The nitrate-nitrite-nitric oxide pathway in physiology and therapeutics

Dietary nitrite prevents hypercholesterolemic microvascular inflammation and reverses endothelial dysfunction

The nitrite anion is an endogenous product of mammalian nitric oxide (NO) metabolism, a key intermediate in the nitrogen cycle in plants, and a constituent of many foods. Research over the past 6 years has revealed surprising biological and cytoprotective activity of this anion. Hypercholesterolemia causes a proinflammatory phenotype in the microcirculation. This phenotype appears to result from a decline in NO bioavailability that results from a reduction in NO biosynthesis, inactivation of NO by superoxide, or both. Since nitrite has been shown to be potently cytoprotective and restore NO biochemical homeostasis, we investigated if supplemental nitrite could attenuate microvascular inflammation caused by a high cholesterol diet. C57Bl/6J mice were fed either a normal diet or a high cholesterol diet for 3 wk to induce microvascular inflammation. Mice on the high cholesterol diet received either nitrite-free drinking water or supplemental nitrite at 33 or 99 mg/l ad libitum in their drinking water. The results from this investigation reveal that mice fed a cholesterol-enriched diet exhibited significantly elevated leukocyte adhesion to and emigration through the venular endothelium as well as impaired endothelium-dependent relaxation in arterioles. Administration of nitrite in the drinking water inhibited the leukocyte adhesion and emigration and prevented the arteriolar dysfunction. This was associated with sparing of reduced tetrahydrobiopterin and decreased levels of C-reactive protein. These data reveal novel anti-inflammatory properties of nitrite and implicate the use of nitrite as a new natural therapy for microvascular inflammation and endothelial dysfunction associated with hypercholesterolemia.

Effects of nitrite on modulating ROS generation following ischemia and reperfusion

It has long been known that the generation of reactive oxygen species (ROS) is a major cause of injury after ischemia/reperfusion. More recently it has emerged that the predominant source of these ROS are the mitochondria, which are specifically damaged during prolonged ischemic episodes. Several strategies have been tested to attenuate mitochondrial damage and reperfusion ROS. Most successful has been ischemic preconditioning, a procedure in which repetitive short periods of ischemia and reperfusion reduce injury from a subsequent prolonged ischemia and reperfusion. Recently, ischemic postconditioning, whereby reperfusion after prolonged ischemia is repetitively interrupted for a short period, has also been shown to equally protect as ischemic preconditioning. Both procedures activate the same down-stream kinase pathways that minimize apoptosis and tissue damage. Endothelial nitric oxide synthase is a target of these kinase pathways and nitric oxide (NO) administration can mimic its protective effect. However, the optimal NO dose is difficult to determine and excess NO levels have been shown to be detrimental. A recently described physiological storage pool of NO, nitrite, has been shown to be a potent mediator of cytoprotection after ischemia/reperfusion that mechanistically reduces mitochondrial ROS generation at reperfusion. Here, we describe the sources, bioactivaton, and mechanisms of action of nitrite and discuss the potential of this simple anion as a therapeutic to protect against ischemia/reperfusion injury.

Hepatocellular protection by nitric oxide or nitrite in ischemia and reperfusion injury

Ischemia and reperfusion (I/R)-induced liver injury occurs in several pathophysiological disorders including hemorrhagic shock and burn as well as resectional and transplantation surgery. One of the earliest events associated with reperfusion of ischemic liver is endothelial dysfunction characterized by the decreased production of endothelial cell-derived nitric oxide (NO). This rapid post-ischemic decrease in NO bioavailability appears to be due to decreased synthesis of NO, enhanced inactivation of NO by the overproduction of superoxide or both. This review presents the most current evidence supporting the concept that decreased bioavailability of NO concomitant with enhanced production of reactive oxygen species initiates hepatocellular injury and that endogenous NO or exogenous NO produced from nitrite play important roles in limiting post-ischemic tissue injury.

Gastroprotective and blood pressure lowering effects of dietary nitrate are abolished by an antiseptic mouthwash

Recently, it has been suggested that the supposedly inert nitrite anion is reduced in vivo to form bioactive nitric oxide with physiological and therapeutic implications in the gastrointestinal and cardiovascular systems. Intake of nitrate-rich food such as vegetables results in increased levels of circulating nitrite in a process suggested to involve nitrate-reducing bacteria in the oral cavity. Here we investigated the importance of the oral microflora and dietary nitrate in regulation of gastric mucosal defense and blood pressure. Rats were treated twice daily with a commercial antiseptic mouthwash while they were given nitrate-supplemented drinking water. The mouthwash greatly reduced the number of nitrate-reducing oral bacteria and as a consequence, nitrate-induced increases in gastric NO and circulating nitrite levels were markedly reduced. With the mouthwash the observed nitrate-induced increase in gastric mucus thickness was attenuated and the gastroprotective effect against an ulcerogenic compound was lost. Furthermore, the decrease in systemic blood pressure seen during nitrate supplementation was now absent. These results suggest that oral symbiotic bacteria modulate gastrointestinal and cardiovascular function via bioactivation of salivary nitrate. Excessive use of antiseptic mouthwashes may attenuate the bioactivity of dietary nitrate.

