Studying the S-nitrosylation of model peptides and eNOS protein by mass spectrometry

NO and Heme Proteins: Cross-Talk between Heme and Cysteine Residues

Heme proteins are a diverse group that includes several unrelated families. Their biological function is mainly associated with the reactivity of the heme group, which-among several other reactions-can bind to and react with nitric oxide (NO) and other nitrogen compounds for their production, scavenging, and transport. The S-nitrosylation of cysteine residues, which also results from the reaction with NO and other nitrogen compounds, is a post-translational modification regulating protein activity, with direct effects on a variety of signaling pathways. Heme proteins are unique in exhibiting this dual reactivity toward NO, with reported examples of cross-reactivity between the heme and cysteine residues within the same protein. In this work, we review the literature on this interplay, with particular emphasis on heme proteins in which heme-dependent nitrosylation has been reported and those for which both heme nitrosylation and S-nitrosylation have been associated with biological functions.

Computational prediction of NO-dependent posttranslational modifications in plants: Current status and perspectives

The perception and transduction of nitric oxide (NO) signal is achieved by NO-dependent posttranslational modifications (PTMs) among which S-nitrosation and tyrosine nitration has biological significance. In plants, 100-1000 S-nitrosated and tyrosine nitrated proteins have been identified so far by mass spectrometry. The determination of NO-modified protein targets/amino acid residues is often methodologically challenging. In the past decade, the growing demand for the knowledge of S-nitrosated or tyrosine nitrated sites has motivated the introduction of bioinformatics tools. For predicting S-nitrosation seven computational tools have been developed (GPS-SNO, SNOSite, iSNO-PseACC, iSNO-AAPAir, PSNO, PreSNO, RecSNO). Four predictors have been developed for indicating tyrosine nitration sites (GPS-YNO2, iNitro-Tyr, PredNTS, iNitroY-Deep), and one tool (DeepNitro) predicts both NO-dependent PTMs. The advantage of these computational tools is the fast provision of large amount of information. In this review, the available software tools have been tested on plant proteins in which S-nitrosated or tyrosine nitrated sites have been experimentally identified. The predictors showed distinct performance and there were differences from the experimental results partly due to the fact that the three-dimensional protein structure is not taken into account by the computational tools. Nevertheless, the predictors excellently establish experiments, and it is suggested to apply all available tools on target proteins and compare their results. In the future, computational prediction must be developed further to improve the precision with which S-nitrosation/tyrosine nitration-sites are identified.

Hydrogen sulfide, an enhancer of vascular nitric oxide signaling: mechanisms and implications

Nitric oxide (NO) vascular signaling has long been considered an independent, self-sufficient pathway. However, recent data indicate that the novel gaseous mediator, hydrogen sulfide (H₂S), serves as an essential enhancer of vascular NO signaling. The current article overviews the multiple levels at which this enhancement takes place. The first level of interaction relates to the formation of biologically active hybrid S/N species and the H₂S-induced stimulation of NO release from its various stable "pools" (e.g., nitrite). The next interactions occur on the level of endothelial calcium mobilization and PI3K/Akt signaling, increasing the specific activity of endothelial NO synthase (eNOS). The next level of interaction occurs on eNOS itself; H₂S directly interacts with the enzyme: sulfhydration of critical cysteines stabilizes it in its physiological, dimeric state, thereby optimizing eNOS-derived NO production and minimizing superoxide formation. Yet another level of interaction, further downstream, occurs at the level of soluble guanylate cyclase (sGC): H₂S stabilizes sGC in its NO-responsive, physiological, reduced form. Further downstream, H₂S inhibits the vascular cGMP phosphodiesterase (PDE5), thereby prolonging the biological half-life of cGMP. Finally, H₂S-derived polysulfides directly activate cGMP-dependent protein kinase (PKG). Taken together, H₂S emerges an essential endogenous enhancer of vascular NO signaling, contributing to vasorelaxation and angiogenesis. The functional importance of the H₂S/NO cooperative interactions is highlighted by the fact that H₂S loses many of its beneficial cardiovascular effects when eNOS is inactive.

