Biochemical characterization of S-nitrosohemoglobin. Mechanisms underlying synthesis, no release, and biological activity

Enhancement of mitochondrial function using NO releasing nanoparticles; a potential approach for therapy of Alzheimer's disease

Alzheimer's disease (AD) is the most common type of dementia. Increasing evidence is showing the important role of mitochondrial dysfunction in AD. Mitochondria based oxidative stress, decrease in respiratory chain activity and ATP production are all associated with AD, hence indicating that the enhancement of mitochondrial function and biogenesis present a promising therapeutic approach for AD. Nitric oxide (NO) is an initiator of mitochondrial biogenesis. However, its gaseous nature and very short half-life limit the realization of its therapeutic potential. Additionally, its uncontrolled in-vivo distribution results in generalized vasodilation, hypotension among other off-target effects. Diazeniumdiolates (NONOates) are NO donors that release NO in physiological temperature and pH. Their encapsulation within a hydrophobic matrix carrier system could control the release of NO, and at the same time enable its delivery to the brain. In this work, PAPANONOate (PN) a NO donor was encapsulated in small (92 ± 7 nm) poly (lactic-co-glycolic acid) (PLGA) NPs. These NPs did not induce hemolysis upon intravenous administration and were able to accumulate in the brains of lipopolysaccharides (LPS) induced neurodegeneration mouse models. The encapsulation of PN within a hydrophobic PLGA matrix enabled the sustained release of NO from NPs (≈ 3 folds slower relative to free PN) and successfully delivered PN to brain. As a result, PN-NPs but not free PN resulted in an enhancement in memory and cognition in animals with neurodegeneration as determined by the Y-maze test. The enhancement in cognition was a result of increased mitochondria function as indicated by the increased production of ATP and Cytochrome C oxidase enzyme activity.

Red Blood Cell-Mediated S-Nitrosohemoglobin-Dependent Vasodilation: Lessons Learned from a β-Globin Cys93 Knock-In Mouse

Significance: Red blood cell (RBC)-mediated vasodilation plays an important role in oxygen delivery. This occurs through hemoglobin actions, at least in significant part, to convert heme-bound nitric oxide (NO) (in tense [T]/deoxygenated-state hemoglobin) into vasodilator S-nitrosothiol (SNO) (in relaxed [R]/oxygenated-state hemoglobin), convey SNO through the bloodstream, and release it into tissues to increase blood flow. The coupling of hemoglobin R/T state allostery, both to NO conversion into SNO and to SNO release (along with oxygen), under hypoxia supports the model of a three-gas respiratory cycle (O2/NO/CO2). Recent Advances: Oxygenation of tissues is dependent on a single, strictly conserved Cys residue in hemoglobin (βCys93). Hemoglobin couples SNO formation/release at βCys93 to O2 binding/release at hemes ("thermodynamic linkage"). Mice bearing βCys93Ala hemoglobin that is unable to generate SNO-βCys93 establish that SNO-hemoglobin is important for R/T allostery-regulated vasodilation by RBCs that couple blood flow to tissue oxygenation. Critical Issues: The model for RBC-mediated vasodilation originally proposed by Stamler et al. in 1996 has been largely validated: SNO-βCys93 forms in vivo, dilates blood vessels, and is hypoxia-regulated, and RBCs actuate vasodilation proportionate to hypoxia. Numerous compensations in βCys93Ala animals to alleviate tissue hypoxia (discussed herein) are predicted to preserve vasodilatory responses of RBCs but impair linkage to R/T transition in hemoglobin. This is borne out by loss of responsivity of mutant RBCs to oxygen, impaired blood flow responses to hypoxia, and tissue ischemia in βCys93-mutant animals. Future Directions: SNO-hemoglobin mediates hypoxic vasodilation in the respiratory cycle. This fundamental physiology promises new insights in vascular diseases and blood disorders.

Gasotransmitter signaling in energy homeostasis and metabolic disorders

Gasotransmitters are small molecules of gases, including nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO). These three gasotransmitters can be endogenously produced and regulate a wide range of pathophysiological processes by interacting with specific targets upon diffusion in the biological media. By redox and epigenetic regulation of various physiological functions, NO, H₂S, and CO are critical for the maintenance of intracellular energy homeostasis. Accumulated evidence has shown that these three gasotransmitters control ATP generation, mitochondrial biogenesis, glucose metabolism, insulin sensitivity, lipid metabolism, and thermogenesis, etc. Abnormal generation and metabolism of NO, H₂S, and/or CO are involved in various abnormal metabolic diseases, including obesity, diabetes, and dyslipidemia. In this review, we summarized the roles of NO, H₂S, and CO in the regulation of energy homeostasis as well as their involvements in the metabolism of dysfunction-related diseases. Understanding the interaction among these gasotransmitters and their specific molecular targets are very important for therapeutic applications.

A comparison of the chemical biology of hydropersulfides (RSSH) with other protective biological antioxidants and nucleophiles

The hydropersulfide (RSSH) functional group has received significant recent interest due to its unique chemical properties that set it apart from other biological species. The chemistry of RSSH predicts that one possible biological role may be as a protectant against cellular oxidative and electrophilic stress. That is, RSSH has reducing and nucleophilic properties that may combat the potentially destructive biochemistry of toxicologically relevant oxidants and electrophiles. However, there are currently numerous other molecules that have established roles in this regard. For example, ascorbate and tocopherols are potent antioxidants that quench deleterious oxidative reactions and glutathione (GSH) is a well-established and highly prevalent biological protectant against electrophile toxicity. Thus, in order to begin to understand the possible role of RSSH species as protectants against oxidative/electrophilic stress, the inherent chemical properties of RSSH versus these other protectants will be discussed and contrasted.

