S-Nitrosothiol measurements in biological systems

Reexamination of the Direct Electrochemical Reduction of S-Nitrosothiols

We report here on the electrochemical reduction of S-nitrosothiol species (RSNO). Nitric oxide (NO) is the reported common product from electrochemically reduced RSNOs at physiological pH. However, studies here at pH 7.4 show that during the reduction of RSNOs (-0.6 V to -0.9 V, vs. Ag/AgCl), no significant amount of NO is detected. Gas analysis suggests RSNO are reduced to nitrous oxide (N2O) at pH 7.4 and can only be converted back to NO at more oxidizing voltages. Interestingly, at pH 4.0, a direct one-electron reduction of RSNOs appears to occur and generates significant amounts of NO from RSNO species.

UPLC-MS/MS measurement of S-nitrosoglutathione (GSNO) in human plasma solves the S-nitrosothiol concentration enigma

We developed and validated a fast UPLC-MS/MS method with positive electrospray ionization (ESI+) for the quantitative determination of S-nitrosoglutathione (GSNO) in human plasma. We used a published protocol for the inactivation of plasma γ-glutamyltransferase (γGT) activity by using the γGT transition inhibitor serine/borate and the chelator EDTA for the stabilization of GSNO, and N-ethylmaleimide (NEM) to block SH groups and to avoid S-transnitrosylation reactions which may diminish GSNO concentration. S-[(15)N]Nitrosoglutathione (GS(15)NO) served as internal standard. Fresh blood was treated with NEM/serine/borate/EDTA, plasma spiked with GS(15)NO (50nM) was ultrafiltered (cut-off 10kDa) and 10μL aliquots of the ultrafiltrate were analyzed by UPLC-MS/MS. Five HILIC columns and an Acquity UPLC BH amide column were tested. The mobile phase was acetonitrile-water (70:30, v/v), contained 20mM ammonium formate, had a pH value of 7, and was pumped isocratically (0.5mL/min). The Nucleoshell column allowed better LC performance and higher MS sensitivity. The retention time of GSNO was about 1.1min. Quantification was performed by selected-reaction monitoring the mass transition m/z 337 ([M+H](+))→m/z 307 ([M+H(14)NO](+)) for GSNO (i.e., GS(14)NO) and m/z 338 ([M+H](+))→m/z 307 ([M+H(15)NO](+)) for GS(15)NO. NEM/serine/borate/EDTA was found to stabilize GSNO in human plasma. The method was validated in human plasma (range, 0-300nM) using 50nM GS(15)NO. Accuracy and precision were in generally acceptable ranges. A considerable matrix effect was observed, which was however outweighed by the internal standard GS(15)NO. In freshly prepared plasma from heparinized blood donated by 10 healthy subjects, no endogenous GSNO was determined above 2.8nM, the limit of quantitation (LOQ) of the method. This study challenges previously reported GSNO plasma concentrations being far above the present method LOQ value and predicts that the concentration of low-molecular-mass and high-molecular-mass S-nitrosothiols are in the upper pM- and lower nM-range, respectively.

Rocket fuel for the quantification of S-nitrosothiols. Highly specific reduction of S-nitrosothiols to thiols by methylhydrazine

Reduction of S-nitrosothiols to the corresponding thiol function is the key step in analyzing S-nitrosocysteinyl residues in proteins. Though it has been shown to give low yields, ascorbate-dependent reduction is commonly performed in the frequently used biotin-switch technique. We demonstrate that the compound methylhydrazine can act as a specific and efficient reducing agent for S-nitrosothiols. The corresponding thiol function is exclusively generated from low molecular weight and proteinaceous S-nitrosothiols while methylhydrazine failed to reduce disulfides. It was possible to optimize the experimental conditions so that thiol autoxidation is excluded, and high reaction yields (>90%) are obtained for the thiol function. The biotin-switch technique performed with methylhydrazine-dependent reduction shows remarkably improved sensitivity compared to the ascorbate-dependent procedure.

Analysis of S-nitrosothiols via copper cysteine (2C) and copper cysteine-carbon monoxide (3C) methods

This chapter summarizes the principles of RSNO measurement in the gas phase, utilizing ozone-based chemiluminescence and the copper cysteine (2C)±carbon monoxide (3C) reagent. Although an indirect method for quantifying RSNOs, this assay represents one of the most robust methodologies available. It exploits the NO detection sensitivity of ozone based chemiluminescence, which is within the range required to detect physiological concentrations of RSNO metabolites. Additionally, the specificity of the copper cysteine (2C and 3C) reagent for RSNOs negates the need for sample pretreatment, thereby minimizing the likelihood of sample contamination (false positive results), or the loss of certain highly labile RSNO species. Herein, we outline the principles of this methodology, summarizing key issues, potential pitfalls and corresponding solutions.

