Nitric oxide, S-nitrosothiols and hemoglobin: is methodology the key?

Artifacts Introduced by Sample Handling in Chemiluminescence Assays of Nitric Oxide Metabolites

We recently developed a combination of four chemiluminescence-based assays for selective detection of different nitric oxide (NO) metabolites, including nitrite, S-nitrosothiols (SNOs), heme-nitrosyl (heme-NO), and dinitrosyl iron complexes (DNICs). However, these NO species (NOx) may be under dynamic equilibria during sample handling, which affects the final determination made from the readout of assays. Using fetal and maternal sheep from low and high altitudes (300 and 3801 m, respectively) as models of different NOx levels and compositions, we tested the hypothesis that sample handling introduces artifacts in chemiluminescence assays of NOx. Here, we demonstrate the following: (1) room temperature placement is associated with an increase and decrease in NOx in plasma and whole blood samples, respectively; (2) snap freezing and thawing lead to the interconversion of different NOx in plasma; (3) snap freezing and homogenization in liquid nitrogen eliminate a significant fraction of NOx in the aorta of stressed animals; (4) A "stop solution" commonly used to preserve nitrite and SNOs leads to the interconversion of different NOx in blood, while deproteinization results in a significant increase in detectable NOx; (5) some reagents widely used in sample pretreatments, such as mercury chloride, acid sulfanilamide, N-ethylmaleimide, ferricyanide, and anticoagulant ethylenediaminetetraacetic acid, have unintended effects that destabilize SNO, DNICs, and/or heme-NO; (6) blood, including the residual blood clot left in the washed purge vessel, quenches the signal of nitrite when using ascorbic acid and acetic acid as the purge vessel reagent; and (7) new limitations to the four chemiluminescence-based assays. This study points out the need for re-evaluation of previous chemiluminescence measurements of NOx, and calls for special attention to be paid to sample handling, as it can introduce significant artifacts into NOx assays.

Measurement of S -Nitrosoglutathione in Plasma by Liquid Chromatography-Tandem Mass Spectrometry

This chapter describes an ultraperformance liquid chromatographic-tandem mass spectrometric (UPLC-MS/MS) method for the quantitative determination of S-nitrosoglutathione (GSNO) in human plasma. S-[¹⁵N]Nitrosoglutathione (GS¹⁵NO) serves as the internal standard. The protocol involves inactivation of plasma γ-glutamyltransferase activity by serine-borate, stabilization of GSNO with EDTA, and avoidance of S-transnitrosylation reactions by blocking SH groups with N-ethylmaleimidide (NEM). Fresh blood is treated with NEM/serine-borate/EDTA, plasma is spiked with GS¹⁵NO (50 nM), ultrafiltered (cutoff 10 kDa) and 10-μL aliquots of ultrafiltrate are analyzed by UPLC-MS/MS in the positive electrospray ionization (ESI+) mode. LC is performed on a Nucleoshell column using isocratic (0.5 mL/min) elution with acetonitrile-20 mM ammonium formate (70:30, v/v), pH 7. Quantification is performed by selected-reaction monitoring the mass transition m/z 337 ([M+H]⁺) → m/z 307 ([M+H-¹⁴NO]^+●) for GSNO and m/z 338 ([M+H]⁺) → m/z 307 ([M+H-¹⁵NO]^+●) for GS¹⁵NO. Matrix effects are outweighed by the internal standard GS¹⁵NO. The lower limit of quantitation (LOQ) is 2.8 nM.

Regulation of protein function by S-nitrosation and S-glutathionylation: processes and targets in cardiovascular pathophysiology

Decades of chemical, biochemical and pathophysiological research have established the relevance of post-translational protein modifications induced by processes related to oxidative stress, with critical reflections on cellular signal transduction pathways. A great deal of the so-called ‘redox regulation’ of cell function is in fact mediated through reactions promoted by reactive oxygen and nitrogen species on more or less specific aminoacid residues in proteins, at various levels within the cell machinery. Modifications involving cysteine residues have received most attention, due to the critical roles they play in determining the structure/function correlates in proteins. The peculiar reactivity of these residues results in two major classes of modifications, with incorporation of NO moieties (S-nitrosation, leading to formation of proteinS-nitrosothiols) or binding of low molecular weight thiols (S-thionylation, i.e. in particularS-glutathionylation,S-cysteinylglycinylation andS-cysteinylation). A wide array of proteins have been thus analyzed in detail as far as their susceptibility to either modification or both, and the resulting functional changes have been described in a number of experimental settings. The present review aims to provide an update of available knowledge in the field, with a special focus on the respective (sometimes competing and antagonistic) roles played by proteinS-nitrosations andS-thionylations in biochemical and cellular processes specifically pertaining to pathogenesis of cardiovascular diseases.

