Nitric oxide-induced S-glutathionylation and inactivation of glyceraldehyde-3-phosphate dehydrogenase

Metal and redox modulation of cysteine protein function

In biological systems, the amino acid cysteine combines catalytic activity with an extensive redox chemistry and unique metal binding properties. The interdependency of these three aspects of the thiol group permits the redox regulation of proteins and metal binding, metal control of redox activity, and ligand control of metal-based enzyme catalysis. Cysteine proteins are therefore able to act as "redox switches," to sense concentrations of oxidative stressors and unbound zinc ions in the cytosol, to provide a "storage facility" for excess metal ions, to control the activity of metalloproteins, and to take part in important regulatory and signaling pathways. The diversity of cysteine's multiple roles in vivo is equally as fascinating as it is promising for future biochemical and pharmacological research.

Glutathione-thiyl radical scavenging and transferase properties of human glutaredoxin (thioltransferase). Potential role in redox signal transduction

Glutaredoxin (GRx, thioltransferase) is implicated in cellular redox regulation, and it is known for specific and efficient catalysis of reduction of protein-S-S-glutathione-mixed disulfides (protein-SSG) because of its remarkably low thiol pK(a) ( approximately 3.5) and its ability to stabilize a catalytic S-glutathionyl intermediate (GRx-SSG). These unique properties suggested that GRx might also react with glutathione-thiyl radicals (GS(.)) and stabilize a disulfide anion radical intermediate (GRx-SSG), thereby facilitating the conversion of GS(.) to GSSG or transfer of GS(.) to form protein-SSG. We found that GRx catalyzes GSSG formation in the presence of GS-thiyl radical generating systems (Fe(2+)/ADP/H(2)O(2) + GSH or horseradish peroxidase/H(2)O(2) + GSH). Catalysis is dependent on O(2) and results in concomitant superoxide formation, and it is distinguished from glutathione peroxidase-like activity. With the horseradish peroxidase system and [(35)S]GSH, GRx enhanced the rate of GS-radiolabel incorporation into GAPDH. GRx also enhanced the rate of S-glutathionylation of glyceraldehyde-3-phosphate dehydrogenase with GSSG or S-nitrosoglutathione, but these glutathionyl donors were much less efficient. Both actin and protein-tyrosine phosphatase-1B were superior substrates for GRx-facilitated S-glutathionylation with GS-radical. These studies characterize GRx as a versatile catalyst, facilitating GS-radical scavenging and S-glutathionylation of redox signal mediators, consistent with a critical role in cellular regulation.

Nitrosative stress on yeast: inhibition of glyoxalase-I and glyceraldehyde-3-phosphate dehydrogenase in the presence of GSNO

Under nitrosative stressed condition intracellular GSNO accumulation is common to all cell types. Conserved NADH-dependent GSNO reductase was reported previously as an important cellular protective measure against this. In spite of the constitutive nature of the enzyme, we observed in vivo inactivation of two important enzymes-glyoxalase-I and glyceraldehyde-3-phosphate dehydrogenase under 5 mM GSNO stress in two budding yeasts, though with difference in their sensitivity. Former was more susceptible to inactivation in in vitro condition, too. In this study, we explored the competitive nature of yeast glyoxalase-I inhibition by GSNO. GSNO actually competes with GSH substrate-binding site of the enzyme.

Inhibition of LPS-induced nitric oxide production in RAW cells by radioprotective thiols

Nitric oxide (NO) is involved in producing damage after exposure to radiation and also in the toxicity associated with bacterial endotoxin (lipopolysaccharide, LPS). We have evaluated different radioprotective thiols for their effects on LPS-stimulated NO production in mouse macrophage cells, RAW 264.7. Our results indicate that although thiols inhibited NO production in general, the degree of inhibition depended upon the thiol compound. Long-chain aminothiols like WR-1065 [N-(2-mercaptoethyl)-1,3-diaminopropane] exerted a strong inhibition; but its parent drug, amifostine, which protects mice against radiation lethality, was not as effective as WR-1065. Diethyl dithiocarbamate, which is less effective than amifostine as a radioprotector, strongly inhibited NO production from macrophages. These results indicate that the radioprotective potential of sulfhydryl compounds is not related to its ability to inhibit NO production by macrophages and suggest that some of the thiol radioprotectors may effectively ameliorate the fatal symptoms of hypotensive shock, associated with endotoxin (LPS)-induced NO production.

Regulation of NF-kappaB activation by protein phosphatase 2B and NO, via protein kinase A activity, in human monocytes

It has been reported previously that a short synthetic immunomodulating peptide (Pa) and the neuropeptide beta-endorphin modulate the immune system. We have found now that NF-kappaB participates in the stimulation of monocytes by both peptides and we investigated the molecular mechanism by which these stimuli activate NF-kappaB. Pa and beta-endorphin induce accumulation of cyclic 3('),5(')-adenosine monophosphate (cAMP) in a calcium/calmodulin-dependent fashion since it was completely inhibited by the calmodulin antagonist W-7. The effect of these complexes seems to be mediated, at least in part, by nitric oxide (NO) synthesized by constitutive NO synthase since the NO synthase inhibitor N-methyl-L-arginine (NMLA) reduced the elevation of cAMP. Furthermore, the NO donor SIN-1 provoked nitration of G(S)alpha, leading to the cAMP elevation that was suppressed by the G(S)alpha-selective antagonist NF-449. Interestingly, the rapid degradation of NF-kappaB inhibitor IkappaBalpha induced by Pa- and beta-endorphin was reversed by a pretreatment with H-89 and cyclosporin A, inhibitors of protein kinase A (PKA) and protein phosphatase 2B (PP2B), respectively. These observations are consistent with the inhibition caused by W-7, NMLA, H-89, and cyclosporin A on NF-kappaB induction by these agonists, indicating the involvement of PKA and PP2B in the regulation of NF-kappaB in human monocytes.

Dopamine biosynthesis is regulated by S-glutathionylation. Potential mechanism of tyrosine hydroxylast inhibition during oxidative stress

Tyrosine hydroxylase (TH), the initial and rate-limiting enzyme in the biosynthesis of the neurotransmitter dopamine, is inhibited by the sulfhydryl oxidant diamide in a concentration-dependent manner. The inhibitory effect of diamide on TH catalytic activity is enhanced significantly by GSH. Treatment of TH with diamide in the presence of [(35)S]GSH results in the incorporation of (35)S into the enzyme. The effect of diamide-GSH on TH activity is prevented by dithiothreitol (DTT), as is the binding of [(35)S]GSH, indicating the formation of a disulfide linkage between GSH and TH protein cysteinyls. Loss of TH catalytic activity caused by diamide-GSH is partially recovered by DTT and glutaredoxin, whereas the disulfide linkage of GSH with TH is completely reversed by both. Treatment of intact PC12 cells with diamide results in a concentration-dependent inhibition of TH activity. Incubation of cells with [(35)S]cysteine, to label cellular GSH prior to diamide treatment, followed by immunoprecipitation of TH shows that the loss of TH catalytic activity is associated with a DTT-reversible incorporation of [(35)S]GSH into the enzyme. A combination of matrix-assisted laser desorption/ionization/mass spectrometry and liquid chromatography/tandem mass spectrometry was used to identify the sites of S-glutathionylation in TH. Six cysteines (177, 249, 263, 329, 330, and 380) of the seven cysteine residues in TH were confirmed as substrates for modification. Only Cys-311 was not S-glutathionylated. These results establish that TH activity is influenced in a reversible manner by S-glutathionylation and suggest that cellular GSH may regulate dopamine biosynthesis under conditions of oxidative stress or drug-induced toxicity.