Chemical methods for the direct detection and labeling of S-nitrosothiols

SIGNIFICANCE

Posttranslational modification of proteins through phosphorylation, glycosylation, and oxidation adds complexity to the proteome by reversibly altering the structure and function of target proteins in a highly controlled fashion.

RECENT ADVANCES

The study of reversible cysteine oxidation highlights a role for this oxidative modification in complex signal transduction pathways. Nitric oxide (NO), and its respective metabolites (including reactive nitrogen species), participates in a variety of these cellular redox processes, including the reversible oxidation of cysteine to S-nitrosothiols (RSNOs). RSNOs act as endogenous transporters of NO, but also possess beneficial effects independent of NO-related signaling, which suggests a complex and versatile biological role. In this review, we highlight the importance of RSNOs as a required posttranslational modification and summarize the current methods available for detecting S-nitrosation.

CRITICAL ISSUES

Given the limitations of these indirect detection methods, the review covers recent developments toward the direct detection of RSNOs by phosphine-based chemical probes. The intrinsic properties that dictate this phosphine/RSNO reactivity are summarized. In general, RSNOs (both small molecule and protein) react with phosphines to yield reactive S-substituted aza-ylides that undergo further reactions leading to stable RSNO-based adducts.

FUTURE DIRECTIONS

This newly explored chemical reactivity forms the basis of a number of exciting potential chemical methods for protein RSNO detection in biological systems.

Nitric oxide in adaptation to altitude

This review summarizes published information on the levels of nitric oxide gas (NO) in the lungs and NO-derived liquid-phase molecules in the acclimatization of visitors newly arrived at altitudes of 2500 m or more and adaptation of populations whose ancestors arrived thousands of years ago. Studies of acutely exposed visitors to high altitude focus on the first 24-48 h with just a few extending to days or weeks. Among healthy visitors, NO levels in the lung, plasma, and/or red blood cells fell within 2h, but then returned toward baseline or slightly higher by 48 h and increased above baseline by 5 days. Among visitors ill with high-altitude pulmonary edema at the time of the study or in the past, NO levels were lower than those of their healthy counterparts. As for highland populations, Tibetans had NO levels in the lung, plasma, and red blood cells that were at least double and in some cases orders of magnitude greater than other populations regardless of altitude. Red blood cell-associated nitrogen oxides were more than 200 times higher. Other highland populations had generally higher levels although not to the degree shown by Tibetans. Overall, responses of those acclimatized and those presumed to be adapted are in the same direction, although the Tibetans have much larger responses. Missing are long-term data on lowlanders at altitude showing how similar they become to the Tibetan phenotype. Also missing are data on Tibetans at low altitude to see the extent to which their phenotype is a response to the immediate environment or expressed constitutively. The mechanisms causing the visitors' and the Tibetans' high levels of NO and NO-derived molecules at altitude remain unknown. Limited data suggest processes including hypoxic upregulation of NO synthase gene expression, hemoglobin-NO reactions, and genetic variation. Gains in understanding will require integrating appropriate methods and measurement techniques with indicators of adaptive function under hypoxic stress.

Direct vasoactive properties of thienopyridine-derived nitrosothiols

Thienopyridines (ticlopidine, clopidogrel, and prasugrel) require in vivo metabolism to exhibit a critical thiol group in the active form that binds to the P2Y12 platelet receptor to inhibit platelet activation. We hypothesized that formation of thienopyridine-derived nitrosothiols (ticlopidine-SNO, clopidogrel-SNO, and prasugrel-SNO) occurs directly from the respective parent drug. Pharmaceutical-grade thienopyridine (ticlopidine, clopidogrel chloride, clopidogrel sulfate, clopidogrel besylate, or prasugrel) was added to nitrite in aqueous solution to form the respective thienopyridine-SNO (Th-SNO). An isolated aortic ring preparation was used to test vasoactivity of the Th-SNO derivatives. Increasing nitrite availability resulted in increased Th-SNO formation for all drugs (other than ticlopidine). Th-SNO induced significant endothelium-independent relaxation of preconstricted aortic rings. Clopidogrel-chloride-SNO displayed rapid-release kinetics in a chemical environment, which was reflected by immediate and transient vasorelaxation when compared with the SNO derivatives of the other thienopyridines. Accounting for differences in yield, clopidogrel-chloride-SNO exhibited the greatest propensity to immediately relax vascular tissue. Th-SNO derivatives exhibit nitrovasodilator properties by supplying NO that can directly activate vascular soluble guanylate cyclase to induce vasorelaxation. Differences in SNO yield and vasoactivity exist between thienopyridine preparations that might be important to our understanding of the direct pharmacological effectiveness of thienopyridines on vascular and platelet function.

