Kinetic feasibility of nitroxyl reduction by physiological reductants and biological implications

Pyruvate dehydrogenase operates as an intramolecular nitroxyl generator during macrophage metabolic reprogramming

M1 macrophages enter a glycolytic state when endogenous nitric oxide (NO) reprograms mitochondrial metabolism by limiting aconitase 2 and pyruvate dehydrogenase (PDH) activity. Here, we provide evidence that NO targets the PDH complex by using lipoate to generate nitroxyl (HNO). PDH E2-associated lipoate is modified in NO-rich macrophages while the PDH E3 enzyme, also known as dihydrolipoamide dehydrogenase (DLD), is irreversibly inhibited. Mechanistically, we show that lipoate facilitates NO-mediated production of HNO, which interacts with thiols forming irreversible modifications including sulfinamide. In addition, we reveal a macrophage signature of proteins with reduction-resistant modifications, including in DLD, and identify potential HNO targets. Consistently, DLD enzyme is modified in an HNO-dependent manner at Cys477 and Cys484, and molecular modeling and mutagenesis show these modifications impair the formation of DLD homodimers. In conclusion, our work demonstrates that HNO is produced physiologically. Moreover, the production of HNO is dependent on the lipoate-rich PDH complex facilitating irreversible modifications that are critical to NO-dependent metabolic rewiring.

The Chemical Biology of NO that Regulates Oncogenic Signaling and Metabolism: NOS2 and Its Role in Inflammatory Disease

Nitric oxide (NO) and the enzyme that synthesizes it, nitric oxide synthase 2 (NOS2), have emerged as key players in inflammation and cancer. Expression of NOS2 in tumors has been correlated both with positive outcomes and with poor prognoses. The chemistry of NO is the major determinate to the biological outcome and the concentration of NO, which can range over five orders of magnitude, is critical in determining which pathways are activated. It is the activation of specific oncogenic and immunological mechanisms that shape the outcome. The kinetics of specific reactions determine the mechanisms of action. In this review, the relevant reactions of NO and related species are discussed with respect to these oncogenic and immunological signals.

Para-Substituted O-Benzyl Sulfohydroxamic Acid Derivatives as Redox-Triggered Nitroxyl (HNO) Sources

Nitroxyl shows a unique biological profile compared to the gasotransmitters nitric oxide and hydrogen sulfide. Nitroxyl reacts with thiols as an electrophile, and this redox chemistry mediates much of its biological chemistry. This reactivity necessitates the use of donors to study nitroxyl’s chemistry and biology. The preparation and evaluation of a small library of new redox-triggered nitroxyl sources is described. The condensation of sulfonyl chlorides and properly substituted O-benzyl hydroxylamines produced O-benzyl-substituted sulfohydroxamic acid derivatives with a 27–79% yield and with good purity. These compounds were designed to produce nitroxyl through a 1, 6 elimination upon oxidation or reduction via a Piloty’s acid derivative. Gas chromatographic headspace analysis of nitrous oxide, the dimerization and dehydration product of nitroxyl, provides evidence for nitroxyl formation. The reduction of derivatives containing nitro and azide groups generated nitrous oxide with a 25–92% yield, providing evidence of nitroxyl formation. The oxidation of a boronate-containing derivative produced nitrous oxide with a 23% yield. These results support the proposed mechanism of nitroxyl formation upon reduction/oxidation via a 1, 6 elimination and Piloty’s acid. These compounds hold promise as tools for understanding nitroxyl’s role in redox biology.

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.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Azanone (HNO): generation, stabilization and detection

HNO (nitroxyl, azanone), joined the 'biologically relevant reactive nitrogen species' family in the 2000s. Azanone is impossible to store due to its high reactivity and inherent low stability. Consequently, its chemistry and effects are studied using donor compounds, which release this molecule in solution and in the gas phase upon stimulation. Researchers have also tried to stabilize this elusive species and its conjugate base by coordination to metal centers using several ligands, like metalloporphyrins and pincer ligands. Given HNO's high reactivity and short lifetime, several different strategies have been proposed for its detection in chemical and biological systems, such as colorimetric methods, EPR, HPLC, mass spectrometry, fluorescent probes, and electrochemical analysis. These approaches are described and critically compared. Finally, in the last ten years, several advances regarding the possibility of endogenous HNO generation were made; some of them are also revised in the present work.