Nitrosothiol formation and S-nitrosation signaling through nitric oxide synthases

Nitric oxide (NO) is a gaseous signaling molecule impacting many biological pathways. NO is produced in mammals by three nitric oxide synthase (NOS) isoforms: neuronal (nNOS), endothelial (eNOS), and inducible (iNOS). nNOS and eNOS produce low concentrations of NO for paracrine signaling; NO produced and released from one cell diffuses to a neighboring cell where it binds and activates soluble guanylyl cyclase (sGC). iNOS produces high concentrations of NO using NO toxicity to amplify the innate immune response. Recent work has also defined protein cysteine S-nitrosation as a pathway of sGC-independent NO signaling. Though many studies have shown that S-nitrosation regulates the activity of NOS isoforms and other proteins in vivo, many issues need to be resolved to establish S-nitrosation as a viable signaling mechanism. Several chemical mechanisms result in S-nitrosation including transition metal-catalyzed pathways, NO oxidation followed by thiolate reaction, and thiyl radical recombination with NO. Once formed, nitrosothiols can be transferred between cellular cysteine residues via transnitrosation reactions. However, it is largely unclear how these chemical processes result in selective S-nitrosation of specific cellular cysteine residues. S-nitrosation site selectivity may be imparted via direct interactions or colocalization with NOS isoforms that focus chemical or transnitrosation mechanisms of nitrosothiol formation or transfer. Here, we discuss chemical mechanisms of nitrosothiol formation, S-nitrosation of NOS isoforms, and potential S-nitrosation signaling cascades resulting from NOS S-nitrosation.

Identification of nuclear target proteins for S-nitrosylation in pathogen-treated Arabidopsis thaliana cell cultures

Nitric oxide (NO) is a significant signalling molecule involved in the regulation of many different physiological processes in plants. One of the most imperative regulatory modes of action of NO is protein S-nitrosylation--the covalent attachment of an NO group to the sulfur atom of cysteine residues. In this study, we focus on S-nitrosylation of Arabidopsis nuclear proteins after pathogen infection. After treatment of Arabidopsis suspension cell cultures with pathogens, nuclear proteins were extracted and treated with the S-nitrosylating agent S-nitrosoglutathione (GSNO). A biotin switch assay was performed and biotin-labelled proteins were purified by neutravidin affinity chromatography and identified by mass spectrometry. A total of 135 proteins were identified, whereas nuclear localization has been described for 122 proteins of them. 117 of these proteins contain at least one cysteine residue. Most of the S-nitrosylated candidates were involved in protein and RNA metabolism, stress response, and cell organization and division. Interestingly, two plant-specific histone deacetylases were identified suggesting that nitric oxide regulated epigenetic processes in plants. In sum, this work provides a new collection of targets for protein S-nitrosylation in Arabidopsis and gives insight into the regulatory function of NO in the nucleus during plant defense response. Moreover, our data extend the knowledge on the regulatory function of NO in events located in the nucleus.

Regulation of eNOS enzyme activity by posttranslational modification

The regulation of endothelial NO synthase (eNOS) employs multiple different cellular control mechanisms impinging on level and activity of the enzyme. This review aims at summarizing the current knowledge on the posttranslational modifications of eNOS, including acylation, nitrosylation, phosphorylation, acetylation, glycosylation and glutathionylation. Sites, mediators and impact on enzyme localization and activity of the single modifications will be discussed. Moreover, interdependence, cooperativity and competition between the different posttranslational modifications will be elaborated with special emphasis on the susceptibility of eNOS to metabolic cues.

Computational prediction of candidate proteins for S-nitrosylation in Arabidopsis thaliana

Nitric oxide (NO) is an important signaling molecule that regulates many physiological processes in plants. One of the most important regulatory mechanisms of NO is S-nitrosylation-the covalent attachment of NO to cysteine residues. Although the involvement of cysteine S-nitrosylation in the regulation of protein functions is well established, its substrate specificity remains unknown. Identification of candidates for S-nitrosylation and their target cysteine residues is fundamental for studying the molecular mechanisms and regulatory roles of S-nitrosylation in plants. Several experimental methods that are based on the biotin switch have been developed to identify target proteins for S-nitrosylation. However, these methods have their limits. Thus, computational methods are attracting considerable attention for the identification of modification sites in proteins. Using GPS-SNO version 1.0, a recently developed S-nitrosylation site-prediction program, a set of 16,610 candidate proteins for S-nitrosylation containing 31,900 S-nitrosylation sites was isolated from the entire Arabidopsis proteome using the medium threshold. In the compartments "chloroplast," "CUL4-RING ubiquitin ligase complex," and "membrane" more than 70% of the proteins were identified as candidates for S-nitrosylation. The high number of identified candidates in the proteome reflects the importance of redox signaling in these compartments. An analysis of the functional distribution of the predicted candidates showed that proteins involved in signaling processes exhibited the highest prediction rate. In a set of 46 proteins, where 53 putative S-nitrosylation sites were already experimentally determined, the GPS-SNO program predicted 60 S-nitrosylation sites, but only 11 overlap with the results of the experimental approach. In general, a computer-assisted method for the prediction of targets for S-nitrosylation is a very good tool; however, further development, such as including the three dimensional structure of proteins in such analyses, would improve the identification of S-nitrosylation sites.