Visible photolysis and amperometric detection of S-nitrosothiols

The concentration of S-nitrosothiols (RSNOs), endogenous transporters of the signaling molecule nitric oxide (NO), fluctuate greatly in physiology often as a function of disease state. RSNOs may be measured indirectly by cleaving the S-N bond and monitoring the liberated NO. While ultraviolet photolysis and reductive-based cleavage both decompose RSNOs to NO, poor selectivity and the need for additional reagents preclude their utility clinically. Herein, we report the coupling of visible photolysis (i.e., 500-550 nm) and amperometric NO detection to quantify RSNOs with greater selectivity and sensitivity. Enhanced sensitivity (up to 1.56 nA μM(-1)) and lowered theoretical detection limits (down to 30 nM) were achieved for low molecular weight RSNOs (i.e., S-nitrosoglutathione, S-nitrosocysteine) by tuning the irradiation exposure. Detection of nitrosated proteins (i.e., S-nitrosoalbumin) was also possible, albeit at a decreased sensitivity (0.11 nA μM(-1)). This detection scheme was used to measure RSNOs in plasma and illustrate the potential of this method for future physiological studies.

S-nitrosation of cellular proteins by NO donors in rat embryonic fibroblast 3Y1 cells: factors affecting S-nitrosation

The mechanism of protein S-nitrosation in cells is not fully understood. Using rat 3Y1 cells, we addressed this issue. Among S-nitrosothiols and NO donors tested, only S-nitrosocysteine (CysNO) induced S-nitrosation when exposed in Hanks' balanced salt solution (HBSS) and not in serum-containing general culture medium. In HBSS, NO release from CysNO was almost completely abolished by sequestering metal ions with a metal chelator without affecting cellular S-nitrosation. In contrast, L-leucine, a substrate of L-type amino acid transporters (LATs), significantly inhibited S-nitrosation. The absence of S-nitrosation with CysNO in general culture medium resulted not only from a competition with amino acids in the medium for LATs but also from transnitrosation of cysteine residues in serum albumin. Collectively, these results suggest that in simple buffered saline, CysNO-dependent S-nitrosation occurs through a cellular incorporation-dependent mechanism, but if it occurs in general culture media, it may be through an NO-dependent mechanism.

Highly sensitive detection of S-nitrosylated proteins by capillary gel electrophoresis with laser induced fluorescence

S-nitrosylated proteins are biomarkers of oxidative damage in aging and Alzheimer's disease (AD). Here, we report a new method for detecting and quantifying nitrosylated proteins by capillary gel electrophoresis with laser induced fluorescence detection (CGE-LIF). Dylight 488 maleimide was used to specifically label thiol group (SH) after switching the S-nitrosothiol (S-NO) to SH in cysteine using the "fluorescence switch" assay. In vitro nitrosylation model-BSA subjected to S-nitrosoglutathione (GSNO) optimized the labeling reactions and characterized the response of the LIF detector. The method proves to be highly sensitive, detecting 1.3 picomolar (pM) concentration of nitrosothiols in nanograms of proteins, which is the lowest limit of detection of nitrosothiols reported to date. We further demonstrated the direct application of this method in monitoring protein nitrosylation damage in MQ mediated human colon adenocarcinoma cells. The nitrosothiol amounts in MQ treated and untreated cells are 14.8±0.2 and 10.4±0.5 pmol/mg of proteins, respectively. We also depicted nitrosylated protein electrophoretic profiles of brain cerebrum of 5-month-old AD transgenic (Tg) mice model. In Tg mice brain, 15.5±0.4 pmol of nitrosothiols/mg of proteins was quantified while wild type contained 11.7±0.3 pmol/mg proteins. The methodology is validated to quantify low levels of S-nitrosylated protein in complex protein mixtures from both physiological and pathological conditions.

