Physiological Levels of S -Nitrosothiols in Human Plasma

Functional evidence that S-nitroso-L-cysteine may be a candidate carotid body neurotransmitter

The primary objective of the present study is to provide further evidence that the endogenous S-nitrosothiol, S-nitroso-L-cysteine (L-CSNO), plays an essential role in signaling the hypoxic ventilatory response (HVR) in rodents. Key findings were that (1) injection of L-CSNO (50 nmol/kg, IV) caused a pronounced increase in frequency of breathing (Freq), tidal volume (TV) and minute ventilation (MV) in naïve C57BL/6 mice, whereas injection of D-CSNO (50 nmol/kg, IV) elicited minimal responses; (2) L-CSNO elicited minor responses in (a) C57BL/6 mice with bilateral carotid sinus nerve transection (CSNX), (b) C57BL/6 mice treated neonatally with capsaicin (CAP) to eliminate small-diameter C-fibers, and (c) C57BL/6 mice receiving continuous infusion of L-CSNO receptor antagonists, S-methyl-L-cysteine and S-ethyl-L-cysteine (L-SMC + L-SEC, both at 5 μmol/kg/min, IV); and (3) injection of S-nitroso-L-glutathione (L-GSNO, 50 nmol/kg, IV) elicited pronounced ventilatory responses that were not inhibited by L-SMC + L-SEC. Subsequent exposure of naïve C57BL/6 mice to a hypoxic gas challenge (HXC; 10% O2, 90% N2) elicited pronounced increases in Freq, TV and MV that were subject to roll-off. These HXC responses were markedly reduced in CSNX, CAP, and L-SMC + L-SEC-infused C57BL/6 mice. Subsequent exposure of all C57BL/6 mice (naïve, CSNX, CAP, and L-SMC + L-SEC) to a hypercapnic gas challenge (5% CO2, 21% O2, 74% N2) elicited similar robust increases in Freq, TV and MV. Taken together, these findings provide evidence that an endogenous factor with pharmacodynamic properties similar to those of L-CSNO, rather than L-GSNO, mediates the HVR in male C57BL/6 mice.

The effect of maternal tobacco smoking and second-hand tobacco smoke exposure on human milk oxidant-antioxidant status

BACKGROUND

Many women who smoke tobacco continue to do so during lactation, and many non-smoking women are exposed to second-hand tobacco smoke (SHS) during the period that she wishes to breastfeed. There are reports documenting the adverse effects of maternal smoking during lactation on their infant's health; however, the pathophysiological mechanisms underlying these effects are incompletely understood.

OBJECTIVES

Our study purpose was to examine the influence of tobacco smoke on biochemical markers reflecting the intensity of oxidative stress using concentration of total protein (TP), trolox equivalent antioxidant capacity (TEAC), S-nitrosothiols (RSNO), nitric oxide (NO), thiobarbituric acid reactive substances (TBARS), reduced glutathione (GSH), glutathione S-transferase (GST), glutathione peroxidase (GPx), superoxide dismutase (SOD), catalase (CAT) in the plasma, colostrum, and mature milk of women who smoke, those only exposed to SHS, and non-smokers.

METHODS

Questionnaire data on the tobacco smoking status were verified based on the determination of cotinine by high performance liquid chromatography with diode array detector (HPLC-DAD). Relevant markers of oxidative stress and biochemical parameters were determined using spectrophotometric methods.

RESULTS

We found that tobacco smoking during lactation increases oxidative stress in the mother's plasma, colostrum, and mature milk, and lesser so in those exposed to SHS. Tobacco smoke significantly increase TBARS and decrease TEAC in colostrum and mature milk. In response to ROS generated by tobacco smoke increase the activity of antioxidant enzymes (SOD, GST, GPx and CAT), p < 0.05.

DISCUSSION

Such exposure to tobacco smoke influences the antioxidant barrier of human colostrum and mature milk that can adversely affect their infant's health. Greater public health awareness of the adverse effects of tobacco smoking during lactation on breast milk quality and its protective effects is urgently needed.

