Assessment of nitric oxide signals by triiodide chemiluminescence

The transfusion problem: role of aberrant S-nitrosylation

Protein S-nitrosylation (the binding of a nitric oxide [NO] group to a cysteine thiol) is a major mechanism through which the ubiquitous cellular influence of NO is exerted. Disruption of S-nitrosylation is associated with a wide range of pathophysiologic conditions. Hemoglobin (Hb) exemplifies both of these concepts. It is the prototypical S-nitrosylated protein in that it binds, activates, and deploys NO. Within red blood cells (RBCs), Hb is S-nitrosylated during the respiratory cycle and thereby conveys NO bioactivity that may be dispensed to regulate local blood flow in the physiologic response known as hypoxic vasodilation. Hb thus both delivers oxygen directly and delivers vasoactivity to potentially optimize tissue perfusion in concert with local metabolic demand. Accordingly, decreased levels of S-nitrosylated Hb (also known as S-nitrosohemoglobin) and/or impaired delivery of RBC-derived NO bioactivity have been observed in a variety of disease states that are characterized by tissue hypoxemia. It has been shown recently that storage of blood depletes S-nitrosylated Hb, accompanied by reduced ability of RBCs to induce vasodilation. This defect appears to account in significant part for the impaired ability of banked RBCs to deliver oxygen. Renitrosylation can correct this impairment and thus may offer a means to ameliorate the disruptions in tissue perfusion produced by transfusion.

Methodologies for the characterization, identification and quantification of S-nitrosylated proteins

BACKGROUND

Protein S-nitrosylation plays a central role in signal transduction by nitric oxide (NO), and aberrant S-nitrosylation of specific proteins is increasingly implicated in disease.

SCOPE OF REVIEW

Here, methodologies for the characterization, identification and quantification of SNO-proteins are reviewed, focusing on techniques suitable for the structural characterization and absolute quantification of isolated SNO-proteins, the identification and relative quantification of SNO-proteins from complex mixtures as well as the mass spectrometry-based identification and relative quantification of sites of S-nitrosylation (SNO-sites) in proteins.

MAJOR CONCLUSIONS

Structural characterization of SNO-proteins by X-ray crystallography is increasingly being utilized to understand both the relationships between protein structure and Cys thiol reactivity as well as the consequences of S-nitrosylation on protein structure and function. New methods for the proteomic identification and quantification of SNO-proteins and SNO-sites have greatly impacted the ability to study protein S-nitrosylation in complex biological systems.

GENERAL SIGNIFICANCE

The ability to identify and quantify SNO-proteins has long been rate-determining for scientific advances in the field of protein S-nitrosylation. Therefore, it is critical that investigators in the field have a good understand the utility and limitations of modern analytical techniques for SNO-protein analysis. This article is part of a Special Issue entitled: Regulation of cellular processes by S-nitrosylation.

Pharmacologically augmented S-nitrosylated hemoglobin improves recovery from murine subarachnoid hemorrhage

BACKGROUND AND PURPOSE

S-nitrosylated hemoglobin (S-nitrosohemoglobin) has been implicated in the delivery of O(2) to tissues through the regulation of microvascular blood flow. This study tested the hypothesis that enhancement of S-nitrosylated hemoglobin by ethyl nitrite inhalation improves outcome after experimental subarachnoid hemorrhage (SAH).

METHODS

A preliminary dosing study identified 20 ppm ethyl nitrite as a concentration that produced a 4-fold increase in S-nitrosylated hemoglobin concentration with no increase in methemoglobin. Mice were subjected to endovascular perforation of the right anterior cerebral artery and were treated with 20 ppm ethyl nitrite in air, or air alone for 72 hours, after which neurologic function, cerebral vessel diameter, brain water content, cortical tissue Po(2), and parenchymal red blood cell flow velocity were measured.