Role of Nitric Oxide Carried by Hemoglobin in Cardiovascular Physiology: Developments on a Three-Gas Respiratory Cycle

A continuous supply of oxygen is essential for the survival of multicellular organisms. The understanding of how this supply is regulated in the microvasculature has evolved from viewing erythrocytes (red blood cells [RBCs]) as passive carriers of oxygen to recognizing the complex interplay between Hb (hemoglobin) and oxygen, carbon dioxide, and nitric oxide-the three-gas respiratory cycle-that insures adequate oxygen and nutrient delivery to meet local metabolic demand. In this context, it is blood flow and not blood oxygen content that is the main driver of tissue oxygenation by RBCs. Herein, we review the lines of experimentation that led to this understanding of RBC function; from the foundational understanding of allosteric regulation of oxygen binding in Hb in the stereochemical model of Perutz, to blood flow autoregulation (hypoxic vasodilation governing oxygen delivery) observed by Guyton, to current understanding that centers on S-nitrosylation of Hb (ie, S-nitrosohemoglobin; SNO-Hb) as a purveyor of oxygen-dependent vasodilatory activity. Notably, hypoxic vasodilation is recapitulated by native S-nitrosothiol (SNO)-replete RBCs and by SNO-Hb itself, whereby SNO is released from Hb and RBCs during deoxygenation, in proportion to the degree of Hb deoxygenation, to regulate vessels directly. In addition, we discuss how dysregulation of this system through genetic mutation in Hb or through disease is a common factor in oxygenation pathologies resulting from microcirculatory impairment, including sickle cell disease, ischemic heart disease, and heart failure. We then conclude by identifying potential therapeutic interventions to correct deficits in RBC-mediated vasodilation to improve oxygen delivery-steps toward effective microvasculature-targeted therapies. To the extent that diseases of the heart, lungs, and blood are associated with impaired tissue oxygenation, the development of new therapies based on the three-gas respiratory system have the potential to improve the well-being of millions of patients.

S-Nitrosothiols: Materials, Reactivity and Mechanisms

The article provides a comprehensive view of S-nitrosothiols, chemical behaviour, the pathways leading to their synthesis, their spectral properties, analytical methods of detection and determination, chemical and photochemical reactivity, kinetic aspects and suggested mechanisms. The structure parameters of S-nitrosothiols and the parent thiols are analysed with respect to their effect on the strengthening or weakening the S–NO bond, and in consequence on the S-nitrosothiol stability. This depends also on the ease of S–S bond formation in the product disulphide. These structural features seem to be crucial both to spontaneous as well as to Cu-catalysed decomposition. Principal emphasis is given here to the S-nitrosothiols’ ability to act as ligands and to the effect of coordination on the ligand properties. The chemical and photochemical behaviours of the complexes are described in more detail and their roles in chemical and biochemical systems are discussed.

The aim of the article is to demonstrate that the contribution of S-nitrosothiols to chemical and biochemical processes is more diverse than supposed hitherto. Nevertheless, their role is predictable and, based on the correlation between structure and reactivity, many important mechanisms of biochemical processes can be interpreted and various applications designed.

Preparation and characterization of SNO-PEG-hemoglobin as a candidate for oxygen transporting material

Acellular hemoglobin (Hb) derivates developed as oxygen carriers are known to cause hypertensive reactions due to their nitric oxide (NO) scavenging action. To modulate this undesired activity, we have developed a new Hb derivative, s-nitrosylated polyethylene glycol (PEG)-modified hemoglobin (SNO-PEG-Hb), which can deliver oxygen and NO. After human Hb was modified with PEG to increase its molecular weight, the free sulfhydryl groups of Hb were s-nitrosylated with s-nitrosoglutathione. Administration of unmodified Hb into anesthetized rats caused a hypertensive reaction, while s-nitrosylated Hb derivatives such as SNO-Hb and SNO-PEG-Hb did not raise blood pressure. The plasma half-lives of heme and NO bound to SNO-PEG-Hb were 11.5 and 2.4 hours respectively, indicating that the s-nitrosylated Hb derivative may act as a slow-releasing agent for NO. Based on these findings, SNO-PEG-Hb is a useful candidate for a blood substitute and tool for oxygen therapeutics.

Regulation of mitochondrial function and endoplasmic reticulum stress by nitric oxide in pluripotent stem cells

Mitochondrial dysfunction and endoplasmic reticulum stress (ERS) are global processes that are interrelated and regulated by several stress factors. Nitric oxide (NO) is a multifunctional biomolecule with many varieties of physiological and pathological functions, such as the regulation of cytochrome c inhibition and activation of the immune response, ERS and DNA damage; these actions are dose-dependent. It has been reported that in embryonic stem cells, NO has a dual role, controlling differentiation, survival and pluripotency, but the molecular mechanisms by which it modulates these functions are not yet known. Low levels of NO maintain pluripotency and induce mitochondrial biogenesis. It is well established that NO disrupts the mitochondrial respiratory chain and causes changes in mitochondrial Ca²⁺ flux that induce ERS. Thus, at high concentrations, NO becomes a potential differentiation agent due to the relationship between ERS and the unfolded protein response in many differentiated cell lines. Nevertheless, many studies have demonstrated the need for physiological levels of NO for a proper ERS response. In this review, we stress the importance of the relationships between NO levels, ERS and mitochondrial dysfunction that control stem cell fate as a new approach to possible cell therapy strategies.

Hemoglobin βCys93 is essential for cardiovascular function and integrated response to hypoxia

Oxygen delivery by Hb is essential for vertebrate life. Three amino acids in Hb are strictly conserved in all mammals and birds, but only two of those, a His and a Phe that stabilize the heme moiety, are needed to carry O2. The third conserved residue is a Cys within the β-chain (βCys93) that has been assigned a role in S-nitrosothiol (SNO)-based hypoxic vasodilation by RBCs. Under this model, the delivery of SNO-based NO bioactivity by Hb redefines the respiratory cycle as a triune system (NO/O2/CO2). However, the physiological ramifications of RBC-mediated vasodilation are unknown, and the apparently essential nature of βCys93 remains unclear. Here we report that mice with a βCys93Ala mutation are deficient in hypoxic vasodilation that governs blood flow autoregulation, the classic physiological mechanism that controls tissue oxygenation but whose molecular basis has been a longstanding mystery. Peripheral blood flow and tissue oxygenation are decreased at baseline in mutant animals and decline excessively during hypoxia. In addition, βCys93Ala mutation results in myocardial ischemia under basal normoxic conditions and in acute cardiac decompensation and enhanced mortality during transient hypoxia. Fetal viability is diminished also. Thus, βCys93-derived SNO bioactivity is essential for tissue oxygenation by RBCs within the respiratory cycle that is required for both normal cardiovascular function and circulatory adaptation to hypoxia.

Regulation of obesity and insulin resistance by nitric oxide

Obesity is a risk factor for developing type 2 diabetes and cardiovascular disease and has quickly become a worldwide pandemic with few tangible and safe treatment options. Although it is generally accepted that the primary cause of obesity is energy imbalance, i.e., the calories consumed are greater than are utilized, understanding how caloric balance is regulated has proven a challenge. Many "distal" causes of obesity, such as the structural environment, occupation, and social influences, are exceedingly difficult to change or manipulate. Hence, molecular processes and pathways more proximal to the origins of obesity-those that directly regulate energy metabolism or caloric intake-seem to be more feasible targets for therapy. In particular, nitric oxide (NO) is emerging as a central regulator of energy metabolism and body composition. NO bioavailability is decreased in animal models of diet-induced obesity and in obese and insulin-resistant patients, and increasing NO output has remarkable effects on obesity and insulin resistance. This review discusses the role of NO in regulating adiposity and insulin sensitivity and places its modes of action into context with the known causes and consequences of metabolic disease.