Regulation of cAMP-dependent protein kinase activity by glutathionylation

The catalytic subunit of cAMP-dependent protein kinase (cAPK) is susceptible to inactivation by a number of thiol-modifying reagents. Inactivation occurs through modification of cysteine 199, which is located near the active site. Because cysteine 199 is reactive at physiological pH, and modification of this site inhibits activity, we hypothesized that cAPK is a likely target for regulation by an oxidative mechanism, specifically glutathionylation. In vitro studies demonstrated the susceptibility of kinase activity to the sulfhydryl oxidant diamide, which inhibited by promoting an intramolecular disulfide bond between cysteines 199 and 343. In the presence of a low concentration of diamide and reduced glutathione, the kinase was rapidly and reversibly inhibited by glutathionylation. Mutant kinase containing an alanine to cysteine mutation at position 199 was resistant to inhibition by both diamide and glutathionylation, thus implicating this as the oxidation-sensitive site. Mouse fibroblast cells treated with diamide showed a reversible decrease in cAPK activity. Inhibition was dramatically enhanced when cells were first treated with cAPK activators. Using biotin-cysteine as means for detecting and purifying thiolated cAPK from cells, we were able to show that, under conditions in which cAPK is inactivated by diamide, it is also readily thiolated.

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.

Glutathionylation of proteins by glutathione disulfide S-oxide

Aqueous solution of S-nitrosoglutathione (GSNO) underwent spontaneous chemical transformation that generated several glutathione derivatives including glutathione sulfonic acid (GSO3H), glutathione disulfide S-oxide (GS(O)SG), glutathione disulfide S-dioxide, and glutathione disulfide. Surprisingly, GS(O)SG (also called glutathione thiosulfinate), which was not identified as a metabolite of GSNO previously, was one of the major products derived from GSNO. This compound was very reactive toward any thiol and the reaction product was a mixed disulfide. The rate of reaction of GS(O)SG with 5-mercapto-2-nitro-benzoate was nearly 20-fold faster than that of GSNO. The mechanism for the formation of GS(O)SG was believed to involve the sulfenic acid (GSOH) and thiosulfinamide (GS(O)NH2) intermediates; the former underwent self-condensation and the latter reacted with GSH to form GS(O)SG. Many reactive oxygen and nitrogen species were also capable of oxidizing GSH or GSSG to form GS(O)SG, which likely played a central role in integrating both the oxidative and nitrosative cellular responses through thionylation of thiols. Treatments of rat brain tissue slices with oxidants resulted in an enhanced thionylation of proteins with a concomitant increase in cellular level of GS(O)SG, suggesting that this compound might play a second messenger role for stimuli that produced a variety of oxidative species.

Thiolation of protein-bound carcinogenic aldehyde. An electrophilic acrolein-lysine adduct that covalently binds to thiols

Acrolein, a representative carcinogenic aldehyde that could be ubiquitously generated in biological systems under oxidative stress, shows facile reactivity with the epsilon-amino group of lysine to form N(epsilon)-(3-formyl-3,4-dehydropiperidino)lysine (FDP-lysine) as the major product (Uchida, K., Kanematsu, M., Morimitsu, Y., Osawa, T., Noguchi, N., and Niki, E. (1998) J. Biol. Chem. 273, 16058-16066). In the present study, we determined the electrophilic potential of FDP-lysine and established a novel mechanism of protein thiolation in which the FDP-lysine generated in the acrolein-modified protein reacts with sulfhydryl groups to form thioether adducts. When a sulfhydryl enzyme, glyceraldehyde-3-phosphate dehydrogenase, was incubated with acrolein-modified bovine serum albumin in sodium phosphate buffer (pH 7.2) at 37 degrees C, a significant loss of sulfhydryl groups, which was accompanied by the loss of enzyme activity and the formation of high molecular mass protein species (>200 kDa), was observed. The FDP-lysine adduct generated in the acrolein-modified protein was suggested to represent a thiol-reactive electrophile based on the following observations. (i) N(alpha)-acetyl-FDP-lysine, prepared from the reaction of N(alpha)-acetyl lysine with acrolein, was covalently bound to glyceraldehyde-3-phosphate dehydrogenase. (ii) The FDP-lysine derivative reacted with glutathione to form a GSH conjugate. (iii) The acrolein-modified bovine serum albumin significantly reacted with GSH to form a glutathiolated protein. Furthermore, the observation that the glutathiolated acrolein-modified protein showed decreased immunoreactivity with an anti-FDP-lysine monoclonal antibody suggested that the FDP-lysine residues in the acrolein-modified protein served as the binding site of GSH. These data suggest that thiolation of the protein-bound acrolein may be involved in redox alteration under oxidative stress, whereby oxidative stress generates the increased production of acrolein and its protein adducts that further potentiate oxidative stress via the depletion of GSH in the cells.

Adenosine and purine nucleosides protect rat primary astrocytes from peroxynitrite-potentiated, glucose deprivation-induced death: preservation of intracellular ATP level

Previously we have reported that immunostimulated astrocytes became highly vulnerable to glucose deprivation. In the present study we examined the effect of various kinds of nucleosides on the augmented death of glucose-deprived immunostimulated astrocytes. Preincubation with interferon-gamma (100 U/ml) and lipopolysaccharide (1 microg/ml) for 48 h and continuous exposure to glucose deprivation (4 h) significantly induced the lactate dehydrogenase (LDH) release, as a marker of cell injury or death, from astrocytes. The glucose deprivation-induced augmented cell death in immunostimulated astrocytes was mimicked by exogenous peroxynitrite generator 3-morpholinosydnonimine (SIN-1). The increased death in immunostimulated or SIN-1-treated astrocytes deprived of glucose was blocked by adenosine and ATP. Other purine nucleos(t)ides, not pyrimidine nucleotides, also showed similar protective effects. Adenosine receptor agonist R(-)-N-(2-phenylisopropyl)-adenosine or N-cyclohexyladenosine did not alter the augmented cell death. Adenosine receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine, xanthine amine congener or 3,7-dimethyl-1-propargylxanthine also did not reverse the protective effect of adenosine. Intracellular ATP levels rapidly decreased prior to the LDH release in glucose-deprived immunostimulated astrocytes. The loss of intracellular ATP was prevented by adenosine and other purine nucleotides. The present results suggest that adenosine and their metabolites may protect astrocytes from peroxynitrite-potentiated, glucose deprivation-induced death by serving as substrates for intracellular ATP generation.

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.

