Martin Feelisch

Correlation between nitric oxide formation during degradation of organic nitrates and activation of guanylate cyclase

Organic nitrates develop their vasodilating potency by stimulating the enzyme guanylate cyclase. There are still several theories concerning the molecular mechanism of enzyme activation, the most likely of which sees nitric oxide (NO.) as the true modulator of the soluble guanylate cyclase. We therefore examined the release of nitric oxide from organic nitrates by means of a difference-spectrophotometric method and found that our results correlated well with the extent of enzyme activation. The more NO. was liberated from the compounds in question, the higher was the enzyme activation observed. When the examined nitrates were used in a concentration which caused a half-maximal enzyme stimulation, the result was a NO. liberation of striking uniformity. This correlation also applied to SIN-1 for which it has been assumed up to now that the intact molecule itself is able to stimulate the enzyme and not the nitric oxide released from it. We found the reaction between organic nitrates and cysteine to be highly dependent on temperature, while the extent of the observed enhancement increased with the number of nitrate groups per molecule. We also studied the potential effects of certain compounds on non-enzymatic NO. release and found that, in addition to methylene blue, thionine and brilliantcresyl blue, but not ferricyanide, were also effective inhibitors. So it seems likely that both an enzymatic and a non-enzymatic mode of inhibition of enzyme activity does exist. Since oxyhemoglobin is an effective scavenger of nitric oxide, its addition can inhibit enzyme activation by nitrovasodilators. Our results stress the important role of the non-enzymatic liberation of NO. from organic nitrates and related compounds as possible, perhaps even as the principal mode of activation of soluble guanylate cyclase by nitrovasodilators.

Plasma nitrite reflects constitutive nitric oxide synthase activity in mammals

Changes in plasma nitrite concentration in the human forearm circulation have recently been shown to reflect acute changes in endothelial nitric oxide synthase (eNOS)-activity. Whether basal plasma nitrite is a general marker of constitutive NOS-activity in vivo is yet unclear. Due to the rapid metabolism of nitrite in blood and the difficulties in its analytical determination literature data on levels of nitrite in mammals are largely inconsistent. We hypothesized that constitutive NOS-activity in the circulatory system is relatively uniform throughout the mammalian kingdom. If true, this should result in comparable systemic plasma nitrite levels in different species. Using three different analytical approaches we determined plasma nitrite concentration to be in a nanomolar range in a variety of species: humans (305 +/- 23 nmol/l), monkeys (367 +/- 62 nmol/l), minipigs (319 +/- 24 nmol/l), dogs (305 +/- 50 nmol/l), rabbits (502 +/- 21 nmol/l), guinea pigs (412 +/- 44 nmol/l), rats (191 +/- 43 nmol/l), and mice (457 +/- 51 nmol/l). Application of different NOS-inhibitors in humans, minipigs, and dogs decreased NOS-activity and thereby increased vascular resistance. This was accompanied by a significant, up to 80%, decrease in plasma nitrite concentration. A comparison of plasma nitrite concentrations between eNOS(-/-) and NOS-inhibited wild-type mice revealed that 70 +/- 5% of plasma nitrite is derived from eNOS. These results provide evidence for a uniform constitutive vascular NOS-activity across mammalian species.

Plasma nitrite rather than nitrate reflects regional endothelial nitric oxide synthase activity but lacks intrinsic vasodilator action