Mechanism and kinetics of inducible nitric oxide synthase auto-S-nitrosation and inactivation

Nitric oxide (NO), the product of the nitric oxide synthase (NOS) reaction, was previously shown to result in S-nitrosation of the NOS Zn(2+)-tetrathiolate and inactivation of the enzyme. To probe the potential physiological significance of NOS S-nitrosation, we determined the inactivation time scale of the inducible NOS isoform (iNOS) and found it directly correlates with an increase in the level of iNOS S-nitrosation. A kinetic model of NOS inactivation in which arginine is treated as a suicide substrate was developed. In this model, NO synthesized at the heme cofactor is partitioned between release into solution (NO release pathway) and NOS S-nitrosation followed by NOS inactivation (inactivation pathway). Experimentally determined progress curves of NO formation were fit to the model. The NO release pathway was perturbed through addition of the NO traps oxymyoglobin (MbO(2)) and β2 H-NOX, which yielded partition ratios between NO release and inactivation of ~100 at 4 μM MbO(2) and ~22000 at saturating trap concentrations. The results suggest that a portion of the NO synthesized at the heme cofactor reacts with the Zn(2+)-tetrathiolate without being released into solution. Perturbation of the inactivation pathway through addition of the reducing agent GSH or TCEP resulted in a concentration-dependent decrease in the level of iNOS S-nitrosation that directly correlated with protection from iNOS inactivation. iNOS inactivation was most responsive to physiological concentrations of GSH with an apparent K(m) value of 13 mM. NOS turnover that leads to NOS S-nitrosation might be a mechanism for controlling NOS activity, and NOS S-nitrosation could play a role in the physiological generation of nitrosothiols.

Post-translational modification in the gas phase: mechanism of cysteine S-nitrosylation via ion-molecule reactions

The gas-phase mechanism of S-nitrosylation of thiols was studied in a quadrupole ion trap mass spectrometer. This was done via ion-molecule reactions of protonated cysteine and many of its derivatives and other thiol ions with neutral tert-butyl nitrite or nitrous acid. Our results showed that the presence of the carboxylic acid functional group, -COOH, in the vicinity of the thiol group is essential for the gas-phase nitrosylation of thiols. When the carboxyl proton is replaced by a methyl group (cysteine methyl ester) no nitrosylation was observed. Other thiols lacking a carboxylic acid functional group displayed no S-nitrosylation, strongly suggesting that the carboxyl hydrogen plays a key role in the nitrosylation process. These results are in excellent agreement with a solution-phase mechanism proposed by Stamler et al. (J. S. Stamler, E. J. Toone, S. A. Lipton, N. J. Sucher. Neuron 1997, 18, 691-696) who suggested a catalytic role for the carboxylic acid group adjacent to cysteine residues and with later additions by Ascenzi et al. (P. Ascenzi, M. Colasanti, T. Persichini, M. Muolo, F. Polticelli, G. Venturini, D. Bordo, M. Bolognesi. Biol. Chem. 2000, 381, 623-627) who postulated that the presence of the carboxyl in the cysteine microenvironment in proteins is crucial for S-nitrosylation. A concerted mechanism for the gas-phase S-nitrosylation was proposed based on our results and was further studied using theoretical calculations. Our calculations showed that this proposed pathway is exothermic by 44.0 kJ mol(-1). This is one of the few recent examples when a gas-phase mechanism matches one in solution.

The role of thioredoxin in the regulation of cellular processes by S-nitrosylation

BACKGROUND

S-nitrosylation (or S-nitrosation) by Nitric Oxide (NO), i.e., the covalent attachment of a NO group to a cysteine thiol and formation of S-nitrosothiols (R-S-N=O or RSNO), has emerged as an important feature of NO biology and pathobiology. Many NO-related biological functions have been directly associated with the S-nitrosothiols and a considerable number of S-nitrosylated proteins have been identified which can positively or negatively regulate various cellular processes including signaling and metabolic pathways.

