A comparison of the chemistry associated with the biological signaling and actions of nitroxyl (HNO) and nitric oxide (NO)

Nitroxyl donating and visualization with a coumarin-based fluorescence probe

Nitroxyl (HNO), the single-electron reduction product of nitric oxide (NO), has attracted great interest in the treatment of congestive heart failure in clinical trials. In this paper, we describe the first coumarin-based compound N-hydroxy-2-oxo-2H-chromene-6-sulfonamide (CD1) as a dualfunctional HNO donor, which can release both an HNO signaling molecule and a fluorescent reporter. Under physiological conditions (pH 7.4 and 37 °C), the CD1 HNO donor can readily decompose with a half-life of ∼90 min. The corresponding stoichiometry HNO from the CD1 donor was confirmed using both Vitamin B12 and phosphine compound traps. In addition to HNO releasing, specifically, the degradation product 2-oxo-2H-chromene-6-sulfinate (CS1) was generated as a fluorescent marker during the decomposition. Therefore, the HNO amount released in situ can be accurately monitored through fluorescence generation. As compared to the CD1 donor, the fluorescence intensity increased by about 4.9-fold. The concentration limit of detection of HNO releasing was determined to be ∼0.13 μM according to the fluorescence generation of CS1 at physiological conditions. Moreover, the bioimaging of the CD1 donor was demonstrated in the cell culture of HeLa cells, where the intracellular fluorescence signals were observed, inferring the site of HNO release. Finally, we anticipate that this novel coumarin-based CD1 donor opens a new platform for exploring the biology of HNO.

Rational design of Near-Infrared fluorescent probe for monitoring HNO in plants

Nitroxyl (HNO), a reactive nitrogen species (RNS), is essential for plant growth. However, the action of HNO in plants has been difficult to understand due to the lack of highly sensitive and real-time in-situ monitoring tools. Herein, we presented a near-infrared fluorescent probe, DCI-HNO, based on dicyanoisophorone fluorophore, for real-time mapping HNO in plants. The introduction of a phosphine moiety as a specific HNO recognition unit can inhibit the intramolecular charge transfer (ICT) of probe DCI-HNO. However, in the presence of HNO, the ICT process occurred, leading to the emission at 665 nm. Probe DCI-HNO exhibited high sensitivity (97 nM), rapid response time (8 min), large Stokes shift (135 nm) for detection of HNO in plants. The novel developed probe has successfully imaged endogenous HNO produced during NO/H2S cross-talk in plant tissues. Additionally, the up-regulated in HNO levels during tobacco aging and in response to stress has been confirmed. Therefore, probe DCI-HNO has provided a reliable method for monitoring the NO/H2S cross-talk and revealing the role of HNO in plants.

Azobenzene-based colorimetric and fluorometric chemosensor for nitroxyl releasing

The precise release and characterization of nitroxyl (HNO) gas signaling molecule remain a challenge due to its short lifetime to date. To solve this issue, an azobenzene-based HNO donor (Azo-D1) was proposed as a colorimetric and fluorometric chemosensor for HNO releasing, to release both HNO and an azobenzene fluorescent reporter together. Specifically, the Azo-D1 has an HNO release half-life of ∼68 min under physiological conditions. The characteristic color change from the original orange to the yellow color indicated the decomposition of the donor molecule. In addition, the stoichiometry release of HNO was qualitatively and quantitatively verified through the classical phosphine compound trap. As compared with the donor molecule by itself, the decomposed product demonstrates a maximum fluorescence emission at 424 nm, where the increase of fluorescence intensity by 6.8 times can be applied to infer the real-time concentration of HNO. Moreover, cellular imaging can also be achieved using this Azo-D1 HNO donor through photoexcitation at 405 and 488 nm, where the real-time monitoring of HNO release was achieved without consuming the HNO source. Finally, the Azo-D1 HNO donor would open a new platform in the exploration of the biochemistry and the biology of HNO.