S-Nitrosation destabilizes glutathione transferase P1-1

Protein S-nitrosation is a post-translational modification that regulates the function of more than 500 human proteins. Despite its apparent physiological significance, S-nitrosation is poorly understood at a molecular level. Here, we investigated the effect of S-nitrosation on the activity, structure, stability, and dynamics of human glutathione transferase P1-1 (GSTP1-1), an important detoxification enzyme ubiquitous in aerobes. S-Nitrosation at Cys47 and Cys101 reduces the activity of the enzyme by 94%. Circular dichroism spectroscopy, acrylamide quenching, and amide hydrogen-deuterium exchange mass spectrometry experiments indicate that the loss of activity is caused by the introduction of local disorder at the active site of GSTP1-1. Furthermore, the modification destabilizes domain 1 of GSTP1-1 against denaturation, smoothing the unfolding energy landscape of the protein and introducing a refolding defect. In contrast, S-nitrosation at Cys101 alone introduces a refolding defect in domain 1 but compensates by stabilizing the domain kinetically. These data elucidate the physical basis for the regulation of GSTP1-1 by S-nitrosation and provide general insight into the consequences of S-nitrosation on protein stability and dynamics.

Oral supplementation with glycine reduces oxidative stress in patients with metabolic syndrome, improving their systolic blood pressure

Reactive oxygen species derived from abdominal fat and uncontrolled glucose metabolism are contributing factors to both oxidative stress and the development of metabolic syndrome (MetS). This study was designed to evaluate the effects of daily administration of an oral glycine supplement on antioxidant enzymes and lipid peroxidation in MetS patients. The study included 60 volunteers: 30 individuals that were supplemented with glycine (15 g/day) and 30 that were given a placebo for 3 months. We analysed thiobarbituric acid reactive substances (TBARS) and S-nitrosohemoglobin (SNO-Hb) in plasma; the enzymatic activities of glucose-6-phosphate dehydrogenase (G6PD), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX) in erythrocytes; and the expression of CAT, GPX, and SOD2 in leukocytes. Individuals treated with glycine showed a 25% decrease in TBARS compared with the placebo-treated group. Furthermore, there was a 20% reduction in SOD-specific activity in the glycine-treated group, which correlated with SOD2 expression. G6PD activity and SNO-Hb levels increased in the glycine-treated male group. Systolic blood pressure (SBP) also showed a significant decrease in the glycine-treated men (p = 0.043). Glycine plays an important role in balancing the redox reactions in the human body, thus protecting against oxidative damage in MetS patients.

Nitric oxide in skeletal muscle: role on mitochondrial biogenesis and function

Nitric oxide (NO) has been implicated in several cellular processes as a signaling molecule and also as a source of reactive nitrogen species (RNS). NO is produced by three isoenzymes called nitric oxide synthases (NOS), all present in skeletal muscle. While neuronal NOS (nNOS) and endothelial NOS (eNOS) are isoforms constitutively expressed, inducible NOS (iNOS) is mainly expressed during inflammatory responses. Recent studies have demonstrated that NO is also involved in the mitochondrial biogenesis pathway, having PGC-1α as the main signaling molecule. Increased NO synthesis has been demonstrated in the sarcolemma of skeletal muscle fiber and NO can also reversibly inhibit cytochrome c oxidase (Complex IV of the respiratory chain). Investigation on cultured skeletal myotubes treated with NO donors, NO precursors or NOS inhibitors have also showed a bimodal effect of NO that depends on the concentration used. The present review will discuss the new insights on NO roles on mitochondrial biogenesis and function in skeletal muscle. We will also focus on potential therapeutic strategies based on NO precursors or analogs to treat patients with myopathies and mitochondrial deficiency.

Superoxide dismutase as a novel macromolecular nitric oxide carrier: preparation and characterization

Nitric oxide (NO) is an important molecule that exerts multiple functions in biological systems. Because of the short-lived nature of NO, S-nitrosothiols (RSNOs) are believed to act as stable NO carriers. Recently, sulfhydryl (SH) containing macromolecules have been shown to be promising NO carriers. In the present study, we aimed to synthesize and characterize a potential NO carrier based on bovine Cu,Zn-superoxide dismutase (bSOD). To prepare S-nitrosated bSOD, the protein was incubated with S-nitrosoglutathione (GSNO) under varied experimental conditions. The results show that significant S-nitrosation of bSOD occurred only at high temperature (50 °C) for prolonged incubation time (>2 h). S-nitrosation efficiency increased with reaction time and reached a plateau at ~4 h. The maximum amount of NO loaded was determined to be about 0.6 mol SNO/mol protein (~30% loading efficiency). The enzymatic activity of bSOD, however, decreased with reaction time. Our data further indicate that NO functionality can only be measured in the presence of extremely high concentrations of Hg²⁺ or when the protein was denatured by guanidine. Moreover, mildly acidic pH was shown to favor S-nitrosation of bSOD. A model based on unfolding and refolding of bSOD during preparation was proposed to possibly explain our observation.

The chemical biology of S-nitrosothiols

SIGNIFICANCE

S-nitrosothiol formation and protein S-nitrosation is an important nitric oxide (NO)-dependent signaling paradigm that is relevant to almost all aspects of cell biology, from proliferation, to homeostasis, to programmed cell death. However, the mechanisms by which S-nitrosothiols are formed are still largely unknown, and there are gaps of understanding between the known chemical biology of S-nitrosothiols and their reported functions.

RECENT ADVANCES

This review attempts to describe the biological chemistry of S-nitrosation and to point out where the challenges lie in matching the known chemical biology of these compounds with their reported functions. The review will detail new discoveries concerning the mechanisms of the formation of S-nitrosothiols in biological systems.

CRITICAL ISSUES

Although S-nitrosothiols may be formed with some degree of specificity on particular protein thiols, through un-catalyzed chemistry, and mechanisms for their degradation and redistribution are present, these processes are not sufficient to explain the vast array of specific and targeted responses of NO that have been attributed to S-nitrosation. Elements of catalysis have been discovered in the formation, distribution, and metabolism of S-nitrosothiols, but it is less clear whether these represent a specific network for targeted NO-dependent signaling.