Channel-like slippage modes in the human anion/proton exchanger ClC-4

The ClC family encompasses two classes of proteins with distinct transport functions: anion channels and transporters. ClC-type transporters usually mediate secondary active anion–proton exchange. However, under certain conditions they assume slippage mode behavior in which proton and anion transport are uncoupled, resulting in passive anion fluxes without associated proton movements. Here, we use patch clamp and intracellular pH recordings on transfected mammalian cells to characterize exchanger and slippage modes of human ClC-4, a member of the ClC transporter branch. We found that the two transport modes differ in transport mechanisms and transport rates. Nonstationary noise analysis revealed a unitary transport rate of 5 × 10⁵ s⁻¹ at +150 mV for the slippage mode, indicating that ClC-4 functions as channel in this mode. In the exchanger mode, unitary transport rates were 10-fold lower. Both ClC-4 transport modes exhibit voltage-dependent gating, indicating that there are active and non-active states for the exchanger as well as for the slippage mode. ClC-4 can assume both transport modes under all tested conditions, with exchanger/channel ratios determined by the external anion. We propose that binding of transported anions to non-active states causes transition from slippage into exchanger mode. Binding and unbinding of anions is very rapid, and slower transitions of liganded and non-liganded states into active conformations result in a stable distribution between the two transport modes. The proposed mechanism results in anion-dependent conversion of ClC-type exchanger into an anion channel with typical attributes of ClC anion channels.

Oral antioxidants and cardiovascular health in the exercise-trained and untrained elderly: a radically different outcome

Both antioxidant supplementation and exercise training have been identified as interventions which may reduce oxidative stress and thus improve cardiovascular health, but the interaction of these interventions on arterial BP (blood pressure) and vascular function has not been studied in older humans. Thus in six older (71+/-2 years) mildly hypertensive men, arterial BP was evaluated non-invasively at rest and during small muscle mass (knee-extensor) exercise with and without a pharmacological dose of oral antioxidants (vitamins C and E, and alpha-lipoic acid). The efficacy of the antioxidant intervention to decrease the plasma free radical concentration was verified via EPR (electron paramagnetic resonance) spectroscopy, while changes in endothelial function in response to exercise training and antioxidant administration were evaluated via FMD (flow-mediated vasodilation). Subjects were re-evaluated after a 6-week aerobic exercise training programme. Prior to training, acute antioxidant administration did not change resting arterial BP or FMD. Six weeks of knee-extensor exercise training reduced systolic BP (from 150+/-8 mmHg at pre-training to 138+/-3 mmHg at post-training) and diastolic BP (from 91+/-5 mmHg at pre-training to 79+/-3 mmHg at post-training), and improved FMD (1.5+/-1 to 4.9+/-1% for pre- and post-training respectively). However, antioxidant administration after exercise training negated these improvements, returning subjects to a hypertensive state and blunting training-induced improvements in FMD. In conclusion, the paradoxical effects of these interventions suggest a need for caution when exercise and acute antioxidant supplementation are combined in elderly mildly hypertensive individuals.

Enhancement of nitric oxide release from nitrosyl hemoglobin and nitrosyl myoglobin by red/near infrared radiation: potential role in cardioprotection

Nitric oxide is an important messenger in numerous biological processes, such as angiogenesis, hypoxic vasodilation, and cardioprotection. Although nitric oxide synthases (NOS) produce the bulk of NO, there is increasing interest in NOS independent generation of NO in vivo, particularly during hypoxia or anoxia, where low oxygen tensions limit NOS activity. Interventions that can increase NO bioavailability have significant therapeutic potential. The use of far red and near infrared light (R/NIR) can reduce infarct size, protect neurons from methanol toxicity, and stimulate angiogenesis. How R/NIR modulates these processes in vivo and in vitro is unknown, but it has been suggested that increases in NO levels are involved. In this study we examined if R/NIR light could facilitate the release of NO from nitrosyl heme proteins. In addition, we examined if R/NIR light could enhance the protective effects of nitrite on ischemia and reperfusion injury in the rabbit heart. We show both in purified systems and in myocardium that R/NIR light can decay nitrosyl hemes and release NO, and that this released NO may enhance the cardioprotective effects of nitrite. Thus, the photodissociation to NO and its synergistic effect with sodium nitrite may represent a noninvasive and site specific means for increasing NO bioavailability.

Regulation of nitrite transport in red blood cells by hemoglobin oxygen fractional saturation

Allosteric regulation of nitrite reduction by deoxyhemoglobin has been proposed to mediate nitric oxide (NO) formation during hypoxia. Nitrite is predominantly an anion at physiological pH, raising questions about the mechanism by which it enters the red blood cell (RBC) and whether this is regulated and coupled to deoxyhemoglobin-mediated reduction. We tested the hypothesis that nitrite transport by RBCs is regulated by fractional saturation. Using human RBCs, nitrite consumption was faster at lower fractional saturations, consistent with faster reactions with deoxyheme. A membrane-based regulation was suggested by slower nitrite consumption with intact versus lysed RBCs. Interestingly, upon nitrite addition, intracellular nitrite concentrations attained a steady state that, despite increased rates of consumption, did not change with decreasing oxygen tensions, suggesting a deoxygenation-sensitive step that either increases nitrite import or decreases the rate of nitrite export. A role for anion exchanger (AE)-1 in the control of nitrite export was suggested by increased intracellular nitrite concentrations in RBCs treated with DIDS. Moreover, deoxygenation decreased steady-state levels of intracellular nitrite in AE-1-inhibited RBCs. Based on these data, we propose a model in which deoxyhemoglobin binding to AE-1 inhibits nitrite export under low oxygen tensions allowing for the coupling between deoxygenation and nitrite reduction to NO along the arterial-to-venous gradient.