Metabolic control of resistance of human epithelial cells to H2O2 and NO stresses

The carbon flux through the oxidative branch of the pentose phosphate pathway (PPP) can be viewed as an integrator of the antioxidant mechanisms via the generation of NADPH. It could therefore be used as a control point of the cellular response to an oxidative stress. Replacement of glucose by galactose sensitized the human epithelial cell line HGT-1 to H2O2 stress. Here we demonstrate that, due to the restricted galactose flux into the PPP, the H2O2 stress led to early cellular blebbing followed by cell necrosis, these changes being associated with a fall in the NADPH/NADP+ ratio and GSH depletion. H2O2 cytotoxicity was prevented by adding 2-deoxyglucose (2dGlc). This protection was associated with an increased flow of 2-deoxyglucose 6-phosphate into the oxidative branch of the PPP together with the prevention of the NADPH/NADP+ fall and the maintenance of intracellular GSH redox homoeostasis. Inhibitors of enzyme pathways connecting the PPP to GSH recycling abolished the 2dGlc protection. In carbohydrate-free culture conditions, 2dGlc dose-dependent protective effect was paralleled by a dose-dependent influx of 2dGlc into the PPP leading to the maintenance of the intracellular redox status. By contrast, in Glc-fed cells, the PPP was not a control point of the cellular resistance to H2O2 stress as they maintained a high NADPH/NADP+ ratio. Both 2dGlc and Glc inhibited, through the maintenance of GSH redox status, NO cytotoxicity on galactose-containing Dulbecco's modified Eagle's medium (Gal-DMEM)-fed cells. 2dGlc did not prevent the fall of ATP content in NO-treated Gal-DMEM-fed cells, indicating that NO cytotoxicity was essentially due to the disruption of GSH redox homoeostasis and not to the alteration of ATP production by the mitochondrial respiratory chain. The maintenance of ATP content in NO-treated glucose-fed cells was due to their ability to derive their energy from anaerobic glycolysis. In conclusion, Gal-DMEM and 2dGlc-supplemented Gal-DMEM provide a useful system to decipher and organize into a hierarchy the targets of several stresses at the level of intact barrier epithelial cells.

The biochemistry and physiology of S-nitrosothiols

S-nitrosothiols are biological metabolites of nitric oxide. It has often been suggested that they represent a more stable metabolite of nitric oxide that can either be stored, or transported, although the evidence for this is sparse. There are many unanswered questions concerning how S-nitrosothiols are formed, how they are metabolized and how they elicit biological responses. These questions are highlighted by the fact that the known chemistry of nitric oxide, thiols, and S-nitrosothiols cannot serve to explain their proposed biological activities. This review attempts to highlight the gulf between our chemical understanding of S-nitrosothiols and the proposed biological activities of these compounds with respect to guanylyl cyclase-independent nitric oxide bioactivity and also the control of vascular tone.

Redox regulation of cytosolic glycerol-3-phosphate dehydrogenase: Cys(102) is the target of the redox control and essential for the catalytic activity

Cytosolic glycerol-3-phosphate dehydrogenase (cG3PDH) occupies the branch point between the glycolytic pathway and triglyceride biosynthesis. However, the regulatory mechanism of the cG3PDH activity has remained obscure. Here we report that cG3PDH is efficiently inhibited by modification of the thiol group through a redox mechanism. In this study, we found that sodium selenite and nitric oxide (NO) donors such as S-nitroso-N-acetylpenicillamine and 3-morpholinosydnonimine inhibited cG3PDH activity, and that similar effects could be achieved with selenium metabolites such as selenocysteine and selenomethionine. Furthermore, we found that reducing agents, such as dithiothreitol and beta-mercaptoethanol, restored the cG3PDH activity suppressed by selenite and NO both in vitro and in cultured cells. Buthionine sulfoximine depleted levels of both reduced glutathione and the oxidized form but had no effect on the suppression of cG3PDH activity by selenite in cultured cells. Moreover, thiol-reactive agents, such as N-ethylmaleimide and o-iodosobenzoic acid, blocked the enzyme activity of cG3PDH through the modification of redox-sensitive cysteine residues in cG3PDH. The inhibitor of NO synthase, L-N(G)-nitro-arginine, restored the cG3PDH activity inhibited by NO in cultured cells, whereas the inhibitor of guanylyl cyclase, 1H-[1,2,4] oxadiazole[4,3-alpha] quinoxalin-1-one (ODQ), has no effect. NO directly inhibits cG3PDH activity not via a cGMP-dependent mechanism. Finally, using site-directed mutagenesis, we found that Cys(102) of cG3PDH was sensitive to both selenite and NO. From the results, we suggest that cG3PDH is a target of cellular redox regulation.

Nitric oxide enhancement of fludarabine cytotoxicity for B-CLL lymphocytes

Fludarabine is active but not curative in the treatment of chronic lymphocytic leukemia (B-CLL). Nitric oxide (NO) supplied from exogenous, NO-donating pro-drugs can also induce apoptosis and death of acute leukemia cells. This study investigated combinations of fludarabine with NO-donating pro-drugs for their cytotoxicity against freshly isolated B-CLL lymphocytes following a 72 h exposure in vitro. The median IC(50)for fludarabine was 2.2 microM (n = 85). The nitric oxide donors DETA-NO, PAPA-NO, and MAHMA-NO were also cytotoxic, and their effects were inversely related to rates of NO release. Neither DETA-NO depleted of NO nor DETA itself was effective, indicating that NO was required for cytotoxicity. Drug interactions were evaluated by a modified combination index method. Synergy was observed in combinations of fludarabine or nelarabine (506U78) with DETA-NO in 52% and 88% of samples, respectively. Interestingly, the combination of fludarabine and DETA-NO was more cytotoxic in B-CLL cells less sensitive to fludarabine. DETA-NO did not enhance the activity of other DNA anti-metabolites, topoisomerase I and II inhibitors, or alkylating agents. Finally, the anti-leukemic activity of fludarabine alone or in combination with DETA-NO was found to correlate with inhibition of cellular RNA synthesis. These results indicate that NO donors could enhance fludarabine therapy for B-CLL.

Is nitric oxide overproduction the target of choice for the management of septic shock?

Sepsis is a heterogeneous class of syndromes caused by a systemic inflammatory response to infection. Septic shock, a severe form of sepsis, is associated with the development of progressive damage in multiple organs, and is a leading cause of patient mortality in intensive care units. Despite important advances in understanding its pathophysiology, therapy remains largely symptomatic and supportive. A decade ago, the overproduction of nitric oxide (NO) had been discovered as a potentially important event in this condition. As a result, great hopes arose that the pharmacological inhibition of NO synthesis could be developed into an efficient, mechanism-based therapeutic approach. Since then, an extraordinary effort by the scientific community has brought a deeper insight regarding the feasibility of this goal. Here we present in summary form the present state of knowledge of the biological chemistry and physiology of NO. We then proceed to a systematic review of experimental and clinical data, indicating an up-regulation of NO production in septic shock; information on the role of NO in septic shock, as provided by experiments in transgenic mice that lack the ability to up-regulate NO production; effects of pharmacological inhibitors of NO production in various experimental models of septic shock; and relevant clinical experience. The accrued evidence suggests that the contribution of NO to the pathophysiology of septic shock is highly heterogeneous and, therefore, difficult to target therapeutically without appropriate monitoring tools, which do not exist at present.