A Near-Infrared Fluorescent Probe for the Rapid Detection of Nitroxyl in Living Cells

Nitroxyl (HNO) plays an important role in various physiological activities. It has the potential to be used as a treatment for certain diseases such as alcohol poisoning, acute hypertension, and atherosclerosis. However, traditional methods for detecting HNO are challenging due to its rapid polymerization and elimination into N2O. Therefore, it is crucial to establish direct and effective HNO detection methods to comprehend these physiological processes better. In this study, a new near-infrared fluorescent probe called HXM-P based on the intramolecular charge transfer (ICT) mechanism was designed and synthesized. This probe employs 2-((6-hydroxy-2,3dihydro-1 H-xanthen-4-yl)methylene)malononitrile as a fluorophore and 2-(diphenylphosphine) benzoate as a recognition group. The results showed that probe HXM-P can detect HNO with high sensitivity (1.07 × 10- 8 M). A good linear correlation was observed between the fluorescence intensities at 640 nm and the concentrations of HNO in the range of 0-80 µM (R2 = 0.997). Moreover, probe HXM-P exhibited a rapid response rate (within 15 s) toward HNO, and the fluorescent intensity reached a plateau within 5 min, making it easier to track the highly reactive and short-lived HNO in living systems. Additionally, HXM-P was successfully employed for imaging HNO in HepG2 cells.

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.

Synthesis and nitroxyl (HNO) donating properties of benzoxadiazole-based Piloty's acids

Developing functional nitroxyl (HNO) donors play a significant role in the further exploration of endogenous HNO in biochemistry and pharmacology. In this work, two novel Piloty's acids (SBD-D1 and SBD-D2) were proposed by incorporating benzoxadiazole-based fluorophores, in order to achieve the dual-function of releasing both HNO and a fluorophore in situ. Under physiological conditions, both SBD-D1 and SBD-D2 efficiently donated HNO (t_1/2 = 10.96 and 8.18 min, respectively). The stoichiometric generation of HNO was determined by both vitamin B₁₂ and phosphine compound traps. Interestingly, due to the different substitution groups on the aromatic ring, SBD-D1 with the chlorine showed no fluorescence emission, but SBD-D2 was strongly fluorescent due to the presence of the dimethylamine group. Specifically, the fluorescent signal would decrease during the release process of HNO. Moreover, theoretical calculations were performed to understand the emission difference. A strong radiation derived from benzoxadiazole with dimethylamine group due to the large transition dipole moment (∼4.3 Debye), while the presence of intramolecular charge transfer process in the donor with chlorine group caused a small transition dipole moment (<0.1 Debye). Finally, these studies would contribute to the future design and application of novel functional HNO donors for the exploration of HNO biochemistry and pharmacology.

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.

Chemoselective Primary Amination of Aryl Boronic Acids by PIII/PV═O-Catalysis: Synthetic Capture of the Transient Nef Intermediate HNO

A catalytic approach to intercept the transient HNO for a chemoselective primary amination of arylboronic acids is reported. A phosphetane-based catalyst operating within P^III/P^V═O redox cycling is shown to capture HNO, generated in situ by Nef decomposition of 2-nitropropane, to selectively install the primary amino group at aryl C_sp2 centers. The method furnishes versatile primary arylamines from arylboronic acid substrates with the preservation of otherwise reactive functional groups.

Hydropersulfides (RSSH) and Nitric Oxide (NO) Signaling: Possible Effects on S-Nitrosothiols (RS-NO)

S-Nitrosothiol (RS-NO) formation in proteins and peptides have been implicated as factors in the etiology of many diseases and as possible regulators of thiol protein function. They have also been proposed as possible storage forms of nitric oxide (NO). However, despite their proposed functions/roles, there appears to be little consensus regarding the physiological mechanisms of RS-NO formation and degradation. Hydropersulfides (RSSH) have recently been discovered as endogenously generated species with unique reactivity. One important reaction of RSSH is with RS-NO, which leads to the degradation of RS-NO as well as the release of NO. Thus, it can be speculated that RSSH can be a factor in the regulation of steady-state RS-NO levels, and therefore may be important in RS-NO (patho)physiology. Moreover, RSSH-mediated NO release from RS-NO may be a possible mechanism allowing RS-NO to serve as a storage form of NO.

