Nitroxyl oxidizes NADPH in a superoxide dismutase inhibitable manner

The Chemistry of HNO: Mechanisms and Reaction Kinetics

Azanone (HNO, also known as nitroxyl) is the protonated form of the product of one-electron reduction of nitric oxide (^•NO), and an elusive electrophilic reactive nitrogen species of increasing pharmacological significance. Over the past 20 years, the interest in the biological chemistry of HNO has increased significantly due to the numerous beneficial pharmacological effects of its donors. Increased availability of various HNO donors was accompanied by great progress in the understanding of HNO chemistry and chemical biology. This review is focused on the chemistry of HNO, with emphasis on reaction kinetics and mechanisms in aqueous solutions.

Nitroxyl (HNO): A Reduced Form of Nitric Oxide with Distinct Chemical, Pharmacological, and Therapeutic Properties

Nitroxyl (HNO), the one-electron reduced form of nitric oxide (NO), shows a distinct chemical and biological profile from that of NO. HNO is currently being viewed as a vasodilator and positive inotropic agent that can be used as a potential treatment for heart failure. The ability of HNO to react with thiols and thiol containing proteins is largely used to explain the possible biological actions of HNO. Herein, we summarize different aspects related to HNO including HNO donors, chemistry, biology, and methods used for its detection.

Nitroxyl accelerates the oxidation of oxyhemoglobin by nitrite

Angeli's salt (Na₂N₂O₃) decomposes into nitroxyl (HNO) and nitrite (NO₂(-)), compounds of physiological and therapeutic interest for their impact on biological signaling both through nitric oxide and nitric oxide independent pathways. Both nitrite and HNO oxidize oxygenated hemoglobin to methemoglobin. Earlier work has shown that HNO catalyzes the reduction of nitrite by deoxygenated hemoglobin. In this work, we have shown that HNO accelerates the oxidation of oxygenated hemoglobin by NO₂(-). We have demonstrated this HNO mediated acceleration of the nitrite/oxygenated hemoglobin reaction with oxygenated hemoglobin being in excess to HNO and nitrite (as would be found under physiological conditions) by monitoring the formation of methemoglobin in the presence of Angeli's salt with and without added NO₂(-). In addition, this acceleration has been demonstrated using the HNO donor 4-nitrosotetrahydro-2H-pyran-4-yl pivalate, a water-soluble acyloxy nitroso compound that does not release NO₂(-) but generates HNO in the presence of esterase. This HNO donor was used both with and without NO₂(-) and acceleration of the NO₂(-) induced formation of methemoglobin was observed. We found that the acceleration was not substantially affected by catalase, superoxide dismutase, c-PTIO, or IHP, suggesting that it is not due to formation of extramolecular peroxide, NO₂ or H₂O₂, or to modulation of allosteric properties. In addition, we found that the acceleration is not likely to be related to HNO binding to free reduced hemoglobin, as we found HNO binding to reduced hemoglobin to be much weaker than has previously been proposed. We suggest that the mechanism of the acceleration involves local propagation of autocatalysis in the nitrite-oxygenated Hb reaction. This acceleration of the nitrite oxyhemoglobin reaction could affect studies aimed at understanding physiological roles of HNO and perhaps nitrite and use of these agents in therapeutics such as hemolytic anemias, heart failure, and ischemia reperfusion injury.

Release of NO from a nitrosyl ruthenium complex through oxidation of mitochondrial NADH and effects on mitochondria