Arginine and nitric oxide synthase: regulatory mechanisms and cardiovascular aspects

L-Arginine (L-Arg) is a conditionally essential amino acid in the human diet. The most common dietary sources of L-Arg are meat, poultry and fish. L-Arg is the precursor for the synthesis of nitric oxide (NO); a key signaling molecule via NO synthase (NOS). Endogenous NOS inhibitors such as asymmetric-dimethyl-L-Arg inhibit NO synthesis in vivo by competing with L-Arg at the active site of NOS. In addition, NOS possesses the ability to be "uncoupled" to produce superoxide anion instead of NO. Reduced NO bioavailability may play an essential role in cardiovascular pathologies and metabolic diseases. L-Arg deficiency syndromes in humans involve endothelial inflammation and immune dysfunctions. Exogenous administration of L-Arg restores NO bioavailability, but it has not been possible to demonstrate, that L-Arg supplementation improved endothelial function in cardiovascular disease such as heart failure or hypertension. L-Arg supplementation may be a novel therapy for obesity and metabolic syndrome. The utility of l-Arg supplementation in the treatment of L-Arg deficiency syndromes remains to be established. Clinical trials need to continue to determine the optimal concentrations and combinations of L-Arg, with other protective compounds such as tetrahydrobiopterin (BH4 ), and antioxidants to combat oxidative stress that drives down NO production in humans.

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.

A multilevel analytical approach for detection and visualization of intracellular NO production and nitrosation events using diaminofluoresceins

Diaminofluoresceins are widely used probes for detection and intracellular localization of NO formation in cultured/isolated cells and intact tissues. The fluorinated derivative 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) has gained increasing popularity in recent years because of its improved NO sensitivity, pH stability, and resistance to photobleaching compared to the first-generation compound, DAF-2. Detection of NO production by either reagent relies on conversion of the parent compound into a fluorescent triazole, DAF-FM-T and DAF-2-T, respectively. Although this reaction is specific for NO and/or reactive nitrosating species, it is also affected by the presence of oxidants/antioxidants. Moreover, the reaction with other molecules can lead to the formation of fluorescent products other than the expected triazole. Thus additional controls and structural confirmation of the reaction products are essential. Using human red blood cells as an exemplary cellular system we here describe robust protocols for the analysis of intracellular DAF-FM-T formation using an array of fluorescence-based methods (laser-scanning fluorescence microscopy, flow cytometry, and fluorimetry) and analytical separation techniques (reversed-phase HPLC and LC-MS/MS). When used in combination, these assays afford unequivocal identification of the fluorescent signal as being derived from NO and are applicable to most other cellular systems without or with only minor modifications.

Preparation and characterization of an improved Cu(2+)-cyclen polyurethane material that catalyzes generation of nitric oxide from S-nitrosothiols

A new, stable and highly efficient Cu(2+)-cyclen-polyurethane material is described and shown to exhibit improved performance compared to prior materials for the catalytic decomposition of S-nitrosothiols to physiologically active nitric oxide.

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.

Study of the effect of thiols on the vasodilatory potency of S-nitrosothiols by using a modified aortic ring assay