Kinetics of Azanone (HNO) Reactions with Thiols: Effect of pH

HNO (nitroxyl, IUPAC name azanone) is an electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present data on the pH-dependent kinetics of azanone reactions with the low molecular thiols glutathione and N-acetylcysteine, as well as with important serum proteins: bovine serum albumin and human serum albumin. The competition kinetics method used is based on two parallel HNO reactions: with RSH/RS⁻ or with O₂. The results provide evidence that the reaction of azanone with the anionic form of thiols (RS⁻) is favored over reactions with the protonated form (RSH). The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with thiolates is provided.

Reaction of Nitroxyl (HNO) with Hydrogen Sulfide and Hydropersulfides

Nitroxyl (HNO) has gained a considerable amount of attention because of its promising pharmacological effects. The biochemical mechanisms of HNO activity are associated with the modification of regulatory thiol proteins. Recently, several studies have suggested that hydropersulfides (RSSH), presumed signaling products of hydrogen sulfide (H₂S)-mediated thiol (RSH) modification, are additional potential targets of HNO. However, the interaction of HNO with reactive sulfur species beyond thiols remains relatively unexplored. Herein, we present characterization of HNO reactivity with H₂S and RSSH. The reaction of H₂S with HNO leads to the formation of hydrogen polysulfides and sulfur (S₈), suggesting a potential role in sulfane sulfur homeostasis. Furthermore, we show that hydropersulfides are more efficient traps for HNO than their thiol counterparts. The reaction of HNO with RSSH at varied stoichiometries has been examined with the observed production of various dialkylpolysulfides (RSS_nSR) and other nitrogen-containing dialkylpolysulfide species (RSS-NH-S_nR). We do not observe evidence of sulfenylsulfinamide (RS-S(O)-NH₂) formation, a pathway expected by analogy with the known reactivity of HNO with thiol.

Updating NO•/HNO interconversion under physiological conditions: A biological implication overview

Azanone (HNO/NO^-), also called nitroxyl, is a highly reactive compound whose biological role is still a matter of debate. A key issue that remains to be clarified regarding HNO and its biological activity is that of its endogenous formation. Given the overlap of the molecular targets and reactivity of nitric oxide (NO•) and HNO, its chemical biology was perceived to be similar to that of NO• as a biological signaling agent. However, despite their closely related reactivity, NO• and HNO's biochemical pathways are quite different. Moreover, the reduction of nitric oxide to azanone is possible but necessarily coupled to other reactions, which drive the reaction forward, overcoming the unfavorable thermodynamic barrier. The mechanism of this NO•/HNO interplay and its downstream effects in different contexts were studied recently, showing that more than fifteen moderate reducing agents react with NO• producing HNO. Particularly, it is known that the reaction between nitric oxide and hydrogen sulfide (H₂S) produces HNO. However, this rate constant was not reported yet. In this work, firstly the NO•/H₂S effective rate constant was measured as a function of the pH. Then, the implications of these chemical (non-enzymatic), biologically compatible, routes to endogenous HNO formation was discussed. There is no doubt that HNO could be (is?) a new endogenously produced messenger that mediates specific physiological responses, many of which were attributed yet to direct NO• effects.

A recent history of nitroxyl chemistry, pharmacology and therapeutic potential

Due to the excitement surrounding the discovery of NO as an endogenously generated signalling molecule, a number of other nitrogen oxides were also investigated as possible physiological mediators. Among these was nitroxyl (HNO). Over the past 25 years or so, a significant amount of work by this laboratory and many others has disclosed that HNO possesses unique chemical properties and important pharmacological utility. Indeed, the pharmacological potential for HNO as a treatment for heart failure, among other uses, has garnered this curious molecule a considerable amount of recent attention. This review summarizes the events that led to this recent attention as well as poses important questions that are still to be answered with regards to understanding the chemistry and biology of HNO. LINKED ARTICLES: This article is part of a themed section on Nitric Oxide 20 Years from the 1998 Nobel Prize. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.2/issuetoc.