Nitrosyl ruthenium complexes are promising NO donor agents with numerous advantages for the biologic applications of NO. We have characterized the NO release from the nitrosyl ruthenium complex [Ru(NO(2))(bpy)(2)(4-pic)](+) (I) and the reactive oxygen/nitrogen species (ROS/RNS)-mediated NO actions on isolated rat liver mitochondria. The results indicated that oxidation of mitochondrial NADH promotes NO release from (I) in a manner mediated by NO(2) formation (at neutral pH) as in mammalian cells, followed by an oxygen atom transfer mechanism (OAT). The NO released from (I) uncoupled mitochondria at low concentrations/incubation times and inhibited the respiratory chain at high concentrations/incubation times. In the presence of ROS generated by mitochondria NO gave rise to peroxynitrite, which, in turn, inhibited the respiratory chain and oxidized membrane protein-thiols to elicit a Ca(2+)-independent mitochondrial permeability transition; this process was only partially inhibited by cyclosporine-A, almost fully inhibited by the thiol reagent N-ethylmaleimide (NEM) and fully inhibited by the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO). These actions correlated with the release of cytochrome c from isolated mitochondria as detected by Western blotting analysis. These events, typically involved in cell necrosis and/or apoptosis denote a potential specific action of (I) and analogs against tumor cells via mitochondria-mediated processes.

The specificity of nitroxyl chemistry is unique among nitrogen oxides in biological systems

The importance of nitric oxide in mammalian physiology has been known for nearly 30 years. Similar attention for other nitrogen oxides such as nitroxyl (HNO) has been more recent. While there has been speculation as to the biosynthesis of HNO, its pharmacological benefits have been demonstrated in several pathophysiological settings such as cardiovascular disorders, cancer, and alcoholism. The chemical biology of HNO has been identified as related to, but unique from, that of its redox congener nitric oxide. A summary of these findings as well as a discussion of possible endogenous sources of HNO is presented in this review.

Kinetic feasibility of nitroxyl reduction by physiological reductants and biological implications

Nitroxyl (HNO), the one-electron reduced and protonated congener of nitric oxide (NO), is a chemically unique species with potentially important biological activity. Although HNO-based pharmaceuticals are currently being considered for the treatment of chronic heart failure or stroke/transplant-derived ischemia, the chemical events leading to therapeutic responses are not established. The interaction of HNO with oxidants results in the well-documented conversion to NO, but HNO is expected to be readily reduced as well. Recent thermodynamic calculations predict that reduction of HNO is biologically accessible. Herein, kinetic analysis suggests that the reactions of HNO with several mechanistically distinct reductants are also biologically feasible. Product analysis verified that the reductants had in fact been oxidized and that in several instances HNO had been converted to hydroxylamine. Moreover, a theoretical analysis suggests that in the reaction of HNO with thiol reductants, the pathway producing sulfinamide is significantly more favorable than that leading to disulfide. Additionally, simultaneous production of HNO and NO yielded a biphasic oxidative capacity.

Solvent stress response of the denitrifying bacterium "Aromatoleum aromaticum" strain EbN1

The denitrifying betaproteobacterium "Aromatoleum aromaticum" strain EbN1 degrades several aromatic compounds, including ethylbenzene, toluene, p-cresol, and phenol, under anoxic conditions. The hydrophobicity of these aromatic solvents determines their toxic properties. Here, we investigated the response of strain EbN1 to aromatic substrates at semi-inhibitory (about 50% growth inhibition) concentrations under two different conditions: first, during anaerobic growth with ethylbenzene (0.32 mM) or toluene (0.74 mM); and second, when anaerobic succinate-utilizing cultures were shocked with ethylbenzene (0.5 mM), toluene (1.2 mM), p-cresol (3.0 mM), and phenol (6.5 mM) as single stressors or as a mixture (total solvent concentration, 2.7 mM). Under all tested conditions impaired growth was paralleled by decelerated nitrate-nitrite consumption. Additionally, alkylbenzene-utilizing cultures accumulated poly(3-hydroxybutyrate) (PHB) up to 10% of the cell dry weight. These physiological responses were also reflected on the proteomic level (as determined by two-dimensional difference gel electrophoresis), e.g., up-regulation of PHB granule-associated phasins, cytochrome cd(1) nitrite reductase of denitrification, and several proteins involved in oxidative (e.g., SodB) and general (e.g., ClpB) stress responses.

