Discriminating formation of HNO from other reactive nitrogen oxide species

Hydroxylamine acutely activates glucose uptake in L929 fibroblast cells

Nitroxyl (HNO) has a unique, but varied, set of biological properties including beneficial effects on cardiac contractility and stimulation of glucose uptake by GLUT1. These biological effects are largely initiated by HNO's reaction with cysteine residues of key proteins. The intracellular production of HNO has not yet been demonstrated, but the small molecule, hydroxylamine (HA), has been suggested as possible intracellular source. We examined the effects of this molecule on glucose uptake in L929 fibroblast cells. HA activates glucose uptake from 2 to 5-fold within two minutes. Prior treatment with thiol-active compounds, such as iodoacetamide (IA), cinnamaldehyde (CA), or phenylarsine oxide (PAO) blocks HA-activation of glucose uptake. Incubation of HA with the peroxidase inhibitor, sodium azide, also blocks the stimulatory effects of HA. This suggests that HA is oxidized to HNO by L929 fibroblast cells, which then reacts with cysteine residues to exert its stimulatory effects. The data suggest that GLUT1 is acutely activated in L929 cells by modification of cysteine residues, possibly the formation of a disulfide bond within GLUT1 itself.

O2-sulfonylethyl protected isopropylamine diazen-1-ium-1,2-diolates as nitroxyl (HNO) donors: synthesis, β-elimination fragmentation, HNO release, positive inotropic properties, and blood pressure lowering studies

New types of nonexplosive O(2)-sulfonylethyl protected (-CH(2)CH(2)SO(2)R; R = OMe, NHOMe, NHOBn, Me) derivatives of isopropylamine diazen-1-ium-1,2-diolate (IPA/NO) (2-5) were developed that are designed to act as novel HNO donors. These compounds, with suitable half-lives (6.6-17.1 h) at pH 7.4, undergo a base-induced β-elimination reaction that releases a methyl vinyl sulfone product and the parent IPA/NO anion which subsequently preferentially releases HNO (46-61% range). Importantly, the O(2)-methylsulfonylethyl compound 5 exhibited a significant in vitro inotropic effect up to 283% of the baseline value and increased the rates of contraction and relaxation but did not induce a chronotropic effect. Furthermore, compound 5 (22.5 mg/kg po dose) provided a significant reduction in blood pressure up to 6 h after drug administration. All these data suggest that O(2)-sulfonylethyl protected derivatives of IPA/NO, which are efficient HNO donors, could have potential applications to treat cardiovascular disease(s) such as congestive heart failure.

Nitroxyl-mediated disulfide bond formation between cardiac myofilament cysteines enhances contractile function

RATIONALE

In the myocardium, redox/cysteine modification of proteins regulating Ca(2+) cycling can affect contraction and may have therapeutic value. Nitroxyl (HNO), the one-electron-reduced form of nitric oxide, enhances cardiac function in a manner that suggests reversible cysteine modifications of the contractile machinery.

OBJECTIVE

To determine the effects of HNO modification in cardiac myofilament proteins.

METHODS AND RESULTS

The HNO-donor, 1-nitrosocyclohexyl acetate, was found to act directly on the myofilament proteins, increasing maximum force (F(max)) and reducing the concentration of Ca(2+) for 50% activation (Ca(50)) in intact and skinned cardiac muscles. The effects of 1-nitrosocyclohexyl acetate are reversible by reducing agents and distinct from those of another HNO donor, Angeli salt, which was previously reported to increase F(max) without affecting Ca50. Using a new mass spectrometry capture technique based on the biotin switch assay, we identified and characterized the formation by HNO of a disulfide-linked actin-tropomyosin and myosin heavy chain-myosin light chain 1. Comparison of the 1-nitrosocyclohexyl acetate and Angeli salt effects with the modifications induced by each donor indicated the actin-tropomyosin and myosin heavy chain-myosin light chain 1 interactions independently correlated with increased Ca(2+) sensitivity and force generation, respectively.

CONCLUSIONS

HNO exerts a direct effect on cardiac myofilament proteins increasing myofilament Ca(2+) responsiveness by promoting disulfide bond formation between critical cysteine residues. These findings indicate a novel, redox-based modulation of the contractile apparatus, which positively impacts myocardial function, providing further mechanistic insight for HNO as a therapeutic agent.

The mechanism underlying nitroxyl and nitric oxide formation from hydroxamic acids

BACKGROUND

The pharmacological effects of hydroxamic acids (RC(O)NHOH, HX) are partially attributed to their ability to serve as HNO and/or NO donors under oxidative stress. Given the development and use of HXs as therapeutic agents, elucidation of the oxidation mechanism is needed for more educated selection of HX-based drugs.