The plasma level of NOx, i.e., the sum of NO2− and NO3−, is frequently used to assess NO bioavailability in vivo. However, little is known about the kinetics of NO conversion to these metabolites under physiological conditions. Moreover, plasma nitrite recently has been proposed to represent a delivery source for intravascular NO. We therefore sought to investigate in humans whether changes in NOx concentration are a reliable marker for endothelial NO production and whether physiological concentrations of nitrite are vasoactive. NO2− and NO3− concentrations were measured in blood sampled from the antecubital vein and brachial artery of 24 healthy volunteers. No significant arterial-venous gradient was observed for either NO2− or NO3−. Endothelial NO synthase (eNOS) stimulation with acetylcholine (1–10 μg/min) dose-dependently augmented venous NO2− levels by maximally 71%. This effect was paralleled by an almost 4-fold increase in forearm blood flow (FBF), whereas an equieffective dose of papaverine produced no change in venous NO2−. Intraarterial infusion of NO2− had no effect on FBF. NOS inhibition (NG-monomethyl-l-arginine; 4–12 μmol/min) dose-dependently reduced basal NO2− and FBF and blunted acetylcholine-induced vasodilation and NO release by more than 80% and 90%, respectively. In contrast, venous NO3− and total NOx remained unchanged as did systemic arterial NO2− and NO3− levels during all these interventions. FBF and NO release showed a positive association (r = 0.85; P < 0.001). These results contradict the current paradigm that plasma NO3− and/or total NOx are generally useful markers of endogenous NO production and demonstrate that only NO2− reflects acute changes in regional eNOS activity. Our results further demonstrate that physiological levels of nitrite are vasodilator-inactive.

Nitrate and nitrite in biology, nutrition and therapeutics

Inorganic nitrate and nitrite from endogenous or dietary sources are metabolized in vivo to nitric oxide (NO) and other bioactive nitrogen oxides. The nitrate-nitrite-NO pathway is emerging as an important mediator of blood flow regulation, cell signaling, energetics and tissue responses to hypoxia. The latest advances in our understanding of the biochemistry, physiology and therapeutics of nitrate, nitrite and NO were discussed during a recent 2-day meeting at the Nobel Forum, Karolinska Institutet in Stockholm.

Cellular targets and mechanisms of nitros(yl)ation: An insight into their nature and kinetics in vivo

There is mounting evidence that the established paradigm of nitric oxide (NO) biochemistry, from formation through NO synthases, over interaction with soluble guanylyl cyclase, to eventual disposal as nitrite/nitrate, represents only part of a richer chemistry through which NO elicits biological signaling. Additional pathways have been suggested that include interaction of NO-derived metabolites with thiols and metals to form S-nitrosothiols (RSNOs) and metal nitrosyls. Despite the overwhelming attention paid in this regard to RSNOs, little is known about the stability of these species, their significance outside the circulation, and whether other nitros(yl)ation products are of equal importance. We here show that N-nitrosation and heme-nitrosylation are indeed as ubiquitous as S-nitrosation in vivo and that the products of these reactions are constitutively present throughout the organ system. Our study further reveals that all NO-derived products are highly dynamic, have fairly short lifetimes, and are linked to tissue oxygenation and redox state. Experimental evidence further suggests that nitroso formation occurs substantially by means of oxidative nitrosylation rather than NO autoxidation, explaining why S-nitrosation can compete effectively with nitrosylation. Moreover, tissue nitrite can serve as a significant extravascular pool of NO during brief periods of hypoxia, and tissue nitrate/nitrite ratios can serve as indicators of the balance between local oxidative and nitrosative stress. These findings vastly expand our understanding of the fate of NO in vivo and provide a framework for further exploration of the significance of nitrosative events in redox sensing and signaling. The findings also raise the intriguing possibility that N-nitrosation is directly involved in the modulation of protein function.