Consequences of MnSOD interactions with nitric oxide: nitric oxide dismutation and the generation of peroxynitrite and hydrogen peroxide

The present study demonstrates that manganese superoxide dismutase (MnSOD) (Escherichia coli), binds nitric oxide (*NO) and stimulates its decay under both anaerobic and aerobic conditions. The results indicate that previously observed MnSOD-catalyzed *NO disproportionation (dismutation) into nitrosonium (NO+) and nitroxyl (NO-) species under anaerobic conditions is also operative in the presence of molecular oxygen. Upon sustained aerobic exposure to *NO, MnSOD-derived NO- species initiate the formation of peroxynitrite (ONOO-) leading to enzyme tyrosine nitration, oxidation and (partial) inactivation. The results suggest that both ONOO- decomposition and ONOO(-)-dependent tyrosine residue nitration and oxidation are enhanced by metal centre-mediated catalysis. We show that the generation of ONOO- is accompanied by the formation of substantial amounts of H2O2. MnSOD is a critical mitochondrial antioxidant enzyme, which has been found to undergo tyrosine nitration and inactivation in various pathologies associated with the overproduction of *NO. The results of the present study can account for the molecular specificity of MnSOD nitration in vivo. The interaction of *NO with MnSOD may represent a novel mechanism by which MnSOD protects the cell from deleterious effects associated with overproduction of *NO.

Oxygen metabolism by neuronal nitric-oxide synthase

Nitric-oxide synthases (NOS) catalyze nitric oxide (NO) formation from the amino acid L-arginine. NOS is known to catalyze more than one reaction: the NO-producing reaction is considered to be the coupled reaction, and the uncoupled reactions are those that produce reactive (reduced) oxygen species (ROS), such as superoxide anion (O-2.) and/or hydrogen peroxide (H2O2). As an oxygenase, NOS has been known for more than two decades, yet there is no complete description of oxygen stoichiometry. The present paper is focused on oxygen stoichiometry and the effects of cofactor binding on the neuronal isoform (nNOS) on oxygen uptake and product formation. Products of the uncoupled reactions are analyzed using diacetyldeuteroheme-substituted horseradish peroxidase as a trapping agent for both O-2. and H2O2. The addition of calmodulin not only stimulated the oxygen uptake rate but also changed the product of the uncoupled reaction, supporting the possibility of two different sites for electron leakage to molecular oxygen. Quantitative analysis of the uncoupled (substrate-free) reaction revealed a stoichiometry close to the theoretical value, and adding L-arginine not only initiates the coupled reaction, but also inhibits oxygen uptake. The presence of tetrahydrobiopterin affects oxygen metabolism by lowering the apparent Km value of nNOS for oxygen in the uncoupled reaction.

Detection of nitrous oxide in the neuronal nitric oxide synthase reaction by gas chromatography–mass spectrometry

Using headspace gas chromatography-mass spectrometry, we detected significant amounts of nitrous oxide in the reaction products of the monooxygenase reaction catalyzed by neuronal nitric oxide synthase. Nitrous oxide is a dimerization product of nitroxyl anion; its presence in the reaction products indicates that the nitroxyl anion is a product of the neuronal nitric oxide synthase-catalyzed reaction.

Inhibition of yeast glycolysis by nitroxyl (HNO): mechanism of HNO toxicity and implications to HNO biology

Nitroxyl (HNO) was found to inhibit glycolysis in the yeast Saccharomyces cerevisiae. The toxicity of HNO in yeast positively correlated with the dependence of yeast on glycolysis for cellular energy. HNO was found to potently inhibit the crucial glycolytic enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH), an effect which is likely to be responsible for the observed inhibition of glycolysis in whole cell preparations. It is proposed that GAPDH inhibition occurs through reaction of HNO with the active site thiolate residue of GAPDH. Significantly, levels of HNO that inhibit GAPDH do not alter the levels or redox status of intracellular glutathione (GSH), indicating that HNO has thiol selectivity. The ability of HNO to inhibit GAPDH in an intracellular environment that contains relatively large concentrations of GSH is an important aspect of HNO pharmacology and possibly, physiology.