In vivo pharmacological activity and biodistribution of S-nitrosophytochelatins after intravenous and intranasal administration in mice

S-nitrosophytochelatins (SNOPCs) are novel analogues of S-nitrosoglutathione (GSNO) with the advantage of carrying varying ratios of S-nitrosothiol (SNO) moieties per molecule. Our aim was to investigate the in vivo pharmacological potency and biodistribution of these new GSNO analogues after intravenous (i.v.) and intranasal (i.n.) administration in mice. SNOPCs with either two or six SNO groups and GSNO were synthesized and characterized for purity. Compounds were administered i.v. or i.n. at 1 μmol NO/kg body weight to CD-1 mice. Blood pressure was measured and biodistribution studies of total nitrate and nitrite species (NOx) and phytochelatins were performed after i.v. administration. At equivalent doses of NO, it was observed that SNOPC-6 generated a rapid and significantly greater reduction in blood pressure (∼60% reduction compared to saline) whereas GSNO and SNOPC-2 only achieved a 30-35% decrease. The reduction in blood pressure was transient and recovered to baseline levels within ∼2 min for all compounds. NOx species were transiently elevated (over 5 min) in the plasma, lung, heart and liver. Interestingly, a size-dependent phytochelatin accumulation was observed in several tissues including the heart, lungs, kidney, brain and liver. Biodistribution profiles of NOx were also obtained after i.n. administration, showing significant lung retention of NOx over 15 min with minor systemic increases observed from 5 to 15 min. In summary, this study has revealed interesting in vivo pharmacological properties of SNOPCs, with regard to their dramatic hypotensive effects and differing biodistribution patterns following two different routes of administration.

Local and systemic vasodilatory effects of low molecular weight S-nitrosothiols

S-nitrosothiols (SNOs) such as S-nitroso-L-cysteine (L-cysNO) are endogenous compounds with potent vasodilatory activity. During circulation in the blood, the NO moiety can be exchanged among various thiol-containing compounds by S-transnitrosylation, resulting in SNOs with differing capacities to enter the cell (membrane permeability). To determine whether the vasodilating potency of SNOs is dependent upon membrane permeability, membrane-permeable L-cysNO and impermeable S-nitroso-D-cysteine (D-cysNO) and S-nitroso-glutathione (GSNO) were infused into one femoral artery of anesthetized adult sheep while measuring bilateral femoral and systemic vascular conductances. L-cysNO induced vasodilation in the infused hind limb, whereas D-cysNO and GSNO did not. L-cysNO also increased intracellular NO in isolated arterial smooth muscle cells, whereas GSNO did not. The infused SNOs remained predominantly in a low molecular weight form during first-passage through the hind limb vasculature, but were converted into high molecular weight SNOs upon systemic recirculation. At systemic concentrations of ~0.6 μmol/L, all three SNOs reduced mean arterial blood pressure by ~50%, with pronounced vasodilation in the mesenteric bed. Pharmacokinetics of L-cysNO and GSNO were measured in vitro and in vivo and correlated with their hemodynamic effects, membrane permeability, and S-transnitrosylation. These results suggest local vasodilation by SNOs in the hind limb requires membrane permeation, whereas systemic vasodilation does not. The systemic hemodynamic effects of SNOs occur after equilibration of the NO moiety amongst the plasma thiols via S-transnitrosylation.

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.

Reductive gas-phase chemiluminescence and flow injection analysis for measurement of the nitric oxide pool in biological matrices

There is growing evidence for nitric oxide (NO.) being involved in cell signaling and pathology. Much effort has been made to elucidate and characterize the different biochemical reaction pathways of NO.in vivo. However, a major obstacle in assessing the significance of nitrosated species and oxidized metabolites often remains: a reliable analytical technique for the detection of NO. in complex biological matrices. This chapter presents refined methodologies, such as chemiluminescence detection and flow injection analysis, compared with adequate sample processing procedures to reliably quantify and assess the circulating and resident NO(.) pool, consisting of nitrite, nitrate, nitroso, and nitrosylated species.

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.