Myocardial protection by nitrite

Nitrite has long been considered to be an inert oxidative metabolite of nitric oxide (NO). Recent work, however, has demonstrated that nitrite represents an important tissue storage form of NO that can be reduced to NO during ischaemic or hypoxic events. This exciting series of discoveries has created an entirely new field of research that involves the investigation of the molecular, biochemical, and physiological activities of nitrite under a variety of physiological and pathophysiological states. This has also led to a re-evaluation of the role that nitrite plays in health and disease. As a result there has been an interest in the use of nitrite as a therapeutic strategy for the treatment of acute myocardial infarction. Nitrite therapy has now been studied in several animal models and has proven to be an effective means to reduce myocardial ischaemia-reperfusion injury. This review article will provide a brief summary of the key findings that have led to the re-evaluation of nitrite and highlight the evidence supporting the cardioprotective actions of nitrite and also highlight the potential clinical application of nitrite therapy to cardiovascular diseases.

Nitrite mediates cytoprotection after ischemia/reperfusion by modulating mitochondrial function

Nitrite, once thought to be an inert biomarker of NO formation, is now recognized as an endocrine storage pool of bioactive NO. While nitrite mediates a number of hypoxic responses, one of its most robust effects is its ability to confer cytoprotection after ischemia/reperfusion in a number of organs and models. The mechanism of this cytoprotection appears to be mediated at the level of the mitochondrion. Here we review the studies demonstrating that nitrite is cytoprotective in the heart and describe the mechanism of this cytoprotection, which involves the post-translational modification of complex I leading to the modulation of mitochondrial reactive oxygen species generation at reperfusion. The mechanism of nitrite-dependent cytoprotection will be compared to other cytoprotective agents including NO and ischemic preconditioning.

The role of nitrite in nitric oxide homeostasis: a comparative perspective

Nitrite is endogenously produced as an oxidative metabolite of nitric oxide, but it also functions as a NO donor that can be activated by a number of cellular proteins under hypoxic conditions. This article discusses the physiological role of nitrite and nitrite-derived NO in blood flow regulation and cytoprotection from a comparative viewpoint, with focus on mammals and fish. Constitutive nitric oxide synthase activity results in similar plasma nitrite levels in mammals and fish, but nitrite can also be taken up across the gills in freshwater fish, which has implications for nitrite/NO levels and nitrite utilization in hypoxia. The nitrite reductase activity of deoxyhemoglobin is a major mechanism of NO generation from nitrite and may be involved in hypoxic vasodilation. Nitrite is readily transported across the erythrocyte membrane, and the transport is enhanced at low O(2) saturation in some species. Also, nitrite preferentially reacts with deoxyhemoglobin rather than oxyhemoglobin at intermediate O(2) saturations. The hemoglobin nitrite reductase activity depends on heme O(2) affinity and redox potential and shows species differences within mammals and fish. The NO forming capacity is elevated in hypoxia-tolerant species. Nitrite-induced vasodilation is well documented, and many studies support a role of erythrocyte/hemoglobin-derived NO. Vasodilation can, however, also originate from nitrite reduction within the vessel wall, and at present there is no consensus regarding the relative importance of competing mechanisms. Nitrite reduction to NO provides cytoprotection in tissues during ischemia-reperfusion events by inhibiting mitochondrial respiration and limiting reactive oxygen species. It is argued that the study of hypoxia-tolerant lower vertebrates and diving mammals may help evaluate mechanisms and a full understanding of the physiological role of nitrite.

Nitrite exerts potent negative inotropy in the isolated heart via eNOS-independent nitric oxide generation and cGMP-PKG pathway activation

The ubiquitous anion nitrite (NO(2)(-)) has recently emerged as an endocrine storage form of nitric oxide (NO) and a signalling molecule that mediates a number of biological responses. Although the role of NO in regulating cardiac function has been investigated in depth, the physiological signalling effects of nitrite on cardiac function have only recently been explored. We now show that remarkably low concentrations of nitrite (1 nM) significantly modulate cardiac contractility in isolated and perfused Langendorff rat heart. In particular, nitrite exhibits potent negative inotropic and lusitropic activities as evidenced by a decrease in left ventricular pressure and relaxation, respectively. Furthermore, we demonstrate that the nitrite-dependent effects are mediated by NO formation but independent of NO synthase (NOS) activity. Specifically, nitrite infusion in the Langendorff system produces NO and cGMP/PKG-dependent negative inotropism, as evidenced by the formation of cellular iron-nitrosyl complexes and inhibition of biological effect by NO scavengers and by PKG inhibitors. These data are consistent with the hypothesis that nitrite represents an eNOS-independent source of NO in the heart which modulates cardiac contractility through the NO-cGMP/PKG pathway. The observed high potency of nitrite supports a physiological function of nitrite as a source of cardiomyocyte NO and a fundamental signalling molecule in the heart.

Mechanistic insights into nitrite-induced cardioprotection using an integrated metabolomic/proteomic approach

Nitrite has recently emerged as an important bioactive molecule, capable of conferring cardioprotection and a variety of other benefits in the cardiovascular system and elsewhere. The mechanisms by which it accomplishes these functions remain largely unclear. To characterize the dose response and corresponding cardiac sequelae of transient systemic elevations of nitrite, we assessed the time course of oxidation/nitros(yl)ation, as well as the metabolomic, proteomic, and associated functional changes in rat hearts following acute exposure to nitrite in vivo. Transient systemic nitrite elevations resulted in: (1) rapid formation of nitroso and nitrosyl species; (2) moderate short-term changes in cardiac redox status; (3) a pronounced increase in selective manifestations of long-term oxidative stress as evidenced by cardiac ascorbate oxidation, persisting long after changes in nitrite-related metabolites had normalized; (4) lasting reductions in glutathione oxidation (GSSG/GSH) and remarkably concordant nitrite-induced cardioprotection, which both followed a complex dose-response profile; and (5) significant nitrite-induced protein modifications (including phosphorylation) revealed by mass spectrometry-based proteomic studies. Altered proteins included those involved in metabolism (eg, aldehyde dehydrogenase 2, ubiquinone biosynthesis protein CoQ9, lactate dehydrogenase B), redox regulation (eg, protein disulfide isomerase A3), contractile function (eg, filamin-C), and serine/threonine kinase signaling (eg, protein kinase A R1alpha, protein phosphatase 2A A R1-alpha). Thus, brief elevations in plasma nitrite trigger a concerted cardioprotective response characterized by persistent changes in cardiac metabolism, redox stress, and alterations in myocardial signaling. These findings help elucidate possible mechanisms of nitrite-induced cardioprotection and have implications for nitrite dosing in therapeutic regimens.