Structural and kinetic modifications of aldose reductase by S-nitrosothiols

Modification of aldose reductase (AR) by the nitrosothiols S-nitroso-N-acetyl penicillamine (SNAP) and N-(beta-glucopyranosyl)-N(2)-acetyl-S-nitrosopenicillamide (glyco-SNAP) resulted in a 3-7-fold increase in its k(cat) and a 25-40-fold increase in its K(m) for glyceraldehyde. In comparison with the native protein, the modified enzyme was less sensitive to inhibition by sorbinil and was not activated by SO(2-)(4) anions. The active-site residue, Cys-298, was identified as the main site of modification, because the site-directed mutant in which Cys-298 was replaced by serine was insensitive to glyco-SNAP. The extent of modification was not affected by P(i) or O(2), indicating that it was not due to spontaneous release of nitric oxide (NO) by the nitrosothiols. Electrospray ionization MS revealed that the modification reaction proceeds via the formation of an N-hydroxysulphenamide-like adduct between glyco-SNAP and AR. In time, the adduct dissociates into either nitrosated AR (AR-NO) or a mixed disulphide between AR and glyco-N-acetylpenicillamine (AR-S-S-X). Removal of the mixed-disulphide form of the protein by lectin-column chromatography enriched the preparation in the high-K(m)-high-k(cat) form of the enzyme, suggesting that the kinetic changes are due to the formation of AR-NO, and that the AR-S-S-X form of the enzyme is catalytically inactive. Modification of AR by the non-thiol NO donor diethylamine NONOate (DEANO) increased enzyme activity and resulted in the formation of AR-NO. However, no adducts between AR and DEANO were formed. These results show that nitrosothiols cause multiple structural and functional changes in AR. Our observations also suggest the general possibility that transnitrosation reactions can generate both nitrosated and thiolated products, leading to non-unique changes in protein structure and function.

Discovery of a new inhibitor lead of adenovirus proteinase: steps toward selective, irreversible inhibitors of cysteine proteinases

Using the computer docking program EUDOC, in silico screening of a chemical database for inhibitors of human adenovirus cysteine proteinase (hAVCP) identified 2,4,5,7-tetranitro-9-fluorenone that selectively and irreversibly inhibits hAVCP in a two-step reaction: reversible binding (Ki = 3.09 microM) followed by irreversible inhibition (ki = 0.006 s(-1)). The reversible binding is due to molecular complementarity between the inhibitor and the active site of hAVCP, which confers the selectivity of the inhibitor. The irreversible inhibition is due to substitution of a nitro group of the inhibitor by the nearby Cys122 in the active site of hAVCP. These findings suggest a new approach to selective, irreversible inhibitors of cysteine proteinases involved in normal and abnormal physiological processes ranging from embryogenesis to apoptosis and pathogen invasions.

The regulatory role of nitric oxide in apoptosis

Nitric oxide (NO) is a multi-faceted molecule with dichotomous regulatory roles in many areas of biology. The complexity of its biological effects is a consequence of its numerous potential interactions with other molecules such as reactive oxygen species (ROS), metal ions, and proteins. The effects of NO are modulated by both direct and indirect interactions that can be dose-dependent and cell-type specific. For example, in some cell types NO can promote apoptosis, whereas in other cells NO inhibits apoptosis. In hepatocytes, NO can inhibit the main mediators of cell death-caspase proteases. Moreover, low physiological concentrations of NO can inhibit apoptosis, but higher concentrations of NO may be toxic. High NO concentrations lead to the formation of toxic reaction products like dinitrogen trioxide or peroxynitrite that induce cell death, if not by apoptosis, then by necrosis. Long-term exposure to nitric oxide in certain conditions like chronic inflammatory states may predispose cells to tumorigenesis through DNA damage, inhibition of DNA repair, alteration in programmed cell death, or activation of proliferative signaling pathways. Understanding the regulatory mechanisms of NO in apoptosis and carcinogenesis will provide important clues to the diagnosis and treatment of tissue damage and cancer. In this article we have reviewed recent discoveries in the regulatory role of NO in specific cell types, mechanisms of pro-apoptotic and anti-apoptotic induction by NO, and insights into the effects of NO on tumor biology.

Nitric oxide in immunity and inflammation

Nitric oxide (NO) is synthesised by many cell types involved in immunity and inflammation. The principal enzyme involved is the inducible type-2 isoform of nitric oxide synthase (NOS-2), which produces high-level sustained NO synthesis. NO is important as a toxic defense molecule against infectious organisms. It also regulates the functional activity, growth and death of many immune and inflammatory cell types including macrophages, T lymphocytes, antigen-presenting cells, mast cells, neutrophils and natural killer cells. However, the role of NO in nonspecific and specific immunity in vivo and in immunologically mediated diseases and inflammation is poorly understood. NO does not act through a receptor-its target cell specificity depends on its concentration, its chemical reactivity, the vicinity of target cells and the way that target cells are programmed to respond. At high concentrations as generated by NOS-2, NO is rapidly oxidised to reactive nitrogen oxide species (RNOS) that mediate most of the immunological effects of NOS-2-derived NO. RNOS can S-nitrosate thiols to modify key signalling molecules such as kinases and transcription factors. Several key enzymes in mitochondrial respiration are also inhibited by RNOS and this leads to a depletion of ATP and cellular energy. A combination of these interactions may explain the multiple actions of NO in the regulation of immune and inflammatory cells.

Metabolism of S-nitrosoglutathione in intact mitochondria

S-nitrosation of protein thiol groups by nitric oxide (NO*) is a widely recognized protein modification. Only few intracellular S-nitrosated proteins have been identified and it has been reported that S-nitrosation/denitrosation can serve as a regulatory process in signal-transduction pathways. Given the potential physiological importance of S-nitrosothiols, and considering that mitochondria are endowed with high levels of thiols and the biochemical requisites for synthesizing NO*, we examined the occurrence of S-nitrosoglutathione (GSNO) in intact, coupled rat liver mitochondria. These organelles contained 0.34 nmol of GSNO/mg of protein, detected by HPLC with UV-visible and electrochemical detections. This concentration was dynamically modulated by the availability of NO*; its decay was affected mainly by GSH and superoxide dismutase in a reaction that entailed the generation of GSSG. On the basis of the relatively long half-life of GSNO and the negligible recovery of NO* during its decay, roles for GSNO as a storage and transport molecule for NO* are discussed. Moreover, the formation of GSNO and its reaction with GSH can be considered to be partly responsible for the catabolism of NO* via a complex mechanism that might result in the formation of hydroxylamine, nitrite or nitrous oxide depending upon the availability of oxygen, superoxide dismutase and glutathione. Finally, the high concentrations of GSH in the cytosol and mitochondria might favour the formation of GSNO by reacting with NO* 'in excess', thereby avoiding damaging side reactions (such as peroxynitrite formation), and facilitate the inactivation of NO* by generating other nitrogen-related species without the chemical properties characteristic of NO*.

Equilibrium and kinetics studies of transnitrosation between S-nitrosothiols and thiols

Using UV-vis spectrometrical measurements, equilibrium constants for NO transfer between S-nitroso-N-acetyl-penicillamine (SNAP) and different thiols as well as kinetic data for NO transfer from S-nitroso bovine serum albumin (BSANO) to thiols have been obtained. NO transfer from SNAP to other primary/secondary thiols are thermodynamically favorable, whereas other S-nitrosothiols exhibit similar NO transfer potential. The obtained Gibbs free energy, enthalpy and entropy data indicated that NO transfer reactions from SNAP to four thiols are exothermic with entropy loss. The kinetic behavior of BSANO/RSH transfer can be related to both the acidity of sulfhydryl group and the electronic structure in thiol.