SCOPE OF THE REVIEW

Taking account of the recent progress in the field of research, this review focuses on the regulation of cellular processes by S-nitrosylation and Trx-mediated cellular homeostasis of S-nitrosothiols.

MAJOR CONCLUSIONS

Thioredoxin (Trx) system in mammalian cells utilizes thiol and selenol groups to maintain a reducing intracellular environment to combat oxidative/nitrosative stress. Reduced glutathione (GSH) and Trx system perform the major role in denitrosylation of S-nitrosylated proteins. However, under certain conditions, oxidized form of mammalian Trx can be S-nitrosylated and then it can trans-S-nitrosylate target proteins, such as caspase 3.

GENERAL SIGNIFICANCE

Investigations on the role of thioredoxin system in relation to biologically relevant RSNOs, their functions, and the mechanisms of S-denitrosylation facilitate the development of drugs and therapies. This article is part of a Special Issue entitled Regulation of Cellular Processes.

Characterization of tyrosine nitration and cysteine nitrosylation modifications by metastable atom-activation dissociation mass spectrometry

The fragmentation behavior of nitrated and S-nitrosylated peptides were studied using collision induced dissociation (CID) and metastable atom-activated dissociation mass spectrometry (MAD-MS). Various charge states, such as 1+, 2+, 3+, 2-, of modified and unmodified peptides were exposed to a beam of high kinetic energy helium (He) metastable atoms resulting in extensive backbone fragmentation with significant retention of the post-translation modifications (PTMs). Whereas the high electron affinity of the nitrotyrosine moiety quenches radical chemistry and fragmentation in electron capture dissociation (ECD) and electron transfer dissociation (ETD), MAD does produce numerous backbone cleavages in the vicinity of the modification. Fragment ions of nitrosylated cysteine modifications typically exhibit more abundant neutral losses than nitrated tyrosine modifications because of the extremely labile nature of the nitrosylated cysteine residues. However, compared with CID, MAD produced between 66% and 86% more fragment ions, which preserved the labile -NO modification. MAD was also able to differentiate I/L residues in the modified peptides. MAD is able to induce radical ion chemistry even in the presence of strong radical traps and therefore offers unique advantages to ECD, ETD, and CID for determination of PTMs such as nitrated and S-nitrosylated peptides.

Mass spectroscopy and molecular modeling predict endothelial nitric oxide synthase dimer collapse by hydrogen peroxide through zinc tetrathiolate metal-binding site disruption

Endothelial nitric oxide synthase (eNOS) is inhibited by hydrogen peroxide (H(2)O(2)), but the mechanism has not been determined. Thus, the purpose of this study was to delineate the mechanism by which H(2)O(2) inhibits eNOS activity. Using mass spectroscopy, we found that the tetrathiolate cysteine residues 94 and 99 were susceptible to oxidation by H(2)O(2). Molecular modeling predicted that these cysteic acid modifications would disrupt the van der Waals interactions and the hydrogen bonding network mediated by the tetrathiolate cysteines 94 and 99 resulting in changes in quaternary structure, zinc release, and dimer collapse. Using recombinant human eNOS (heNOS) to test the predictions of the molecular modeling we found that H(2)O(2) caused disruption of the heNOS dimer and this was accompanied by zinc release and decreased NO generation. We also found that H(2)O(2) increased the oxidation of tetrahydrobiopterin (BH(4)) to dihydrobiopterin (BH(2)), whereas preincubation of heNOS with excess BH(4) prevented the destruction of zinc tetrathiolate and dimer collapse and preserved activity. Interestingly, we found that the dimmer-stabilizing effect of BH(4) is due to its ability to act as a catalase mimetic. Further, we confirmed that, in ovine aortic endothelial cells, H(2)O(2) could also induce dimer collapse and that increasing cellular BH(4) levels could maintain eNOS in its dimeric form and NO signaling when cells were challenged with H(2)O(2). This study links the inhibitory action of H(2)O(2) on heNOS through the destruction of zinc tetrathiolate metal-binding site and dimer collapse both in vitro and in vivo.