Molecular hydrogen is protective against 6-hydroxydopamine-induced nigrostriatal degeneration in a rat model of Parkinson's disease

Molecular hydrogen serves as an antioxidant that reduces hydroxyl radicals, but not the other reactive oxygen and nitrogen species. In the past year, molecular hydrogen has been reported to prevent or ameliorate eight diseases in rodents and one in human associated with oxidative stress. In Parkinson's disease, mitochondrial dysfunction and the associated oxidative stress are major causes of dopaminergic cell loss in the substantia nigra. We examined effects of approximately 50%-saturated molecular hydrogen in drinking water before or after the stereotactic surgery on 6-hydroxydopamine-induced nigrostrital degeneration in a rat model of Parkinson's disease. Methamphetamine-induced behavioral analysis showed that molecular hydrogen prevented both the development and progression of the nigrostrital degeneration. Tyrosine hydroxylase staining of the substantia nigra and striatum also demonstrated that pre- and post-treatment with hydrogen prevented the dopaminergic cell loss. Our studies suggest that hydrogen water is likely able to retard the development and progression of Parkinson's disease.

Porphyrins containing nitric oxide donors: Synthesis and cancer cell-oriented NO release

Four novel porphyrins containing nitric oxide (NO) donors were synthesized, and the structures of all the products were characterized by IR, UV-vis, (1)H NMR, and elementary analysis. Interestingly, these new compounds not only were able to release NO, but also showed cancer cell-oriented accumulation. Higher accumulation of these new porphyrins containing NO donors in BEL-7402 liver cancer cells than in L-02 liver normal cells was corroborated by UV-vis spectroscopy. The biological activity of these porphyrins against BEL-7402 liver cancer cells was tested with a MTT assay. The studies indicated that they had more effective killing of BEL-7402 liver cancer cells than that of L-02 liver normal cells, and they had similar activity against MCF-7 breast cancer cells when compared to 5-fluorouracil in the absence of light.

Prevention of the pulmonary vasoconstrictor effects of HBOC-201 in awake lambs by continuously breathing nitric oxide

BACKGROUND

Hemoglobin-based oxygen-carrying solutions (HBOC) provide emergency alternatives to blood transfusion to carry oxygen to tissues without the risks of disease transmission or transfusion reaction. Two primary concerns hampering the clinical acceptance of acellular HBOC are the occurrence of systemic and pulmonary vasoconstriction and the maintenance of the heme-iron in the reduced state (Fe2+). We recently demonstrated that pretreatment with inhaled nitric oxide prevents the systemic hypertension induced by HBOC-201 (polymerized bovine hemoglobin) infusion in awake mice and sheep without causing methemoglobinemia. However, the impact of HBOC-201 infusion with or without inhaled nitric oxide on pulmonary vascular tone has not yet been examined.

METHODS

The pulmonary and systemic hemodynamic effects of breathing nitric oxide both before and after the administration of HBOC-201 were determined in healthy, awake lambs.

RESULTS

Intravenous administration of HBOC-201 (12 ml/kg) induced prolonged systemic and pulmonary vasoconstriction. Pretreatment with inhaled nitric oxide (80 parts per million [ppm] for 1 h) prevented the HBOC-201--induced increase in mean arterial pressure but not the increase of pulmonary arterial pressure, systemic vascular resistance, or pulmonary vascular resistance. Pretreatment with inhaled nitric oxide (80 ppm for 1 h) followed by breathing a lower concentration of nitric oxide (5 ppm) during and after HBOC-201 infusion prevented systemic and pulmonary vasoconstriction without increasing methemoglobin levels.

CONCLUSIONS

These findings demonstrate that pretreatment with inhaled nitric oxide followed by breathing a lower concentration of the gas during and after administration of HBOC-201 may enable administration of an acellular hemoglobin substitute without vasoconstriction while preserving its oxygen-carrying capacity.

PPAR-alpha activation protects the type 2 diabetic myocardium against ischemia-reperfusion injury: involvement of the PI3-Kinase/Akt and NO pathway