Fluorescent probes for the detection of nitroxyl (HNO)

Nitroxyl (HNO), which according to the IUPAC recommended nomenclature should be named azanone, is the protonated one-electron reduction product of nitric oxide. Recently, it has gained a considerable attention due to the interesting pharmacological effects of its donors. Although there has been great progress in the understanding of HNO chemistry and chemical biology, it still remains the most elusive reactive nitrogen species, and its selective detection is a real challenge. The development of reliable methodologies for the direct detection of azanone is essential for the understanding of important signaling properties of this reactive intermediate and its pharmacological potential. Over the last decade, there has been considerable progress in the development of low-molecular-weight fluorogenic probes for the detection of HNO, and therefore, in this review, we have focused on the challenges and limitations of and perspectives on nitroxyl detection based on the use of such probes.

Deciphering Stability of Five-Membered Heterocyclic Radicals: Balancing Act Between Delocalization and Ring Strain

Computational studies on five-membered heterocycles with single heteroatom and their isomeric dehydro-borole 1a-1c, cyclopentadiene 2a-2c, pyrrole 3a-3c, furan 4b-4c, phosphole 5a-5c, and thiophene 6b-6c radicals have been carried out. Geometrical aspects through ground state electronic structures and stability aspects using bond dissociation energies (BDE) and radical stabilization energies (RSE) have been envisaged in this regard. Spin densities, electrostatic potentials (ESP), and natural bond orbital (NBO) analysis unveiled the extent of spin delocalization. The estimated nucleus-independent chemical shifts (NICS) values revealed the difference in aromaticity characteristics of radicals. Particularly the heteroatom centered radicals exhibit odd electron π-delocalized systems with a quasi-antiaromatic character. Various factors such as, the relative position of the radical center with respect to heteroatoms, resonance, ring strain and orbital interactions influence the stability that follows the order: heteroatom centered > β-centered > α-centered radicals. Among the influences of various factors, we confirmed the existence of a competition between delocalization and the ring strain, and the interplay of both decides the overall stability order.

Development and cellular application of visible-light-controllable HNO releasers based on caged Piloty's acid

Novel visible-light-controllable HNO releasers was developed based on a caged form of Piloty's acid, and applied for cellular systems.

Selenols are resistant to irreversible modification by HNO

The discovery of nitric oxide (NO) as an endogenously generated signaling species in mammalian cells has spawned a vast interest in the study of the chemical biology of nitrogen oxides. Of these, nitroxyl (azanone, HNO) has gained much attention for its potential role as a therapeutic for cardiovascular disease. Known targets of HNO include hemes/heme proteins and thiols/thiol-containing proteins. Recently, due to their roles in redox signaling and cellular defense, selenols and selenoproteins have also been speculated to be additional potential targets of HNO. Indeed, as determined in the current work, selenols are targeted by HNO. Such reactions appear to result only in formation of diselenide products, which can be easily reverted back to the free selenol. This characteristic is distinct from the reaction of HNO with thiols/thiolproteins. These findings suggest that, unlike thiolproteins, selenoproteins are resistant to irreversible oxidative modification, support that Nature may have chosen to use selenium instead of sulfur in certain biological systems for its enhanced resistance to electrophilic and oxidative modification.

The chemical biology of HNO signaling

Nitroxyl (HNO) is a simple molecule with significant potential as a pharmacological agent. For example, its use in the possible treatment of heart failure has received recent attention due to its unique therapeutic properties. Recent progress has been made on the elucidation of the mechanisms associated with its biological signaling. Importantly, the biochemical mechanisms described for HNO bioactivity are consistent with its unique and novel chemical properties/reactivity. To date, much of the biology of HNO can be associated with interactions and modification of important regulatory thiol proteins. Herein will be provided a description of HNO chemistry and how this chemistry translates to some of its reported biological effects.