Quantification of cysteinyl S-nitrosylation by fluorescence in unbiased proteomic studies

Cysteinyl S-nitrosylation has emerged as an important post-translational modification affecting protein function in health and disease. Great emphasis has been placed on global, unbiased quantification of S-nitrosylated proteins because of physiologic and oxidative stimuli. However, current strategies have been hampered by sample loss and altered protein electrophoretic mobility. Here, we describe a novel quantitative approach that uses accurate, sensitive fluorescence modification of cysteine S-nitrosylation that leaves electrophoretic mobility unaffected (SNOFlo) and introduce unique concepts for measuring changes in S-nitrosylation status relative to protein abundance. Its efficacy in defining the functional S-nitrosoproteome is demonstrated in two diverse biological applications: an in vivo rat hypoxia-ischemia/reperfusion model and antimicrobial S-nitrosoglutathione-driven transnitrosylation of an enteric microbial pathogen. The suitability of this approach for investigating endogenous S-nitrosylation is further demonstrated using Ingenuity Pathways analysis that identified nervous system and cellular development networks as the top two networks. Functional analysis of differentially S-nitrosylated proteins indicated their involvement in apoptosis, branching morphogenesis of axons, cortical neurons, and sympathetic neurites, neurogenesis, and calcium signaling. Major abundance changes were also observed for fibrillar proteins known to be stress-responsive in neurons and glia. Thus, both examples demonstrate the technique's power in confirming the widespread involvement of S-nitrosylation in hypoxia-ischemia/reperfusion injury and in antimicrobial host responses.

Membrane transfer of S-nitrosothiols

The distinctive function of nitric oxide (NO) in biology is to transmit cellular signals through membranes and regulate cellular functions in adjacent cells. NO conveys signals as a second messenger from a cell where NO is generated to contiguous cells in two ways; one is as gaseous molecule by free diffusion resulting in an activation of soluble guanylate cyclase (NO/cGMP pathway), and another form is by binding with a molecule such as cysteine or protein thiol through S-nitrosylation (SNO pathway). Both pathways transmit much of the biological influence of NO from cell where other messenger molecules but NO are confined, through the plasma membrane to the adjacent cells. Since SNO pathway cannot utilize free-diffusion mechanism to get through the membrane as the molecular size is significantly larger than NO molecule, it utilizes amino acid transporter to convey signals as a form of S-nitrosylated cysteine (CysNO). Although S-nitrosylated glutathione (GSNO) is the molecule which act as a determinant of the total S-nitrosothiol level in cell, transnitrosylation reaction from GSNO to CysNO is an initial requirement to pass through signal through the membrane. Thus, multiplexed combination of these steps and the regulatory factors involved in this system conform and modify the outcome from stimulus-response coupling via the SNO pathway.

Site-specific proteomics approach for study protein S-nitrosylation

Here we present a novel and robust method for the identification of protein S-nitrosylation sites in complex protein mixtures. The approach utilizes the cysteinyl affinity resin to selectively enrich S-nitrosylated peptides reduced by ascorbate followed by nanoscale liquid chromatography tandem mass spectrometry. Two alkylation agents with different added masses were employed to differentiate the S-nitrosylation sites from the non-S-nitrosylation sites. We applied this approach to MDA-MB-231 cells treated with Angeli's salt, a nitric oxide donor that has been shown to inhibit breast tumor growth and angiogenesis. A total of 162 S-nitrosylation sites were identified and an S-nitrosylation motif was revealed in our study. The 162 sites are significantly more than the number reported by previous methods, demonstrating the efficiency of our approach. Our approach will further facilitate the functional study of protein S-nitrosylation in cellular processes and may reveal new therapeutic targets.

Reductive ligation mediated one-step disulfide formation of S-nitrosothiols

A one-step reductive ligation mediated disulfide formation of S-nitrosothiols was developed. This reaction involves the reaction of the S-nitroso group with phosphine-thioesters to form sulfenamide and thiolate intermediates, which then undergo a fast intermolecular disulfide formation to form stable conjugates. This reaction can be used to design new biosensors of S-nitrosated proteins.

Screening systems for the identification of S-nitrosylated proteins

S-nitrosylation is a well-characterized reaction involving the covalent binding of nitric oxide (NO) to cysteine residues (Cys) in a protein. Similar to protein phosphorylation, S-nitrosylation is a post-translational modification involved in the regulation of a large number of intracellular functions and signaling events. Moreover, like phosphorylation, S-nitrosylation is precisely regulated in time and space. A procedure known as the biotin-switch method that specifically detects S-nitrosylated proteins (SNO-P) was recently developed by Snyder's group. They found that many proteins are substrates for NO, and several groups have attempted to identify other SNO-P by improving this method. In this review, we describe the SNO-P identified using modified versions of the biotin-switch method.