Several clinical studies have shown the beneficial cardiovascular effects of fibrates in patients with diabetes and insulin resistance. The ligands of peroxisome proliferator-activated receptor-alpha (PPAR-alpha) reduce ischemia-reperfusion injury in nondiabetic animals. We hypothesized that the activation of PPAR-alpha would exert cardioprotection in type 2 diabetic Goto-Kakizaki (GK) rats, involving mechanisms related to nitric oxide (NO) production via the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. GK rats and age-matched Wistar rats (n >or= 7) were given either 1) the PPAR-alpha agonist WY-14643 (WY), 2) dimethyl sulfoxide (DMSO), 3) WY and the NO synthase inhibitor N(G)-nitro-l-arginine (l-NNA), 4) l-NNA, 5) WY and the PI3K inhibitor wortmannin, or 6) wortmannin alone intravenously before a 35-min period of coronary artery occlusion followed by 2 h of reperfusion. Infarct size (IS), expression of endothelial NO synthase (eNOS), inducible NO synthase, and Akt as well as nitrite/nitrate were determined. The IS was 75 +/- 3% and 72 +/- 4% of the area at risk in the Wistar and GK DMSO groups, respectively. WY reduced IS to 56 +/- 3% in Wistar (P < 0.05) and to 46 +/- 5% in GK rats (P < 0.001). The addition of either l-NNA or wortmannin reversed the cardioprotective effect of WY in both Wistar (IS, 70 +/- 5% and 65 +/- 5%, respectively) and GK (IS, 66 +/- 4% and 64 +/- 4%, P < 0.05, respectively) rats. The expression of eNOS and eNOS Ser1177 in the ischemic myocardium from both strains was increased after WY. The expression of Akt, Akt Ser473, and Akt Thr308 was also increased in the ischemic myocardium from GK rats following WY. Myocardial nitrite/nitrate levels were reduced in GK rats (P < 0.05). The results suggest that PPAR-alpha activation protects the type 2 diabetic rat myocardium against ischemia-reperfusion injury via the activation of the PI3K/Akt and NO pathway.

Reaction of the Five-Coordinate O-Nitrito Complex Fe(Por)(ONO) (Por = meso-tetra-arylporphyrinato) with THF Gives Two Six-Coordinate Isomers

The effect of the proximal ligand on the coordination of the nitrite ligand to the heme model systems Fe(Por)(η1-ONO) (Por = meso-tetraarylporphyrinato dianion) was investigated by FTIR and UV-vis spectra in solvent free, low temperature, porous layered solids and by density functional computations. The reaction of the five-coordinate complex Fe(Por)(η1-ONO) with the ether tetrahydrofuran gives a mixture of the O-nitrito and N-nitrito isomers Fe(Por)(THF)(η1-ONO) and Fe(Por)(THF)(NO2), respectively. This observation is in contrast to earlier studies with nitrogen donor Lewis bases where the N-nitrito isomers were clearly the more stable of the six-coordinated complexes. The adduct formation is reversible; the five-coordinate O-nitrito complexes Fe(Por)(η1-ONO) were largely restored upon warming under vacuum pumping.

Nitric oxide mechanism of protection in ischemia and reperfusion injury

In 1992 nitric oxide (NO) was declared molecule of the year by Science magazine, and ever since research on this molecule continues to increase. Following this award, NO was shown to be a mediator/protector of ischemia and reperfusion injury in many organs, such as the heart, liver, lungs, and kidneys. Controversy has existed concerning the actual protective effects of NO. However, literature from the past 15 years seems to reinforce the consensus that NO is indeed protective. Some of the protective actions of NO in ischemia and reperfusion are due to its potential as an antioxidant and anti-inflammatory agent, along with its beneficial effects on cell signaling and inhibition of nuclear proteins, such as NF-kappa B and AP-1. New therapeutic potentials for this drug are also continuously emerging. Exogenous NO and endogenous NO may both play protective roles during ischemia and reperfusion injury. Sodium nitroprusside and nitroglycerin have been used clinically with much success; though only recently have they been tested and proven effective in attenuating some of the injuries associated with ischemia and reperfusion. NO inhalation has, in the past, mostly been used for its pulmonary effects, but has also recently been shown to be protective in other organs. The potential of NO in the treatment of ischemic disease is only just being realized. Elucidation of the mechanism by which NO exerts its protective effects needs further investigation. Therefore, this paper will focus on the mechanistic actions of NO in ischemia and reperfusion injury, along with the compound's potential therapeutic benefits.

Therapeutic antioxidant medical gas

Medical gases are pharmaceutical gaseous molecules which offer solutions to medical needs and include traditional gases, such as oxygen and nitrous oxide, as well as gases with recently discovered roles as biological messenger molecules, such as carbon monoxide, nitric oxide and hydrogen sulphide. Medical gas therapy is a relatively unexplored field of medicine; however, a recent increasing in the number of publications on medical gas therapies clearly indicate that there are significant opportunities for use of gases as therapeutic tools for a variety of disease conditions. In this article, we review the recent advances in research on medical gases with antioxidant properties and discuss their clinical applications and therapeutic properties.

Myocardial protection by nitrite: evidence that this reperfusion therapeutic will not be lost in translation

The circulating anion nitrite (NO(2)(-)), previously thought to be an inert product of nitric oxide (NO) oxidation, has now been identified as an important storage reservoir of bioavailable NO in the blood and tissues. Reduction of NO(2)(-) to NO over the physiologic pH and oxygen gradient by deoxyhemoglobin, myoglobin, xanthine oxidoreductase, and by nonenzymatic acidic disproportionation has been demonstrated to confer cytoprotection against ischemia-reperfusion injury in the heart, liver, brain, and kidney. Here, we review the mechanisms that have been established to regulate hypoxic NO(2)(-) reduction to NO, analyze the preclinical and clinical evidence supporting NO(2)(-)-mediated cytoprotection after ischemia-reperfusion injury, and examine the therapeutic potential of NO(2)(-) for cardiovascular disease. Evidence is accumulating that suggests NO(2)(-) has surmounted many of the direct challenges to reperfusion therapeutics summarized by the National Heart, Lung, and Blood Institute Working Group in "Myocardial protection at a crossroads: the need for translation into clinical therapy." In this context, we discuss important considerations in designing human clinical trials to test the efficacy of NO(2)(-) in the setting of ischemia-reperfusion injury, with particular attention to the study of ST-segment elevation myocardial infarction.