Comparison of Reductive Ligation-Based Detection Strategies for Nitroxyl (HNO) and S-Nitrosothiols

Phosphine-based detection strategies for both nitroxyl (HNO) and S-nitrosothiols (RSNO) were investigated and compared. Phosphorus NMR studies show that azaylides derived from HNO or organic RSNO efficiently participate in subsequent reductive ligation required for fluorescence generation in properly substituted substrates. S-Azaylides derived from biological RSNO containing free amine and carboxylic acid groups primarily yield phosphine oxides suggesting these groups facilitate nonligation pathways such as hydrolysis. The fluorescence response of a phosphine-based fluorophore toward the same RSNO confirms these differences and indicates that these probes selectively react with HNO. Flow cytometry experiments in HeLa cells reinforce the reactivity difference and offer a potential fast screening approach for endogenous HNO sources.

Mechanistic studies of the reactions of the reduced vitamin B12 derivatives with the HNO donor Piloty's acid: further evidence for oxidation of cob(I)alamin by (H)NO

There is accumulating evidence for the existence of HNO in biological systems. Compared with NO (˙NO), much less is known about the chemical and biochemical reactivity of HNO. Kinetic and mechanistic studies have been carried out on the reaction between the vitamin B12-derived radical complex cob(II)alamin (Cbl(II)˙, Cbl(II)) with the widely used HNO donor Piloty's acid (PA). A stoichiometry of 1 : 2 Cbl(II) : PA was obtained and PA decomposition to HNO and benzenesulfinate (C6H5SO2(-)) is the rate-determining step. No evidence was found for nitrite (Griess assay), ammonia (Nessler's test) or NH2OH (indooxine test) in the product solution, and it is likely that HNO is instead reduced to N2. A mechanism is proposed in which reduction of Cbl(II) by (H)NO results in formation of cob(I)alamin (Cbl(I)(-)) and ˙NO. The Cbl(I)(-) intermediate is subsequently oxidized back to Cbl(II) by a second (H)NO molecule, and Cbl(II) reacts rapidly with ˙NO to form nitroxylcobalamin (NOCbl). Separate studies on the reaction between Cbl(I)(-) and PA shows that this system involves an additional step in which Cbl(I)(-) is first oxidized by (H)NO to Cbl(II), which reacts further with (H)NO to form NOCbl, with an overall stoichiometry of 1 : 3 Cbl(I)(-) : PA. Experiments in the presence of nitrite for both systems support the involvement of a Cbl(I)(-) intermediate in the Cbl(II)/PA reaction. These systems provide the second example of oxidation of cob(I)alamin by (H)NO.

Ring expansions of acyloxy nitroso compounds

Treatment of cyclopentanone and cyclobutanone-derived oximes with lead (IV) tetraacetate gives the bright blue acyloxy nitroso compounds, which upon basic hydrolysis yields the ring expansion product cyclic hydroxamic acids in 12-81% yield. Reactions of substituted cyclopentanones provide ring expanded products where the -NOH group regioselectively inserts to the more substituted position and gives a better yield compared to the treatment of the same ketone with a basic solution of Piloty's acid. Reaction of phosphines with acyloxy nitroso compounds generally generates a ring-expanded Beckmann rearrangement product that can be hydrolyzed to the corresponding lactam. Acyloxy nitroso compounds that undergo rapid hydrolysis to HNO do not show this ring expansion reactivity. These results further demonstrate the versatility of acyloxy nitroso compound to yield structurally complex materials.

Breathing new life into nitric oxide signaling: A brief overview of the interplay between oxygen and nitric oxide

Nitric oxide (^•NO, nitrogen monoxide) is one of the most unique biological signaling molecules associated with a multitude of physiologic and pathological conditions. In order to fully appreciate its numerous roles, it is essential to understand its basic biochemical properties. Most signaling effector molecules such as steroids or proteins have a significant life-span and function through classical receptor–ligand interactions. ^•NO, however, is a short-lived free-radical gas that only reacts with two types of molecules under biological conditions; metals and other free radicals. These simple interactions can lead to a myriad of complex intermediates which in turn have their own phenotypic effects. For these reasons, responses to ^•NO often appear to be random or contradictory when outcomes are compared across various experimental settings. This article will serve as a brief overview of the chemical, biological, and microenvironmental factors that dictate ^•NO signaling with an emphasis on ^•NO metabolism. The prominent role that oxygen (dioxygen, O₂) plays in ^•NO metabolism and how it influences the biological effects of ^•NO will be highlighted. This information and these concepts are intended to help students and investigators think about the interpretation of data from experiments where biological effects of ^•NO are being elucidated.