FUTURE DIRECTIONS

Much recent work has uncovered new targets for S-nitrosation through either targeted or proteome-wide approaches There is a need to understand which of these modifications represent concerted and targeted signaling processes and which is an inevitable consequence of living with NO. There is still much to be learned about how NO transduces signals in cells and the role played by protein S-nitrosation.

Augmentation of aerobic respiration and mitochondrial biogenesis in skeletal muscle by hypoxia preconditioning with cobalt chloride

High altitude/hypoxia training is known to improve physical performance in athletes. Hypoxia induces hypoxia inducible factor-1 (HIF-1) and its downstream genes that facilitate hypoxia adaptation in muscle to increase physical performance. Cobalt chloride (CoCl₂), a hypoxia mimetic, stabilizes HIF-1, which otherwise is degraded in normoxic conditions. We studied the effects of hypoxia preconditioning by CoCl₂ supplementation on physical performance, glucose metabolism, and mitochondrial biogenesis using rodent model. The results showed significant increase in physical performance in cobalt supplemented rats without (two times) or with training (3.3 times) as compared to control animals. CoCl₂ supplementation in rats augmented the biological activities of enzymes of TCA cycle, glycolysis and cytochrome c oxidase (COX); and increased the expression of glucose transporter-1 (Glut-1) in muscle showing increased glucose metabolism by aerobic respiration. There was also an increase in mitochondrial biogenesis in skeletal muscle observed by increased mRNA expressions of mitochondrial biogenesis markers which was further confirmed by electron microscopy. Moreover, nitric oxide production increased in skeletal muscle in cobalt supplemented rats, which seems to be the major reason for peroxisome proliferator activated receptor-gamma coactivator-1α (PGC-1α) induction and mitochondrial biogenesis. Thus, in conclusion, we state that hypoxia preconditioning by CoCl₂ supplementation in rats increases mitochondrial biogenesis, glucose uptake and metabolism by aerobic respiration in skeletal muscle, which leads to increased physical performance. The significance of this study lies in understanding the molecular mechanism of hypoxia adaptation and improvement of work performance in normal as well as extreme conditions like hypoxia via hypoxia preconditioning.

Nitric oxide in myogenesis and therapeutic muscle repair

Nitric oxide is a short-lived intracellular and intercellular messenger. The first realisation that nitric oxide is important in physiology occurred in 1987 when its identity with the endothelium-derived relaxing factor was discovered. Subsequent studies have shown that nitric oxide possesses a number of physiological functions that are essential not only to vascular homeostasis but also to neurotransmission, such as in the processes of learning and memory and endocrine gland regulation, as well as inflammation and immune responses. The discovery in 1995 that a splice variant of the neuronal nitric oxide synthase is localised at the sarcolemma via the dystrophin-glycoprotein complex and of its displacement in Duchenne muscular dystrophy has stimulated a host of studies exploring the role of nitric oxide in skeletal muscle physiology. Recently, nitric oxide has emerged as a relevant messenger also of myogenesis that it regulates at several key steps, especially when the process is stimulated for muscle repair following acute and chronic muscle injuries. Here, we will review briefly the mechanisms and functions of nitric oxide in skeletal muscle and discuss its role in myogenesis, with specific attention to the promising nitric oxide-based approaches now being explored at the pre-clinical and clinical level for the therapy of muscular dystrophy.

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

BACKGROUND

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

SCOPE OF THE REVIEW

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

MAJOR CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

Nitric oxide production pathways in erythrocytes and plasma

Nitric oxide (NO) is a potent regulator of vascular tone and hemorheology. The signaling function of NO was largely unappreciated until approximately 30 years ago, when the endothelium-derived relaxing factor (EDRF) was identified as NO. Since then, NO from the endothelium has been considered the major source of NO in the vasculature and a contributor to the paracrine regulation of blood hemodynamics. Because NO is highly reactive, and its half-life in vivo is only a few seconds (even less in the bloodstream), any NO bioactivity derived from the intraluminal region has traditionally been considered insignificant. However, the availability and significance of NO signaling molecules derived from intraluminal sources, particularly erythrocytes, have gained attention in recent years. Multiple potential sources of NO bioactivity have been identified in the blood, but unresolved questions remain concerning these proposed sources and how the NO released via these pathways actually interacts with intravascular and extravascular targets. Here we review the hypotheses that have been put forward concerning blood-borne NO and its contribution to hemorheological properties and the regulation of vascular tone, with an emphasis on the quantitative aspects of these processes.

Amino acids and mitochondrial biogenesis

Mitochondria are sources of energy production through their role in producing adenosine triphosphate for cell metabolism. Defective mitochondrial biogenesis and function play relevant roles in the pathophysiology of relevant diseases, including obesity, diabetes mellitus, myopathies, and neurodegenerative diseases. Their function is the product of synthesis of macromolecules within the mitochondria and import of proteins and lipids synthesized outside the organelles. Both are required for mitochondrial proliferation and may also facilitate the growth of preexisting mitochondria. Recent evidence indicates that these events are regulated in a complex way by several agonists and environmental conditions, through activation of specific signaling pathways and transcription factors. Nitric oxide (NO) appears to be a novel modulator of mitochondrial biogenesis. High levels of NO acutely inhibit cell respiration by binding to cytochrome c oxidase. Conversely, chronic, low-grade increases of NO stimulate mitochondrial biogenesis in diverse cell types. Here, we suggest that some types of nutrients, including specific mixtures of amino acids, may improve mitochondrial biogenesis and energy production in energy-defective conditions by increasing endothelial NO synthase expression.

An electron paramagnetic resonance investigation of the oxygen dependence of the arterial-venous gradient of nitrosyl hemoglobin in blood circulation

Whether there is a nitrosyl hemoglobin (HbNO) gradient between the venous and the arterial parts of the circulatory system is a very controversial issue in nitric oxide research. We have carefully evaluated the measurement of HbNO concentration in blood using EPR generated in vivo by the NO donor DEANO under various oxygen tensions. We found that the absolute concentrations of HbNO in venous and arterial blood were the same within experimental error, independent of hemoglobin saturation; only the ratios of 5-coordinate and 6-coordinate HbNO differed. The HbNO concentration increased when the oxygen concentration breathed by the rats decreased in a manner that was linear in hemoglobin saturation. These results do not support the existence of an arterial-venous gradient of HbNO under our experimental conditions.