Both low-molecular-mass thiols (LMM-SH) and protein thiols (P-SH) can modulate the biological activity of S-nitrosothiols (RSNO) via S-transnitrosation reactions. It has been difficult to evaluate the entity of this effect in blood circulation by in vitro assays with isolated aorta rings so far, because media rich in proteins cannot be used due to the foaming as a consequence of the needed gas bubbling. We have modified the original apparatus for organ bioassay in order to minimize foaming and to increase analytical performance. By using this modified bioassay we investigated the vasodilatory potency of various endogenous RSNOs in the presence of physiologically relevant concentrations of albumin and LMM-SH. Our results show that the sulfhydryl group of the cysteine moiety of albumin and LMM-SH has a dramatic effect on the vasodilatory potency of RSNO. Considering the equilibrium constants for S-transnitrosation reactions and the concentration of P-SH and LMM-SH we measured in healthy humans (aged 18-85 years), we infer that the age-dependency of hematic levels of LMM-SH may have a considerable impact in RSNO-mediated vasodilation. S-Nitrosoproteins such as S-nitrosoalbumin may constitute a relatively silent and constant amount of circulating RSNO. On the other hand, LMM-SH may mediate and control the biological actions of S-nitrosoproteins via S-transnitrosation reactions, by forming more potent nitric oxide-releasing LMM-S-nitrosothiols. Lifestyle habits, status of health and individual age are proven factors that, in turn, may influence the concentration of these compounds. These aspects should be taken into consideration when testing the vasodilatory effects of RSNO in pre-clinical studies.

Preservation of endothelium nitric oxide release during beating heart surgery with respect to continuous flow cardiopulmonary bypass

A correlation between perfusion modality and vascular dilation induced by endothelial nitric oxide (NO) release has been pointed out in the literature; nevertheless, only a few studies deal with the analysis of patients treated by cardiac surgery. The aim of this work is to analyze endothelial NO release in patients undergoing cardiac surgery under continuous flow cardiopulmonary bypass (CPB) or pulsatile perfusion. Pulsatile devices approved for clinical CPB do not accurately reproduce the physiological flow waveform provided by the left ventricle while, on the other hand, it is important to analyze pulsatile perfusion under both physiological flow waveform and pulsatile flow CPB. Physiological pulsatile perfusion (supplied by the left ventricle) was examined in this study. A total of 16 patients undergoing cardiac surgery were enrolled in the study and divided into two groups: 8 patients were put on continuous flow CPB while the others underwent beating heart surgery. Venous blood samples were withdrawn to quantify endothelial NO release through its bioactive forms in blood. Plasma was used for the chemiluminescent detection of nitrite (NO2—) and nitrate (NO3—), and the cellular component for electron spin resonance detection of nitrosylhemoglobin. Significant reduction in the intraoperative concentration with respect to the preoperative was observed only in the continuous group for both NO2— and NOx (NO2— + NO3—) concentration (p=0.003 and p=0.016, respectively). A significant difference in the intraoperative nitrite concentration was also observed between the groups (p=0.006). Nitrosylhemoglobin concentration, although not instrumentally detectable, resulted as negligible with respect to the other NO metabolites. Despite the small number of patients belonging to each group, this significant reduction of NO2— concentration under continuous flow CPB revealed a strong dependence on endothelial NO release and plasma nitrite concentration on perfusion modality.

Adaptation of the Griess Reaction for Detection of Nitrite in Human Plasma

The determination of nitrite in human plasma or serum has been most frequently used as a marker of nitric oxide (NO) production. In addition, it has recently been suggested that nitrite could act as a vasodilating agent at physiological concentrations by NO delivery. Therefore, nitrite determination in biological fluids is becoming increasingly important. The most frequently used method to measure nitrite is based on the spectrophotometric analysis of the azo dye obtained after reaction with the Griess reagent. This method has some limitations regarding detection limit and sensitivity, thus resulting unsuitable for nitrite detection in plasma. We have identified some drawbacks and modified the original procedure to overcome these problems. By the use of the newly developed method, we measured 221+/-72 nM nitrite in human plasma from healthy donors.

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.

Exogenous vs. endogenous gamma-glutamyltransferase activity: Implications for the specific determination of S-nitrosoglutathione in biological samples