NITROXYL (HNO): Chemistry, Biochemistry, and Pharmacology

Recent discoveries of novel and potentially important biological activity have spurred interest in the chemistry and biochemistry of nitroxyl (HNO). It has become clear that, among all the nitrogen oxides, HNO is unique in its chemistry and biology. Currently, the intimate chemical details of the biological actions of HNO are not well understood. Moreover, many of the previously accepted chemical properties of HNO have been recently revised, thus requiring reevaluation of possible mechanisms of biological action. Herein, we review these developments in HNO chemistry and biology.

Mechanism of aerobic decomposition of Angeli's salt (sodium trioxodinitrate) at physiological pH

The recent determination that Angeli's salt may have clinical application as a nitrogen oxide donor for treatment of cardiovascular diseases such as heart failure has led to renewed interest in the mechanism and products of thermal decomposition of Angeli's salt under physiological conditions. In this report, several mechanisms are evaluated experimentally and by quantum mechanical calculations to determine whether HNO is in fact released from Angeli's salt in neutral, aerobic solution. The mechanism of product autoxidation is also considered.

Reduced cGMP signaling associated with neointimal proliferation and vascular dysfunction in late-stage atherosclerosis

Atherosclerosis is associated with alterations in nitric oxide (NO)/cGMP signaling. In early stages of the disease, inflammatory and possibly other cells produce reactive oxygen species that scavenge vasoprotective NO. In addition to the oxidative stress, expression and activity of enzymes downstream to NO formation may also be affected. Here, we show in the aortas of chronically hypercholesterolemic rabbits (a model of late-stage atherosclerosis), both subunits and specific activity of the NO receptor soluble guanylyl cyclase (sGC) were significantly reduced, whereas overall NO synthase activity was unaffected. These changes were most prominent in the neointimal layer, wherein cGMP-dependent protein kinase I (cGK) levels also were reduced. Additionally, a protein (p38(nt)) that was constitutively tyrosine-nitrated was detected, and its expression was significantly reduced in atherosclerotic aorta. Phosphorylation of the cGK substrate vasodilator-stimulated phosphoprotein (VASP) at Ser-239, an established biochemical endpoint of NO/cGMP signaling, also was reduced. Thus, late-stage atherosclerosis is associated not only with enhanced NO breakdown but also with altered NO reception and cGMP signaling. Preferential down-regulation in neointima suggests a direct connection of these changes to neointimal proliferation and vascular dysfunction and provides a rationale for future pharmacotherapy using classical and novel sGC activators.

Superoxide dismutase and catalase are required to detect (.-)NO from both coupled and uncoupled neuronal no synthase

Despite numerous approaches to measuring nitric oxide ((.-)NO) formation from purified NO synthase (NOS), it is still not clear whether (.-)NO is a direct or indirect product of the NO synthase reaction. The direct detection of catalytically formed (.-)NO is complicated by side reactions with reactive oxide species like H(2)O(2) and superoxide. The aim of the present study was therefore to reinvestigate these reactions both electrochemically and by chemiluminescence detection with particular emphasis on the requirement for cofactors and their interference with (.-)NO detection. Flavins were found to generate large amounts of H(2)O(2) and were therefore excluded from subsequent incubations. Under conditions of both coupled and uncoupled catalysis, SOD was absolutely required to detect (.-)NO from NOS. H(2)O(2) formation took place also in the presence of SOD and gave a smaller yet significant interfering signal. Similar data were obtained when the proposed intermediate N(omega)-hydroxy-l-arginine was utilized as substrate. In conclusion, standard Clark-type ()NO electrodes are cross-sensitive to H(2)O(2) and therefore both SOD and catalase are absolutely required to specifically detect (.-)NO from NOS.