RESULTS

At 72 hours after hemorrhage, air- and ethyl nitrite-exposed mice had similarly sized blood clots. Ethyl nitrite improved neurologic score and rotarod performance; abated SAH-induced constrictions in the ipsilateral anterior, middle cerebral, and internal carotid arteries; and prevented an increase in ipsilateral brain water content. Ethyl nitrite inhalation increased red blood cell flow velocity and cortical tissue Po(2) in the ipsilateral cortex with no effect on systemic blood pressure.

CONCLUSIONS

Targeted S-nitrosylation of hemoglobin improved outcome parameters, including vessel diameter, tissue blood flow, cortical tissue Po(2), and neurologic function in a murine SAH model. Augmenting endogenous Po(2)-dependent delivery of NO bioactivity to selectively dilate the compromised cerebral vasculature has significant clinical potential in the treatment of SAH.

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.

Erythrocyte-dependent regulation of human skeletal muscle blood flow: role of varied oxyhemoglobin and exercise on nitrite, S-nitrosohemoglobin, and ATP

The erythrocyte is proposed to play a key role in the control of local tissue perfusion via three O(2)-dependent signaling mechanisms: 1) reduction of circulating nitrite to vasoactive NO, 2) S-nitrosohemoglobin (SNO-Hb)-dependent vasodilatation, and 3) release of the vasodilator and sympatholytic ATP; however, their relative roles in vivo remain unclear. Here we evaluated each mechanism to gain insight into their roles in the regulation of human skeletal muscle blood flow during hypoxia and hyperoxia at rest and during exercise. Arterial and femoral venous hemoglobin O(2) saturation (O(2)Hb), plasma and erythrocyte NO and ATP metabolites, and leg and systemic hemodynamics were measured in 10 healthy males exposed to graded hypoxia, normoxia, and graded hyperoxia both at rest and during submaximal one-legged knee-extensor exercise. At rest, leg blood flow and NO and ATP metabolites in plasma and erythrocytes remained unchanged despite large alterations in O(2)Hb. During exercise, however, leg and systemic perfusion and vascular conductance increased in direct proportion to decreases in arterial and venous O(2)Hb (r(2) = 0.86-0.98; P = 0.01), decreases in venous plasma nitrite (r(2) = 0.93; P < 0.01), increases in venous erythrocyte nitroso species (r(2) = 0.74; P < 0.05), and to a lesser extent increases in erythrocyte SNO (r(2) = 0.59; P = 0.07). No relationship was observed with plasma ATP (r(2) = 0.01; P = 0.99) or its degradation compounds. These in vivo data indicate that, during low-intensity exercise and hypoxic stress, but not hypoxic stress alone, plasma nitrite consumption and formation of erythrocyte nitroso species are associated with limb vasodilatation and increased blood flow in the human skeletal muscle vasculature.

Increased circulating cell-free hemoglobin levels reduce nitric oxide bioavailability in preeclampsia

Contrasting with increased nitric oxide (NO) formation during healthy pregnancy, reduced NO bioavailability plays a role in preeclampsia. However, no study has examined whether increased NO consumption by enhanced circulating levels of cell-free hemoglobin plays a role in preeclampsia. We studied 82 pregnant women (38 healthy pregnant and 44 with preeclampsia). To assess NO bioavailability, we measured plasma and whole blood nitrite concentrations using an ozone-based chemiluminescence assay. Plasma ceruloplasmin concentrations and plasma NO consumption (pNOc) were assessed and plasma hemoglobin (pHb) concentrations were measured with a commercial immunoassay. We found lower whole blood and plasma nitrite concentrations in preeclamptic patients (-48 and -39%, respectively; both P<0.05) compared with healthy pregnant women. Plasma samples from preeclamptic women consumed 63% more NO (P=0.003) and had 53% higher pHb and 10% higher ceruloplasmin levels than those found in healthy pregnant women (P<0.01). We found significant positive correlations between pHb and pNOc (r=0.61; P<0.0001), negative correlations between pNOc and whole blood or plasma nitrite concentrations (P=0.02; r=-0.32 and P=0.01; r=-0.34, respectively), and negative correlations between pHb and whole blood or plasma nitrite concentrations (P=0.03; r=-0.36 and P=0.01; r=-0.38, respectively). These findings suggest that increased pHb levels lead to increased NO consumption and lower NO bioavailability in preeclamptic compared with healthy pregnant women.