The determination of S-nitrosoglutathione (GSNO) levels in biological fluids is controversial, partly due to the laborious sample handling and multiple pretreatment steps required by current techniques. GSNO decomposition can be effected by the enzyme gamma-glutamyltransferase (GGT), whose involvement in GSNO metabolism has been suggested. We have set up a novel analytical method for the selective determination and speciation of GSNO and its metabolite S-nitrosocysteinylglycine, based on liquid chromatography separation coupled to on-line enzymatic hydrolysis of GSNO by commercial GGT. In a post-column reaction coil, GGT allows the specific hydrolysis of the gamma-glutamyl moiety of GSNO, and the S-nitrosocysteinylglycine (GCNO) thus formed is decomposed by copper ions originating oxidized cysteinylglycine and nitric oxide (NO). NO immediately reacts with 4,5-diaminofluorescein (DAF-2) forming a triazole derivative, which is detected fluorimetrically. The limit of quantitation (LOQc) for GSNO and GCNO in plasma ultrafiltrate was 5 nM, with a precision (CV) of 1-6% within the 5-1500 nM dynamic linear range. The method was applied to evaluate the recovery of exogenous GSNO after addition of aliquots to human plasma samples presenting with different total GGT activities. By inhibiting GGT activity in a time dependent manner, it was thus observed that the recovery of GSNO is inversely correlated with plasmatic levels of endogenous GGT, which indicates the need for adequate inhibition of endogenous GGT activity for the reliable determination of endogenous GSNO.

Molecular mechanisms and potential clinical significance of S-glutathionylation

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

GC–MS assay for hepatic DDAH activity in diabetic and non-diabetic rats by measuring dimethylamine (DMA) formed from asymmetric dimethylarginine (ADMA): Evaluation of the importance of S-nitrosothiols as inhibitors of DDAH activity in vitro and in vivo in humans

Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthesis, is hydrolyzed to dimethylamine (DMA) and L-citrulline by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). In the present article we report on a GC-MS assay for DDAH activity in rat liver homogenate in phosphate buffered saline. The method is based on the quantitative determination of ADMA-derived DMA by GC-MS as the pentafluorobenzamide derivative. Quantification was performed by selected-ion monitoring of the protonated molecules at m/z 240 for DMA and m/z 246 for the internal standard (CD3)2NH in the positive-ion chemical ionization mode. The assay was applied to determine the enzyme kinetics in rat liver, the hepatic DDAH activity in streptozotocin-induced (50 mg/kg) diabetes in rats, and to evaluate the importance of S-nitrosothiols as DDAH inhibitors. The KM and Vmax values were determined to be 60 microM ADMA and 12.5 pmol DMA/minmg liver corresponding to 166 pmol DMA/minmg protein. Typical DDAH activity values measured in rat liver homogenate were 8.7 pmol DMA/minmg liver at added ADMA concentration of 100 microM. DDAH activity was found to be 1.7-fold elevated in diabetic as compared to non-diabetic rats (P=0.01). The SH-specific agents HgCl2, S-nitrosocysteine ethyl ester (SNACET), a synthetic lipophilic S-nitrosothiol, S-nitrosoglutathione (GSNO), S-nitrosocysteine (CysNO) and S-nitrosohomocysteine (HcysNO) were found to inhibit DDAH activity in rat liver homogenate. The IC50 values for HcysNO, SNACET, CysNO and GSNO were estimated to be 300, 500, 700 and 1000 microM, respectively. Oral administration of 15N-labelled SNACET to two healthy volunteers (1 micromol/kg) resulted in elevated urinary excretion of 15N-labelled nitrite and nitrate, but did not reduce creatinine-corrected excretion of DMA in the urine. Our results suggest that inhibition of DDAH activity on the basis of reversible nitros(yl)ation or irreversible N-thiosulfoximidation of the sulfhydryl group of the cysteine moiety involved in the catalytic process is most likely not a rationale design of DDAH inhibitors. A major advantage of the present GC-MS assay over other assays is that DDAH activity is assessed by measuring the formation of the specific enzymatic product DMA but not the formation of unlabelled or (radio)labelled L-citrulline or the decay of the substrate ADMA. The GC-MS assay reported here should be suitable to probe for DDAH activity in various disease models.

Transport and peripheral bioactivities of nitrogen oxides carried by red blood cell hemoglobin: role in oxygen delivery