METHODS

Acetohydroxamic and glycine-hydroxamic acids were oxidized at pH 7.0 by a continuous flux of radiolytically generated (·)OH or by metmyoglobin and H(2)O(2) reactions system. Gas chromatography and spectroscopic methods were used to monitor the accumulation of N(2)O, N(2), nitrite and hydroxylamine.

RESULTS

Oxidation of HXs by (·)OH under anoxia yields N(2)O, but not nitrite, N(2) or hydroxylamine. Upon the addition of H(2)O(2) to solutions containing HX and metmyoglobin, which is instantaneously and continuously converted into compound II, nitrite and, to a lesser extent, N(2)O are accumulated under both anoxia and normoxia.

CONCLUSIONS

Oxidation of HXs under anoxia by a continuous flux of (·)OH, which solely oxidizes the hydroxamate moiety to RC(O)NHO(·), forms HNO. This observation implies that bimolecular decomposition of RC(O)NHO(·) competes efficiently with unimolecular decomposition processes such as internal disproportionation, hydrolysis or homolysis. Oxidation by metmyoglobin/H(2)O(2) involves relatively mild oxidants (compounds I and II). Compound I reacts with HX forming RC(O)NHO(·) and compound II, which oxidizes HX, RC(O)NHO(·), HNO and NO. The latter reaction is the main source of nitrite.

GENERAL SIGNIFICANCE

HXs under oxidative stress release HNO, but can be considered as NO-donors provided that HNO oxidation is more efficient than its reaction with other biological targets.

Reactivity of nitroxyl-derived sulfinamides

Sulfinamide [RS(O)NH(2)] formation is known to occur upon exposure of cysteine residues to nitroxyl (HNO), which has received recent attention as a potential heart failure therapeutic. Because this modification can alter protein structure and function, we have examined the reactivity of sulfinamides in several systems, including a small organic molecule, peptides, and a protein. Although it has generally been assumed that this thiol to sulfinamide modification is irreversible, we show that sulfinamides can be reduced back to the free thiol in the presence of excess thiol at physiological pH and temperature. We have examined this sulfinamide reduction both in peptides, where a cyclic intermediate analogous to that proposed for asparagine deamidation reactions potentially can contribute, and in a small organic molecule, where the mechanism is restricted to a direct thiolysis. These studies suggest that the contribution from the cyclic intermediate becomes more important in environments with lower dielectric constants. In addition, although sulfinic acid [RS(O)OH] formation is observed upon prolonged incubations in water, reduction of sulfinamides is found to dominate in the presence of thiols. Finally, studies with the cysteine protease, papain, suggest that the reduction of sulfinamide to the free thiol is viable in a protein environment.

Nitroxyl (HNO) acutely activates the glucose uptake activity of GLUT1

Nitroxyl (HNO) is a molecule of significant interest due to its unique pharmacological properties, particularly within the cardiovascular system. A large portion of HNO biological effects can be attributed to its reactivity with protein thiols, where it can generate disulfide bonds. Evidence from studies in erythrocytes suggests that the activity of GLUT1 is enhanced by the formation of an internal disulfide bond. However, there are no reports that document the effects of HNO on glucose uptake. Therefore, we examined the acute effects of Angeli's salt (AS), a HNO donor, on glucose uptake activity of GLUT1 in L929 fibroblast cells. We report that AS stimulates glucose uptake with a maximum effective concentration of 5.0 mM. An initial 7.2-fold increase occurs within 2 min, which decreases and plateaus to a 4.0-fold activation after 10 min. About 60% of the 4.0-fold activation recovers within 10 min, and 40% remains after an hour. The activation is blocked by the pretreatment of cells with thiol-reactive compounds, iodoacetamide (0.75 mM), cinnamaldehyde (2.0 mM), and phenylarsine oxide (10 μM). The effects of AS are not additive to the stimulatory effects of other acute activators of glucose uptake in L929 cells, such as azide (5 mM), berberine (50 μM), or glucose deprivation. These data suggest that GLUT1 is acutely activated in L929 cells by the formation of a disulfide bond, likely within GLUT1 itself.