Flavanols and Cardiovascular Health: Effects on the circulating NO Pool in Humans

Atherosclerosis is the major cause for chronic vascular diseases. The key event in the pathogenesis of atherosclerosis is believed to be dysfunction of the endothelium and disruption of endothelial homeostasis, leading to vasoconstriction, inflammation, leukocyte adhesion, thrombosis, and proliferation of vascular smooth muscle cells. Endothelium-derived nitric oxide (NO) plays a major role in vascular homeostasis and a decrease in NO-bioavailability accelerates the development of atherosclerosis. Given that endothelial dysfunction is at least in part reversible, the characterization of endothelial function and therapeutical approaches have gained much attention over the past years. Recent studies demonstrated that especially the consumption of plant-derived foods rich in certain flavonoids can improve endothelial function in both compromised and healthy humans. Furthermore, various physiologic and biochemical measures have been used previously as biomarkers for the assessment of the proposed beneficial effects of flavonoids in this context. More recently, the analysis of plasma nitros(yl)ated species (RXNOs), referred to as the circulating NO pool, has gained recognition, especially as a marker for endothelial function. This review is aimed at evaluating the suitability of quantifying this NO pool as a biomarker for cardiovascular function in humans, in particular during dietary interventions with flavonoid-rich foods.

Circulating NO Pool in Humans

Nitric oxide (NO) generated from L-arginine by NO synthases in the endothelium and in other cells plays a central role in several aspects of vascular biology and has been linked to many regulatory functions in mammalian cells. Whereas for a long time the signaling actions of NO in the vasculature have been thought to be short-lived as a result of the rapid reaction of NO with hemoglobin, recent studies changed the biochemical thinking of NO. NO is not anymore the paracrine agent with only local effects, but, like a hormone, it disseminates throughout the body. Thus, a circulating pool of NO exists, opening new considerable pharmacological and therapeutical avenues in the diagnosis and therapy of cardiovascular diseases. In this review we briefly discuss the major routes of NO metabolism and transport in the mammalian circulation, considering plasma, red blood cell and tissue compartments separately, with a special focus on the implication of the circulating NO pool in clinical research.

Plasma nitroso compounds are decreased in patients with endothelial dysfunction

OBJECTIVES

We investigated whether plasma nitros(yl)ated species (RXNOs) that mediate systemic nitric oxide (NO) bioactivity are depleted in individuals with cardiovascular risk factors and endothelial dysfunction.

BACKGROUND

Endothelium-derived NO acts not only as a regional messenger but exerts significant systemic effects via formation of circulating RXNOs delivering NO to sites of impaired production.

METHODS

Endothelial function was assessed in 68 patients with one to four major cardiovascular risk factors (RF) and 39 healthy control subjects (C) by measurement of flow-mediated dilation (FMD) of the brachial artery using high-resolution ultrasound. In parallel, plasma RXNOs were determined by reductive gas phase chemiluminescence.

RESULTS

Increasing numbers of risk factors were accompanied by a progressive decrease in FMD: 6.5 +/- 0.4% (C); 4.7 +/- 0.5% (one RF); 2.8 +/- 0.4% (two RF); 2.2 +/- 0.4% (three RF); and 1.0 +/- 0.3% (four RF). Progressively impaired vascular function was associated with a concomitant decrease in plasma RXNOs (p < 0.01): 39 +/- 2 nmol/l (C); 30 +/- 2 nmol/l (one RF); 24 +/- 3 nmol/l (two RF); 22 +/- 3 nmol/l (three RF); and 15 +/- 2 nmol/l (four RF), with univariate correlation between FMD and RXNO (r = 0.41, p < 0.001). In a multivariate regression model, RXNO was an independent predictor of endothelial function.

CONCLUSIONS

Endothelial dysfunction in patients with cardiovascular risk factors is associated with decreased levels of circulating RXNOs. Plasma RXNOs may be diagnostically useful markers of NO bioavailability and a surrogate index of endothelial function. Whether the observed decrease in concentration reflects impaired NO formation, accelerated decomposition, and/or consumption of RXNOs and whether these processes play a causal role in the pathophysiology of arteriosclerosis remain to be investigated.

Acute consumption of flavanol-rich cocoa and the reversal of endothelial dysfunction in smokers

OBJECTIVES

This study was designed to assess the effect of flavanol-rich food on the circulating pool of bioactive nitric oxide (NO) and endothelial dysfunction in smokers.

BACKGROUND

Studies suggest that smoking-related vascular disease is caused by impaired NO synthesis and that diets rich in flavanols can increase bioactive NO in plasma.

METHODS

In smokers (n = 11), the effects of flavanol-rich cocoa on circulating NO species in plasma (RXNO) measured by reductive gas-phase chemiluminescence and endothelial function as assessed by flow-mediated dilation (FMD) were characterized in a dose-finding study orally administering cocoa containing 88 to 370 mg flavanols and in a randomized double-blind crossover study using 100 ml cocoa drink with high (176 to 185 mg) or low (<11 mg) flavanol content on two separate days. In addition to cocoa drink, ascorbic acid and NO-synthase inhibitor L-NMMA (n = 4) were applied.