Inclusion of a nitric oxide congener in the insufflation gas repletes S-nitrosohemoglobin and stabilizes physiologic status during prolonged carbon dioxide pneumoperitoneum

A method to maintain organ blood flow during laparoscopic surgery has not been developed. Here we determined if ethyl nitrite, an S-nitrosylating agent that would maintain nitric oxide bioactivity (the major regulator of tissue perfusion), might be an effective intervention to preserve physiologic status during prolonged pneumoperitoneum. The study was conducted on appropriately anesthetized adult swine; the period of pneumoperitoneum was 240 minutes. Cohorts consisted of an anesthesia control group and groups insufflated with CO2 alone or CO2 containing fixed amounts of ethyl nitrite (1-300 ppm). Insufflation with CO2 alone produced declines in splanchnic organ blood flows and it reduced circulating levels of S-nitrosohemoglobin (i.e., nitric oxide bioactivity); these reductions were obviated by ethyl nitrite. In a specific example, preservation of kidney blood flow with ethyl nitrite kept serum creatinine and blood urea nitrogen concentrations constant whereas in the CO2 alone group both increased as kidney blood flow declined. The data indicate ethyl nitrite can effectively attenuate insufflation-induced decreases in organ blood flow and nitric oxide bioactivity leading to reductions in markers of acute tissue injury. This simple intervention provides a method for controlling a major source of laparoscopic-related morbidity and mortality: tissue ischemia and altered postoperative organ function.

Transport rather than diffusion-dependent route for nitric oxide gas activity in alveolar epithelium

The pathway by which inhaled NO gas enters pulmonary alveolar epithelial cells has not been directly tested. Although the expected mechanism is diffusion, another route is the formation of S-nitroso-L-cysteine, which then enters the cell through the L-type amino acid transporter (LAT). To determine if NO gas also enters alveolar epithelium this way, we exposed alveolar epithelial-rat type I, type II, L2, R3/1, and human A549-cells to NO gas at the air liquid interface in the presence of L- and D-cysteine+/-LAT competitors. NO gas exposure concentration dependently increased intracellular NO and S-nitrosothiol levels in the presence of L- but not D-cysteine, which was inhibited by LAT competitors, and was inversely proportional to diffusion distance. The effect of L-cysteine on NO uptake was also concentration dependent. Without preincubation with L-cysteine, NO uptake was significantly reduced. We found similar effects using ethyl nitrite gas in place of NO. Exposure to either gas induced activation of soluble guanylyl cylase in a parallel manner, consistent with LAT dependence. We conclude that NO gas uptake by alveolar epithelium achieves NO-based signaling predominantly by forming extracellular S-nitroso-L-cysteine that is taken up through LAT, rather than by diffusion. Augmenting extracellular S-nitroso-L-cysteine formation may augment pharmacological actions of inhaled NO gas.

Direct and indirect detection methods for the analysis of S-nitrosylated peptides and proteins

Covalent binding of nitric oxide to specific cysteine residues in proteins is a key event in cellular redox signal transduction. This modification influences both physiological and pathological processes, such as cardiovascular, neurological, and cancer-associated events. Even though, since its introduction, the biotin switch technique is the most used indirect method for the study of S-nitrosylation both in vivo and in vitro, during the last years modifications of this method have emerged, allowing more efficient sample enrichment and the precise identification of the modified aminoacidic sites. At the same time, to bypass the difficulties generated by the multiple chemical reaction steps required by these labeling methods, the direct identification of the SNO groups by mass spectrometry is emerging as a useful tool in this field, although, until now, it has been limited to the study of synthetic or purified recombinant proteins. Here we present two different techniques, developed in our laboratories, for detection of S-nitrosylation: the first is based on a modification of the biotin switch technique and is called His-tag switch, and the second is a direct mass spectrometry-based method used to detect in vivo generated SNO groups.