Rapid and selective nitroxyl (HNO) trapping by phosphines: kinetics and new aqueous ligations for HNO detection and quantitation

Recent studies distinguish the biological and pharmacological effects of nitroxyl (HNO) from its oxidized/deprotonated product nitric oxide (·NO), but the lack of HNO detection methods limits the understanding its in vivo mechanisms and the identification of endogenous sources. We previously demonstrated that reaction of HNO with triarylphosphines provides aza-ylides and HNO-derived amides, which may serve as stable HNO biomarkers. We now report a kinetic analysis for the trapping of HNO by phosphines, ligations of enzyme-generated HNO, and compatibility studies illustrating the selectivity of phosphines for HNO over other physiologically relevant nitrogen oxides. Quantification of HNO using phosphines is demonstrated using an HPLC-based assay and ligations of phosphine carbamates generate HNO-derived ureas. These results further demonstrate the potential of phosphine probes for reliable biological detection and quantification of HNO.

Aorta from angiotensin II hypertensive mice exhibit preserved nitroxyl anion mediated relaxation responses

Hypertension is a disorder affecting millions worldwide, and is a leading cause of death and debilitation in the United States. It is widely accepted that during hypertension and other cardiovascular diseases the vasculature exhibits endothelial dysfunction; a deficit in the relaxatory ability of the vessel, attributed to a lack of nitric oxide (NO) bioavailability. Recently, the one electron redox variant of NO, nitroxyl anion (NO(-)) has emerged as an endothelium-derived relaxing factor (EDRF) and a candidate for endothelium-derived hyperpolarizing factor (EDRF). NO(-) is thought to exist protonated (HNO) in vivo, which would make this species more resistant to scavenging. However, no studies have investigated the role of this redox species during hypertension, and whether the vasculature loses the ability to relax to HNO. Thus, we hypothesize that aorta from angiotensin II (AngII)-hypertensive mice will exhibit a preserved relaxation response to Angeli's Salt, an HNO donor. Male C57Bl6 mice, aged 12-14 weeks were implanted with mini-osmotic pumps containing AngII (90ng/min, 14 days plus high salt chow) or sham surgery. Aorta were excised, cleaned and used to perform functional studies in a myograph. We found that aorta from AngII-hypertensive mice exhibited a significant endothelial dysfunction as demonstrated by a decrease in acetylcholine (ACh)-mediated relaxation. However, vessels from hypertensive mice exhibited a preserved response to Angeli's Salt (AS), the HNO donor. To confirm that relaxation responses to HNO were maintained, concentration response curves (CRCs) to ACh were performed in the presence of scavengers to both NO and HNO (carboxy-PTIO and L-cys, resp.). We found that ACh-mediated relaxation responses were significantly decreased in aorta from sham and almost completely abolished in aorta from AngII-treated mice. Vessels incubated with l-cys exhibited a modest decrease in ACh-mediated relaxations responses. These data demonstrate that aorta from AngII-treated hypertensive mice exhibit a preserved relaxation response to AS, an HNO donor, regardless of a significant endothelial dysfunction.

Playing with cardiac "redox switches": the "HNO way" to modulate cardiac function

The nitric oxide (NO(•)) sibling, nitroxyl or nitrosyl hydride (HNO), is emerging as a molecule whose pharmacological properties include providing functional support to failing hearts. HNO also preconditions myocardial tissue, protecting it against ischemia-reperfusion injury while exerting vascular antiproliferative actions. In this review, HNO's peculiar cardiovascular assets are discussed in light of its unique chemistry that distinguish HNO from NO(•) as well as from reactive oxygen and nitrogen species such as the hydroxyl radical and peroxynitrite. Included here is a discussion of the possible routes of HNO formation in the myocardium and its chemical targets in the heart. HNO has been shown to have positive inotropic/lusitropic effects under normal and congestive heart failure conditions in animal models. The mechanistic intricacies of the beneficial cardiac effects of HNO are examined in cellular models. In contrast to β-receptor/cyclic adenosine monophosphate/protein kinase A-dependent enhancers of myocardial performance, HNO uses its "thiophylic" nature as a vehicle to interact with redox switches such as cysteines, which are located in key components of the cardiac electromechanical machinery ruling myocardial function. Here, we will briefly review new features of HNO's cardiovascular effects that when combined with its positive inotropic/lusitropic action may render HNO donors an attractive addition to the current therapeutic armamentarium for treating patients with acutely decompensated congestive heart failure.

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.