Exploring the Potential of 2-(2-Nitrophenyl)ethyl-Caged N-Hydroxysulfonamides for the Photoactivated Release of Nitroxyl (HNO)

The emergence of nitroxyl (HNO) as a biological signaling molecule is attracting increasing attention. HNO-based prodrugs show considerable potential in treating congestive heart failure, with HNO reacting rapidly with metal centers and protein-bound and free thiols. A new class of 2-(2-nitrophenyl)ethyl (2-NPE)-photocaged N-hydroxysulfonamides has been developed, and the mechanisms of photodecomposition have been investigated. Three photodecomposition pathways are observed: the desired concomitant C-O/N-S bond cleavage to generate HNO, sulfinate, and 2-nitrostyrene, C-O bond cleavage to give the parent sulfohydroxamic acid and 2-nitrostyrene, and O-N bond cleavage to release a sulfonamide and 2-nitrophenylacetaldehyde. Laser flash photolysis experiments provide support for a Norrish type II mechanism involving 1,5-hydrogen atom abstraction to generate an aci-nitro species. A mechanism is proposed in which the (Z)-aci-nitro intermediate undergoes either C-O bond cleavage to release RSO₂NHO(H), concerted C-O/N-S bond cleavage to generate sulfinate and HNO, or isomerization to the (E)-isomer prior to O-N bond cleavage. The pK_a of the N(H) of the N-hydroxysulfonamide plays a key role in determining whether C-O or concerted C-O/N-S bond cleavage occurs. Deprotonating this site favors the desired C-O/N-S bond cleavage at the expense of an increased level of undesired O-N bond cleavage. Triplet state quenchers have no effect on the observed photoproducts.

Vasoprotective Actions of Nitroxyl (HNO): A Story of Sibling Rivalry

ABSTRACT

Nitroxyl (HNO), the 1 electron-reduced and protonated form of nitric oxide (NO•), has emerged as a nitrogen oxide with a suite of vasoprotective properties and therapeutic advantages over its redox sibling. Although HNO has garnered much attention due to its cardioprotective actions in heart failure, its ability to modulate vascular function, without the limitations of tolerance development and NO• resistance, is desirable in the treatment of vascular disease. HNO serves as a potent vasodilator and antiaggregatory agent and has an ability to limit vascular inflammation and reactive oxygen species generation. In addition, its resistance to scavenging by reactive oxygen species and ability to target distinct vascular signaling pathways (Kv, KATP, and calcitonin gene-related peptide) contribute to its preserved efficacy in hypertension, diabetes, and hypercholesterolemia. In this review, the vasoprotective actions of HNO will be compared with those of NO•, and the therapeutic utility of HNO donors in the treatment of angina, acute cardiovascular emergencies, and chronic vascular disease are discussed.

Nitric Oxide Reacts Very Fast with Hydrogen Sulfide, Alcohols, and Thiols to Produce HNO: Revised Rate Constants

The chemical reactivity of NO and its role in several biological processes seem well established. Despite this, the chemical reduction of ^•NO toward HNO has been historically discarded, mainly because of the negative reduction potential of NO. However, this value and its implications are nowadays under revision. The last reported redox potential, E'(NO,H⁺/HNO), at micromolar and picomolar concentrations of ^•NO and HNO, respectively, is between -0.3 and 0 V at pH 7.4. This potential implies that the one-electron-reduction process for NO is feasible under biological conditions and could be promoted by well-known biological reductants with reduction potentials of around -0.3 to -0.5 V. Moreover, the biologically compatible chemical reduction of ^•NO (nonenzymatic), like direct routes to HNO by alkylamines, aromatic and pseudoaromatic alcohols, thiols, and hydrogen sulfide, has been extensively explored by our group during the past decade. The aim of this work is to use a kinetic modeling approach to analyze electrochemical HNO measurements and to report for the first-time direct reaction rate constants between ^•NO and moderate reducing agents, producing HNO. These values are between 5 and 30 times higher than the previously reported k_eff values. On the other hand, we also showed that reaction through successive attack by two NO molecules to biologically compatible compounds could produce HNO. After over 3 decades of intense research, the ^•NO chemistry is still there, ready to be discovered.