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.

Detection of protein S-nitrosylation with the biotin-switch technique

Protein S-nitrosylation, the posttranslational modification of cysteine thiols to form S-nitrosothiols, is a principle mechanism of nitric oxide-based signaling. Studies have demonstrated myriad roles for S-nitrosylation in organisms from bacteria to humans, and recent efforts have greatly advanced our scientific understanding of how this redox-based modification is dynamically regulated during physiological and pathophysiological conditions. The focus of this review is the biotin-switch technique (BST), which has become a mainstay assay for detecting S-nitrosylated proteins in complex biological systems. Potential pitfalls and modern adaptations of the BST are discussed, as are future directions for this assay in the burgeoning field of protein S-nitrosylation.

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.

Altered free radical metabolism in acute mountain sickness: implications for dynamic cerebral autoregulation and blood-brain barrier function

We tested the hypothesis that dynamic cerebral autoregulation (CA) and blood-brain barrier (BBB) function would be compromised in acute mountain sickness (AMS) subsequent to a hypoxia-mediated alteration in systemic free radical metabolism. Eighteen male lowlanders were examined in normoxia (21% O(2)) and following 6 h passive exposure to hypoxia (12% O(2)). Blood flow velocity in the middle cerebral artery (MCAv) and mean arterial blood pressure (MAP) were measured for determination of CA following calculation of transfer function analysis and rate of regulation (RoR). Nine subjects developed clinical AMS (AMS+) and were more hypoxaemic relative to subjects without AMS (AMS-). A more marked increase in the venous concentration of the ascorbate radical (A(*-)), lipid hydroperoxides (LOOH) and increased susceptibility of low-density lipoprotein (LDL) to oxidation was observed during hypoxia in AMS+ (P < 0.05 versus AMS-). Despite a general decline in total nitric oxide (NO) in hypoxia (P < 0.05 versus normoxia), the normoxic baseline plasma and red blood cell (RBC) NO metabolite pool was lower in AMS+ with normalization observed during hypoxia (P < 0.05 versus AMS-). CA was selectively impaired in AMS+ as indicated both by an increase in the low-frequency (0.07-0.20 Hz) transfer function gain and decrease in RoR (P < 0.05 versus AMS-). However, there was no evidence for cerebral hyper-perfusion, BBB disruption or neuronal-parenchymal damage as indicated by a lack of change in MCAv, S100beta and neuron-specific enolase. In conclusion, these findings suggest that AMS is associated with altered redox homeostasis and disordered CA independent of barrier disruption.

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.

Hypoxic modulation of exogenous nitrite-induced vasodilation in humans

BACKGROUND

It has been proposed that under hypoxic conditions, nitrite may release nitric oxide, which causes potent vasodilation. We hypothesized that nitrite would have a greater dilator effect in capacitance than in resistance vessels because of lower oxygen tension and that resistance-vessel dilation should become more pronounced during hypoxemia. The effect of intra-arterial infusion of nitrite on forearm blood flow and forearm venous volumes was assessed during normoxia and hypoxia.