One-pot thioether formation from S-nitrosothiols

Protein S-nitrosation is an important post-translational modification. However, the detection of S-nitrosation is still problematic because S-nitrosation products, that is, S-nitrosothiols, are unstable species. Here a new reaction which can selectively convert unstable S-nitrosothiols to stable thioethers in one-pot under very mild conditions is reported. This reaction has the potential to be applied in the detection of protein S-nitrosation.

Chemical methods to detect S-nitrosation

Nitric oxide (NO) is a cell-signaling molecule involved in a number of physiological and pathophysiological processes. Modification of cysteine residues by NO (or NO metabolites), that is S-nitrosation, changes the function of a broad spectrum of proteins. This reaction represents an important post-translational modification that transduces NO-dependent signals. However, the detection and quantification of S-nitrosation in biological samples remain a challenge mainly because of the lability of S-nitrosation products: S-nitrosothiols (SNO). In this review we summarize recent developments of the methods to detect S-nitrosation. Our focus is on the methods which can be used to directly conjugate the site(s) of S-nitrosation.

Direct regulation of striated muscle myosins by nitric oxide and endogenous nitrosothiols

BACKGROUND

Nitric oxide (NO) has long been recognized to affect muscle contraction, both through activation of guanylyl cyclase and through modification of cysteines in proteins to yield S-nitrosothiols. While NO affects the contractile apparatus directly, the identities of the target myofibrillar proteins remain unknown. Here we report that nitrogen oxides directly regulate striated muscle myosins.

PRINCIPAL FINDINGS

Exposure of skeletal and cardiac myosins to physiological concentrations of nitrogen oxides, including the endogenous nitrosothiol S-nitroso-L-cysteine, reduced the velocity of actin filaments over myosin in a dose-dependent and oxygen-dependent manner, caused a doubling of force as measured in a laser trap transducer, and caused S-nitrosylation of cysteines in the myosin heavy chain. These biomechanical effects were not observed in response to S-nitroso-D-cysteine, demonstrating specificity for the naturally occurring isomer. Both myosin heavy chain isoforms in rats and cardiac myosin heavy chain from human were S-nitrosylated in vivo.

SIGNIFICANCE

These data show that nitrosylation signaling acts as a molecular "gear shift" for myosin--an altogether novel mechanism by which striated muscle and cellular biomechanics may be regulated.

Water-soluble triarylphosphines as biomarkers for protein S-nitrosation

S-Nitrosothiols (RSNOs) represent an important class of post-translational modifications that preserve and amplify the actions of nitric oxide and regulate enzyme activity. Several regulatory proteins are now verified targets of cellular S-nitrosation, and the direct detection of S-nitrosated residues in proteins has become essential to better understand RSNO-mediated signaling. Current RSNO detection depends on indirect assays that limit their overall specificity and reliability. Herein, we report the reaction of S-nitrosated cysteine, glutathione, and a mutated C165S alkyl hydroperoxide reductase with the water-soluble phosphine tris(4,6-dimethyl-3-sulfonatophenyl)phosphine trisodium salt hydrate (TXPTS). A combination of NMR and MS techniques reveals that these reactions produce covalent S-alkylphosphonium ion adducts (with S-P(+) connectivity), TXPTS oxide, and a TXPTS-derived aza-ylide. Mechanistically, this reaction may proceed through an S-substituted aza-ylide or the direct displacement of nitroxyl from the RSNO group. This work provides a new means for detecting and quantifying S-nitrosated species in solution and suggests that phosphines may be useful tools for understanding the complex physiological roles of S-nitrosation and its implications in cell signaling and homeostasis.

Facile formation of dehydroalanine from S-nitrosocysteines

A reductive elimination of S-nitrosocysteines using phosphine substrates has been developed. This reaction converts unstable S-nitrosocysteines to dehydroalanine derivatives under very mild conditions. It can potentially be applied for the detection of protein S-nitrosation.