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Oxygen is a major determinant of the rates of nitric oxide synthesis and metabolism.

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Under biological conditions nitric oxide only reacts with metals and other free radicals.

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Oxygen determines the half-life, concentration, and diffusional distance of nitric oxide.

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Proteins respond to nitric oxide in a concentration and time-dependent manner.

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Oxygen and the redox environment will greatly influence signaling responses to nitric oxide.

Redox properties of the nitronyl nitroxide antioxidants studied via their reactions with nitroxyl and ferrocyanide

Nitronyl nitroxides (NNs) are the paramagnetic probes that are capable of scavenging physiologically relevant reactive oxygen (ROS) and nitrogen (RNS) species, namely superoxide, nitric oxide (NO), and nitroxyl (HNO). NNs are increasingly considered as potent antioxidants and potential therapeutic agents. Understanding redox chemistry of the NNs is important for their use as antioxidants and as paramagnetic probes for discriminative detection of NO and HNO by electron paramagnetic resonance (EPR) spectroscopy. Here we investigated the redox properties of the two most commonly used NNs, including determination of the equilibrium and rate constants of their reduction by HNO and ferrocyanide, and reduction potential of the couple NN/hydroxylamine of nitronyl nitroxide (hNN). The rate constants of the reaction of the NNs with HNO were found to be equal to (1-2) × 10(4) M(-1)s(- 1) being close to the rate constants of scavenging superoxide and NO by NNs. The reduction potential of the NNs and iminonitroxides (INs, product of NNs reaction with NO) were calculated based on their reaction constants with ferrocyanide. The obtained values of the reduction potential for NN/hNN (E'0 ≈ 285 mV) and IN/hIN (E' ≈ 495 mV) are close to the corresponding values for vitamin C and vitamin E, correspondingly. The "balanced" scavenging rates of the NNs towards superoxide, NO, and HNO, and their low reduction potential being thermodynamically close to the bottom of the pecking order of oxidizing radicals, might be important factors contributing into their antioxidant activity.

The concomitant coronary vasodilator and positive inotropic actions of the nitroxyl donor Angeli's salt in the intact rat heart: contribution of soluble guanylyl cyclase-dependent and -independent mechanisms

BACKGROUND AND PURPOSE

The NO redox sibling nitroxyl (HNO) elicits soluble guanylyl cyclase (sGC)-dependent vasodilatation. HNO has high reactivity with thiols, which is attributed with HNO-enhanced left ventricular (LV) function. Here, we tested the hypothesis that the concomitant vasodilatation and inotropic actions induced by a HNO donor, Angeli's salt (sodium trioxodinitrate), were sGC-dependent and sGC-independent respectively.

EXPERIMENTAL APPROACH

Haemodynamic responses to Angeli's salt (10 pmol-10 μmol), alone and in the presence of scavengers of HNO (L-cysteine, 4 mM) or of NO [hydroxocobalamin (HXC), 100 μM] or a selective inhibitor of sGC [1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), 10 μM], a CGRP receptor antagonist (CGRP8-37 , 0.1 μM) or a blocker of voltage-dependent potassium channels [4-aminopyridine (4-AP), 1 mM] were determined in isolated hearts from male rats.

KEY RESULTS

Angeli's salt elicited concomitant, dose-dependent increases in coronary flow and LV systolic and diastolic function. Both L-cysteine and ODQ shifted (but did not abolish) the dose-response curve of each of these effects to the right, implying contributions from HNO and sGC in both the vasodilator and inotropic actions. In contrast, neither HXC, CGRP8-37 nor 4-AP affected these actions.