The mode of decomposition of Angeli's salt (Na2N2O3) and the effects thereon of oxygen, nitrite, superoxide dismutase, and glutathione

The classical view of the aerobic decomposition of Angeli's salt is that it releases NO(2)(-) + NO(-)/HNO the latter then reacting with O(2) to yield ONOO(-). An alternative that has recently been proposed envisions electron transfer to O(2) followed by decomposition to NO(2)(-) + NO. The classical view is now strongly supported by the observation that the rates of decomposition of Angeli's salt under 20% O(2) or 100% O(2) were equal. Moreover, NO(2)(-), which inhibits this decomposition by favoring the back reaction, was more effective in the absence of agents that scavenge NO(-)/HNO. It is thus clear that Angeli's salt is a useful source of NO(-)/HNO for use in defined aqueous systems. The measurements made in the course of this work allowed approximation of the rate constants for the reactions of NO(-)/HNO with NO(2)(-), O(2), glutathione, or Cu, Zn superoxide dismutase. The likelihood of the formation of NO(-)/HNO in vivo is also discussed.

Spin-forbidden deprotonation of aqueous nitroxyl (HNO)

The first mechanistic study of a spin-forbidden proton-transfer reaction in aqueous solution is reported. Laser flash photolysis of alkaline trioxodinitrate (N(2)O(3)(2)(-), Angeli's anion) is used to generate a nitroxyl anion in its excited singlet state ((1)NO(-)). Through rapid partitioning between protonation by water and electronic relaxation, (1)NO(-) produces (1)HNO (ground state, yield 96%) and (3)NO(-) (ground state, yield 4%), which comprise a unique conjugate acid-base couple with different ground-state multiplicities. Using the large difference between reactivities of (1)HNO and (3)NO(-) in the peroxynitrite-forming reaction with (3)O(2), the kinetics of spin-forbidden deprotonation reaction (1)HNO + OH(-) --> (3)NO(-) + H(2)O is investigated in H(2)O and D(2)O. Consistent with proton transfer, this reaction exhibits primary kinetic hydrogen isotope effect k(H)/k(D) = 3.1 at 298 K, which is found to be temperature-dependent. Arrhenius pre-exponential factors and activation energies of the second-order rate constant are found to be: log(A, M(-)(1) s(-)(1)) = 10.0 +/- 0.2 and E(a) = 30.0 +/- 1.1 kJ/mol for proton transfer and log(A, M(-)(1) s(-)(1)) = 10.4 +/- 0.1 and E(a) = 35.1 +/- 0.7 kJ/mol for deuteron transfer. Collectively, these data are interpreted to show that the nuclear reorganization requirements arising from the spin prohibition necessitate significant activation before spin change can take place, but the spin change itself must occur extremely rapidly. It is concluded that a synergy between the spin prohibition and the reaction energetics creates an intersystem barrier and is responsible for slowness of the spin-forbidden deprotonation of (1)HNO by OH(-); the spin prohibition alone plays a minor role.

Bacterial [Cu,Zn]-cofactored superoxide dismutase protects opsonized, encapsulated Neisseria meningitidis from phagocytosis by human monocytes/macrophages

Superoxide dismutase cofactored by copper and zinc ([Cu,Zn]-SOD) contributes to the protection of opsonized serogroup B Neisseria meningitidis against phagocytosis by human monocytes/macrophages, with sodC mutant organisms being endocytosed in significantly higher numbers than are wild-type organisms. The influence of [Cu,Zn]-SOD was found to be exerted at the stage of phagocytosis, rather than at earlier (modulating surface association) or later (intracellular killing) stages.