Hemoglobin, nitric oxide and molecular mechanisms of hypoxic vasodilation

The protected transport of nitric oxide (NO) by hemoglobin (Hb) links the metabolic activity of working tissue to the regulation of its local blood supply through hypoxic vasodilation. This physiologic mechanism is allosterically coupled to the O(2) saturation of Hb and involves the covalent binding of NO to a cysteine residue in the beta-chain of Hb (Cys beta93) to form S-nitrosohemoglobin (SNO-Hb). Subsequent S-transnitrosation, the transfer of NO groups to thiols on the RBC membrane and then in the plasma, preserves NO vasodilator activity for delivery to the vascular endothelium. This SNO-Hb paradigm provides insight into the respiratory cycle and a new therapeutic focus for diseases involving abnormal microcirculatory perfusion. In addition, the formation of S-nitrosothiols in other proteins may regulate an array of physiological functions.

Unraveling the S-nitrosoproteome: tools and strategies

One of the major tasks to be accomplished in the postgenomic era is the characterization of PTMs in proteins. The S-nitrosation of protein thiols is a redox-based PTM that modulating enzymatic activity, subcellular localization, complex formation, and degradation of proteins, largely contributes to the complexity of cellular proteomes. Although the detection of S-nitrosated proteins is problematical due to the lability of S-nitrosothiols, with the improvement of molecular tools an increasing range of proteins has been shown to undergo S-nitrosation. We here review recent proteomic approaches for the systematic assessment of potential targets for protein S-nitrosation. The development of new analytical methods and strategies over the past several years now allows us to investigate the nitrosoproteome on a global scale.

GC-MS analysis of S-nitrosothiols after conversion to S-nitroso-N-acetyl cysteine ethyl ester and in-injector nitrosation of ethyl acetate

S-Nitrosothiols from low-molecular-mass and high-molecular-mass thiols, including glutathione, albumin and hemoglobin, are endogenous potent vasodilators and inhibitors of platelet aggregation. By utilizing the S-transnitrosation reaction and by using the lipophilic (pK(L) 0.78) and strong nucleophilic synthetic thiol N-acetyl cysteine ethyl ester (NACET) we have developed a GC-MS method for the analysis of S-nitrosothiols and their (15)N- or (2)H-(15)N-labelled analogs as S-nitroso-N-acetyl cysteine ethyl ester (SNACET) and S(15)NACET or d(3)-S(15)NACET derivatives, respectively, after their extraction with ethyl acetate. Injection of ethyl acetate solutions of S-nitrosothiols produced two main reaction products, compound X and compound Y, within the injector in dependence on its temperature. Quantification was performed by selected-ion monitoring of m/z 46 (i.e., [NO(2)](-)) for SNACET and m/z 47 (i.e., [(15)NO(2)](-)) for S(15)NACET/d(3)-S(15)NACET for compound X, and m/z 157 for SNACET and m/z 160 for d(3)-S(15)NACET for compound Y. In this article we describe the development, validation and in vitro and in vivo applications of the method to aqueous buffered solutions, human and rabbit plasma. Given the ester functionality of SNACET/S(15)NACET/d(3)-S(15)NACET, stability studies were performed using metal chelators and esterase inhibitors. The method was found to be suitable for the quantitative determination of various S-nitrosothiols including SNACET externally added to human plasma (0-10microM). Nitrite contamination in ethyl acetate was found to interfere. Our results suggest that the concentration of endogenous S-nitrosothiols in human plasma does not exceed about 200nM in total. Oral administration of S(15)NACET to rabbits (40-63micromol/kg body weight) resulted in formation of ALB-S(15)NO, [(15)N]nitrite and [(15)N]nitrate in plasma.

An unexpected Bis-ligation of S-nitrosothiols

A bis-ligation reaction of S-nitrosothiols using triaryl substituted phosphine-thioester substrates has been developed. This reaction converts unstable primary S-nitrosothiols to stable disulfide-iminophosphorane products in high yields under mild conditions. It can potentially be applied for the detection of S-nitrosation in protein systems.

Exploration of the "traceless" reductive ligation of S-nitrosothiols

The first "traceless" reductive ligation of S-nitrosothiols using phosphine ester/thioester conjugates is reported. Experiments also show that stable thioimidate compounds could be formed in the reaction between S-nitrosothiols and some phosphine-thioester substrates.