PA-824 kills nonreplicating Mycobacterium tuberculosis by intracellular NO release

Bicyclic nitroimidazoles, including PA-824, are exciting candidates for the treatment of tuberculosis. These prodrugs require intracellular activation for their biological function. We found that Rv3547 is a deazaflavin-dependent nitroreductase (Ddn) that converts PA-824 into three primary metabolites; the major one is the corresponding des-nitroimidazole (des-nitro). When derivatives of PA-824 were used, the amount of des-nitro metabolite formed was highly correlated with anaerobic killing of Mycobacterium tuberculosis (Mtb). Des-nitro metabolite formation generated reactive nitrogen species, including nitric oxide (NO), which are the major effectors of the anaerobic activity of these compounds. Furthermore, NO scavengers protected the bacilli from the lethal effects of the drug. Thus, these compounds may act as intracellular NO donors and could augment a killing mechanism intrinsic to the innate immune system.

Increased nitrite reductase activity of fetal versus adult ovine hemoglobin

Growing evidence indicates that nitrite, NO2-, serves as a circulating reservoir of nitric oxide (NO) bioactivity that is activated during physiological and pathological hypoxia. One of the intravascular mechanisms for nitrite conversion to NO is a chemical nitrite reductase activity of deoxyhemoglobin. The rate of NO production from this reaction is increased when hemoglobin is in the R conformation. Because the mammalian fetus exists in a low-oxygen environment compared with the adult and is exposed to episodes of severe ischemia during the normal birthing process, and because fetal hemoglobin assumes the R conformation more readily than adult hemoglobin, we hypothesized that nitrite reduction to NO may be enhanced in the fetal circulation. We found that the reaction was faster for fetal than maternal hemoglobin or blood and that the reactions were fastest at 50-80% oxygen saturation, consistent with an R-state catalysis that is predominant for fetal hemoglobin. Nitrite concentrations were similar in blood taken from chronically instrumented normoxic ewes and their fetuses but were elevated in response to chronic hypoxia. The findings suggest an augmented nitrite reductase activity of fetal hemoglobin and that the production of nitrite may participate in the regulation of vascular NO homeostasis in the fetus.

Effect of iron-quercetin complex on reduction of nitrite in in vitro and in vivo systems

This study investigated whether reducing agents such as quercetin and iron(II) facilitate formation of nitric oxide (NO) gas from orally ingested nitrite in an vivo study. When 3 mg/kg Na (15)NO2 was orally administered to rats with or without iron(II) or quercetin, Hb (15)NO, which is indicative of systemic (15)NO, was detected in the blood, with the maximum blood concentration of Hb (15)NO at 15 min after nitrite or nitrite plus quercetin treatment, whereas after administration of nitrite plus iron(II) or nitrite plus iron(II) and quercetin, the time was shortened to 10 min. Interestingly, iron(II), quercetin, or iron(II) plus quercetin did not affect the total amount of Hb (15)NO generated from orally administered Na (15)NO2. However, the systemic nitrite concentration was significantly decreased in the presence of iron(II) or iron(II) plus quercetin. These results may indicate that iron(II) is critical to the generation of NO gas from nitrite, whereas quercetin contributed little under the in vivo experimental conditions.

Dietary doses of nitrite restore circulating nitric oxide level and improve renal injury inl-NAME-induced hypertensive rats

We have reported that pharmacological doses of oral nitrite increase circulating nitric oxide (NO) and exert hypotensive effects in Nω-nitro-l-arginine methyl ester (l-NAME)-induced hypertensive rats. In this study, we examined the effect of a chronic dietary dose of nitrite on the hypertension and renal damage induced by chronic l-NAME administration in rats. The animals were administered tap water containing l-NAME (1 g/l) or l-NAME + nitrite (low dose: 0.1 mg/l, medium dose: 1 mg/l, high dose: 10 mg/l) for 8 wk. We evaluated blood NO levels as hemoglobin-NO adducts (iron-nitrosyl-hemoglobin), using an electron paramagnetic resonance method. Chronic administration of l-NAME for 8 wk induced hypertension and renal injury and reduced the blood iron-nitrosyl-hemoglobin level (control 38.8 ± 8.9 vs. l-NAME 6.0 ± 3.1 arbitrary units). Coadministration of a low dose of nitrite with l-NAME did not change the reduced iron-nitrosyl-hemoglobin signal and did not improve the l-NAME-induced renal injury. The blood iron-nitrosyl-hemoglobin signals of the medium dose and high dose of nitrite were significantly higher than that of l-NAME alone. Chronic administration of a medium dose of nitrite attenuated l-NAME-induced renal histological changes and proteinuria. A high dose of nitrite also attenuated l-NAME-induced renal injury. These findings suggest that dietary doses of nitrite that protect the kidney are associated with significant increase in iron-nitrosyl-hemoglobin levels. We conclude that dietary nitrite-derived NO generation may serve as a backup system when the nitric oxide synthase/l-arginine-dependent NO generation system is compromised.