Nitroxyl in the central nervous system

Nitroxyl (HNO) is the one-electron-reduced and protonated congener of nitric oxide (NO). Compared to NO, it is far more reactive with thiol groups either in proteins or in small antioxidant molecules either converting those into sulfinamides or inducing disulfide bond formation. HNO might mediate cytoprotective changes of protein function through thiol modifications. However, HNO is a strong oxidant that in vitro reacts with glutathione to form glutathione disulfide and glutathione sulfinamide. The resulting oxidative stress might aggravate tissue damage in inflammatory diseases. In this review, we will summarize the current knowledge of how exogenous HNO affects the central nervous system, especially nerve cells and glia in health and disease. Unlike most other organs, the brain is separated from the circulation by the blood-brain barrier, which limits access of many pharmacological compounds. Given that, we will review what is known about the ability of currently used HNO donors to cross the blood-brain barrier. Moreover, considering that the physiology and composition of the brain has unique properties, for example, expression of brain-specific enzymes like neuronal NO synthase, its high iron content, and increased energy metabolism, we will discuss possible sources of endogenous HNO in the brain.

Ethanesulfohydroxamic acid ester prodrugs of nonsteroidal anti-inflammatory drugs (NSAIDs): synthesis, nitric oxide and nitroxyl release, cyclooxygenase inhibition, anti-inflammatory, and ulcerogenicity index studies

The carboxylic acid group of the anti-inflammatory (AI) drugs indo-methacin, (S)-naproxen and ibuprofen was covalently linked via a two-carbon ethyl spacer to a sulfohydroxamic acid moiety (CH(2)CH(2)SO(2)NHOH) to furnish a group of hybrid ester prodrugs that release nitric oxide (NO) and nitroxyl (HNO). Biological data acquired for this hitherto unknown class of ethanesulfohydroxamic acid ester prodrugs showed (i) all compounds exhibited superior NO, but similar HNO, release properties relative to arylsulfohydroxamic acids, (ii) the (S)-naproxen and ibuprofen prodrug esters are more potent AI agents than their parent NSAID, (iii) the indomethacin prodrug ester, in contrast to indomethacin which is highly ulcerogenic, showed no visible stomach lesions [ulcer index (UI) = 0 for a 80 μmol/kg oral dose] while retaining potent AI activity, and iv) that the indomethacin prodrug ester, unlike indomethacin which is an ulcerogenic selective COX-1 inhibitor, is a selective COX-2 inhibitor (COX-2 selectivity index = 184) devoid of ulcerogenicity that is attributed to its high COX-2 SI and/or ability to release cytoprotective NO.

Biochemical and neurochemical sequelae following mild traumatic brain injury: summary of experimental data and clinical implications

Although numerous studies have been carried out to investigate the pathophysiology of mild traumatic brain injury (mTBI), there are still no standard criteria for the diagnosis and treatment of this peculiar condition. The dominant theory that diffuse axonal injury is the main neuropathological process behind mTBI is being revealed as weak at best or inconclusive, given the current literature and the fact that neuronal injury inherent to mTBI improves, with few lasting clinical sequelae in the vast majority of patients. Clinical and experimental evidence suggests that such a course, rather than being due to cell death, is based on temporal neuronal dysfunction, the inevitable consequence of complex biochemical and neurochemical cascade mechanisms directly and immediately triggered by the traumatic insult. This report is an attempt to summarize data from a long series of experiments conducted in the authors' laboratories and published during the past 12 years, together with an extensive analysis of the available literature, focused on understanding the biochemical damage produced by an mTBI. The overall clinical implications, as well as the metabolic nature of the post-mTBI brain vulnerability, are discussed. Finally, the application of proton MR spectroscopy as a possible tool to monitor the full recovery of brain metabolic functions is emphasized.

Acyclic triaryl olefins possessing a sulfohydroxamic acid pharmacophore: synthesis, nitric oxide/nitroxyl release, cyclooxygenase inhibition, and anti-inflammatory studies

Nitric oxide (NO) and its reduced form nitroxyl (HNO), effective vasodilation agents that can inhibit platelet aggregation and adhesion, could suppress adverse cardiovascular effects associated with the use of selective COX-2 inhibitors. In this regard, a sulfohydroxamic acid (SO(2)NHOH) substituent, that can act as a dual NO/HNO donor moiety, was inserted at the para-position of the C2 phenyl ring of acyclic 2-alkyl-1,1,2-triaryl olefins previously shown to be potent and highly selective COX-2 inhibitors. Although this new group of 1,1-diaryl-2-(4-hydroxyaminosulfonylphenyl)alk-1-enes exhibited weak inhibition of the constitutive cyclooxygenase-1 (COX-1) and inducible COX-2 isozymes, in vivo studies showed anti-inflammatory potencies that were generally intermediate between that of the reference drugs aspirin and ibuprofen. All compounds released NO (5.6-13.5% range) upon incubation with phosphate buffer which was increased further (8.3-25.6% range) in the presence of the oxidant K(3)(FeCN(6)).The low release of HNO in MeOH-buffer (< 2% at 24 h incubation) was much higher at alkaline pH (11-37% range). The concept of designing better anti-inflammatory drugs possessing either an effective HNO, or dual NO/HNO, donor moiety that are devoid of adverse ulcerogenic and/or cardiovascular side effects warrants further investigation.