Gas-phase fragmentation of long-lived cysteine radical cations formed via NO loss from protonated S-nitrosocysteine

In this work, we describe two different methods for generating protonated S-nitrosocysteine in the gas phase. The first method involves a gas-phase reaction of protonated cysteine with t-butylnitrite, while the second method uses a solution-based transnitrosylation reaction of cysteine with S-nitrosoglutathione followed by transfer of the resulting S-nitrosocysteine into the gas phase by electrospray ionization mass spectrometry (ESI-MS). Independent of the way it was formed, protonated S-nitrosocysteine readily fragments via bond homolysis to form a long-lived radical cation of cysteine (Cys(*+)), which fragments under collision-induced dissociation (CID) conditions via losses in the following relative abundance order: *COOH CH(2)S >> *CH(2)SH approximately = H(2)S. Deuterium labeling experiments were performed to study the mechanisms leading to these pathways. DFT calculations were also used to probe aspects of the fragmentation of protonated S-nitrosocysteine and the radical cation of cysteine. NO loss is found to be the lowest energy channel for the former ion, while the initially formed distonic Cys(*+) with a sulfur radical site undergoes proton and/or H atom transfer reactions that precede the losses of CH(2)S, *COOH, *CH(2)SH, and H(2)S.

Proteomic analysis of protein S-nitrosylation

Nitric oxide (NO) produces covalent PTMs of specific cysteine residues, a process known as S-nitrosylation. This route is dynamically regulated and is one of the major NO signalling pathways known to have strong and dynamic interactions with redox signalling. In agreement with this scenario, binding of NO to key cysteine groups can be linked to a broad range of physiological and pathological cellular events, such as smooth muscle relaxation, neurotransmission and neurodegeneration. The characterization of S-nitrosylated residues and the functional relevance of this protein modification are both essential information needed to understand the action of NO in living organisms. In this review, we focus on recent advances in this field and on state-of-the-art proteomic approaches which are aimed at characterizing the S-nitrosylome in different biological backgrounds.

A strategy for direct identification of protein S-nitrosylation sites by quadrupole time-of-flight mass spectrometry

S-nitrosylation of proteins serves an important role in regulating diverse cellular processes including signal transduction, DNA repair, and neurotransmission. Identification of S-nitrosylation sites is crucial for understanding the significance of this post-translational modification (PTM) in modulating the function of a protein. However, it is challenging to identify S-nitrosylation sites directly by mass spectrometric (MS) methods due to the labile nature of the S-NO bond. Here we describe a strategy for direct identification of protein S-nitrosylation sites in an electrospray ionization (ESI) quadrupole time-of-flight (QTOF) mass spectrometer without prior chemical derivatization of S-nitrosylated peptides. Both sample buffer composition and MS hardware parameters were carefully adjusted to ensure that S-nitrosylated peptide ions could be analyzed by the QTOF MS with optimal signal/noise ratios. It was crucial that the proteins were preserved in a sample solution containing 1 mM EDTA and 0.1 mM neocuproine at neutral pH. Proteins dissolved in this solution are amenable to in-solution tryptic digestion, which is important for the analysis of biological samples. S-nitrosylated peptides were effectively analyzed by LC/MS/MS on QTOF MS, with an optimized cone voltage of 20 V and collision energy of 4 V. We have successfully applied this method to thioredoxin, a key antioxidant protein, and identified within it an S-nitrosylation site at Cys73.

Identification of the cysteine nitrosylation sites in human endothelial nitric oxide synthase

S-nitrosylation, or the replacement of the hydrogen atom in the thiol group of cysteine residues by a -NO moiety, is a physiologically important posttranslational modification. In our previous work we have shown that S-nitrosylation is involved in the disruption of the endothelial nitric oxide synthase (eNOS) dimer and that this involves the disruption of the zinc (Zn) tetrathiolate cluster due to the S-nitrosylation of Cysteine 98. However, human eNOS contains 28 other cysteine residues whose potential to undergo S-nitrosylation has not been determined. Thus, the goal of this study was to identify the cysteine residues within eNOS that are susceptible to S-nitrosylation in vitro. To accomplish this, we utilized a modified biotin switch assay. Our modification included the tryptic digestion of the S-nitrosylated eNOS protein to allow the isolation of S-nitrosylated peptides for further identification by mass spectrometry. Our data indicate that multiple cysteine residues are capable of undergoing S-nitrosylation in the presence of an excess of a nitrosylating agent. All these cysteine residues identified were found to be located on the surface of the protein according to the available X-ray structure of the oxygenase domain of eNOS. Among those identified were Cys 93 and 98, the residues involved in the formation of the eNOS dimer through a Zn tetrathiolate cluster. In addition, cysteine residues within the reductase domain were identified as undergoing S-nitrosylation. We identified cysteines 660, 801, and 1113 as capable of undergoing S-nitrosylation. These cysteines are located within regions known to bind flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and nicotinamide adenine dinucleotide (NADPH) although from our studies their functional significance is unclear. Finally we identified cysteines 852, 975/990, and 1047/1049 as being susceptible to S-nitrosylation. These cysteines are located in regions of eNOS that have not been implicated in any known biochemical functions and the significance of their S-nitrosylation is not clear from this study. Thus, our data indicate that the eNOS protein can be S-nitrosylated at multiple sites other than within the Zn tetrathiolate cluster, suggesting that S-nitrosylation may regulate eNOS function in ways other than simply by inducing dimer collapse.