Glutathiolation of proteins by glutathione disulfide S-oxide derived from S-nitrosoglutathione. Modifications of rat brain neurogranin/RC3 and neuromodulin/GAP-43

S-Nitrosoglutathione (GSNO) undergoes spontaneous degradation that generates several nitrogen-containing compounds and oxidized glutathione derivatives. We identified glutathione sulfonic acid, glutathione disulfide S-oxide (GS(O)SG), glutathione disulfide S-dioxide, and GSSG as the major decomposition products of GSNO. Each of these compounds and GSNO were tested for their efficacies to modify rat brain neurogranin/RC3 (Ng) and neuromodulin/GAP-43 (Nm). Among them, GS(O)SG was found to be the most potent in causing glutathiolation of both proteins; four glutathiones were incorporated into the four Cys residues of Ng, and two were incorporated into the two Cys residues of Nm. Ng and Nm are two in vivo substrates of protein kinase C; their phosphorylations by protein kinase C attenuate the binding affinities of both proteins for calmodulin. When compared with their respective unmodified forms, the glutathiolated Ng was a poorer substrate and glutathiolated Nm a better substrate for protein kinase C. Glutathiolation of these two proteins caused no change in their binding affinities for calmodulin. Treatment of [(35)S]cysteine-labeled rat brain slices with xanthine/xanthine oxidase or a combination of xanthine/xanthine oxidase with sodium nitroprusside resulted in an increase in cellular level of GS(O)SG. These treatments, as well as those by other oxidants, all resulted in an increase in thiolation of proteins; among them, thiolation of Ng was positively identified by immunoprecipitation. These results show that GS(O)SG is one of the most potent glutathiolating agents generated upon oxidative stress.

Evidence that Alzheimer's disease is a microvascular disorder: the role of constitutive nitric oxide

Evidence is fast accumulating which indicates that Alzheimer's disease is a vascular disorder with neurodegenerative consequences rather than a neurodegenerative disorder with vascular consequences. It is proposed that two factors need to be present for AD to develop: (1) advanced ageing, (2) presence of a condition that lowers cerebral perfusion, such as a vascular-risk factor. The first factor introduces a normal but potentially insidious process that lowers cerebral blood flow in inverse relation to increased ageing; the second factor adds a crucial burden which further lowers brain perfusion and places vulnerable neurons in a state of high energy compromise leading to a cascade of neuronal metabolic turmoil. Convergence of the two factors above will culminate in a critically attained threshold of cerebral hypoperfusion (CATCH). CATCH is a hemodynamic microcirculatory insufficiency that will destabilize neurons, synapses, neurotransmission and cognitive function, creating in its wake a neurodegenerative state characterized by the formation of senile plaques, neurofibrillary tangles, amyloid angiopathy and in some cases, Lewy bodies. Since any of a considerable number of vascular-related conditions must be present in the ageing individual for cognition to be disturbed, CATCH identifies an important aspect of the heterogeneic disease profile assumed to be present in the AD syndrome. It is proposed that CATCH initiates AD by distorting regional brain capillary structure involving endothelial cell shape changes and impairment of nitric oxide (NO) release which affect signaling between the immune, cardiovascular and nervous systems. Evidence is presented that in many tissues there is a basal level of NO being produced and that the actions of several signaling molecules may initiate increases in basal NO levels. Moreover, these temporary increases in basal NO levels exert inhibitory cellular actions, via cellular conformational changes. Findings indicate that (a) constitutive NO is responsible for a basal or 'tonal' level of NO; (b) this NO keeps particular types of cells in a state of inhibition and (c) activation of these cells occurs through disinhibition. Consequently, tissues not maintaining a basal NO level are more prone to excitatory, immune, vascular and neural influences. Under such circumstances, these tissues cannot be down-regulated to normal basal levels, thus prolonging their excitatory state. Thus, the clinical convergence of advanced ageing in the presence of a chronic, pre-morbid vascular risk factor, can, in time, contribute to an endotheliopathy involving basal NO deficit, to the degree where regional metabolic dysfunction leads to cognitive meltdown and to progressive neurodegeneration characteristic of Alzheimer's disease.

Cadmium-dependent enzyme activity alteration is not imputable to lipid peroxidation

The effect of cadmium on the liver-specific activities of NADPH-cytochrome P450 reductase (CPR), malic dehydrogenase (MDH), glyceraldehyde-3-phosphate dehydrogenase (GADPH), and sorbitol dehydrogenase (SDH) was assessed 6, 24, and 48 h after administration of the metal to rats (2.5 mg/kg of body weight, as CdCl2, single ip injection). CPR specific activity increased after 6 h and afterward decreased significantly, while MDH specific activity increased up to 24 h and then remained unchanged. Both SDH and GADPH specific activities reduced after 6 h, the former only a little but the latter much more, and after 24 and 48 h were strongly inhibited. In vitro experiments, by incubating rat liver microsomes, mitochondria, or cytosol with CdCl2 in the pH range 6.0-8.0, excluded cadmium-induced lipid peroxidation as the cause of the reduction in enzyme activity. In addition, from these experiments, we obtained indications on the type of interactions between cadmium and the enzymes studied. In the case of CPR, the inhibitory effect is probably due to Cd2+ binding to the histidine residue of the apoenzyme, which, at physiological pH, acts as a nucleophilic group. In vitro, mitochondrial MDH was not significantly affected by cadmium at any pH, indicating that this enzyme is probably not involved in the decrease in mitochondrial respiration caused by this metal. As for GADPH specific activity, its inhibition at pH 7.4 and above is imputable to the binding of cadmium to the SH groups present in the enzyme active site, since in the presence of dithiothreitol this inhibition was removed. SDH was subjected to a dual effect when cytosol was exposed to cadmium. At pH 6.0 and 6.5, its activity was strongly stimulated up to 75 microM CdCl2 while at higher metal concentrations it was reduced. At pH 7.4 and 8.0, a stimulation up to 50 microM CdCl2 occurred but above this concentration, a reduction was found. These data seem to indicate that cadmium can bind to different enzyme sites. One, at low cadmium concentration, stimulates the SDH activity while the other, at higher metal concentrations, substitutes for zinc, thus causing inhibition. This last possibility seems to occur in vivo essentially at least 24 h after intoxication. The cadmium-induced alterations of the investigated enzymes are discussed in terms of the metabolic disorders produced which are responsible for several pathological conditions.

Nitric-oxide-induced necrosis and apoptosis in PC12 cells mediated by mitochondria

Nitric oxide (NO) can trigger either necrotic or apoptotic cell death. We have used PC12 cells to investigate the extent to which NO-induced cell death is mediated by mitochondria. Addition of NO donors, 1 mM S-nitroso-N-acetyl-DL-penicillamine (SNAP) or 1 mM diethylenetriamine-NO adduct (NOC-18), to PC12 cells resulted in a steady-state level of 1-3 microM: NO, rapid and almost complete inhibition of cellular respiration (within 1 min), and a rapid decrease in mitochondrial membrane potential within the cells. A 24-h incubation of PC12 cells with NO donors (SNAP or NOC-18) or specific inhibitors of mitochondrial respiration (myxothiazol, rotenone, or azide), in the absence of glucose, caused total ATP depletion and resulted in 80-100% necrosis. The presence of glucose almost completely prevented the decrease in ATP level and the increase in necrosis induced by the NO donors or mitochondrial inhibitors, suggesting that the NO-induced necrosis in the absence of glucose was due to the inhibition of mitochondrial respiration and subsequent ATP depletion. However, in the presence of glucose, NO donors and mitochondrial inhibitors induced apoptosis of PC12 cells as determined by nuclear morphology. The presence of apoptotic cells was prevented completely by benzyloxycarbonyl-Val-Ala-fluoromethyl ketone (a nonspecific caspase inhibitor), indicating that apoptosis was mediated by caspase activation. Indeed, both NO donors and mitochondrial inhibitors in PC12 cells caused the activation of caspase-3- and caspase-3-processing-like proteases. Caspase-1 activity was not activated. Cyclosporin A (an inhibitor of the mitochondrial permeability transition pore) decreased the activity of caspase-3- and caspase-3-processing-like proteases after treatment with NO donors, but was not effective in the case of the mitochondrial inhibitors. The activation of caspases was accompanied by the release of cytochrome c from mitochondria into the cytosol, which was partially prevented by cyclosporin A in the case of NO donors. These results indicate that NO donors (SNAP or NOC-18) may trigger apoptosis in PC12 cells partially mediated by opening the mitochondrial permeability transition pores, release of cytochrome c, and subsequent caspase activation. NO-induced apoptosis is blocked completely in the absence of glucose, probably due to the lack of ATP. Our findings suggest that mitochondria may be involved in both types of cell death induced by NO donors: necrosis by respiratory inhibition and apoptosis by opening the permeability transition pore. Further, our results indicate that the mode of cell death (necrosis versus apoptosis) induced by either NO or mitochondrial inhibitors depends critically on the glycolytic capacity of the cell.