RESULTS

There were significant increases in RXNO (21 +/- 3 nmol/l to 29 +/- 5 nmol/l) and FMD (4.5 +/- 0.8% to 6.9 +/- 0.9%, each p < 0.05) at 2 h after ingestion of 176 to 185 mg flavanols, a dose potentially exerting maximal effects. These changes correlated with increases in flavanol metabolites. Cocoa-associated increases in RXNO and FMD were reversed by L-NMMA. Ascorbic acid had no effect.

CONCLUSIONS

The circulating pool of bioactive NO and endothelium-dependent vasodilation is acutely increased in smokers following the oral ingestion of a flavanol-rich cocoa drink. The increase in circulating NO pool may contribute to beneficial vascular health effects of flavanol-rich food.

Measurement of S-nitrosothiols in extracellular fluids from healthy human volunteers and rheumatoid arthritis patients, using electron paramagnetic resonance spectrometry

In human tissues, S-nitrosothiols (RSNOs) are generated by the nitric oxide (NO.)-dependent S-nitrosation of thiol-containing species. Here, a novel electron paramagnetic resonance spectrometry assay for RSNOs is described, together with its application to studies of human health and disease. The assay involves degrading RSNOs using N-methyl-d-glucamine dithiocarbamate (MGD) at high pH and spin trapping the NO. released using (MGD)2-Fe2+. Because dietary nitrate might contribute to tissue RSNOs, the assay was used to monitor the effect of Na15NO3 ingestion on plasma and gastric juice RSNOs in healthy human volunteers. Na15NO3 ingestion (2 mmol) increased gastric RS15NO concentrations (p<0.01), but there was no significant effect on plasma RS15NO concentrations. Having established that dietary nitrate was not a confounding factor, we applied the RSNO assay to matched plasma and knee-joint synovial fluid (SF) from rheumatoid arthritis (RA) patients, with healthy subjects as controls. Clinical markers of RA inflammatory disease activity were quantified, as were plasma and SF NO2- and NO3-. Median RSNO concentrations were 0 (interquartile range 68) nM, 109 (282) nM, and 309 (470) nM in normal plasma, RA plasma, and SF, respectively. The median RSNO concentration was significantly elevated in RA SF compared with RA plasma (p<0.05) and in RA plasma compared with normal plasma (p<0.05). SF RSNO concentrations correlated positively with SF neutrophil counts (rs=0.55, p<0.05) and inversely with blood hemoglobin concentrations (rs=-0.52, p<0.05), but not with NO2- or NO3-. Thus the raised levels of RSNOs in RA SF correlate with some established markers of inflammation, suggesting the described RSNO assay may have applications in rapid clinical monitoring of NO metabolism in human inflammatory conditions.

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.

S-Nitrosothiols: cellular formation and transport

This review will focus on the transport and intracellular formation of S-nitrosothiols in cell culture models. The major points made in this article are: (1) S-Nitrosothiols are actively metabolized by cells. (2) S-Nitrosothiols affect cells in ways distinctly different from those of nitric oxide and can act through mechanisms that do not involve the intermediacy of nitric oxide. (3) Some S-nitrosothiols (S-nitrosocysteine, S-nitrosohomocysteine) can be taken up into cells via amino acid transport system L, whereas others (S-nitrosoglutathione, S-nitroso-N-acetylpenicillamine) are not directly transported, but require the presence of cysteine and/or cystine before the nitroso functional group is transported. (4) Proteomic detection of intracellular S-nitrosothiols is currently possible only if cells are loaded with high levels of S-nitrosothiols, and methodological advances are required in order to examine the S-nitrosated proteome after exposure of cells to physiological levels of nitric oxide.

Nitric oxide and blood: a review

Nitric oxide (NO) was identified as a physiological mediator of vascular tone in 1987. NO produced by endothelial cells causes vasodilatation and also inhibits platelet aggregation and leucocyte adhesion. Red cells metabolize NO to nitrate but may possibly carry and release, or even produce, NO in hypoxic conditions. NO physiology may have important implications for transfusion medicine, ranging from adverse effects of haemoglobin substitutes to preservation of stored platelets and to detrimental effects of stored red cells.