The biology of NO (nitric oxide) is poorly explained by the activity of the free radical NO ((.)NO) itself. Although (.)NO acts in an autocrine and paracrine manner, it is also in chemical equilibrium with other NO species that constitute stable stores of NO bioactivity. Among these species, S-nitrosylated hemoglobin (S-nitrosohemoglobin; SNO-Hb) is an evolved transducer of NO bioactivity that acts in a responsive and exquisitely regulated manner to control cardiopulmonary and vascular homeostasis. In SNO-Hb, O(2) sensing is dynamically coupled to formation and release of vasodilating SNOs, endowing the red blood cell (RBC) with the capacity to regulate its own principal function, O(2) delivery, via regulation of blood flow. Analogous, physiological actions of RBC SNO-Hb also contribute to central nervous responses to blood hypoxia, the uptake of O(2) from the lung to blood, and baroreceptor-mediated control of the systemic flow of blood. Dysregulation of the formation, export, or actions of RBC-derived SNOs has been implicated in human diseases including sepsis, sickle cell anemia, pulmonary arterial hypertension, and diabetes mellitus. Delivery of SNOs by the RBC can be harnessed for therapeutic gain, and early results support the logic of this approach in the treatment of diseases as varied as cancer and neonatal pulmonary hypertension.

Measurement of circulating nitrite and S-nitrosothiols by reductive chemiluminescence

Considerable disparities in the reported levels of basal human nitrite and S-nitrosothiols (RSNO) in blood have brought methods of quantifying these nitric oxide (NO) metabolites to the forefront of NO biology. Ozone-based chemiluminescence is commonly used and is a robust method for measuring these species when combined with proper reductive chemistry. The goal of this article is to review existing methodologies for the measurement of nitrite and RSNO by reductive chemiluminescence. Specifically, we discuss in detail the measurement of nitrite and RSNO in biological matrices using tri-iodide and copper(I)/cysteine-based reduction methods coupled to chemiluminescence. The underlying reaction mechanisms, as well as the potential pitfalls of each method are discussed.

Age-related influence on thiol, disulfide, and protein-mixed disulfide levels in human plasma

In this study, plasma levels of both low-molecular-mass sulfhydryls/disulfides and mixed disulfides with proteins in 41 healthy humans aged 21-92 years were measured, with the aim of assessing whether there is a shift of the thiol/disulfide balance during aging and verifying some of the possible effects of the thiol imbalance. Our data suggest that aging is strictly correlated to a decrease in plasma glutathione and cysteinylglycine with the concomitant increase of most oxidized forms of thiols and a parallel increase in total cysteine and total homocysteine, probably due to an augmented efflux of these amino acids from various organs. The occurrence of two distinct regulatory systems for plasmatic pools of glutathione/cysteinylglycine on the one hand and cysteine/homocysteine on the other hand is hypothesized.

Detection of S-nitrosothiols in biological fluids: a comparison among the most widely applied methodologies

Many different methodologies have been applied for the detection of S-nitrosothiols (RSNOs) in human biological fluids. One unsatisfactory outcome of the last 14 years of research focused on this issue is that a general consensus on reference values for physiological RSNO concentration in human blood is still missing. Consequently, both RSNO physiological function and their role in disease have not yet been clarified. Here, a summary of the values measured for RSNOs in erythrocytes, plasma, and other biological fluids is provided, together with a critical review of the most widely used analytical methods. Furthermore, some possible methodological drawbacks, responsible for the highlighted discrepancies, are evidenced.

Measurement of nitric oxide levels in the red cell: validation of tri-iodide-based chemiluminescence with acid-sulfanilamide pretreatment

The tri-iodide-based chemiluminescence assay is the most widely used methodology for the detection of S-nitrosothiols (RSNOs) in biological samples. Because of the low RSNO levels detected in a number of biological compartments using this assay, criticism has been raised that this method underestimates the true values in biological samples. This claim is based on the beliefs that (i) acidified sulfanilamide pretreatment, required to remove nitrite, leads to RSNO degradation and (ii) that there is auto-capture of released NO by heme in the reaction vessel. Because our laboratories have used this assay extensively without ever encountering evidence that corroborated these claims, we sought to experimentally address these issues using several independent techniques. We find that RSNOs of glutathione, cysteine, albumin, and hemoglobin are stable in acidified sulfanilamide as determined by the tri-iodide method, copper/cysteine assay, Griess-Saville assay and spectrophotometric analysis. Quantitatively there was no difference in S-nitroso-hemoglobin (SNOHb) or S-nitroso-albumin (SNOAlb) using the tri-iodide method and a recently described modified assay using a ferricyanide-enhanced reaction mix at biologically relevant NO:heme ratios. Levels of SNOHb detected in human blood ranged from 20-100 nM with no arterial-venous gradient. We further find that 90% of the total NO-related signal in blood is caused by erythrocytic nitrite, which may partly be bound to hemoglobin. We conclude that all claims made thus far that the tri-iodide assay underestimates RSNO levels are unsubstantiated and that this assay remains the "gold standard" for sensitive and specific measurement of RSNOs in biological matrices.