Vascular smooth muscle NO exposure from intraerythrocytic SNOHb: a mathematical model

We previously constructed computational models based on the biochemical pathway analysis of different nitric oxide (NO) synthase isoforms and found a large discrepancy between our predictions and perivascular NO measurements, suggesting the existence of nonenzymatic sources of NO. S-nitrosohemoglobin (SNOHb) has been suggested as a major source to release NO in the arteriolar lumen and induce hypoxic vasodilation. In the present study, we formulated a multicellular computational model to quantify NO exposure in arteriolar smooth muscle when the NO released by intraerythrocytic SNOHb is the sole NO source in the vasculature. Our calculations show an NO exposure of approximately 0.25-6 pM in the smooth muscle region. This amount does not account for the large discrepancy we encountered regarding perivascular NO levels. We also found that the amount of NO delivered by SNOHb to smooth muscle strongly depends on the SNOHb concentration and half-life, which further determine the rate of NO release, as well as on the membrane permeability of red blood cells (RBCs) to NO. In conclusion, our mathematical model predicts that picomolar amounts of NO can be delivered to the vascular smooth muscle by intraerythrocytic SNOHb; this amount of NO alone appears not sufficient to induce the hypoxic vasodilation.

Blue laser light increases perfusion of a skin flap via release of nitric oxide from hemoglobin

It has recently been shown that nitrosyl complexes of hemoglobin (NO-Hb) are sensitive to low-level blue laser irradiation, suggesting that laser irradiation can facilitate the release of biologically active nitric oxide (NO), which can affect tissue perfusion. The aim of this study was to evaluate the therapeutic value of blue laser irradiation for local tissue perfusion after surgical intervention. Blood was withdrawn from a rat, exposed to NO and infused back to the same rat or used for in vitro experiments. In vitro, an increase of NO-Hb levels (electron paramagnetic resonance spectroscopy) up to 15 microM in rat blood did not result in the release of detectable amounts of NO (NO selective electrode). Blue laser irradiation of NO-Hb in blood caused decomposition of NO-Hb complexes and release of free NO. Systemic infusion of NO-Hb in rats affected neither systemic circulation (mean arterial pressure) nor local tissue perfusion (Doppler blood flow imaging system). In contrast, a clear enhancement of local tissue perfusion was observed in epigastric flap when elevated NO-Hb levels in blood were combined with local He-Cd laser irradiation focused on the left epigastric artery. The enhancement of regional tissue perfusion was not accompanied by any detectable changes in systemic circulation. This study demonstrates that blue laser irradiation improves local tissue perfusion in a controlled manner stimulating NO release from NO-Hb complexes.

Regulation of NF-kappaB activation and nuclear translocation by exogenous nitric oxide (NO) donors in TNF-alpha activated vascular endothelial cells

Nitric oxide (NO) is a unique mediator which may promote or suppress inflammation. In this study, we examine the effect of exogenous NO on nuclear translocation of nuclear factor-kappa B (NF-kappaB) in quiescent human umbilical vein endothelial cells (HUVECs) subsequently activated by tumor necrosis factor-alpha (TNF-alpha), and in HUVECs previously activated by TNF-alpha, a model of vascular inflammation. Quiescent and activated HUVECs are exposed to exogenous NO donors of varying half-lives and the degree of NF-kappaB translocation into the nucleus determined by unique application of immunofluorescence image analysis in whole cells and correlative biochemical analysis of activated NF-kappaB proteins in the nucleus. NO donors with shorter half-lives are more effective in blocking the activation and translocation of NF-kappaB, when added to quiescent HUVECs prior to cellular activation by TNF-alpha. However, in previously activated HUVECs where NF-kappaB had relocated into the cytoplasm, addition of short half-life NO donors, but not TNF-alpha, induced re-translocation of NF-kappaB back into the nucleus sustaining the inflamed cell phenotype. These data suggest that NO as an inhibitor or activator of NF-kappaB may depend on the state of activation of vascular endothelial cells in which it contacts. Additionally, in activated cells, NO may modulate expression of NF-kappaB-dependent gene products, when cytokines are ineffective.

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.

Extrapulmonary effects of inhaled nitric oxide: role of reversible S-nitrosylation of erythrocytic hemoglobin

Early applications of inhaled nitric oxide (iNO), typically in the treatment of diseases marked by acute pulmonary hypertension, were met by great enthusiasm regarding the purported specificity of iNO: vasodilation by iNO was specific to the lung (without a change in systemic vascular resistance), and within the lung, NO activity was said to be confined spatially and temporally by Hb within the vascular lumen. Underlying these claims were classical views of NO as a short-lived paracrine hormone that acts largely through the heme groups of soluble guanylate cyclase, and whose potential activity is terminated on encountering the hemes of red blood cell (RBC) Hb. These classical views are yielding to a broader paradigm, in which NO-related signaling is achieved through redox-related NO adducts that endow NO synthase products with the ability to act at a distance in space and time from NO synthase itself. Evidence supporting the biological importance of such stable NO adducts is probably strongest for S-nitrosothiols (SNOs), in which NO binds to critical cysteine residues in proteins or peptides. The circulating RBC is a major SNO reservoir, and RBC Hb releases SNO-related bioactivity peripherally on O2 desaturation. These new paradigms describing NO transport also provide a plausible mechanistic understanding of the increasingly recognized peripheral effects of inhaled NO. An explanation for the peripheral actions of inhaled NO is discussed here, and the rationale and results of attempts to exploit the "NO delivery" function of the RBC are reviewed.

Effects of S-nitrosation on hemoglobin-induced microvascular damage

Blood substitutes, such as diaspirin cross-linked hemoglobin (Hb), cause microvascular leakiness to macromolecules. Because of the potentially stabilizing effects of nitric acid (NO) on endothelium, experiments were performed to determine whether S-nitrosohemoglobin (SNO-Hb), a potential NO-donor Hb-based blood substitute, would not cause microvascular damage. Release of NO, or its metabolites, from the SNO-Hb was facilitated by addition of glutathione, which aids in the decomposition of S-nitrosothiols. In anesthetized rats, the mesenteric microvasculature was perfused with SNO-Hb with glutathione (six rats), SNO-Hb alone (six rats), or saline (eight rats) for 10 min, followed by fluorescein isothiocyanate (FITC)-albumin for 1 min, and finally fixed for epifluorescence microscopic examination. When comparing the SNO-Hb group with saline, both the numbers and areas of leaks were significantly increased [0.019 +/- 0.003 (SEM) microm vs. 0.0030 +/- 0.0004 and 7.36 +/- 1.50 vs. 0.156 +/- 0.035 (p < 0.005)]. With the addition of glutathione, leakage was still high (0.005 +/- 0.00005 microm and 5.086 +/- 0.064 microm) but decreased compared with SNO-Hb alone (p < 0.005). In conclusion, NO, or a related vasodilator, when released from SNO-Hb, significantly reduces but does not eliminate microvascular damage. Further improvements may result by S-nitrosating a more stable form of modified hemoglobin.