Amperometric S-nitrosothiol sensor with enhanced sensitivity based on organoselenium catalysts

A new S-nitrosothiol (RSNO) detection strategy based on an electrochemical sensor is described for rapidly estimating levels of total RSNOs in blood and other biological samples. The sensor employs a cellulose dialysis membrane covalently modified with an organoselenium catalyst that converts RSNOs to NO at the distal tip of an amperometric NO sensor. The sensor is characterized by very low detection limits (<20 nM), good long-term stability, and can be employed for the rapid detection of total low-molecular-weight (LMW) RSNO levels in whole blood samples using a simple standard addition method. A strategy for detecting macromolecular RSNOs is also demonstrated via use of a transnitrosation reaction with added LMW thiols allowing the estimation of total RSNO levels in blood. The sensor is shown to exhibit high selectivity over nitrosamines and nitrite. Such RSNO detection is potentially useful to reveal correlation between blood RSNO levels and endothelial cell dysfunction, which often is associated with cardiovascular diseases.

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.

Assessment of the nitrosylation process

PURPOSE OF REVIEW

To understand the principles and limits of the methodologies used for the measurement of S-nitrosylated proteins.

RECENT FINDINGS

Among methods for studying protein S-nitrosylation, chemoluminescence and biotin switch assay have rapidly gained popularity. However, recent findings have attempted to highlight potential pitfalls for these methods. Many assays for biological S-nitrosylated proteins are used near the limit of detection and pretreatment of the biological samples can modify the S-NO bond. These results suggest that additional controls are essential in order to identify S-nitrosylated proteins and results should be quantitatively validated using more than one methodology.

SUMMARY

Protein S-nitrosylation is emerging as a key mechanism by which nitric oxide regulates cell signalling. This review focuses on existing methodologies for the measurement of S-nitrosylated proteins in biological matrices and the potential pitfalls of each method.

Novel method for measuring S-nitrosothiols using hydrogen sulfide

Recent advances in techniques that allow sensitive and specific measurement of S-nitrosothiols (RSNOs) have provided evidence for a role for these compounds in various aspects of nitric oxide (NO) biology. The most widely used approach is to couple reaction chemistry that selectively reduces RSNOs by one electron to produce NO, with the sensitive detection of the latter under anaerobic conditions using ozone based chemiluminescence in NO analyzers. Herein, we report a novel reaction that is readily adaptable for commercial NO analyzers that utilizes hydrogen sulfide (H2S), a gas that can reduce RSNO to NO and, analogous to NO, is produced by endogenous metabolism and has effects on diverse biological functions. We discuss factors that affect H2S based methods for RSNO measurement and discuss the potential of H2S as an experimental tool to measure RSNO.

Photoinstability of S-nitrosothiols during sampling of whole blood: a likely source of error and variability in S-nitrosothiol measurements

BACKGROUND

The determination of reference intervals for the concentration of total S-nitrosothiols (RSNOs) in blood is a highly controversial topic, likely because of the inherent instability of these species. Most currently available techniques to quantify RSNOs in blood require considerable sample handling and multiple pretreatment steps during which light exposure is difficult to completely eliminate. We investigated the effect of brief light exposure on the stability of RSNO species in blood during the initial sampling process.

METHODS

A novel amperometric RSNO sensor, based on an immobilized organoselenium catalyst at the distal tip of an electrochemical nitric oxide detector, was used to determine RSNO species in diluted whole blood without centrifugation or pretreatment. Porcine blood was collected into aluminum foil-wrapped syringes via a 12-inch butterfly needle tube assembly. Two blood samples were collected from the same animal -- one with the butterfly needle tubing wrapped in aluminum foil and one with the tubing exposed to ambient room light. The RSNO concentrations in these sequential blood samples were determined by a standard addition procedure.

RESULTS

Eight sets of measurements were made in 6 animals. Samples exposed to light yielded RSNO concentrations only 23.6% (7.2%) [mean (SD)] of the RSNO concentrations determined in samples that were shielded from light and obtained from the same animals.

CONCLUSIONS

These results suggest significant photoinstablity of RSNOs in whole blood and indicate the critical importance of proper light protection during sampling and processing of blood samples for the accurate determinations of endogenous RSNO concentrations.

S-nitrosothiol assays that avoid the use of iodine

S-Nitrosylation is a ubiquitous signaling process in biological systems. Research regarding this signaling has been hampered, however, by assays that lack sensitivity and specificity. In particular, iodine-based assays for S-nitrosothiols (1) produce nitrosyliodide, a potent nitrosating agent that can be lost to reactions in the biological sample being studied; (2) require pretreatment of biological samples with several reagents that react with proteins, artifactually forming or breaking S-NO bonds before the assay; and (3) are not sensitive or specific for nitrogen oxides in biological samples, reporting a wide range of different concentrations and falsely reporting NO-modified proteins, to be nitrite. These data, therefore, suggest that iodine-based assays should never be used for biological S-nitrosothiols. There are other assays that provide reasonably sensitive and accurate data regarding biological S-nitrosothiols, including assays based on mass spectrometry, spectrophotometry, chemiluminescence, fluorescence, and immunostaining. Each assay, however, has limitations and should be quantitatively complemented by separate assays. Continued improvement in assays will facilitate improved understanding of S-nitrosylation signaling.