New methods to evaluate endothelial function: A search for a marker of nitric oxide (NO) in vivo: re-evaluation of NOx in plasma and red blood cells and a trial to detect nitrosothiols

Although plasma NOx (NO(2)(-) and NO(3)(-)) has been used as an index of nitric oxide (NO) formation in vivo, many unreasonable results appeared even after active elimination of NOx contamination from laboratory ware. For example, plasma NOx concentrations did not increase during vasodilation mediated by the NO/cGMP pathway or after organ perfusion. A possible shift of NOx from plasma to erythrocytes (RBCs) as a cause of these phenomena has been excluded, leaving the destination of NOx (after leaving plasma) unknown. Kinetic analyses have revealed that steady state NOx concentrations in plasma and whole blood did not correlate with the NOx formation rate, but rather with the NOx elimination rate. Therefore, the supposition that the NO status is directly reflected by plasma NOx concentrations appears untenable. As nitrosothiols (R-SNOs), possible carriers of NO bioactivity, have been flagged as alternative indices of NO status in vivo, efforts have been made to detect these substances. When interference by ultrafiltration was eliminated, low molecular weight R-SNOs such as nitrosocystein and nitrosogluthathione were undetectable. However, a high-molecular weight R-SNO, nitrosoalbumin, was detected in human blood. Further research is required into the significance and practical use of nitrosoalbumin as a marker of NO in vivo.

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

Two main hypotheses describe the role of hemoglobin in the regulation of nitric oxide (NO) bioavailability. It has been suggested that hemoglobin interacts with circulating NO, forming Fe-nitrosyl hemoglobin and then S-nitrosothiols, which deliver NO extracellularly by an allosterically regulated mechanism. Alternatively, the existence of diffusional barriers that protect NO from hemoglobin-mediated degradation has been proposed. The reliability of each model in vivo is supported by the detection of physiological hematic levels of S-nitrosohemoglobin. However, the measured concentrations of S-nitrosohemoglobin are largely divergent between the two models. Moreover, recent reports suggest that circulating levels of S-nitrosohemoglobin in human blood could be significantly lower than assessed previously. We suggest that solving the methodological controversies that make the field of NO research a 'minefield', even for skilled analysts, is fundamental to understanding the role of S-nitrosothiols in the vasculature.

Emerging role of nitrite in human biology

Nitric oxide (NO) plays a fundamental role in maintaining normal vascular function. NO is produced by endothelial cells and diffuses both into smooth muscle causing vasodilation and into the vessel lumen where the majority of this highly potent gas is rapidly inactivated by dioxygenation reaction with oxyhemoglobin to form nitrate. Diffusional barriers for NO around the erythrocyte and along the endothelium in laminar flowing blood reduce the inactivation reaction of NO by hemoglobin, allowing sufficient NO to escape for vasodilation and also to react in plasma and tissues to form nitrite anions (NO(2)(-)) and NO-modified peptides and proteins (RX-NO). Several recent studies have highlighted the importance of the nitrite anion in human biology. These studies have shown that measurement of plasma nitrite is a sensitive index of constitutive NO synthesis, suggesting that it may be useful as a marker of endothelial function. Additionally, recent evidence suggests that nitrite represents a circulating storage pool of NO and may selectively donate NO to hypoxic vascular beds. The conversion of nitrite to NO requires a reaction with a deoxygenated heme protein, suggesting a novel function of hemoglobin as a deoxygenation-dependent nitrite reductase. This review focuses on the role of nitrite as a circulating NO donor, its potential as an index of NO synthase (NOS) activity and endothelial function, and discusses potential diagnostic and therapeutic applications.

Circulating NO pool: assessment of nitrite and nitroso species in blood and tissues

The formation of nitric oxide (NO) has been linked to many regulatory functions in mammalian cells. With the appreciation that NO-mediated nitrosation reactions are involved in cell signaling and pathology there is a need to elucidate and better characterize the different biochemical pathways of NO in vivo. Despite significant methodological advances over the years one major obstacle in assessing the significance of nitrosated species and other NO-related metabolites remains: their reliable measurement in complex biological matrices. In this review we briefly discuss the major routes of NO metabolism and transport in the mammalian circulation, considering plasma, red blood cell, and tissue compartments separately. In addition, we attempt to give a recommendation as to the most appropriate analytical technique and sample processing procedures for the reliable quantification of either species.