S-nitrosylated proteins of a medicinal CAM plant Kalanchoe pinnata- ribulose-1,5-bisphosphate carboxylase/oxygenase activity targeted for inhibition

Nitric oxide (NO) is a signaling molecule that affects a myriad of processes in plants. However, the mechanistic details are limited. NO post-translationally modifies proteins by S-nitrosylation of cysteines. The soluble S-nitrosoproteome of a medicinal, crassulacean acid metabolism (CAM) plant, Kalanchoe pinnata, was purified using the biotin switch technique. Nineteen targets were identified by MALDI-TOF mass spectrometry, including proteins associated with carbon, nitrogen and sulfur metabolism, the cytoskeleton, stress and photosynthesis. Some were similar to those previously identified in Arabidopsis thaliana, but kinesin-like protein, glycolate oxidase, putative UDP glucose 4-epimerase and putative DNA topoisomerase II had not been identified as targets previously for any organism. In vitro and in vivo nitrosylation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), one of the targets, was confirmed by immunoblotting. Rubisco plays a central role in photosynthesis, and the effect of S-nitrosylation on its enzymatic activity was determined using NaH14CO3. The NO-releasing compound S-nitrosoglutathione inhibited its activity in a dose-dependent manner suggesting Rubisco inactivation by nitrosylation for the first time.

S-Nitrosylation - another biological switch like phosphorylation?

Nitric oxide (NO) has emerged as a key-signaling molecule affecting plant growth and development right from seed germination to cell death. It is now being considered as a new plant hormone. NO is predominantly produced by nitric oxide synthase (NOS) in animal systems. NOS converts L-arginine (substrate) to citrulline and NO is a byproduct of the reaction. However, a similar biosynthetic mechanism is still not fully established in plants as NOS is still to be purified. First plant NOS gene (AtNOS1) was cloned from Arabidopsis suggesting the existence of NOS in plants. It was shown to be involved in hormonal signaling, stomatal closure, flowering, pathogen defense response, oxidative stress, senescence and salt tolerance. However, recent studies have raised critical questions/concerns about its substantial role in NO biosynthesis. Despite the ever increasing number of NO responses observed, little is known about the signal transduction pathway(s) and mechanisms by which NO interacts with different components and results in altered cellular activities. A brief overview is presented here. Proteins are one of the major bio-molecule besides DNA, RNA and lipids which are modified by NO and its derivatives. S-nitrosylation is a ubiquitous NO mediated posttranslational modification that might regulate broad spectrum of proteins. In this review S-nitrosylation formation, catabolism and its biological significance is discussed to present the current scenario of this modification in plants.

Identification of the tyrosine nitration sites in human endothelial nitric oxide synthase by liquid chromatography-mass spectrometry

The formation of nitric oxide (NO) in biological systems has led to the discovery of a number of post- translational protein modifications that can affect biological conditions such as vasodilation. Studies both from our laboratory and others have shown that beside its effect on cGMP generation from soluble guanylate cylcase, NO can produce protein modifications through both S-nitrosylation of cysteine residues. Previously, we have identified the potential S-nitrosylation sites on endothelial NO synthase (eNOS). Thus, the goal of this study was to further increase our understanding of reactive nitrogen protein modifications of eNOS by identifing tyrosine residues within eNOS that are susceptible to nitration in vitro. To accomplish this, nitration was carried out using tetranitromethane followed by tryptic digest of the protein. The resulting tryptic peptides were analyzed by liquid chromatography/mass spectrometry (LC/MS) and the position of nitrated tyrosines in eNOS were identified. The eNOS sequence contains 30 tyrosine residues and our data indicate that multiple tyrosine residues are capable of being nitrated. We could identify 25 of the 30 residues in our tryptic digests and 19 of these were susceptible to nitration. Interstingly, our data identified four tyrosine residues that can be modified by nitration that are located in the region of eNOS responsible for the binding to heat shock protein 90 (Hsp90), which is responsible for ensuring efficient coupling of eNOS.