Nitroxyl affords thiol-sensitive myocardial protective effects akin to early preconditioning

Nitric oxide (NO) donors mimic the early phase of ischemic preconditioning (IPC). The effects of nitroxyl (HNO/NO(-)), the one-electron reduction product of NO, on ischemia/reperfusion (I/R) injury are unknown. Here we investigated whether HNO/NO(-), produced by decomposition of Angeli's salt (AS; Na(2)N(2)O(3)), has a cardioprotective effect in isolated perfused rat hearts. Effects were examined after intracoronary perfusion (19 min) of either AS (1 microM), the NO donor diethylamine/NO (DEA/NO, 0.5 microM), vehicle (100 nM NaOH) or buffer, followed by global ischemia (30 min) and reperfusion (30 min or 120 min in a subset of hearts). IPC was induced by three cycles of 3 min ischemia followed by 10 min reperfusion prior to I/R. The extent of I/R injury under each intervention was assessed by changes in myocardial contractility as well as lactate dehydrogenase (LDH) release and infarct size. Postischemic contractility, as indexed by developed pressure and dP/dt(max), was similarly improved with IPC and pre-exposure to AS, as opposed to control or DEA/NO-treated hearts. Infarct size and LDH release were also significantly reduced in IPC and AS groups, whereas DEA/NO was less effective in limiting necrosis. Co-infusion in the triggering phase of AS and the nitroxyl scavenger, N-acetyl-L-cysteine (4 mM) completely reversed the beneficial effects of AS, both at 30 and 120 min reperfusion. Our data show that HNO/NO(-) affords myocardial protection to a degree similar to IPC and greater than NO, suggesting that reactive nitrogen oxide species are not only necessary but also sufficient to trigger myocardial protection against reperfusion through species-dependent, pro-oxidative, and/or nitrosative stress-related mechanisms.

NO and the vasculature: where does it come from and what does it do?

Nitric oxide (NO) is involved in a large number of cellular processes and dysfunctions in NO production have been implicated in many different disease states. In the vasculature NO is released by endothelial cells where it modulates the underlying smooth muscle to regulate vascular tone. Due to the unique chemistry of NO, such as its reactive and free radical nature, it can interact with many different cellular constituents such as thiols and transition metal ions, which determine its cellular actions. In this review we also discuss many of the useful pharmacological tools that have been developed and used extensively to establish the involvement of NO in endothelium-derived relaxations. In addition, the recent literature identifying a potential source of NO in endothelial cells, which is not directly derived from endothelial nitric oxide synthase is examined. Finally, the photorelaxation phenomena, which mediates the release of NO from a vascular smooth muscle NO store, is discussed.

Nitroxyl (NO-): a substrate for superoxide dismutase

The interactions of Cu, Zn superoxide dismutase (SOD) with nitroxyl (NO-) and nitric oxide (NO), both of which are thought to be biologically significant, have been studied but remain undefined. Having previously noted that NO- can reduce Cu (II), Zn SOD aerobically, we now report that it also can do so anaerobically and that Cu, Zn SOD can catalyze the elimination of NO(-) in the absence of O2.NO- acts as a reductant of ferricytochrome c anaerobically, but in the presence of O2 causes the oxidation of ferrocytochrome c and NADPH. Equivalent fluxes of NO-, and NO + O2- were able to comparably oxidize NADPH, but the oxidation by NO + O2- was more than fivefold more sensitive to inhibition by Cu, Zn SOD than was the oxidation by NO-. Thus Cu, Zn SOD inhibited NADPH oxidation by NO- by a route independent of catalyzing the dismutation of O2. Plausible mechanisms for those observations are offered and rate constants are estimated.

Nitroxyl and its anion in aqueous solutions: spin states, protic equilibria, and reactivities toward oxygen and nitric oxide