Metabolism of oxidants by blood from different mouse strains

Haemoglobins bearing reactive sulfhydryl groups have been shown to be able to interplay with glutathione in some detoxification processes. Blood from different mouse strains commonly used as experimental animal models, i.e., C57, DBA and ICR, was treated with oxidants with the aim of evaluating: (i) the involvement of protein SH groups in oxido-reductive reactions that are commonly carried out by glutathione and (ii) the impact of this phenomenon on blood-mediated metabolism of thiol reactants. All the main forms of glutathione (reduced, disulfide, and mixed disulfide with haemoglobin) were measured after oxidant treatment. Significant differences were found among the studied strains: DBA mice formed preferably mixed disulfides instead of glutathione disulfide, whereas the opposite behaviour was shown by C57 mice. Unexpectedly, the ICR strain resulted to be composed of three different subgroups (ICRa, ICRb, and ICRc), with the ICRa behaving similarly to the DBA strain, ICRc to the C57 strain, and ICRc showing an intermediate behaviour. These results are due to the different number of haemoglobin SH groups in the studied mouse strains. In particular, additional fast-reacting SH groups were found in haemoglobin from DBA, ICRa, and ICRb mice, but not in the C57 and ICRc strain. These differences were also reflected in the susceptibility of haemoglobin to dimerize and in its ability to react with S-nitrosocysteine. Because of the widely different reactivity of haemoglobin cysteinyl residues, the mouse strains examined are an interesting but complicated model in which to study the pharmacological and toxicological action of some drugs.

Hemoglobin effects in the Saville assay

There is a great need to establish accurate, sensitive methods for measuring the concentration of nitrosothiols. Although some progress may have been made recently, differing methodologies have lead to reports of basal levels of nitrosothiols in human plasma that differ by three orders of magnitude. The Saville assay has been widely accepted as an accurate method for measuring nitrosothiols, but one that suffers from sensitivity below that of some other methods. Recently, it has been suggested that when hemoglobin is included in reaction mixtures used for the Saville assay, the sensitivity can be increased by an order of magnitude. Here we show that, on the contrary, the presence of sufficient hemoglobin in the Saville assay decreases its sensitivity.

Lack of allosterically controlled intramolecular transfer of nitric oxide from the heme to cysteine in the beta subunit of hemoglobin

The SNO-Hb hypothesis holds that heme-bound nitric oxide (NO) present in the beta subunits of T-state hemoglobin (Hb) will be transferred to the beta-93 cysteine upon conversion to R-state Hb, thereby forming SNO-Hb. A deficiency in the ability of Hb to facilitate this intramolecular transfer has recently been purported to play a role in pulmonary hypertension and sickle cell disease. We prepared deoxygenated Hb samples with small amounts of heme-bound NO and then oxygenated the samples. Electron paramagnetic resonance (EPR) spectroscopy was used to (1) determine the concentration of iron nitrosyl Hb (Fe-NO Hb), (2) show that the NO is evenly distributed among alpha and beta subunits, and (3) show that the Hb undergoes a change in its quaternary state (T to R) upon oxygenation. We did not observe a decrease in the concentration of Fe-NO Hb on oxygenation, which is inconsistent with the prediction of the SNO-Hb hypothesis.

Specific transport of S-nitrosocysteine in human red blood cells: Implications for formation of S-nitrosothiols and transport of NO bioactivity within the vasculature

The transport of various S-nitrosothiols, NO and NO donors in human red blood cells (RBC) and the formation of erythrocytic S-nitrosoglutathione were investigated. Of the NO species tested only S-nitrosocysteine was found to form S-nitrosoglutathione in the RBC cytosol. L-Serine, L-cysteine and L-lysine inhibited formation of S-nitrosoglutathione. Incubation of RBC pre-incubated with S-[15N]nitroso-L-cysteine with native plasma or platelet-rich plasma led to formation of S-[15N]nitrosoalbumin and inhibited platelet aggregation, respectively. The specific transporter system of S-nitroso-L-cysteine in the RBC membrane may have implications for formation of S-nitrosoalbumin and S-nitrosohemoglobin and for transport of NO bioactivity within the vasculature.