An improved HPLC measurement for GSH and GSSG in human blood

The pathophysiological sequelae of oxidative/nitrosative stress are notoriously difficult to quantify. Despite these impediments, the medical significance of oxidative/nitrosative stress has become increasingly recognized to the point that it is now considered to be a component of virtually every disease. The level of oxidative stress can be quantified in blood by the measurement of the increase in glutathione disulfide (GSSG) and the decrease in the GSH/GSSG ratio, which has been shown to be altered in a variety of human diseases such as lung inflammation, amyotrophic lateral sclerosis, chronic renal failure, malignant disorders, and diabetes. Among the proposed methods for GSH/GSSG detection, the amino group derivatization with 2,4-dinitrofluorobenzene followed by HPLC separation has the advantage of allowing evaluation of both parameters within a single run contemporaneously. However, it has been shown that the application of this method on blood samples is not reproducible. In this report, we offer an explanation for these experimental limits and suggest some modifications that allow the application of this method to blood samples. The modified method has a low detection limit (0.5 microM, i.e., 1.4 pmoles) and a high reproducibility with a within-run imprecision of less than 2%. It could have a wide application as it is simple, virtually artifact-free, and not time-consuming, especially for large-scale screening studies.

S-nitrosothiols inhibit uterine smooth muscle cell proliferation independent of metabolism to NO and cGMP formation

S-nitrosothiols (RSNOs) are important mediators of nitric oxide (NO) biology. The two mechanisms that appear to dominate in their biological effects are metabolism leading to the formation of NO and S-nitrosation of protein thiols. In this study we demonstrate that RSNOs inhibit uterine smooth muscle cell proliferation independent of NO. The antiproliferative effects of NO on vascular smooth muscle are well defined, with the classic NO-dependent production of cGMP being demonstrated as the active pathway. However, less is known on the role of NO in mediating uterine smooth muscle cell function, a process that is important during menstruation and pregnancy. The RSNOs S-nitrosoglutathione and S-nitroso-N-acetyl pencillamine inhibited growth factor-dependent proliferation of human and rat uterine smooth muscle cells (ELT-3). Interestingly, these cells reduced RSNOs to generate NO. However, use of NO donors and other activators of the cGMP pathway failed to inhibit proliferation. These findings demonstrate the tissue-specific nature of responses to NO and demonstrate the presence of a RSNO-dependent but NO-independent pathway of inhibiting DNA synthesis in uterine smooth muscle cells.

Vasoactivity of S-nitrosohemoglobin: role of oxygen, heme, and NO oxidation states

The mechanisms by which S-nitrosohemoglobin (SNOHb) stimulates vasodilation are unclear and underlie the controversies surrounding the proposal that this S-nitrosothiol modulates blood flow in vivo. Among the mechanistic complexities are the nature of vasoactive species released from SNOHb and the role heme and oxygen play in this process. This is important to address since hemoglobin inhibits NO-dependent vasodilation. We compared the vasodilatory properties of distinct oxidation and ligation states of SNOHb at different oxygen tensions. The results show that SNOHb in the oxygenated state (SNOoxyHb) is significantly less efficient than SNOHb in the ferric or met oxidation state (SNOmetHb) at stimulating relaxation of isolated rat aortic rings. Using pharmacologic approaches to modulate nitrogen monoxide radical (.NO)-dependent relaxation, our data suggest that SNOoxyHb promotes vasodilation in a.NO-independent manner. In contrast, both SNOmetHb and S-nitrosoglutathione (GSNO), a putative intermediate in SNOHb reactivity, elicit vasodilation in a.NO-dependent process. Consistent with previous observations, an increase in sensitivity of SNOHb vasodilation at low oxygen tensions also was observed. However, this was not exclusive for this protein but applied to a range of nitrosovasodilators (including a.NO donor [DeaNonoate], an S-nitrosothiol [GSNO], and the nitroxyl anion donor, Angelis salt). This suggests that oxygen-dependent modulation of SNOHb vasoactivity does not occur by controlling the allosteric state of Hb but is a property of vessel responsiveness to nitrosovasodilators at low oxygen tensions.