METHODS AND RESULTS

Forty healthy volunteers were studied. After baseline infusion of 0.9% saline, sodium nitrite was infused at incremental doses from 40 nmol/min to 7.84 mumol/min. At each stage, forearm blood flow was measured by strain-gauge plethysmography. Forearm venous volume was assessed by radionuclide plethysmography. Changes in forearm blood flow and forearm venous volume in the infused arm were corrected for those in the control arm. The peak percentage of venodilation during normoxia was 35.8+/-3.4% (mean+/-SEM) at 7.84 micromol/min (P<0.001) and was similar during hypoxia. In normoxia, arterial blood flow, assessed by the forearm blood flow ratio, increased from 1.04+/-0.09 (baseline) to 1.62+/-0.18 (nitrite; P<0.05) versus 1.07+/-0.09 (baseline) to 2.37+/-0.15 (nitrite; P<0.005) during hypoxia. This result was recapitulated in vitro in vascular rings.

CONCLUSIONS

Nitrite is a potent venodilator in normoxia and hypoxia. Arteries are modestly affected in normoxia but potently dilated in hypoxia, which suggests the important phenomenon of hypoxic augmentation of nitrite-mediated vasodilation in vivo. The use of nitrite as a selective arterial vasodilator in ischemic territories and as a potent venodilator in heart failure has therapeutic implications.

S-nitrosohemoglobin deficiency: a mechanism for loss of physiological activity in banked blood

RBCs distribute oxygen to tissues, but, paradoxically, blood transfusion does not always improve oxygen delivery and is associated with ischemic events. We hypothesized that storage of blood would result in loss of NO bioactivity, impairing RBC vasodilation and thus compromising blood flow, and that repleting NO bioactivity would restore RBC function. We report that S-nitrosohemoglobin (SNO-Hb) concentrations declined rapidly after storage of fresh venous blood and that hypoxic vasodilation by banked RBCs correlated strongly with the amounts of SNO-Hb (r(2) = 0.90; P < 0.0005). Renitrosylation of banked blood during storage increased the SNO-Hb content and restored its vasodilatory activity. In addition, canine coronary blood flow was greater during infusion of renitrosylated RBCs than during infusion of S-nitrosothiol-depleted RBCs, and this difference in coronary flow was accentuated by hypoxemia (P < 0.001). Our findings indicate that NO bioactivity is depleted in banked blood, impairing the vasodilatory response to hypoxia, and they suggest that SNO-Hb repletion may improve transfusion efficacy.

Methods to detect nitric oxide and its metabolites in biological samples

Nitric oxide (NO) methodology is a complex and often confusing science and the focus of many debates and discussion concerning NO biochemistry. NO is involved in many physiological processes including regulation of blood pressure, immune response, and neural communication. Therefore its accurate detection and quantification are critical to understanding health and disease. Due to the extremely short physiological half-life of this gaseous free radical, alternative strategies for the detection of reaction products of NO biochemistry have been developed. The quantification of NO metabolites in biological samples provides valuable information with regard to in vivo NO production, bioavailability, and metabolism. Simply sampling a single compartment such as blood or plasma may not always provide an accurate assessment of whole body NO status, particularly in tissues. Therefore, extrapolation of plasma or blood NO status to specific tissues of interest is no longer a valid approach. As a result, methods continue to be developed and validated which allow the detection and quantification of NO and NO-related products/metabolites in multiple compartments of experimental animals in vivo. The methods described in this review is not an exhaustive or comprehensive discussion of all methods available for the detection of NO but rather a description of the most commonly used and practical methods which allow accurate and sensitive quantification of NO products/metabolites in multiple biological matrices under normal physiological conditions.

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.

S-Nitrosothiol measurements in biological systems

S-Nitrosothiol (SNO) cysteine modifications are regulated signaling reactions that dramatically affect, and are affected by, protein conformation. The lability of the SNO bond can make SNO-modified proteins cumbersome to measure accurately. Here, we review methodologies for detecting SNO modifications in biology. There are three caveats. (1) Many assays for biological SNOs are used near the limit of detection: standard curves must be in the biologically relevant concentration range. (2) The assays that are most reliable are those that modify SNO protein or peptide chemistry the least. (3) Each result should be quantitatively validated using more than one assay. Improved assays are needed and are in development.