CONCLUSIONS AND IMPLICATIONS

Both vasodilator and inotropic actions of the HNO donor Angeli's salt were mediated in part by sGC-dependent mechanisms, representing the first evidence that sGC contributes to the inotropic and lusitropic action of HNO in the intact heart. Thus, HNO acutely enhances LV contraction and relaxation, while concomitantly unloading the heart, potentially beneficial actions in failing hearts.

A proposed abiotic reaction scheme for hydroxylamine and monochloramine under chloramination relevant drinking water conditions

Drinking water monochloramine (NH2Cl) use may promote ammonia-oxidizing bacteria (AOB). AOB use (i) ammonia monooxygenase for biological ammonia (NH3) oxidation to hydroxylamine (NH2OH) and (ii) hydroxylamine oxidoreductase for NH2OH oxidation to nitrite. NH2Cl and NH2OH may react, providing AOB potential benefits and detriments. The NH2Cl/NH2OH reaction would benefit AOB by removing the disinfectant (NH2Cl) and releasing their growth substrate (NH3), but the NH2Cl/NH2OH reaction would also provide a possible additional inactivation mechanism besides direct NH2Cl reaction with cells. Because biological NH2OH oxidation supplies the electrons required for biological NH3 oxidation, the NH2Cl/NH2OH reaction provides a direct mechanism for NH2Cl to inhibit NH3 oxidation, starving the cell of reductant by preventing biological NH2OH oxidation. To investigate possible NH2Cl/NH2OH reaction implications on AOB, an understanding of the underlying abiotic reaction is first required. The present study conducted a detailed literature review and proposed an abiotic NH2Cl/NH2OH reaction scheme (RS) for chloramination relevant drinking water conditions (μM concentrations, air saturation, and pH 7-9). Next, RS literature based kinetics and end-products were evaluated experimentally between pHs 7.7 and 8.3, representing (i) the pH range for future experiments with AOB and (ii) mid-range pHs typically found in chloraminated drinking water. In addition, a (15)N stable isotope experiment was conducted to verify nitrous oxide and nitrogen gas production and their nitrogen source. Finally, the RS was slightly refined using the experimental data and an AQUASIM implemented kinetic model. A chloraminated drinking water relevant RS is proposed and provides the abiotic reaction foundation for future AOB biotic experiments.

The effects of nitroxyl (HNO) on H₂O₂ metabolism and possible mechanisms of HNO signaling

Nitroxyl (HNO) possesses unique and potentially important biological/physiological activity that is currently mechanistically ill-defined. Previous work has shown that the likely biological targets for HNO are thiol proteins, oxidized metalloproteins (i.e. ferric heme proteins) and, most likely, selenoproteins. Interestingly, these are the same classes of proteins that interact with H2O2. In fact, these classes of proteins not only react with H2O2, and thus potentially responsible for the signaling actions of H2O2, but are also responsible for the degradation of H2O2. Therefore, it is not unreasonable to speculate that HNO can affect H2O2 degradation by interacting with H2O2-degrading proteins possibly leading to an increase in H2O2-mediated signaling. Moreover, considering the commonality between HNO and H2O2 biological targets, it also seems likely that HNO-mediated signaling can also be due to reactivity at otherwise H2O2-reactive sites. Herein, it is found that HNO does indeed inhibit H2O2 degradation via inhibition of H2O2-metaboilizing proteins. Also, it is found that in a system known to be regulated by H2O2 (T cell activation), HNO behaves similarly to H2O2, indicating that HNO- and H2O2-signaling may be similar and/or intimately related.