Tissue processing of nitrite in hypoxia: an intricate interplay of nitric oxide-generating and -scavenging systems

Although nitrite (NO(2)(-)) and nitrate (NO(3)(-)) have been considered traditionally inert byproducts of nitric oxide (NO) metabolism, recent studies indicate that NO(2)(-) represents an important source of NO for processes ranging from angiogenesis through hypoxic vasodilation to ischemic organ protection. Despite intense investigation, the mechanisms through which NO(2)(-) exerts its physiological/pharmacological effects remain incompletely understood. We sought to systematically investigate the fate of NO(2)(-) in hypoxia from cellular uptake in vitro to tissue utilization in vivo using the Wistar rat as a mammalian model. We find that most tissues (except erythrocytes) produce free NO at rates that are maximal under hypoxia and that correlate robustly with each tissue's capacity for mitochondrial oxygen consumption. By comparing the kinetics of NO release before and after ferricyanide addition in tissue homogenates to mathematical models of NO(2)(-) reduction/NO scavenging, we show that the amount of nitrosylated products formed greatly exceeds what can be accounted for by NO trapping. This difference suggests that such products are formed directly from NO(2)(-), without passing through the intermediacy of free NO. Inhibitor and subcellular fractionation studies indicate that NO(2)(-) reductase activity involves multiple redundant enzymatic systems (i.e. heme, iron-sulfur cluster, and molybdenum-based reductases) distributed throughout different cellular compartments and acting in concert to elicit NO signaling. These observations hint at conserved roles for the NO(2)(-)-NO pool in cellular processes such as oxygen-sensing and oxygen-dependent modulation of intermediary metabolism.

Effect of inhaled nitric oxide on cerebrospinal fluid and blood nitrite concentrations in newborn lambs

Inhaled nitric oxide (iNO) has many extrapulmonary effects. As the half-life of nitric oxide (NO) in blood is orders of magnitude less than the circulation time from lungs to the brain, the mediator of systemic effects of iNO is unknown. We hypothesized that concentrations of nitrite, a circulating byproduct of NO with demonstrated NO bioactivity, would increase in blood and cerebrospinal fluid (CSF) during iNO therapy. iNO (80 ppm) was given to six newborn lambs and results compared with six control lambs. Blood and CSF nitrite concentrations increased 2-fold in response to iNO. cGMP increased in blood but not CSF suggesting brain guanylate cyclase activity was not increased. When sodium nitrite was infused i.v. blood and CSF nitrite levels increased within 10 min and reached similar levels of 14.6 +/- 1.5 microM after 40 min. The reactivity of nitrite in Hb-free brain homogenates was investigated, with the findings that nitrite did not disappear nor did measurable amounts of s-nitroso, n-nitroso, or iron-nitrosyl-species appear. We conclude that although nitrite diffuses freely between blood and CSF, due to its lack of reactivity in the brain, nitrite's putative role as the mediator of the systemic effects of iNO is limited to intravascular reactions.

Nitrite-nitric oxide control of mitochondrial respiration at the frontier of anoxia

Actively respiring animal and plant tissues experience hypoxia because of mitochondrial O(2) consumption. Controlling oxygen balance is a critical issue that involves in mammals hypoxia-inducible factor (HIF) mediated transcriptional regulation, cytochrome oxidase (COX) subunit adjustment and nitric oxide (NO) as a mediator in vasodilatation and oxygen homeostasis. In plants, NO, mainly derived from nitrite, is also an important signalling molecule. We describe here a mechanism by which mitochondrial respiration is adjusted to prevent a tissue to reach anoxia. During pea seed germination, the internal atmosphere was strongly hypoxic due to very active mitochondrial respiration. There was no sign of fermentation, suggesting a down-regulation of O(2) consumption near anoxia. Mitochondria were found to finely regulate their surrounding O(2) level through a nitrite-dependent NO production, which was ascertained using electron paramagnetic resonance (EPR) spin trapping of NO within membranes. At low O(2), nitrite is reduced into NO, likely at complex III, and in turn reversibly inhibits COX, provoking a rise to a higher steady state level of oxygen. Since NO can be re-oxidized into nitrite chemically or by COX, a nitrite-NO pool is maintained, preventing mitochondrial anoxia. Such an evolutionarily conserved mechanism should have an important role for oxygen homeostasis in tissues undergoing hypoxia.

The gastric mucus layers: constituents and regulation of accumulation

The mucus layer continuously covering the gastric mucosa consists of a loosely adherent layer that can be easily removed by suction, leaving a firmly adherent mucus layer attached to the epithelium. These two layers exhibit different gastroprotective roles; therefore, individual regulation of thickness and mucin composition were studied. Mucus thickness was measured in vivo with micropipettes in anesthetized mice [isoflurane; C57BL/6, Muc1-/-, inducible nitric oxide synthase (iNOS)-/-, and neuronal NOS (nNOS)-/-] and rats (inactin) after surgical exposure of the gastric mucosa. The two mucus layers covering the gastric mucosa were differently regulated. Luminal administration of PGE(2) increased the thickness of both layers, whereas luminal NO stimulated only firmly adherent mucus accumulation. A new gastroprotective role for iNOS was indicated since iNOS-deficient mice had thinner firmly adherent mucus layers and a lower mucus accumulation rate, whereas nNOS did not appear to be involved in mucus secretion. Downregulation of gastric mucus accumulation was observed in Muc1-/- mice. Both the firmly and loosely adherent mucus layers consisted of Muc5ac mucins. In conclusion, this study showed that, even though both the two mucus layers covering the gastric mucosa consist of Muc5ac, they are differently regulated by luminal PGE(2) and NO. A new gastroprotective role for iNOS was indicated since iNOS-/- mice had a thinner firmly adherent mucus layer. In addition, a regulatory role of Muc1 was demonstrated since downregulation of gastric mucus accumulation was observed in Muc1-/- mice.