Nitroxyl (HNO) signaling

Nitroxyl (HNO) has become a nitrogen oxide of significant interest due to its reported biological activity. The actions of HNO in the cardiovascular system appear to make it a good candidate for therapeutic applications for cardiovascular disorders and other potentially important effects have been noted as well. Although the chemistry associated with this activity has not been firmly established, the propensity for HNO to react with thiols and metals are likely mechanisms. Herein, are described the biological activity of HNO and some of the chemistry of HNO that may be responsible for its biological effects.

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.

Comparing the chemical biology of NO and HNO

For the past couple of decades nitric oxide (NO) and nitroxyl (HNO) have been extensively studied due to the important role they play in many physiological and/or pharmacological processes. Many researchers have reported important signaling pathways as well as mechanisms of action of these species, showing direct and indirect effects depending on the environment. Both NO and HNO can react with, among others, metals, proteins, thiols and heme proteins via unique and distinct chemistry leading to improvement of some clinical conditions. Understanding the basic chemistry of NO and HNO and distinguishing their mechanisms of action as well as methods of detection are crucial for understanding the current and potential clinical applications. In this review, we summarize some of the most important findings regarding NO and HNO chemistry, revealing some of the possible mechanisms of their beneficial actions.

Nitroxyl exacerbates ischemic cerebral injury and oxidative neurotoxicity

Nitroxyl (HNO) donor compounds function as potent vasorelaxants, improve myocardial contractility and reduce ischemia-reperfusion injury in the cardiovascular system. With respect to the nervous system, HNO donors have been shown to attenuate NMDA receptor activity and neuronal injury, suggesting that its production may be protective against cerebral ischemic damage. Hence, we studied the effect of the classical HNO-donor, Angeli's salt (AS), on a cerebral ischemia/reperfusion injury in a mouse model of experimental stroke and on related in vitro paradigms of neurotoxicity. I.p. injection of AS (40 mumol/kg) in mice prior to middle cerebral artery occlusion exacerbated cortical infarct size and worsened the persistent neurological deficit. AS not only decreased systolic blood pressure, but also induced systemic oxidative stress in vivo indicated by increased isoprostane levels in urine and serum. In vitro, neuronal damage induced by oxygen-glucose-deprivation of mature neuronal cultures was exacerbated by AS, although there was no direct effect on glutamate excitotoxicity. Finally, AS exacerbated oxidative glutamate toxicity - that is, cell death propagated via oxidative stress in immature neurons devoid of ionotropic glutamate receptors. Taken together, our data indicate that HNO might worsen cerebral ischemia-reperfusion injury by increasing oxidative stress and decreasing brain perfusion at concentrations shown to be cardioprotective in vivo.

The emergence of nitroxyl (HNO) as a pharmacological agent

Once a virtually unknown nitrogen oxide, nitroxyl (HNO) has emerged as a potential pharmacological agent. Recent advances in the understanding of the chemistry of HNO has led to the an understanding of HNO biochemistry which is vastly different from the known chemistry and biochemistry of nitric oxide (NO), the one-electron oxidation product of HNO. The cardiovascular roles of NO have been extensively studied, as NO is a key modulator of vascular tone and is involved in a number of vascular related pathologies. HNO displays unique cardiovascular properties and has been shown to have positive lusitropic and ionotropic effects in failing hearts without a chronotropic effect. Additionally, HNO causes a release of CGRP and modulates calcium channels such as ryanodine receptors. HNO has shown beneficial effects in ischemia reperfusion injury, as HNO treatment before ischemia-reperfusion reduces infarct size. In addition to the cardiovascular effects observed, HNO has shown initial promise in the realm of cancer therapy. HNO has been demonstrated to inhibit GAPDH, a key glycolytic enzyme. Due to the Warburg effect, inhibiting glycolysis is an attractive target for inhibiting tumor proliferation. Indeed, HNO has recently been shown to inhibit tumor proliferation in mouse xenografts. Additionally, HNO inhibits tumor angiogenesis and induces cancer cell apoptosis. The effects seen with HNO donors are quite different from NO donors and in some cases are opposite. The chemical nature of HNO explains how HNO and NO, although closely chemically related, act so differently in biochemical systems. This also gives insight into the potential molecular motifs that may be reactive towards HNO and opens up a novel field of pharmacological development.

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.