NO synthase and NO-dependent signal pathways in brain aging and neurodegenerative disorders: the role of oxidant/antioxidant balance

Nitric oxide and other reactive nitrogen species appear to play several crucial roles in the brain. These include physiological processes such as neuromodulation, neurotransmission and synaptic plasticity, and pathological processes such as neurodegeneration and neuroinflammation. There is increasing evidence that glial cells in the central nervous system can produce nitric oxide in vivo in response to stimulation by cytokines and that this production is mediated by the inducible isoform of nitric oxide synthase. Although the etiology and pathogenesis of the major neurodegenerative and neuroinflammatory disorders (Alzheimer's disease, amyothrophic lateral sclerosis, Parkinson's disease, Huntington's disease and multiple sclerosis) are unknown, numerous recent studies strongly suggest that reactive nitrogen species play an important role. Furthermore, these species are probably involved in brain damage following ischemia and reperfusion, Down's syndrome and mitochondrial encephalopathies. Recent evidence also indicates the importance of cytoprotective proteins such as heat shock proteins (HSPs) which appear to be critically involved in protection from nitrosative and oxidative stress. In this review, evidence for the involvement of nitrosative stress in the pathogenesis of the major neurodegenerative/ neuroinflammatory diseases and the mechanisms operating in brain as a response to imbalance in the oxidant/antioxidant status are discussed.

Depletion of intracellular glutathione reduces mutations by nitric oxide-donating drugs

The Mutatect system is a mouse tumor line in which mutations at the hypoxanthine phosphoribosyltransferase (Hprt) locus can be readily detected both in vitro and in vivo. We have previously shown that the nitric oxide-generating drugs, glyceryl trinitrate (GTN) and sodium nitroprusside (SNP), can induce mutations that are readily detected in these cells. In the present report, we have tested the effect of glutathione depletion by buthionine sulfoximine (BSO) on cytotoxicity and mutagenicity by these two drugs. Exposure for 24 h to either drug (123 microM GTN; 500 microM SNP) induced mutations with relatively little cytotoxicity. Pretreatment with 50 microM BSO for 24 h, and then removal at the time of GTN or SNP addition, enhanced cytotoxicity to a modest extent. However, mutagenicity induced by both GTN and SNP was largely abolished. BSO did not affect nitrite accumulation in the medium over a 24-h period, indicating no inhibition of bioactivation of GTN or SNP. Maintaining BSO in the medium for 24 h prior and throughout the period of exposure to GTN or SNP produced a similar effect on mutations. N-Acetylcysteine and oxothiazolidine-4-carboxylate, drugs that are used to increase intracellular glutathione, also blocked mutations. We postulate that a product of the reaction between nitric oxide and intracellular glutathione, such as GSNO or some species derived from it, is promutagenic.

S-nitrosothiols as novel, reversible inhibitors of human rhinovirus 3C protease

Human rhinovirus (HRV) 3C protease was inactivated by a series of S-nitrosothiols. These compounds exhibited different inhibitory activities in a time- and concentration-dependent manner with second-order rate constants (kinact/K(I)) ranging from 131 to 5360 M(-1) min(-1). The inactive enzyme could be re-activated by DTT, GSH and ascorbate, which indicated the inactivation mechanism was through an S-transnitrosylation process.

Nitric oxide modulates stretch activation of mitogen-activated protein kinases in mesangial cells

BACKGROUND

In vivo, intraglomerular hypertension results in resident cell hypertrophy, proliferation and matrix protein production, leading to glomerulosclerosis. Mesangial cells (MCs) exposed to in vitro stretch also proliferate and produce matrix. We have shown activation of Jun N-terminal kinase/stress-activated protein kinase (SAPK) and p42/44 mitogen-activated protein kinase (MAPK) in stretched MCs and have also demonstrated that L-arginine decreases resident cell proliferation and protects against glomerulosclerosis in remnant kidney glomeruli, presumably by increasing nitric oxide (NO) production. Consequently, we studied whether NO could affect SAPK and p42/44 MAPK activation in stretched MCs.

METHODS

MCs (passages 5 to 10) cultured on type 1 collagen-coated, flexible-bottom plates were exposed to 0 to 30 minutes of cyclic strain (60 cycles per minute) by computer-driven generation of vacuum of -27 kPa, inducing 28% elongation in the diameter of the surface. Control MCs were grown on coated, flexible-bottom plates. Protein levels (by Western blot) and activity assays for SAPK/JNK and p42/44 MAPK were performed under these conditions. As maximal activation was at 10 minutes, with decay by 30 minutes, the effect of NO on kinase activation was studied at 0, 2, 5, and 10 minutes by preincubation with 70 micromol/L s-nitroso-n-acetylpenicillamine (SNAP; an NO donor) or 1 mmol/L 8-bromo cyclic guanosine monophosphate (8-bromo-cGMP). Downstream events in response to stretch and NO were studied at the time of maximal response (10 minutes) by examining nuclear translocation of SAPK with immunofluorescence microscopy and transcription factor activator protein-1 nuclear protein binding by gel mobility shift assay. The effect of kinase inhibition by NO donors on MC proliferation was studied by Western blotting for proliferating cell nuclear antigen (PCNA).

RESULTS

Cyclic MC stretch led to prompt SAPK and p42/44 MAPK activation, which was maximal at 10 minutes. Preincubation with either SNAP or 8-bromo-cGMP decreased this by 50 and 70%, respectively (N = 4), suggesting that the effect of NO was through cGMP generation. Nuclear translocation of both phosphorylated kinases was seen after 10 minutes of stretch and was largely prevented by 8-bromo-cGMP. Increased DNA binding of activator protein-1 proteins was observed in the nuclei of stretched MCs at 10 minutes by mobility shift assay (N = 4), which was also largely prevented by 8-bromo-cGMP. Stretch increased PCNA expression by MCs, and this was inhibited by 8-bromo-cGMP.

CONCLUSIONS

Stretch-induced activation of SAPK and p42/44 MAPK in MCs can be inhibited by NO. The effect of NO is mediated by the generation of cGMP. These mechanisms may be responsible, at least in part, for the protective effect of NO in animal models of glomerular injury characterized by glomerular capillary hypertension.