S-nitrosation versus S-glutathionylation of protein sulfhydryl groups by S-nitrosoglutathione

S-Nitrosation of protein sulfhydryl groups is an established response to oxidative/nitrosative stress. The transient nature and reversibility of S-nitrosation, as well as its specificity, render this posttranslational modification an attractive mechanism of regulation of protein function and signal transduction, in analogy to S-glutathionylation. Several feasible mechanisms for protein S-nitrosation have been proposed, including transnitrosation by S-nitrosothiols, such as S-nitrosoglutathione (GSNO), where the nitrosonium moiety is directly transferred from one thiol to another. The reaction between GSNO and protein sulfhydryls can also produce a mixed disulfide by S-glutathionylation, which involves the nucleophilic attack of the sulfur of GSNO by the protein thiolate anion. In this study, we have investigated the possible occurrence of S-glutathionylation during reaction of GSNO with papain, creatine phosphokinase, glyceraldehyde-3-phosphate dehydrogenase, alcohol dehydrogenase, bovine serum albumin, and actin. Our results show that papain, creatine phosphokinase, and glyceraldehyde-3-phosphate dehydrogenase were significantly both S-nitrosated and S-glutathionylated by GSNO, whereas alcohol dehydrogenase, bovine serum albumin, and actin appeared nearly only S-nitrosated. The susceptibility of the modified proteins to denitrosation and deglutathionylation by reduced glutathione was also investigated.

Speciation and Quantification of Thiols by Reversed-Phase Chromatography Coupled with On-Line Chemical Vapor Generation and Atomic Fluorescence Spectrometric Detection: Method Validation and Preliminary Application for Glutathione Measurements in Human Whole Blood

Background: We developed a sensitive, specific method for the low–molecular-mass thiols cysteine, cysteinylglycine, glutathione, and homocysteine and validated the method for measurement of glutathione in blood.

Methods: The technique was based on reversed-phase chromatography (RPC) coupled on line with cold vapor generation atomic fluorescence spectrometry (CVGAFS). Thiols were derivatized before introduction on the column by use of a p-hydroxymercuribenzoate (PHMB) mercurial probe and separated as thiol-PHMB complexes on a Vydac C4 column. Postcolumn on-line reaction of derivatized thiols with bromine allowed rapid conversion of the thiol-PHMB complexes to inorganic mercury with recovery of 100 (2)% of the sample. HgII was selectively detected by atomic fluorescence spectrometry in an Ar/H2 miniaturized flame after sodium borohydride reduction to Hg0.

Results: The relationship between thiol-PHMB complex concentration and peak area (CVGAFS signal) was linear over the concentration range 0.01–1400 μmol/L (injected). The detection limits were 1, 1, 0.6, and 0.8 nmol/L for cysteine, cysteinylglycine, homocysteine, and glutathione in the injected sample, respectively. The CVs for thiols were 1.5%–2.2% for calibrator solutions and 2.1% and 3.0% for real samples. The RPC-CVGAFS method allowed speciation of glutathione (reduced and oxidized) in human whole blood from healthy donors and from the coronary sinus of patients with idiopathic dilated cardiomyopathy during and after chronotropic stress.

Conclusion: The RPC-CVGAFS method could be used to measure reduced and oxidized glutathione in human whole blood as disease biomarkers.

Detection of human red blood cell-bound nitric oxide

Major disparities in reported levels of basal human nitric oxide metabolites have resulted in a recent literature focusing almost exclusively on methods. We chose to analyze triiodide chemiluminescence, drawn by the prospect of identifying why the most commonly employed assay in nitric oxide biology typically yielded lower metabolite values, compared with several other techniques. We found that the sensitivity of triiodide was greatly affected by the auto-capture of nitric oxide by deoxygenated cell-free heme in the reaction chamber. Potential contaminants and signal losses were also associated with standard sample purification procedures and the chemistry involved in nitrite removal. To inhibit heme nitric oxide auto-capture, we added potassium ferricyanide to the triiodide reagent, reasoning this would provide a more complete detection of any liberated nitric oxide. From human venous blood samples, we established nitric oxide levels ranging from 0.000178 to 0.00024 mol nitric oxide/mol hemoglobin. We went on to find significantly elevated nitric oxide levels in venous blood taken from diabetic patients in comparison to healthy controls (p < 0.0001). We concluded that the lack of signals reported of late by several groups using triiodide chemiluminescence for the detection of hemoglobin-bound nitric oxide may not represent levels on the border of assay sensitivity but rather underestimated values because of methodological limitations. We therefore stress the need for assay systems to be developed that differentiate between individual nitric oxide metabolite species and overcome the limitations we outline, allowing accurate conclusions to be drawn regarding physiological nitric oxide metabolite levels.