Discriminative EPR detection of NO and HNO by encapsulated nitronyl nitroxides

Nitric oxide, •NO, is one of the most important molecules in the biochemistry of living organisms. By contrast, nitroxyl, NO-, one-electron reduced analog of •NO which exists at physiological conditions in its protonated form, HNO, has been relatively overlooked. Recent data show that HNO might be produced endogenously and display unique biological effects. However, there is a lack of specific and quantitative methods of detection of endogenous HNO production. Here we present a new method for discriminative •NO and HNO detection by nitronyl nitroxides (NNs) using electron paramagnetic resonance (EPR). It was found that NNs react with •NO and HNO with similar rate constants of about 10(4) M(-1) s(-1) but yield different products: imino nitroxides and the hydroxylamine of imino nitroxides, correspondingly. An EPR approach for discriminative •NO and HNO detection using liposome-encapsulated NNs was developed. The membrane barrier of liposomes protects NNs against reduction in biological systems while is permeable to both analytes, •NO and HNO. The sensitivity of this approach for the detection of the rates of •NO/HNO generation is about 1 nM/s. The application of encapsulated NNs for real-time discriminative •NO/HNO detection might become a valuable tool in nitric oxide-related studies.

The use of cyclic nitroxide radicals as HNO scavengers

Reduction of cyclic stable nitroxides (RNO) by HNO to the respective hydroxylamines (RNO-H) has been demonstrated using EPR spectrometry. HNO shows low reactivity toward piperidine, pyrrolidine and nitronyl nitroxides with rate constants below 1.4 × 10(5)M(-1)s(-1) at pH 7.0, despite the high driving force for these reactions. The rate constants can be predicted assuming that the reactions take place via a concerted proton-electron transfer pathway and significantly low self-exchange rate constants for HNO/NO and RNO-H/RNO. NO does not react with piperidine and pyrrolidine nitroxides, but does add to HNO forming the highly oxidizing and moderately reducing hyponitrite radicals. In this work, the radicals are produced by pulse radiolysis and the rate constants of their reactions with 2,2,6,6,-tetramethylpiperidine-1-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl (TEMPOL) and 3-carbamoyl-PROXYL have been determined at pH 6.8 to be (2.4 ± 0.2)× 10(6), (9.8 ± 0.2)× 10(5), (5.9 ± 0.5)× 10(5)M(-1)s(-1), respectively. This low reactivity implies that NO competes efficiently with these nitroxides for the hyponitrite radical. The ability of TEMPOL and 2-(4-carboxyphenyl)-4,4,5,5,-tetramethyl-imidazoline-1-oxyl-3-oxide (C-PTIO) to oxidize HNO and their different reactivity toward NO are used to quantify HNO formed via acetohydroxamic acid oxidation. The extent of TEMPOL or C-PTIO reduction was similar to the yield of HNO formed upon oxidation by ()OH under anoxia, but not by the metmyoglobin and H(2)O(2) reaction system where both nitroxides catalytically facilitate H(2)O(2) depletion and nitrite accumulation. In this system the conversion of C-PTIO into 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (C-PTI) is a minor reaction, which does not provide any mechanistic insight.

Small molecule signaling agents: the integrated chemistry and biochemistry of nitrogen oxides, oxides of carbon, dioxygen, hydrogen sulfide, and their derived species

Several small molecule species formally known primarily as toxic gases have, over the past 20 years, been shown to be endogenously generated signaling molecules. The biological signaling associated with the small molecules NO, CO, H₂S (and the nonendogenously generated O₂), and their derived species have become a topic of extreme interest. It has become increasingly clear that these small molecule signaling agents form an integrated signaling web that affects/regulates numerous physiological processes. The chemical interactions between these species and each other or biological targets is an important factor in their roles as signaling agents. Thus, a fundamental understanding of the chemistry of these molecules is essential to understanding their biological/physiological utility. This review focuses on this chemistry and attempts to establish the chemical basis for their signaling functions.

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.

HNO signaling mechanisms

Due to recent discoveries of important and novel biological activity, nitroxyl (HNO) has become a molecule of significant interest. Although it has been used in the past as a treatment for alcoholism, it is currently being touted as a treatment for heart failure. It is becoming increasingly clear that many of the biological actions of HNO can be attributed to its ability to react with specific thiol- and, possibly, heme-proteins. Herein is discussed the chemistry of HNO with likely biological targets. A particular focus is given to targets associated with the pharmacological utility of HNO as a cardiovascular agent and for the treatment of alcoholism.

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.