced oxygen species in reaction mixtures containing reduced glutathione. Both proteins were extensively modified at similar rates under conditions in which the oxidation of glutathione was inadequate to cause S-thiolation by thiol-disulfide exchange. Creatine kinase was both S-thiolated and non-reducibly oxidized at the same time at low glutathione concentration. The amount of each modification was decreased by adding additional reduced glutathione, and with adequate glutathione oxidation was prevented while S-thiolation was still very active. S-thiolation of glycogen phosphorylase b was not significantly affected by glutathione concentration and non-reducible oxidation of glycogen phosphorylase b was not observed. These experiments suggest that oxyradical or H2O2-initiated processes may be an important mechanism of protein S-thiolation during oxidative stress, and that the cellular concentration of glutathione may be an important factor in S-thiolation of different proteins. Both creatine kinase and glycogen phosphorylase b competed favorably with ferricytochrome c for superoxide anion in the standard xanthine oxidase system for the generation of oxyradicals and H2O2. These proteins were as effective as ascorbate and much more effective than reduced glutathione in this regard. Ascorbate was also an effective inhibitor of oxyradical-initiated S-thiolation of creatine kinase, suggesting a role of superoxide anion in protein S-thiolation. Other experiments showed that both catalase and superoxide dismutase could partially inhibit protein S-thiolation. Thus, reduced oxygen species may react with protein sulfhydryls resulting in S-thiolation by a mechanism that involves the reaction of an activated protein thiol with reduced glutathione.