Nitric oxide regulation of MMP-9 activation and its relationship to modifications of the cysteine switch

Matrix metalloproteases (MMPs) are Zn-containing endopeptidases involved in the degradation of extracellular matrix components and are typically secreted in a latent (pro-MMP) form and activated either by proteolytic or oxidative disruption of a conserved cysteine switch. Several recent studies have suggested that nitric oxide (NO) can contribute to the activation of MMPs, but the mechanisms involved are incompletely understood. We investigated the ability of NO to regulate the activation of (pro)MMP-9 using a variety of NO-donor compounds and characterized modifications of the cysteine switch using a synthetic peptide (PRCGVPDLGR) representing the cysteine switch domain of MMP-9. Among the NO-donors used, only S-nitrosocysteine (SNOC) was found to be capable of modest activation of proMMP-9, but S-nitrosoglutathione (GSNO) or the NONOates, DEA-NO, SPER-NO, or DETA-NO, were ineffective. In fact, high concentrations of DETA-NO were found to inhibit MMP-9 activity, presumably by direct interaction with the active-site Zn (2+). Analysis of chemical modifications within the Cys-containing peptide, PRCGVPDLGR, revealed rapid and transient S-nitrosylation by SNOC and GSNO, and formation of mixed disulfides and dimerized peptide as major final products. Similarly, NONOates induced transient S-nitrosylation and primarily peptide dimerization. Coordination of the peptide Cys with a synthetic Zn (2+) complex, to more closely mimic the structure of the active site in proMMP-9, reduced peptide nitrosylation and oxidation by NONOates, but enhanced peptide nitrosylation by SNOC and GSNO. Collectively, our results demonstrate that NO is incapable of directly activating proMMP-9 and that S-nitrosylation of MMP-9 propeptide by NO-donors is unrelated to their ability to regulate MMP-9 activity.

Nitric oxide and cardiobiology-methods for intact hearts and isolated myocytes

The cross talk between reactive oxygen species (ROS) and reactive nitrogen species (RNS) plays a pivotal role in the regulation of myocardial and vascular function. Both nitric oxide and redox-based signaling involve the posttranslational modification of proteins through S-nitrosylation and oxidation of specific cysteine residues. Disruption of this cross talk between ROS and RNS contributes to the pathogenesis of heart failure. Therefore, the elucidation of these complex chemical interactions may improve our understanding of cardiovascular pathophysiology. This chapter discusses the significant role of spatial confinement of nitric oxide synthases, NADPH oxidase, and xanthine oxidoreductase in the regulation of myocardial excitation-contraction coupling. This chapter describes techniques for assessing oxidative and nitrosative stress. A variety of assays have been developed that quantify S-nitrosylated proteins. Among them, the biotin-switch method directly evaluates endogenously nitrosylated proteins in a reproducible way. Identification of the biotinylated or S-nitrosylated proteins subjected to the biotin-switch assay are described and evaluated with a one-dimensional gel (Western blot) or with the newly developed two-dimensional fluorescence difference gel electrophoresis proteomic analysis. Quantifying the number of free thiols with the monobromobimane assay in a protein of interest allows estimation of cysteine oxidation and, in turn, the state of nitroso-redox balance of effector molecules. In summary, this chapter reviews the biochemical methods that assess the impact of nitroso/redox signaling in the cardiovascular system.

Low-molecular-weight S-nitrosothiols and blood vessel injury

S-nitrosothiol signaling reactions are argued to play key modulatory roles in mediating the actions of NOS in health and disease. A report by Palmer et al. in this issue of the JCI provides new insight into the in vivo biology of S-nitrosothiols (see the related article beginning on page 2592). The authors examine the chronic effects of exogenous nitrosothiol therapy and demonstrate that the commonly used antioxidant N-acetylcysteine (NAC) induces pulmonary arterial hypertension in mice. Importantly, the authors argue that the vascular pathology they observe in the lungs of these animals is functionally and morphologically equivalent to that observed in chronic hypoxia. These findings raise the concern that chronic NAC therapy may induce similar vascular pathology in patients.