The regulation and pharmacology of endothelial nitric oxide synthase

Nitric oxide (NO) is a small, diffusible, lipophilic free radical gas that mediates significant and diverse signaling functions in nearly every organ system in the body. The endothelial isoform of nitric oxide synthase (eNOS) is a key source of NO found in the cardiovascular system. This review summarizes the pharmacology of NO and the cellular regulation of endothelial NOS (eNOS). The molecular intricacies of the chemistry of NO and the enzymology of NOSs are discussed, followed by a review of the biological activities of NO. This information is then used to develop a more global picture of the pharmacological control of NO synthesis by NOSs in both physiologic conditions and pathophysiologic states.

Assessing NO-dependent vasodilatation using vessel bioassays at defined oxygen tensions

Results from vessel bioassays have provided the foundation for much of our understanding of the mechanisms that control vascular homeostasis and blood flow. The seminal observations that led to the discovery that nitric oxide (NO) is a critical mediator of vascular relaxation were made with the use of such methodology, and many studies have used NO-dependent vessel relaxation as an experimental readout for understanding mechanisms that regulate vascular NO function. Studies have coupled controlling oxygen tensions within vessel bioassay chambers to begin to understand how oxygen-specifically hypoxia-regulate NO function, and this context has identified red cells-specifically hemoglobin within-as critical modulators. Alone, vessel bioassays or measuring oxygen partial pressures (pO2) is relatively straightforward, but the combination necessitates consideration of several factors. We use the example of deoxygenated red cells/hemoglobin-dependent potentiation of nitrite-dependent dilation to illustrate the salient factors that are critical to consider in designing and interpreting experiments aimed at understanding the interplay between oxygen and NO function in the vasculature.

Red blood cells and hemoglobin in hypoxic pulmonary vasoconstriction

Nitric oxide (NO) plays an important role in the modulation of hypoxic pulmonary vasoconstriction; in turn, red blood cells (RBCs) augment HPV by hemoglobin-mediated oxidation and inactivation of NO. In addition, scavenging of reactive oxygen species by RBCs may play a role in augmentation of HPV. NO delivery and/or production by RBCs does not appear to be important in the control of pulmonary vasomotor tone. This review will discuss regulation of HPV by RBCs with an emphasis on hemoglobin-NO interactions. In addition, the review will discuss how biologic (S-nitrosation) or pharmacologic (cross-linking) modification of hemoglobin may affect pulmonary circulatory-hemoglobin interactions.

Inorganic nitric oxide metabolites participating in no-dependent modifications of biopolymers

Biogenous nitric(II) oxide (NO), the higher nitrogen oxides (NO2, isomeric N2O3 and N2O4, ONOO-, etc.) that are NO-derived in vivo, and the products of their transformations are active compounds capable of reactions with biopolymers and low-molecular metabolites. The products of these reactions are often considered to be various NO-dependent modifications (NODMs). The nitrated, nitrosylated, nitrosated, and other NODMs play key roles in the regulation of the most important biochemical processes. In this review, we briefly discuss the metabolic reactions of nitrogen oxides that supply active intermediates for NODMs, the NODM reaction products, and some mechanisms of NODM reparation that allow the recovery of chemically intact biopolymer molecule from a modified (chemically damaged) NODM. For example, residues of 3-nitrotyrosine arising due to the NODM reactions of proteins can be reduced to unsubstituted Tyr residues as a result of alternative NODM reactions through intermediate diazotyrosine derivatives. The heterogeneity of a medium in vivo is an important factor controlling the proceeding of NODM reactions. We showed that many processes determining NODM efficiency proceed differently in the heterogeneous media of organisms and in homogeneous aqueous solutions.

Hypoxia, red blood cells, and nitrite regulate NO-dependent hypoxic vasodilation

Local vasodilation in response to hypoxia is a fundamental physiologic response ensuring oxygen delivery to tissues under metabolic stress. Recent studies identify a role for the red blood cell (RBC), with hemoglobin the hypoxic sensor. Herein, we investigate the mechanisms regulating this process and explore the relative roles of adenosine triphosphate, S-nitrosohemoglobin, and nitrite as effectors. We provide evidence that hypoxic RBCs mediate vasodilation by reducing nitrite to nitric oxide (NO) and ATP release. NO dependence for nitrite-mediated vasodilation was evidenced by NO gas formation, stimulation of cGMP production, and inhibition of mitochondrial respiration in a process sensitive to the NO scavenger C-PTIO. The nitrite reductase activity of hemoglobin is modulated by heme deoxygenation and heme redox potential, with maximal activity observed at 50% hemoglobin oxygenation (P(50)). Concomitantly, vasodilation is initiated at the P(50), suggesting that oxygen sensing by hemoglobin is mechanistically linked to nitrite reduction and stimulation of vasodilation. Mutation of the conserved beta93cys residue decreases the heme redox potential (ie, decreases E(1/2)), an effect that increases nitrite reductase activity and vasodilation at any given hemoglobin saturation. These data support a function for RBC hemoglobin as an allosterically and redox-regulated nitrite reductase whose "enzyme activity" couples hypoxia to increased NO-dependent blood flow.

Nitric oxide and mitochondrial biogenesis: a key to long-term regulation of cellular metabolism

Mitochondria, the site of oxidative energy metabolism in eukariotic cells, are a highly organised structure endowed with different enzymes and reactions localized in discrete membranes and aqueous compartments. Mitochondrial function is regulated in complex ways by several agonists and environmental conditions, through activation of specific transcription factors and signalling pathways. A key player in this scenario is nitric oxide (NO). Its binding to cytochrome c oxidase in the mitochondrial respiratory chain, which is reversible and in competition with oxygen, plays a role in acute oxygen sensing and in the cell response to hypoxia. Evidence of the last two years showed that NO has also long-term effects, leading to biogenesis of functionally active mitochondria, that complement its oxygen sensing function. Mitochondrial biogenesis is triggered by NO through activation of guanylate cyclase and generation of cyclic GMP, and yields formation of functionally active mitochondria. Thus, the combined action of NO at its two known intracellular receptors, cytochrome c oxidase and guanylate cyclase, appears to play a role in coupling energy generation with energy demand. This may explain why dysregulation of the NO signalling pathway is often associated with the pathogenesis of metabolic disorders.

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.