Oxidant-induced S-glutathiolation inactivates protein kinase C-alpha (PKC-alpha): a potential mechanism of PKC isozyme regulation

Protein kinase C (PKC) isozymes are subject to inactivation by reactive oxygen species (ROS) through as yet undefined oxidative modifications of the isozyme structure. We previously reported that Cys-containing, Arg-rich peptide-substrate analogues spontaneously form disulfide-linked complexes with PKC isozymes, resulting in isozyme inactivation. This suggested that PKC might be inactivated by oxidant-induced S-glutathiolation, i.e., disulfide linkage of the endogenous molecule glutathione (GSH) to PKC. Protein S-glutathiolation is a reversible oxidative modification that has profound effects on the activity of certain enzymes and binding proteins. To directly examine whether PKC could be inactivated by S-glutathiolation, we used the thiol-specific oxidant diamide because its oxidant activity is restricted to induction of disulfide bridge formation. Diamide weakly inactivated purified recombinant cPKC-alpha, and this was markedly potentiated to nearly full inactivation by 100 microM GSH, which by itself was without effect on cPKC-alpha activity. Diamide inactivation of cPKC-alpha and its potentiation by GSH were both fully reversed by DTT. Likewise, GSH markedly potentiated diamide inactivation of a PKC isozyme mixture purified from rat brain (alpha, beta, gamma, epsilon, zeta) in a DTT-reversible manner. GSH potentiation of diamide-induced cPKC-alpha inactivation was associated with S-glutathiolation of the isozyme. cPKC-alpha S-glutathiolation was demonstrated by the DTT-reversible incorporation of [(35)S]GSH into the isozyme structure and by an associated change in the migration position of cPKC-alpha in nonreducing SDS-PAGE. Diamide treatment of NIH3T3 cells likewise induced potent, DTT-reversible inactivation of cPKC-alpha in association with [(35)S] S-thiolation of the isozyme. Taken together, the results indicate that PKC isozymes can be oxidatively inactivated by S-thiolation reactions involving endogenous thiols such as GSH.

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.

Novel application of S-nitrosoglutathione-Sepharose to identify proteins that are potential targets for S-nitrosoglutathione-induced mixed-disulphide formation

Site-specific S-glutathionylation is emerging as a novel mechanism by which S-nitrosoglutathione (GSNO) may modify functionally important protein thiols. Here, we show that GSNO-Sepharose mimicks site-specific S-glutathionylation of the transcription factors c-Jun and p50 by free GSNO in vitro. Both c-Jun and p50 were found to bind to immobilized GSNO through the formation of a mixed disulphide, involving a conserved cysteine residue located in the DNA-binding domains of these transcription factors. Furthermore, we show that c-Jun, p50, glycogen phosphorylase b, glyceraldehyde-3-phosphate dehydrogenase, creatine kinase, glutaredoxin and caspase-3 can be precipitated from a mixture of purified thiol-containing proteins by the formation of a mixed-disulphide bond with GSNO-Sepharose. With few exceptions, protein binding to this matrix correlated well with the susceptibility of the investigated proteins to undergo GSNO- but not diamide-induced mixed-disulphide formation in vitro. Finally, it is shown that covalent GSNO-Sepharose chromatography of HeLa cell nuclear extracts results in the enrichment of proteins which incorporate glutathione in response to GSNO treatment. As suggested by DNA-binding assays, this group of nuclear proteins include the transcription factors activator protein-1, nuclear factor-kappaB and cAMP-response-element-binding protein. In conclusion, we introduce GSNO-Sepharose as a probe for site-specific S-glutathionylation and as a novel and potentially useful tool to isolate and identify proteins which are candidate targets for GSNO-induced mixed-disulphide formation.

Inhibition of papain by S-nitrosothiols. Formation of mixed disulfides

S-Nitrosylation of protein thiols is one of the cellular regulatory mechanisms induced by NO. The cysteine protease papain has a critical thiol residue (Cys(25)). It has been demonstrated that NO or NO donors such as sodium nitroprusside and N-nitrosoaniline derivatives can reversibly inhibit this enzyme by S-NO bond formation in its active site. In this study, a different regulated mechanism of inactivation was reported using S-nitrosothiols as the NO donor. Five S-nitroso compounds, S-nitroso-N-acetyl-dl-penicillamine, S-nitrosoglutathione, S-nitrosocaptopril, glucose-S-nitroso-N-acetyl-dl-penicillamine-2, and the S-nitroso tripeptide acetyl-Phe-Gly-S-nitrosopenicillamine, exhibited different inhibitory activities toward the enzyme in a time- and concentration-dependent manner with second-order rate constants (k(i)/K(I)) ranging from 8.9 to 17.2 m(-1) s(-1). The inhibition of papain by S-nitrosothiol was rapidly reversed by dithiothreitol, but not by ascorbate, which could reverse the inhibition of papain by NOBF(4). Incubation of the enzyme with a fluorescent S-nitroso probe (S-nitroso-5-dimethylaminonaphthalene-1-sulfonyl) resulted in the appearance of fluorescence of the protein, indicating the formation of a thiol adduct. Moreover, S-transnitrosylation in the incubation of S-nitroso inactivators with papain was excluded. These results suggest that inactivation of papain by S-nitrosothiols is due to a direct attack of the highly reactive thiolate (Cys(25)) in the enzyme active site on the sulfur of S-nitrosothiols to form a mixed disulfide between the inactivator and papain.

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.

Biological chemistry and clinical potential of S-nitrosothiols

S-Nitrosothiols are endogenous metabolites of nitric oxide that have been detected in extra- and intracellular spaces. Many biological functions of S-nitrosothiols have been described that can be categorized as being due to one or more of the following: (i) nitric oxide release, (ii) transnitrosation, (iii) S-thiolation, and (iv) direct action. This emphasizes the fact that S-nitrosothiols are more than simply nitric oxide donors. Many of the biological functions that have been described for S-nitrosothiols have clinical correlates. This review describes the biological chemistry, biological actions, and clinical potential of these compounds.

Biology of nitric oxide signaling

The free radical nitric oxide (NO) has emerged in recent years as a fundamental signaling molecule for the maintenance of homeostasis, as well as a potent cytotoxic effector involved in the pathogenesis of a wide range of human diseases. Although this paradoxical fate has generated confusion, separating the biological actions of NO on the basis of its physiologic chemistry provides a conceptual framework which helps to distinguish between the beneficial and toxic consequences of NO, and to envision potential therapeutic strategies for the future. Under normal conditions, NO produced in low concentration acts as a messenger and cytoprotective (antioxidant) factor, via direct interactions with transition metals and other free radicals. Alternatively, when the circumstances allow the formation of substantial amounts of NO and modify the cellular microenvironment (formation of the superoxide radical), the chemistry of NO will turn into indirect effects consecutive to the formation of dinitrogen trioxide and peroxynitrite. These "reactive nitrogen species" will, in turn, mediate both oxidative and nitrosative stresses, which form the basis of the cytotoxicity generally attributed to NO, relevant to the pathophysiology of inflammation, circulatory shock, and ischemia-reperfusion injury.