Persistent susceptibility of cathepsin B to irreversible inhibition by nitroxyl (HNO) in the presence of endogenous nitric oxide

Nitrosation of enzyme regulatory cysteines is one of the key posttranslational modification mechanisms of enzyme function. Frequently such modifications are readily reversible; however, cysteine proteases, such as cathepsin B, have been shown to be covalently and permanently inactivated by nitroxyl (HNO), the one-electron reduction product of NO. Owing to the high reactivity of HNO with NO, endogenous NO production could provide direct protection for the less reactive protein cysteines by scavenging HNO. Additionally, endogenous cellular production of NO could rescue enzyme function by protective nitrosation of cysteines prior to exposure to HNO. Thus, we studied the effect of endogenous NO production, induced by LPS or IFN-gamma, on inhibition of cysteine protease cathepsin B in RAW macrophages. Both LPS and IFN-gamma induce iNOS with generation of nitrate up to 9 muM in the media after a 24-h stimulation, while native RAW 264.7 macrophages neither express iNOS nor generate nitrate. After the 24-h stimulation, the HNO-releasing Angeli's salt (0-316 microM) caused dose-dependent and DTT-irreversible loss of cathepsin B activity, and induction of iNOS activity did not protect the enzyme. The lack of protection was also verified in an in vitro setup, where papain, a close structural analogue of cathepsin B, was inhibited by Angeli's salt (2.7 microM) in the presence of the NO donor DEA/NO (0-316 microM). This clearly showed that a high molar excess of DEA/NO (EC(50) 406 microM) is needed to protect papain from the DTT-irreversible covalent modification by HNO. Our results provide first evidence on a cellular level for the remarkably high sensitivity of active-site cysteines in cysteine proteases for modification by HNO.

Generation of nitroxyl by heme protein-mediated peroxidation of hydroxylamine but not N-hydroxy-L-arginine

The chemical reactivity, toxicology, and pharmacological responses to nitroxyl (HNO) are often distinctly different from those of nitric oxide (NO). The discovery that HNO donors may have pharmacological utility for treatment of cardiovascular disorders such as heart failure and ischemia reperfusion has led to increased speculation of potential endogenous pathways for HNO biosynthesis. Here, the ability of heme proteins to utilize H2O2 to oxidize hydroxylamine (NH2OH) or N-hydroxy-L-arginine (NOHA) to HNO was examined. Formation of HNO was evaluated with a recently developed selective assay in which the reaction products in the presence of reduced glutathione (GSH) were quantified by HPLC. Release of HNO from the heme pocket was indicated by formation of sulfinamide (GS(O)NH2), while the yields of nitrite and nitrate signified the degree of intramolecular recombination of HNO with the heme. Formation of GS(O)NH2 was observed upon oxidation of NH2OH, whereas NOHA, the primary intermediate in oxidation of L-arginine by NO synthase, was apparently resistant to oxidation by the heme proteins utilized. In the presence of NH2OH, the highest yields of GS(O)NH2 were observed with proteins in which the heme was coordinated to a histidine (horseradish peroxidase, lactoperoxidase, myeloperoxidase, myoglobin, and hemoglobin) in contrast to a tyrosine (catalase) or cysteine (cytochrome P450). That peroxidation of NH2OH by horseradish peroxidase produced free HNO, which was able to affect intracellular targets, was verified by conversion of 4,5-diaminofluorescein to the corresponding fluorophore within intact cells.

Oxygen- and NssR-dependent globin expression and enhanced iron acquisition in the response of campylobacter to nitrosative stress

Pathogenic bacteria experience nitrosative stress from NO generated in the host and from nitrosating species such as S-nitrosoglutathione. The food-borne pathogen Campylobacter jejuni responds by activating gene expression from a small regulon under the control of the NO-sensitive regulator, NssR. Here, we describe the full extent of the S-nitrosoglutathione response using transcriptomic and proteomic analysis of batch- and chemostat-cultured C. jejuni. In addition to the NssR regulon, which includes two hemoglobins (Cgb and Ctb), we identify more than 90 other up-regulated genes, notably those encoding heat shock proteins and proteins involved in oxidative stress tolerance and iron metabolism/transport. Up-regulation of a subset of these genes, including cgb, is also elicited by NO-releasing compounds. Mutation of the iron-responsive regulator Fur results in insensitivity of growth to NO, suggesting that derepression of iron-regulated genes and augmentation of iron acquisition is a physiological response to nitrosative damage. We describe the effect of oxygen availability on nitrosative stress tolerance; cells cultured at higher rates of oxygen diffusion have elevated levels of hemoglobins, are more resistant to inhibition by NO of both growth and respiration, and consume NO more rapidly. The oxygen response is mediated by NssR. Thus, in addition to NO detoxification catalyzed by the hemoglobins Cgb and possibly Ctb, C. jejuni mounts an extensive stress response. We suggest that inhibition of respiration by NO may increase availability of oxygen for Cgb synthesis and function.