Hemoglobin conformation couples erythrocyte S-nitrosothiol content to O2 gradients

It is proposed that the bond between nitric oxide (NO) and the Hb thiol Cys-beta(93) (SNOHb) is favored when hemoglobin (Hb) is in the relaxed (R, oxygenated) conformation, and that deoxygenation to tense (T) state destabilizes the SNOHb bond, allowing transfer of NO from Hb to form other (vasoactive) S-nitrosothiols (SNOs). However, it has not previously been possible to measure SNOHb without extensive Hb preparation, altering its allostery and SNO distribution. Here, we have validated an assay for SNOHb that uses carbon monoxide (CO) and cuprous chloride (CuCl)-saturated Cys. This assay is specific for SNOs and sensitive to 2-5 pmol. Uniquely, it measures the total SNO content of unmodified erythrocytes (RBCs) (SNO(RBC)), preserving Hb allostery. In room air, the ratio of SNO(RBC) to Hb in intact RBCs is stable over time, but there is a logarithmic loss of SNO(RBC) with oxyHb desaturation (slope, 0.043). This decay is accelerated by extraerythrocytic thiol (slope, 0.089; P < 0.001). SNO(RBC) stability is uncoupled from O(2) tension when Hb is locked in the R state by CO pretreatment. Also, SNO(RBC) is increased approximately 20-fold in human septic shock (P = 0.002) and the O(2)-dependent vasoactivity of RBCs is affected profoundly by SNO content in a murine lung bioassay. These data demonstrate that SNO content and O(2) saturation are tightly coupled in intact RBCs and that this coupling is likely to be of pathophysiological significance.

S-nitroso-albumin carries a thiol-labile pool of nitric oxide, which causes venodilation in the rat

It is now established that S-nitroso-albumin (SNO-albumin) circulates at low nanomolar concentrations under physiological conditions, but concentrations may increase to micromolar levels during disease states (e.g., cirrhosis or endotoxemia). This study tested the hypothesis that high concentrations of SNO-albumin observed in some diseases modulate vascular function and that it acts as a stable reservoir of nitric oxide (NO), releasing this molecule when the concentrations of low-molecular-weight thiols are increased. SNO-albumin was infused into rats to increase the plasma concentration from <50 nmol/l to approximately 4 micromol/l. This caused a 29 +/- 6% drop in blood pressure, 20 +/- 4% decrease in aortic blood flow, and a 25 +/- 14% reduction of renal blood flow within 10 min. These observations were in striking contrast to those of an infused arterial vasodilator (hydralazine), which increased aortic blood flow, and suggested that SNO-albumin acts primarily as a venodilator in vivo. This was confirmed by the observations that glyceryl trinitrate (a venodilator) led to similar hemodynamic changes and that the hemodynamic effects of SNO-albumin are reversed by infusion of colloid. Infusion of N-acetylcysteine into animals with artificially elevated plasma SNO-albumin concentrations led to the rapid decomposition of SNO-albumin in vivo and reproduced the hemodynamic effects of SNO-albumin infusion. These data demonstrate that SNO-albumin acts primarily as a venodilator in vivo and represents a stable reservoir of NO that can release NO when the concentrations of low-molecular-weight thiols are elevated.

Mechanisms of vasodilatation induced by nitrite instillation in intestinal lumen: possible role of hemoglobin

It has been shown that nitrite can be reduced to nitric oxide (NO) in intestine and a number of other tissues and released into the blood to form nitrosylhemoglobin (NO-Hb), existing in an equilibrium with S-nitrosohemoglobin. The latter has been suggested to be an NO transporter to distant organs. The aim of this study was to define the pathway of nitrite reduction to form NO in intestinal wall and to estimate whether this pathway has an effect on peripheral circulation. We have shown that in rat intestine at pH 7.0 70% of nitrite is converted to NO in mitochondria. At pH 6.0, nonenzymatic nitrite reduction becomes as efficient as the mitochondrial pathway. To prove whether the NO formed from nitrite in intestine can induce vasodilatation, sodium nitrite was instilled into intestinal lumen and the concentration of NO formed and diffused into the blood was followed by measuring of NO-Hb complex formation. We found that the concentration of NO-Hb gradually increases with the increase of nitrite concentration in intestinal lumen. However, it was not always accompanied by a decrease in systemic blood pressure. Blood pressure dropped down only after NO-Hb reached a threshold concentration of approximately 10 microM. These data show that NO-Hb cannot provide enough NO for vasodilatation if the concentration of NO bound to Hb is < 10 microM. The exact mechanism underlying vasodilatation observed when the concentration of NO-bound Hb was > 10 microM is, however, not clear yet and requires further studies.

Critical redox and allosteric aspects of nitric oxide interactions with hemoglobin

Nitric oxide (NO) is an important signaling molecule. Relatively long-lived NO adducts at the heme and SH groups of hemoglobin (Hb) could enable NO to carry out long-range signaling functions. In spite of significant advances, there remain as yet unresolved issues regarding the possible role of Hb in moderating NO-signaling events that affect blood pressure regulation. In this review, we summarize recent reports concerning the redox and allosteric aspects of NO/Hb interactions that have advanced our understanding of the physiological significance of NO binding to heme groups (forming NO-Hb) and of reactions promoting formation of S-nitrosated Hb (SNO-Hb). Allosteric mechanisms modify the bioactivity of NO/Hb complexes by altering the lifetime of NO-Hb and the properties of SNO-Hb. Redox reactions are significant because of the complex chemistry possible for NO and its oxidation products. Reactions at ferrous and ferric heme sites have differing consequences and affinities for interactions with NO. Moreover, redox changes at heme groups affect reactivity of SH groups and vice versa. In spite of low levels of NO-Hb and SNO-Hb found in vivo, recent findings do not rule out participation of NO-Hb or SNO-Hb in NO-dependent signaling reactions.

Pulmonary vascular effects of red blood cells containing S-nitrosated hemoglobin

The role of S-nitrosated hemoglobin (SNO-Hb) in the regulation of blood flow is a central and controversial question in cardiopulmonary physiology. In the present study, we investigate whether intact human red blood cells (RBCs) synthesized to contain high SNO-Hb levels are able to export nitric oxide bioactivity and vasodilate the pulmonary circulation, and whether SNO-Hb dependent vasodilation occurs secondary to an intrinsic oxygen-linked, allosteric function of Hb. RBCs containing supraphysiological concentrations (100-1,000x normal) of SNO-Hb (SNO-RBCs) were synthesized and added to isolated, perfused rat lungs during anoxic or normoxic ventilation, and during normoxic ventilation with pulmonary hypertension induced by the thromboxane mimetic U-46619. SNO-RBCs produced dose-dependent pulmonary vasodilation compared with control RBCs during conditions of both normoxic (U-46619) and hypoxic pulmonary vasoconstriction. These effects were associated with a simultaneous, rapid, and temperature-dependent loss of SNO from Hb. Both vasodilatory effects and the rate of SNO-Hb degradation were independent of oxygen tension and Hb oxygen saturation. Furthermore, these effects were not affected by inhibition of the RBC membrane band 3 protein (anion exchanger-1), a putative membrane facilitator of NO export from RBCs. Whereas these data support observations by multiple groups that synthesized SNO-Hb can vasodilate, this effect is not under intrinsic oxygen-dependent allosteric control, nor likely to be relevant in the pulmonary circulation at normal physiological concentrations.