The thermodynamic properties of aqueous nitroxyl (HNO) and its anion (NO(-)) have been revised to show that the ground state of NO(-) is triplet and that HNO in its singlet ground state has much lower acidity, pKa((1)HNO/(3)NO(-)) approximately 11.4, than previously believed. These conclusions are in accord with the observed large differences between (1)HNO and (3)NO(-) in their reactivities toward O(2) and NO. Laser flash photolysis was used to generate (1)HNO and (3)NO(-) by photochemical cleavage of trioxodinitrate (Angeli's anion). The spin-allowed addition of (3)O(2) to (3)NO(-) produced peroxynitrite with nearly diffusion-controlled rate (k = 2.7 x 10(9) M(-1) x s(-1)). In contrast, the spin-forbidden addition of (3)O(2) to (1)HNO was not detected (k << 3 x 10(5) M(-1) x s(-1)). Both (1)HNO and (3)NO(-) reacted sequentially with two NO to generate N(3)O as a long-lived intermediate; the rate laws of N(3)O formation were linear in concentrations of NO and (1)HNO (k = 5.8 x 10(6) M(-1) x s(-1)) or NO and (3)NO(-) (k = 2.3 x 10(9) M(-1) x s(-1)). Catalysis by the hydroxide ion was observed for the reactions of (1)HNO with both O(2) and NO. This effect is explicable by a spin-forbidden deprotonation by OH(-) (k = 4.9 x 10(4) M(-1) x s(-1)) of the relatively unreactive (1)HNO into the extremely reactive (3)NO(-). Dimerization of (1)HNO to produce N(2)O occurred much more slowly (k = 8 x 10(6) M(-1) x s(-1)) than previously suggested. The implications of these results for evaluating the biological roles of nitroxyl are discussed.

Formation of peroxynitrite from reaction of nitroxyl anion with molecular oxygen

Peroxynitrite (ONOO(-)/ONOOH) is generally expected to be formed in vivo from the diffusion-controlled reaction between superoxide (O(2)) and nitric oxide ((*)NO). In the present paper we show that under aerobic conditions the nitroxyl anion (NO(-)), released from Angeli's salt (disodium diazen-1-ium-1,2,2-triolate, (-)ON=NO(2)(-)), generated peroxynitrite with a yield of about 65%. Simultaneously, hydroxyl radicals are formed from the nitroxyl anion with a yield of about 3% via a minor, peroxynitrite-independent pathway. Further experiments clearly underline that the chemistry of NO(-) in the presence of oxygen is mainly characterized by peroxynitrite and not by HO( small middle dot) radicals. Quantum-chemical calculations predict that peroxynitrite formation should proceed via intermediary formation of (*)NO and O(2), probably by an electron-transfer mechanism. This prediction is supported by the fact that H(2)O(2) is formed during the decay of NO(-) in the presence of superoxide dismutase (Cu(II),Zn-SOD). Since the nitroxyl anion may be released endogenously by a variety of biomolecules, substantial amounts of peroxynitrite might be formed in vivo via NO(-) in addition to the "classical" ( small middle dot)NO + O(2)() pathway.

Copper,zinc superoxide dismutase as a univalent NO(-) oxidoreductase and as a dichlorofluorescin peroxidase

Nitroxyl (NO(-)) may be produced by nitric-oxide synthase and by the reduction of NO by reduced Cu,Zn-SOD. The ability of NO(-) to cause oxidations and of SOD to inhibit such oxidations was therefore explored. The decomposition of Angeli's salt (AS) produces NO(-) and that in turn caused the aerobic oxidation of NADPH, directly or indirectly. O(2) was produced concomitant with the aerobic oxidation of NADPH by AS, as evidenced by the SOD-inhibitable reduction of cytochrome c. Both Cu,Zn-SOD and Mn-SOD inhibited the aerobic oxidation of NADPH by AS, but the amounts required were approximately 100-fold greater than those needed to inhibit the reduction of cytochrome c. This inhibition was not due to a nonspecific protein effect or to an effect of those large amounts of the SODs on the rate of decomposition of AS. NO(-) caused the reduction of the Cu(II) of Cu,Zn-SOD, and in the presence of O(2), SOD could catalyze the oxidation of NO(-) to NO. The reverse reaction, i.e. the reduction of NO to NO(-) by Cu(I),Zn-SOD, followed by the reaction of NO(-) with O(2) would yield ONOO(-) and that could explain the oxidation of dichlorofluorescin (DCF) by Cu(I),Zn-SOD plus NO. Cu,Zn-SOD plus H(2)O(2) caused the HCO(3)(-)-dependent oxidation of DCF, casting doubt on the validity of using DCF oxidation as a reliable measure of intracellular H(2)O(2) production.