A novel approach to identify proteins modified by nitric oxide: the HIS-TAG switch method

S-nitrosylation is emerging as an important signaling mechanism that regulates a broad range of cellular functions. The recognition of Cysteine residues that undergo S-nitrosylation is crucial to elucidate how this modification modulates protein activity. We report here a novel strategy, defined His-tag switch, which allows the purification and identification of S-nitrosylated proteins and the unambiguous localization of the modified cysteine residues by mass spectrometry analysis.

Extracellular S-nitrosoglutathione, but not S-nitrosocysteine or N(2)O(3), mediates protein S-nitrosation in rat spinal cord slices

There is evidence that protein S-nitrosothiols (PrSNOs) accumulate in inflammatory demyelinating disorders like multiple sclerosis and experimental allergic encephalomyelitis. However, very little is known regarding the mechanism by which PrSNOs are formed in target cells. The present study compares the ability of potential intercellular mediators of nitrosative damage including S-nitrosoglutathione (GSNO), S-nitrosocysteine and N(2)O(3) to induce protein S-nitros(yl)ation in the spinal cord, a CNS region that is commonly affected in multiple sclerosis and experimental allergic encephalomyelitis. The results clearly demonstrate that while all three NO-donors cause S-nitrosation of proteins in cell-free systems, only GSNO is a viable S-nitrosating agent in rat spinal cord slices. Generation of PrSNOs with GSNO occurs by S-transnitrosation as the process was not inhibited by either the NO-scavenger rutin or the N(2)O(3)-scavenger azide. Contrary to other cell types, nerve cells incorporate intact GSNO and neither functional l-amino acid transporters nor cell-surface thiols are required. We also found that there is a restricted number of proteins available for S-nitrosation, even at high, non-physiological concentrations of GSNO. These proteins are highly concentrated in mitochondria and mitochondria-rich subcellular compartments. This study is relevant to those CNS disorders characterized by excessive nitric oxide production.

Inhibition of MEK/ERK1/2 signalling alters endothelial nitric oxide synthase activity in an agonist-dependent manner

eNOS (endothelial nitric oxide synthase) activity is post-translationally regulated in a complex fashion by acylation, protein-protein interactions, intracellular trafficking and phosphorylation, among others. Signalling pathways that regulate eNOS activity include phosphoinositide 3-kinase/Akt, cyclic nucleotide-dependent kinases [PKA (protein kinase A) and PKG], PKC, as well as ERKs (extracellular-signal-regulated kinases). The role of ERKs in eNOS activation remains controversial. In the present study, we have examined the role of ERK1/2 in eNOS activation in HUVEC-CS [transformed HUVEC (human umbilical-vein endothelial cells)] as well as a widely used model for eNOS study, transiently transfected COS-7 cells. U0126 pretreatment of HUVEC-CS potentiated ATP-stimulated eNOS activity, independent of changes in intracellular Ca2+ concentration ([Ca2+]i). In COS-7 cells transiently expressing ovine eNOS, U0126 potentiated A23187-stimulated eNOS activity, but inhibited ATP-stimulated activity. Compensatory changes in phosphorylation of five key eNOS residues did not account for changes in A23187-stimulated activity. However, in the case of ATP, altered phosphorylation and changes in [Ca2+]i may partially contribute to U0126 inhibition of activity. Finally, seven eNOS alanine mutants of putative ERK1/2 targets were generated and the effects of U0126 pretreatment on eNOS activity were gauged with A23187 and ATP treatment. T97A-eNOS was the only construct significantly different from wild-type after U0126 pretreatment and ATP stimulation of eNOS activation. In the present study, eNOS activity was either potentiated or inhibited in COS-7 cells, suggesting agonist dependence for MEK/ERK1/2 signalling [where MEK is MAPK (mitogen-activated protein kinase)/ERK kinase] to eNOS and a complex mechanism including [Ca2+]i, phosphorylation and, possibly, intracellular trafficking.