Nitroxyl (HNO): the Cinderella of the nitric oxide story

Until recently, most of the biological effects of nitric oxide (NO) have been attributed to its uncharged state (NO*), yet NO can also exist in the reduced state as nitroxyl (HNO or NO(-)). Putatively generated from both NO synthase (NOS)-dependent and -independent sources, HNO is rapidly emerging as a novel entity with distinct pharmacology and therapeutic advantages over its redox sibling, NO*. Thus, unlike NO*, HNO can target cardiac sarcoplasmic ryanodine receptors to increase myocardial contractility, can interact directly with thiols and is resistant to both scavenging by superoxide (*O2-) and tolerance development. HNO donors are protective in the setting of heart failure in which NO donors have minimal impact. Here, we discuss the unique pharmacology of HNO versus NO* and highlight the therapeutic potential of HNO donors in the treatment of cardiovascular disease.

eNOS and stroke: prevention, treatment and recovery

It is common knowledge that ischemic stroke has major social and economic consequences. However, until now, translation of experimental studies into clinical reality has been sorely lacking. So far, most studies have focused on acute stroke outcome and early treatment paradigms affording neuroprotection. It is increasingly recognized that it will be necessary to harness the capacity of the brain for neuroregeneration to improve longer-term outcome. Endothelial nitric oxide synthase (eNOS) is emerging as a key target in molecular stroke research. eNOS ameliorates acute ischemic injury and promotes recovery following cerebral ischemia. This review summarizes the effects of eNOS on the regulation of cerebral blood flow, hemostasis, inflammation, angiogenesis as well as neurogenesis. The possible impact on stroke prevention as well as on strategies aimed at improving long-term stroke outcome are discussed.

The increase in plasma nitrite after a dietary nitrate load is markedly attenuated by an antibacterial mouthwash

Recent studies surprisingly show that dietary inorganic nitrate, abundant in vegetables, can be metabolized in vivo to form nitrite and then bioactive nitric oxide. A reduction in blood pressure was recently noted in healthy volunteers after dietary supplementation with nitrate; an effect consistent with formation of vasodilatory nitric oxide. Oral bacteria have been suggested to play a role in bioactivation of nitrate by first reducing it to the more reactive anion nitrite. In a cross-over designed study in seven healthy volunteers we examined the effects of a commercially available chlorhexidine-containing antibacterial mouthwash on salivary and plasma levels of nitrite measured after an oral intake of sodium nitrate (10mg/kg dissolved in water). In the control situation the salivary and plasma levels of nitrate and nitrite increased greatly after the nitrate load. Rinsing the mouth with the antibacterial mouthwash prior to the nitrate load had no effect on nitrate accumulation in saliva or plasma but abolished its conversion to nitrite in saliva and markedly attenuated the rise in plasma nitrite. We conclude that the acute increase in plasma nitrite seen after a nitrate load is critically dependent on nitrate reduction in the oral cavity by commensal bacteria. The removal of these bacteria with an antibacterial mouthwash will very likely attenuate the NO-dependent biological effects of dietary nitrate.

Dietary nitrite restores NO homeostasis and is cardioprotective in endothelial nitric oxide synthase-deficient mice

Endothelial production of nitric oxide (NO) is critical for vascular homeostasis. Nitrite and nitrate are formed endogenously by the stepwise oxidation of NO and have, for years, been regarded as inactive degradation products. As a result, both anions are routinely used as surrogate markers of NO production, with nitrite as a more sensitive marker. However, both nitrite and nitrate are derived from dietary sources. We sought to determine how exogenous nitrite affects steady-state concentrations of NO metabolites thought to originate from nitric oxide synthase (NOS)-derived NO as well as blood pressure and myocardial ischemia-reperfusion (I/R) injury. Mice deficient in endothelial nitric oxide synthase (eNOS-/-) demonstrated decreased blood and tissue nitrite, nitrate, and nitroso proteins, which were further reduced by low-nitrite (NOx) diet for 1 week. Nitrite supplementation (50 mg/L) in the drinking water for 1 week restored NO homeostasis in eNOS-/- mice and protected against I/R injury. Nitrite failed to alter heart rate or mean arterial blood pressure at the protective dose. These data demonstrate the significant influence of dietary nitrite intake on the maintenance of steady-state NO levels. Dietary nitrite and nitrate may serve as essential nutrients for optimal cardiovascular health and may provide a novel prevention/treatment modality for disease associated with NO insufficiency.

The mechanism of oleic acid nitration by *NO(2)

Fatty acid nitration is a recently discovered process that generates biologically active nitro lipids; however, its mechanism has not been fully characterized. For example, some structural details such as vinyl and allyl isomers of the nitro fatty acids have not been established. To characterize lipids that originated from a biomimetic reaction of *NO(2) with oleic acid, we synthesized several isomers of nitro oleic acids and studied their chromatography and mass spectra by various techniques of mass spectrometry. LC/MS analysis performed on a high resolution micro column detected molecular carboxylic anions of various oleic acid nitro isomers (NO(2)OA). Esterification of NO(2)OA with pentafluorobenzyl bromide and diisopropylethylamine as a catalyst produced a unique isoxazole ester derivative exclusively from allyl NO(2)OA isomers via dehydration of the nitro group at ambient temperatures. This new analytical procedure revealed that *NO(2) generated two vinyl and two allyl isomers of NO(2)OA. The vinyl isomers showed high regioselectivity with the 1.8:1 preference for the 10-NO(2)OA isomer that was absent among allylic isomers. The nitration also generated elaidic acid via cis-trans isomerization and diatereoisomers of vicinal nitro hydroxy, nitro keto and alpha-nitro epoxy stearic acids with high stereo and regioselectivity. Nitration of small unilamelar phospholipid vesicles resulted in several phospholipids containing nitro lipids and elaidic acid amenable to hydrolysis by phospholipase A(2).