Chemical physiology of blood flow regulation by red blood cells: the role of nitric oxide and S-nitrosohemoglobin

Blood flow in the microcirculation is regulated by physiological oxygen (O2) gradients that are coupled to vasoconstriction or vasodilation, the domain of nitric oxide (NO) bioactivity. The mechanism by which the O2 content of blood elicits NO signaling to regulate blood flow, however, is a major unanswered question in vascular biology. While the hemoglobin in red blood cells (RBCs) would appear to be an ideal sensor, conventional wisdom about its chemistry with NO poses a problem for understanding how it could elicit vasodilation. Experiments from several laboratories have, nevertheless, very recently established that RBCs provide a novel NO vasodilator activity in which hemoglobin acts as an O2 sensor and O2-responsive NO signal transducer, thereby regulating both peripheral and pulmonary vascular tone. This article reviews these studies, together with biochemical studies, that illuminate the complexity and adaptive responsiveness of NO reactions with hemoglobin. Evidence for the pivotal role of S-nitroso (SNO) hemoglobin in mediating this response is discussed. Collectively, the reviewed work sets the stage for a new understanding of RBC-derived relaxing activity in auto-regulation of blood flow and O2 delivery and of RBC dysfunction in disorders characterized by tissue O2 deficits, such as sickle cell disease, sepsis, diabetes, and heart failure.

Nitrosylation of thiols in vascular homeostasis and disease

Post-translational modification of thiols to form a nitrosothiol (S-nitrosylation) is gaining attention as a mechanism by which nitric oxide can exert some of its effects in the cardiovascular system, as well as in other systems. It has been proposed that this modification would have a particularly important role in cell signaling. However, its study has been hampered by many technical difficulties. In this paper, we summarize current achievements in the field that may help to answer the questions that have been posed about the functional role of this modification. Some of these achievements have arisen from methodologic improvements, such as proteomic studies or approaches to identify subcellular localization and tissue distribution of the modification. New functional consequences of the modification of individual proteins are also summarized.

Control of Plasma Nitric Oxide Bioactivity by Perfluorocarbons

Background— Perfluorocarbons (PFCs) are promising blood substitutes because of their chemical inertness and unparalleled ability to transport and upload O 2 and CO 2 . Here, we report that PFC emulsions also efficiently absorb and transport nitric oxide (NO).

Methods and Results— Accumulation of NO and O 2 in PFC micelles results in rapid NO oxidation and generation of reactive NO x species. Such micellar catalysis of NO oxidation leads to formation of vasoactive S -nitrosothiols (RSNO) in vitro and in vivo as detected electrochemically. The efficiency of PFC-mediated S -nitrosation depends on the amount of PFC in aqueous solution. The optimal PFC concentration that produced the maximum level of RSNO was ≈1% (vol/vol). Larger PFC amounts were progressively less efficient in generating RSNO and functioned simply as NO sink. These results explain the characteristic hemodynamic effects of PFCs. Intravenous bolus application of PFC (0.14 g/kg, ≈1% vol/vol) to Wistar-Kyoto rats decreased mean arterial pressure significantly (−10 mm Hg over 40 minutes). PFC-induced hypotension could be further stimulated (−17 mm Hg over 140 minutes) by exogenous thiols (cysteine and glutathione). In contrast, a larger amount of PFC (1 g/kg, ≈7% vol/vol) exhibited a strong hypertensive effect (11 mm Hg over 40 minutes).

Conclusions— The present study reveals a physiologically significant pool of endogenous plasma NO and underscores the crucial role of the circulating hydrophobic phase in modulating its bioactivity. The results also establish PFC as a conceptually new pharmacological tool for various cardiovascular complications associated with NO imbalance.