Nitroxyl inhibits breast tumor growth and angiogenesis

Nitroxyl (HNO) can inhibit the glycolytic enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Because of the importance of glycolysis in many malignant cells, we thus propose that HNO can adversely affect tumor growth. This hypothesis was tested using in vitro and in vivo models of breast cancer. We report here for the first time that HNO suppresses the proliferation of both estrogen receptor (ER)-positive and ER-negative human breast cancer cell lines, in a dose dependent manner. Mice treated with HNO either injected into the tumor itself or via the intraperitoneal approach had smaller xenograft tumor size. In addition to significantly decreased blood vessel density in the HNO-treated tumors, we observed lower levels of circulating serum vascular endothelial growth factor (VEGF). Accordingly, there was a decrease in total HIF-1alpha (hypoxia-inducible factor) protein in HNO-treated tumor cells. Further studies showed inhibition of GAPDH activity in HNO-treated human breast cancer cell lines and in HNO-treated tumor tissue derived from xenografts. One explanation for the multiplicity of actions observed after HNO treatment could be the effect from the initial inhibition of GAPDH, providing a potential therapeutic avenue based upon blocking glycolysis resulting in decreased HIF-1alpha, thus leading to angiogenesis inhibition. Therefore, HNO appears to act via mechanism(s) different from those of existing breast cancer drugs, making it a potential candidate to overcome known and emerging drug resistance pathways.

Xerogel optical sensor films for quantitative detection of nitroxyl

Xerogel sensing films were synthesized via sol-gel chemistry were used to fabricate optical nitroxyl (HNO) sensors [corrected] Selective detection of HNO in solution was achieved by monitoring the rates of manganese(III) meso-tetrakis(4-sulfonatophenyl) porphyrinate (MnIIITPPS) reductive nitrosylation in the anaerobic interior of aminoalkoxysilane-derived xerogel films. Nitroxyl permeability in sensor films deposited in round-bottom 96-well plates was enhanced via incorporation of trimethoxysilyl-terminated poly(amidoamine-organosilicon) dendrimers in the xerogel network. The selectivity of MnIIITPPS for HNO, the overall sensitivity, and the working dynamic range of the resulting sensors were characterized. The HNO-sensing microtiter plates were used to quantify pH-dependent HNO generation by the recently described HNO-donor sodium 1-(isopropylamino)diazene-1-ium-1,2-diolate (IPA/NO), and compare HNO production efficiency between IPA/NO and Angeli's salt, a traditional HNO-donor.

Temporal window of metabolic brain vulnerability to concussions: oxidative and nitrosative stresses--part II

OBJECTIVE

In the present study, we investigated the occurrence of oxidative and nitrosative stresses in rats undergoing repeat mild traumatic brain injury (mTBI) delivered with increasing time intervals.

METHODS

Rats were subjected to two diffuse mTBIs (450 g/1 m height), with the second mTBI delivered after 1 (n = 6), 2 (n = 6), 3 (n = 6), 4 (n = 6), or 5 days (n = 6). The rats were sacrificed 48 hours after the last mTBI. Sham-operated animals were used as controls (n = 6). Concentrations of biochemical indices of oxidative stress (malondialdehyde, ascorbic acid, reduced and oxidized glutathione) and nitrosative stress (nitrite, nitrate) were synchronously measured by high-performance liquid chromatography in deproteinized tissue extracts (three right + three left hemispheres for each group of animals).

RESULTS

Increase of malondialdehyde, reduced/oxidized glutathione ratio, nitrite, nitrate, and decrease of ascorbic acid and glutathione were dependent on the interval between impacts with maximal changes recorded when mTBIs were spaced by 3 days. Biochemical markers of oxidative and nitrosative stresses were near control levels only in animals receiving mTBIs 5 days apart.

CONCLUSION

This study shows the remarkable negative contribution of reactive oxygen species overproduction and activation of inducible nitric oxide synthase in repeat mTBI. Because these effects were maximal when mTBIs were spaced by 3 days, it can be inferred that occurrence of a second mTBI within the temporal window of brain vulnerability not only causes profound derangement of mitochondrial functions, but also induces sustained oxidative and nitrosative stresses. Both phenomena certainly play a major role in the overall brain tissue damage occurring under these pathological conditions.