Nitric oxide and S-nitrosothiols in human blood

The hypothesis that endothelial-derived relaxing factor (EDRF) is nitric oxide has stimulated a wealth of research into the significance of this novel intriguing molecule. Given its short life, many storage forms of NO as well as targets have been postulated. Among these, a pool of derivatives of NO (S-nitrosothiols, RSNOs) covalently bound to SH groups of proteins and low molecular weight thiols (e.g., glutathione) have been identified in various biological systems. The importance of RSNOs results from the very similar biological actions exhibited by both NO and RSNOs in vivo as well as in vitro. In particular, it has been observed that in the bloodstream, these molecules are able to provoke vasodilatation with a consequent fall in blood pressure and an antithrombotic effect by inhibition of platelet aggregation. Many hypotheses have been postulated about the biochemical species and the mechanisms involved in these processes, but many aspects have not yet been clarified. In addition, some RSNOs have been recently proposed to be clinical parameters, whose levels may vary under some pathological conditions. The therapeutic utility of RSNOs as an alternative to classic NO donors has also been suggested.Here, we provide a critical analysis of the main reports about the biochemical, physiological, pathological and therapeutic properties of RSNOs in the cardiovascular system. Particular attention is addressed to conflicting results and to discrepancies in the methodologies and models utilized. The numerous unanswered questions concerning the role of RSNOs in the control of vascular tone are discussed.

Concomitant presence of N-nitroso and S-nitroso proteins in human plasma

Nitric oxide (NO)-mediated nitrosation reactions are involved in cell signaling and pathology. Recent efforts have focused on elucidating the role of S-nitrosothiols (RSNO) in different biological systems, including human plasma, where they are believed to represent a transport and buffer system that controls intercellular NO exchange. Although RSNOs have been implicated in cardiovascular disease processes, it is yet unclear what their true physiological concentration is, whether a change in plasma concentration is causally related to the underlying pathology or purely epiphenomenological, and to what extent other nitrosyl adducts may be formed under the same conditions. Therefore, using gas phase chemiluminescence and liquid chromatography we sought to quantify the basal plasma levels of NO-related metabolites in 18 healthy volunteers. We find that in addition to the oxidative products of NO metabolism, nitrite (0.20 +/- 0.02 micromol/l nitrite) and nitrate (14.4 +/- 1.7 micromol/l), on average human plasma contains an approximately 5-fold higher concentration of N-nitroso species (32.3 +/- 5.0 nmol/l) than RSNOs (7.2 +/- 1.1 nmol/l). Both N- and S-nitroso moieties appear to be associated with the albumin fraction. This is the first report on the constitutive presence of a high-molecular-weight N-nitroso compound in the human circulation, raising the question as to its origin and potential physiological role. Our findings may not only have important implications for the transport of NO in vivo, but also for cardiovascular disease diagnostics and the risk assessment of nitrosamine-related carcinogenesis in man.

S-Nitrosohemoglobin is unstable in the reductive erythrocyte environment and lacks O2/NO-linked allosteric function

Our previous results run counter to the hypothesis that S-nitrosohemoglobin (SNO-Hb) serves as an in vivo reservoir for NO from which NO release is allosterically linked to oxygen release. We show here that SNO-Hb undergoes reductive decomposition in erythrocytes, whereas it is stable in purified solutions and in erythrocyte lysates treated with an oxidant such as ferricyanide. Using an extensively validated methodology that eliminates background nitrite and stabilizes erythrocyte S-nitrosothiols, we find the levels of SNO-Hb in the basal human circulation, including red cell membrane fractions, were 46 +/- 17 nm in human arterial erythrocytes and 69 +/- 11 nm in venous erythrocytes, incompatible with the postulated reservoir function of SNO-Hb. Moreover, we performed experiments on human red blood cells in which we elevated the levels of SNO-Hb to 10,000 times the normal in vivo levels. The elevated levels of intra-erythrocytic SNO-Hb fell rapidly, independent of oxygen tension and hemoglobin saturation. Most of the NO released during this process was oxidized to nitrate. A fraction (25%) was exported as S-nitrosothiol, but this fraction was not increased at low oxygen tensions that favor the deoxy (T-state) conformation of Hb. Results of these studies show that, within the redox-active erythrocyte environment, the beta-globin cysteine 93 is maintained in a reduced state, necessary for normal oxygen affinity, and incapable of oxygen-linked NO storage and delivery.