The use of nitric oxide donors in pharmacological studies

Abstract A growing appreciation of the involvement of nitric oxide (NO) in numerous bioregulatory pathways has not only opened up new therapeutic avenues for organic nitrates and other NO donors but also led to an increased use of such compounds in pharmacological studies. By definition, all NO donors produce NO-related activity when applied to biological systems and are thus principally suited to either mimic an endogenous NO-related response or substitute for an endogenous NO deficiency. However, the pathways leading to enzymatic and/or non-enzymatic formation of NO differ greatly among individual compound classes, as do their chemical reactivities and kinetics of NO release. Moreover, since the reaction of NO with oxygen is a function of its concentration, the same absolute amounts of NO generated over different periods of time may lead to substantially different rates of NOx formation and, consequently, to varying extents of side reactions, such as nitration and/or nitrosation of biomolecules. Matters are further complicated by compound-specific formation of by-products, which may arise during decomposition or metabolism, sometimes in amounts far exceeding those of NO. The term “NO donor” implies that the compound releases the active mediator, NO. Ultimately, this may be true for many different chemical classes of compound, since the principal NO-related species generated may be converted to NO, if not directly released as such. However, in a biological system, the redox form of nitrogen monoxide (NO+, NO· or NO–) that is actually released makes a substantial difference to the NO donor’s reactivity towards other biomolecules, the profile of by-products, and the bioresponse. Such considerations are likely to account for much of the discrepancy in experimental results obtained using the same cell or tissue preparation but different NO mimetics. Thus, compound selection is not a trivial issue and the investigator should be aware of the key properties and differences between various NO donor classes in order to avoid misinterpretation of experimental results.

Quantitative and kinetic characterization of nitric oxide and EDRF released from cultured endothelial cells.

Endothelial cells (EC) contribute to the control of local vascular diameter by formation of an endothelium derived relaxant factor (EDRF) (1). Whether nitric oxide (NO) is identical with (EDRF) or might represent only one species of several EDRFs has not been decided as yet (2-5). Therefore, we have directly compared in cultured EC the kinetics of NO formation determined in a photometric assay with the vasodilatory effect of EDRF and NO in a bioassay. Basal release of NO was 16, 4 pmol/min/ml packed EC column. After stimulation with bradykinin (BK) and ATP onset of endothelial NO release and maximal response preceded the EDRF-mediated relaxation. Concentrations of NO formed by stimulated EC were quantitatively sufficient to fully explain the smooth muscle relaxation determined in the bioassay. Our data provide convincing evidence that under basal, BK and ATP-stimulated conditions 1. endothelial cells release nitric oxide as free radical, 2. nitric oxide is solely responsible for the vasodilatory properties of EDRF.

On the mechanism of NO release from sydnonimines

SUMMARY The vasodilator and antiaggregatory properties of sydnonimines like SIN-1 are thought to be due to their marked stimulatory action on soluble guanylate cyclase. Enzyme activation and consecutive cyclic GMP accumulation is mediated by the liberation of nitric oxide (NO) from the open-ring A forms of sydnonimines. The purpose of the present study was to investigate the mechanism of NO release from sydnonimines in direct comparison to their stimulatory effect at the target enzyme, soluble guanylate cyclase. All sydnonimines tested were found to spontaneously liberate NO, the rate of which closely correlated with the extent of enzyme activation. NO release occurred nonlinearly with time and became maximal at high sydnonimine concentration. The in vitro stability of the A forms neither correlated with the measured rate of NO release nor with enzyme activation, indicating that a direct stimulation of guanylate cyclase by the A forms is rather unlikely. Besides NO, all sydnonimines generated NO2− and NO3− at a nearly equimolar rate. The addition of cysteine induced a marked shift from NO3− to NO2− with a small reduction in NO release, which is paralleled by a weak rightward shift of the EC50 at the guanylate cyclase. All tested sydnonimines were found to consume molecular oxygen at rates that closely corresponded to the measured rates of NO formation. By a molar comparison, the amounts of consumed oxygen are clearly higher, as would be expected for the oxidative conversion of NO to NO2− and NO3−. Oxygen seems to be additionally involved in the induction of NO formation while being converted to superoxide (O2−). In accordance with an autocatalytic processs, O2− further enhances sydnonimine decomposition, since in the presence of superoxide dismutase (SOD) the rate of SIN-1C and NO2−/NO3− formation from SIN-1 A was reduced, whereas the rate of NO liberation seemingly increased. O2− has, however, no influence on the rate of hydrolysis of SIN-1 to SIN-1 A. At the level of guanylate cyclase, the presence of SOD induced a leftward shift of the concentration-response curve to SIN-1, in agreement with an enhancement of efficacy of NO by blocking the NO-scavenging effect of O2−. An additional O2− generation markedly enhanced SIN-1 A decomposition to NO2−/NO3− and reduced the apparent rate of NO formation. We conclude from our results that oxygen plays a key role in the decomposition of sydnonimines and thus in the formation of NO as their pharmacodynamically active principle. Oxygen attack most probably occurs by one-electron abstraction from the A form of the respective sydnonimine compound. Oxygen is thereby converted to O2− while the radical cation of the sydnonimine A form decomposes with the release of NO. Sydnonimines such as SIN-1 thus not only appear to be donors of EDRF by releasing NO but also of EDCF while simultaneously producing O2−, a behavior that has recently been demonstrated for endothelial cells, activated macrophages, and neutrophils.