S-nitrosylated human alpha(1)-protease inhibitor

Alpha(1)-protease inhibitor (alpha(1)PI) is an acute phase plasma protein, and possesses a single cysteine residue at position 232. A single cysteinyl sulfhydryl of human alpha(1)PI is found to be readily S-nitrosylated by nitric oxide (NO) in vitro without affecting the inhibitory capacity against bovine trypsin or elastase, a major target protease of alpha(1)PI in vivo. S-nitroso-alpha(1)PI (S-NO-alpha(1)PI) was also formed by the reaction of alpha(1)PI with NO produced excessively by a murine macrophage cell line (RAW264 cells) upon infection with Salmonella typhimurium and in an ex vivo perfusion system of the liver obtained from lipopolysaccharide-treated rats. S-NO-alpha(1)PI (10(-9)-10(-6) M) induces a dose-dependent relaxation of the ring preparation of rabbit aorta. Also, S-NO-alpha(1)PI but not alpha(1)PI shows a potent inhibitory effect on platelet aggregation. Unprecedented observation is that S-NO-alpha(1)PI showed a potent bacteriostatic effect against a wide range of bacteria at the concentration of 1-10 microM, which was 10-1000-fold stronger than that of NO and other S-nitrosylated compounds including S-nitrosylated albumin and S-nitrosylated glutathione. These results suggest that S-NO-alpha(1)PI is produced as an NO sink under inflammatory conditions, where production of both alpha(1)PI and NO is highly up-regulated, and it may function as a soluble factor which consists of an innate defense system through not only the protease inhibitory activity but also its antibacterial activity and facilitating the peripheral blood flow. Therefore, S-nitrosylation of alpha(1)PI occurring under physiological conditions in vivo should diversify the biological functions contributing to cytoprotective effects of alpha(1)PI.

Inhibition of protein tyrosine phosphatases by low-molecular-weight S-nitrosothiols and S-nitrosylated human serum albumin

The homogeneous recombinant mammalian protein tyrosine phosphatase 1B (PTP1B) and Yersinia protein tyrosine phosphatase (PTPase) are inactivated by a series of low-molecular-weight S-nitrosothiols. These compounds exhibited different inhibitory activities in a time- and concentration-dependent manner with second-order rate constants (k(inact)/K(I)) ranging from 37 to 113 M(-1) min(-1) against mammalian PTP1B and from 66 to 613 M(-1) min(-1) against Yersinia PTPase. Furthermore, the inactivation of Yersinia PTPase by S-nitrosylated protein:S-nitroso human serum albumin was investigated. Both single-S-nitrosylated and poly-S-nitrosylated human serum albumin show good inhibitory ability to Yersinia PTPase. The second-order rate constants are 472 and 1188 M(-1) min(-1), respectively. This result indicates a possibility that S-nitrosylated albumin in vivo may function as an inhibitor for a variety of cysteine-dependent enzymes.

Responses to peroxynitrite in yeast: glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as a sensitive intracellular target for nitration and enhancement of chaperone expression and ubiquitination

Peroxynitrite (ONOO-), a potent oxidizing and nitrating species, has been linked to covalent modifications of biomolecules in a number of pathological conditions. In S. cerevisiae, a model eukaryotic cell system, ONOO- was found to be more potent than hydrogen peroxide in oxidizing thiols, inducing heat shock proteins (Hsp70) and enhancing the ubiquitination of proteins. As identified by microsequence analysis following immunoprecipitation with anti-nitrotyrosine antibodies, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was especially susceptible to nitration by ONOO- in yeast cells. The activity of this enzyme was strongly inhibited upon steady-state exposure of the cells to low doses of ONOO- in yeast and in cultured rat astrocytes. Thus, ONOO- is a potent stressor in yeast capable of inducing oxidative damage and protein nitration, with GAPDH being a preferential target protein that is efficiently inactivated.

Nitric oxide nitrates tyrosine residues of tumor-suppressor p53 protein in MCF-7 cells

It has been reported that mammalian cells incubated with excess nitric oxide (NO) accumulate p53 protein but concomitantly this p53 loses its capacity for binding to its DNA consensus sequence. As nitration of tyrosine residues in various proteins has been shown to inhibit their functions, we examined whether NO nitrates tyrosine residues in p53 protein. MCF-7 cells expressing wild-type p53 were incubated with S-nitrosoglutathione for 4 h and cellular extracts were immunoprecipitated with an anti-p53 antibody. Western blot analyses of immunoprecipitates for p53 or for nitrotyrosine revealed low levels of nitrotyrosine in p53 from untreated cells. Incubation with 2 mM S-nitrosoglutathione induced a significant increase in the nitrotyrosine level in p53 protein compared to nontreated cells. These results suggest that excess NO produced in inflamed tissues could nitrate p53 protein, playing a role in carcinogenesis by impairing functions of this tumor-suppressor protein.

Glutathione and its role in cellular functions

Glutathione (GSH) is the major cellular thiol participating in cellular redox reactions and thioether formation. This article serves as introduction to the FRBM Forum on glutathione and emphasizes cellular functions: What is GSH? Where does it come from? Where does it go? What does it do? What is new and noteworthy? Research tools, historical remarks, and links to current trends.

Inhibition of cathepsin K by nitric oxide donors: evidence for the formation of mixed disulfides and a sulfenic acid

The cysteine protease cathepsin K is believed to play a key role in bone resorption as it has collagenolytic activity and is expressed predominantly and in high levels in bone resorbing osteoclast cells. The addition of nitric oxide (NO) and NO donors to osteoclasts in vitro results in a reduction of bone resorption, although the mechanism of this effect is not fully understood. The S-nitroso derivatives of glutathione (GSNO) and N-acetylpenicillamine (SNAP) and the non-thiol NO donors NOR-1 and NOR-3 all inhibited the activity of purified cathepsin K in a time- and concentration-dependent manner (IC(50) values after 15 min of preincubation at pH 7.5 of 28, 105, 0.4, and 10 microM, respectively). Cathepsin K activity in Chinese hamster ovary cells stably transfected with cathepsin K was also inhibited by the above NO donors with similar potencies. GSNO at 100 microM also completely inhibited the autocatalytic maturation at pH 4.0 of procathepsin K to cathepsin K. The inhibition of cathepsin K by GSNO was rapidly reversed by DTT, but inhibition by NOR-1 was not reversed by DTT, and analysis of the inhibited cathepsin K for S-nitrosylation using the Greiss reaction gave negative results in both cases. Analysis of the protein by electrospray liquid chromatography/mass spectrometry showed that the inhibition of cathepsin K by GSNO resulted in a mass increase of 306 +/- 2 Da, consistent with the formation of a glutathione adduct. Prior inhibition of cathepsin K by the active site thiol-modifying inhibitor E-64 blocked the modification by GSNO, indicating that the glutathione adduct is likely formed at the active site cysteine. Treatment of cathepsin K with NOR-1 resulted in a mass increase of between 30 and 50 Da, corresponding to the oxidation of a cysteine to sulfinic and sulfonic acids. Cotreatment of cathepsin K with NOR-1 plus the sulfenic acid reagent dimedone resulted in a mass increase of approximately 141 Da, which is consistent with the formation of a dimedone adduct. This result demonstrates that the NOR-1-dependent formation of cathepsin K sulfinic and sulfonic acids occurs via a sulfenic acid. These results show that inhibition of cathepsin K activity and its autocatalytic maturation represent two potential mechanisms by which NO can exert its inhibitory effect on bone resorption. This work also shows that oxidative thiol modifications besides S-nitrosylation should be considered when the effects of NO and NO donors on critical thiol-containing proteins are investigated.