Hydrolysis of acyloxy nitroso compounds yields nitroxyl (HNO)

Nitroxyl (HNO/NO(-)), the reduced form of nitric oxide, has gained attention based on its separate chemistry and biology from nitric oxide. The inherent reactivity of HNO requires new and mechanistically unique donors for the detailed study of HNO chemistry and biology. Oxidation of cyclohexanone oxime with lead tetraacetate yields 1-nitrosocyclohexyl acetate, whereas oxidation of oximes in the presence of excess carboxylic acid gives various acyloxy nitroso compounds. These bright blue compounds exist as monomers as indicated by their infrared, proton, and carbon NMR spectra, and X-ray crystallographic analysis reveals the nitroso groups possess a "nitroxyl-like" bent configuration. Hydrolysis of these compounds produces nitrous oxide, the dimerization and dehydration product of HNO, and provides evidence for the intermediacy of HNO. Both thiols and oxidative metal complexes inhibit nitrous oxide formation. Hydrolysis of these compounds in the presence of ferric heme complexes forms ferrous nitrosyl complexes providing further evidence for the intermediacy of HNO. Kinetic analysis shows that the rate of hydrolysis depends on pH and the structure of the acyl group of the acyloxy nitroso compound. These compounds relax pre-constricted rat aortic rings similar to known HNO donors. Together, these results identify acyloxy nitroso compounds as a new class of HNO donors.

Redox in redux: Emergent roles for glutathione S-transferase P (GSTP) in regulation of cell signaling and S-glutathionylation

Glutathione (GSH) provides a major source of thiol homeostasis critical to the maintenance of a reduced cellular environment that is conducive to cell survival. Mammals have accumulated a significant cadre of sulfur containing proteins, the interactive significance of which has become clear in recent times. Glutathione transferases (GST) are prevalent in eukaryotes and have been ascribed catalytic functions that involve detoxification of electrophiles through thioether bond formation with the cysteine thiol of GSH. The neutralizing impact of these reactions on products of reactive oxygen has contributed to the significant evolutionary conservation and adaptive functional redundancy of the multifaceted GSH system. Amongst the GSTs, GSTP has been implicated in tumorigenesis and in anticancer drug resistance. Emerging studies indicate that GSTP has ligand binding properties and contributes in the regulation of signaling kinases through direct protein:protein interactions. Furthermore, S-glutathionylation is a post-translational modification of low pK(a) cysteine residues in target proteins. The forward rate of the S-glutathionylation reaction can be influenced by GSTP, whereas the reverse rate is affected by a number of redox sensitive proteins including glutaredoxin, thioredoxin and sulfiredoxin. The functional importance of these reactions in governing how cells respond to oxidative or nitrosative stress exemplifies the broad importance of GSH/GST homeostasis in conditions such as cancer, ageing and neurodegenerative diseases. GSTP has also provided a platform for therapeutic drug development where some agents have completed preclinical testing and are in clinical trial for the management of cancer.

The pharmacology of nitroxyl (HNO) and its therapeutic potential: not just the Janus face of NO

Nitroxyl (HNO), the 1-electron reduced and protonated congener of nitric oxide (NO), has received recent attention as a potential pharmacological agent for the treatment of heart failure and as a preconditioning agent for the mitigation of ischemia-reperfusion injury. Interest in the pharmacology and biology of HNO has prompted examination, or in some instances reexamination, of many of its chemical properties. Such studies have provided insight into the chemical basis for the biological effects of HNO, although the biochemical mechanisms for many of these effects remain to be established. In this review, a brief description of the biologically relevant chemistry of HNO is given, followed by a more detailed discussion of the pharmacology and potential toxicology of HNO.

Ischemia-reperfusion and cardioprotection: a delicate balance between reactive oxygen species generation and redox homeostasis

Ischemia-reperfusion injury of the myocardium has long been a subject of intense research. Cardiac preconditioning, an associated phenomenon, has also been critically investigated over the past two decades. Although the biochemistry of ischemia-reperfusion and its association with oxidative metabolism has long been established, recent studies have further revealed a more intricate role of a number of reactive oxygen-nitrogen species in those processes. Emerging evidence suggests that an elaborate network of enzymes (and other biomolecules) dedicated to the generation, utilization, and diminution of reactive oxygen-nitrogen species maintains the redox homeostasis in the myocardium, and any perturbation of its status has distinctive effects. It thus appears that while excessive generation of reactive species leads to cellular injury, their regulated generation may cause transient and reversible modifications of cellular proteins leading the transmission of intracellular signals with specific effects. Taken together, generation of reactive oxygen-nitrogen species in the myocardium plays a nodal role in mediating both ischemic injury and cardioprotection.