Nitrite as Regulator of Hypoxic Signaling in Mammalian Physiology

In this review we consider the effects of endogenous and pharmacological levels of nitrite under conditions of hypoxia. In humans, the nitrite anion has long been considered as metastable intermediate in the oxidation of nitric oxide radicals to the stable metabolite nitrate. This oxidation cascade was thought to be irreversible under physiological conditions. However, a growing body of experimental observations attests that the presence of endogenous nitrite regulates a number of signaling events along the physiological and pathophysiological oxygen gradient. Hypoxic signaling events include vasodilation, modulation of mitochondrial respiration, and cytoprotection following ischemic insult. These phenomena are attributed to the reduction of nitrite anions to nitric oxide if local oxygen levels in tissues decrease. Recent research identified a growing list of enzymatic and nonenzymatic pathways for this endogenous reduction of nitrite. Additional direct signaling events not involving free nitric oxide are proposed. We here discuss the mechanisms and properties of these various pathways and the role played by the local concentration of free oxygen in the affected tissue.

Nitroxyl anion exerts redox-sensitive positive cardiac inotropy in vivo by calcitonin gene-related peptide signaling

Nitroxyl anion (NO−) is the one-electron reduction product of nitric oxide (NO⋅) and is enzymatically generated by NO synthase in vitro. The physiologic activity and mechanism of action of NO− in vivo remains unknown. The NO− generator Angelis salt (AS, Na2N2O3) was administered to conscious chronically instrumented dogs, and pressure–dimension analysis was used to discriminate contractile from peripheral vascular responses. AS rapidly enhanced left ventricular contractility and concomitantly lowered cardiac preload volume and diastolic pressure (venodilation) without a change in arterial resistance. There were no associated changes in arterial or venous plasma cGMP. The inotropic response was similar despite reflex blockade with hexamethonium or volume reexpansion, indicating its independence from baroreflex stimulation. However, reflex activation did play a major role in the selective venodilation observed under basal conditions. These data contrasted with the pure NO donor diethylamine/NO, which induced a negligible inotropic response and a more balanced veno/arterial dilation. AS-induced positive inotropy, but not systemic vasodilatation, was highly redox-sensitive, being virtually inhibited by coinfusion of N-acetyl-l-cysteine. Cardiac inotropic signaling by NO− was mediated by calcitonin gene-related peptide (CGRP), as treatment with the selective CGRP-receptor antagonist CGRP(8–37) prevented this effect but not systemic vasodilation. Thus, NO− is a redox-sensitive positive inotrope with selective venodilator action, whose cardiac effects are mediated by CGRP-receptor stimulation. This fact is evidence linking NO− to redox-sensitive cardiac contractile modulation by nonadrenergic/noncholinergic peptide signaling. Given its cardiac and vascular properties, NO− may prove useful for the treatment of cardiovascular diseases characterized by cardiac depression and elevated venous filling pressures.