Electrochemical assay for determining nitrosyl derivatives of human hemoglobin: nitrosylhemoglobin and S-nitrosylhemoglobin

Nitric oxide (NO) is an important biological regulator. It can bind to heme iron and form NO+, involved in the synthesis of S-nitrosothiols (-SNOs). NO reacts with human hemoglobin (Hb) to produce the derivatives: S-nitrosylhemoglobin (-SNOHb) and nitrosylhemoglobin (HbNO). At neutral pH values, free NO does not react directly with the -SH groups of Hb. The reductive nitrosylation of Fe(III) heme upon reaction with NO has long been studied, but it is not yet completely known. To quantify the reaction of NO with Hb, we developed a new, sensitive (nanomolar concentration range) electrochemical assay to selectively measure HbNO and -SNOHb. The assay also allows the monitoring of free NO during the reaction with human Fe(III)Hb and Fe(II)HbO(2).

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.

Transduction of NO-bioactivity by the red blood cell in sepsis: novel mechanisms of vasodilation during acute inflammatory disease

Sepsis is an acute inflammatory disease characterized by dysfunctional blood flow and hypotension. Nitric oxide (NO) is elevated during sepsis and plays an integral role in the associated vascular pathology. However, precise mechanisms and functions of NO in sepsis remain unclear. In this study, we show that red blood cells (RBCs) are foci for nitrosative reactions during acute inflammation, resulting in the formation of cells that can promote systemic vascular relaxation in an uncontrolled manner. Specifically, using experimental models of endotoxemia and surgical sepsis, NO adducts were found in the RBCs, including S-nitrosohemoglobin (SNOHb). These RBCs, referred to as septic RBCs, spontaneously stimulated vasodilation in a manner consistent with elevated SNOHb concentrations. Moreover, relaxation was cyclic guanosine monophosphate (cGMP) dependent and was inhibited by RBC lysis and glutathione but not by the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5 tetramethylimidazoline 1-oxyl 3-oxide (C-PTIO). The potential mechanism of septic RBC-mediated vasorelaxation is discussed and may involve the intermediate, nitroxyl (HNO). Coupled with data showing that NO adducts in septic RBCs were dependent on the inducible nitric oxide synthase and correlated with plasma nitrite, these findings provide a novel framework to understand mechanisms underlying dysfunctional blood flow responses during sepsis. Specifically, the concept that RBCs directly mediate systemic hypotension through NO-dependent mechanisms is discussed.

S-nitrosohemoglobin: a biochemical perspective

It has been suggested that S-nitrosohemoglobin (HbSNO) is an oxygen-dependent mediator of nitric oxide delivery to vascular smooth muscle cells, thus regulating vascular tone and blood flow. Central to this much-debated hypothesis is the concept that our previous understanding of the interaction between nitric oxide and ferrous hemoglobin was deficient. In this review we will examine the chemical and biochemical mechanisms for the formation of HbSNO, the properties of HbSNO, and the release of nitric oxide from HbSNO. This review concludes that although novel reactions of nitric oxide, nitrite, and S-nitrosothiols with hemoglobin have been uncovered, there is little evidence to support the notion that the interaction of nitric oxide with ferrous hemoglobin is more complex than had been previously established.

Mitochondrial biogenesis as a cellular signaling framework

The identification, more than 50 years ago, of mitochondria as the site of oxidative energy metabolism has prompted studies that have unraveled the complexity of the numerous biosynthetic and degradative reactions, fundamental to cell function, carried out by these organelles. These activities depend on a distinctive mitochondrial structure, with different enzymes and reactions localized in discrete membranes and aqueous compartments. The characteristic mitochondrial structural organization is the product of both synthesis of macromolecules within the mitochondria and the import of proteins and lipids synthesized outside the organelle. Synthesis and import of mitochondrial components are required for mitochondrial proliferation, but rather than producing new organelles, these processes may facilitate the growth of pre-existing mitochondria. Recent evidence indicates that these events are regulated in a complex way by several agonists and environmental conditions, through activation of specific transcription factors and signaling pathways. Some of these are now being elucidated. Generation of nitric oxide (NO) appears to be a novel player in this scenario, possibly acting as a unifying molecular switch to trigger the whole mitochondriogenic process.

Can endogenous gaseous messengers control mitochondrial biogenesis in mammalian cells?

Mitochondria have been identified as the site of oxidative energy metabolism and of numerous biosynthetic and degradative reactions, which depend on a distinctive mitochondrial structure, with different enzymes and reactions localised in discrete membranes and aqueous compartments. Synthesis and import of mitochondrial components are required for mitochondrial proliferation, but rather than producing new organelles, these processes may facilitate the growth of preexisting mitochondria. Recent evidence indicates that these events are regulated in a complex way by several agonists and environmental conditions, through activation of specific transcription factors and signaling pathways. Some of these are now being elucidated. Generation of nitric oxide (NO) appears to be a novel player in this scenario, possibly acting as a unifying molecular switch to trigger the whole process of the mitochondrial biogenesis.

Oxidation and nitrosylation of oxyhemoglobin by S-nitrosoglutathione via nitroxyl anion

The reaction between low molecular weight S-nitrosothiols and hemoglobin is often used to synthesize S-nitrosohemoglobin, a form of hemoglobin suggested to be involved in the regulation of vascular oxygen delivery. However, this reaction has not been studied in detail, and several groups have reported a variable co-formation of oxidized methemoglobin (metHb) during synthesis. This study examines the mechanism of metHb formation and shows that nitrosylhemoglobin (HbNO) can also be formed. Generation of metHb and HbNO is largely dependent on the presence of protein thiol groups. We present evidence for a mechanism for the formation of metHb and HbNO involving the intermediacy of nitroxyl anion. Specifically, the reaction of nitroxyl with S-nitrosothiols to liberate nitric oxide and reduced thiol is proposed to be central to the reaction mechanism.