Helpful correlations to estimate the pKa of coordinated HNO: a potential-pH exploration in a pendant-arm cyclam-based ruthenium nitroxyl

The acid-base speciation of coordinated azanone (HNO) remains a highly relevant topic in bioinorganic chemistry. Ruthenium nitroxyl complexes with sufficient robustness towards ligand loss have gained significance as operating platforms to delve into such studies. In this work, we revisit an octahedral {RuNO}⁶ complex containing the cyclam-based pentadentate ligand L^py = 1-(pyridine-2-ylmethyl)-1,4,8,11-tetraazacyclotetradecane and explore the thermodynamic and spectroscopic aspects of its reduced states in aqueous media. Upon in situ electro-generation of the bound HNO moiety, we have undertaken different strategies to determine both its acidity and electrochemical properties. This robust HNO complex does not undergo deprotonation in a wide pH range. We have found pK_a ([Ru(L^py)(HNO)]²⁺) = 13.0 ± 0.1 and . There are indications that pK_a (HNO) values in several ruthenium-based species correlate with the redox potential associated with the {RuNO}^6,7 and {RuNO}^7,8 couples. The present pK_a extends the range of acidity of bound HNO to more than five pH units, confirming a remarkable sensitivity to the nature of the coordination sphere. This result lays new foundations to continue rational ligand design that may contribute to a better understanding of the different biological roles of both HNO and NO^- by investigating key chemical aspects of model complexes.

A novel NIR fluorescent probe for the highly sensitive detection of HNO and its application in bioimaging

A “naked-eye” HNO probe based on xanthene was obtained.

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.

Cardiovascular Therapeutic Potential of the Redox Siblings, Nitric Oxide (NO•) and Nitroxyl (HNO), in the Setting of Reactive Oxygen Species Dysregulation

Reactive oxygen species (ROS) dysregulation is a hallmark of cardiovascular disease, characterised by an imbalance in the synthesis and removal of ROS. ROS such as superoxide (•O₂^-), hydrogen peroxide (H₂O₂), hydroxyl (OH•) and peroxynitrite (ONOO^-) have a marked impact on cardiovascular function, contributing to the vascular impairment and cardiac dysfunction associated with diseases such as angina, hypertension, diabetes and heart failure. Central to the vascular dysfunction is a reduction in bioavailability and/or physiological effects of vasoprotective nitric oxide (NO•), leading to vasoconstriction, inflammation and vascular remodelling. In a cardiac context, increased ROS generation can also lead to modification of key proteins involved in cardiac contractility. Whilst playing a key role in the pathogenesis of cardiovascular disease, ROS dysregulation also limits the clinical efficacy of current therapies, such as nitrosovasodilators. As such, alternate therapies are sought. This review will discuss the impact of ROS dysregulation on the therapeutic utility of NO• and its redox sibling, nitroxyl (HNO). Both nitric oxide (NO) and nitroxyl (HNO) donors signal through soluble guanylyl cyclase (sGC). NO binds to the Fe(II) form of sGC and nitroxyl possibly to both sGC heme and thiol groups. In the vasculature, nitroxyl can also signal through voltage-dependent (K_v) and ATP-sensitive (K_ATP) K⁺ channels as well as calcitonin gene-related peptide (CGRP). In the heart, HNO directly targets critical thiols to increase myocardial contractility, an effect not seen with NO. The qualitative effects via elevation of cGMP are similar, i.e. lusitropic in the heart and inhibitory on vasoconstriction, inflammation, aggregation and vascular remodelling. Of pathophysiological significance is the fact the efficacy of NO donors is impaired by ROS, e.g. through chemical scavenging of NO, to generate reactive nitrogen oxide species (RNOS), whilst nitroxyl is apparently not.

A caged 2-hydroxyethyl luciferin for bioluminescence imaging of nitroxyl in living cells

Nitroxyl (HNO), a one-electron reduction product of nitric oxide, demonstrates distinct biological and pharmacological activities. Here we designed a bioluminescent turn-on probe, HNO-8, that could be used to visualize HNO without the need for excitation light. HNO-8 was prepared by caging 2-hydroxyethyl luciferin with a triphenylphosphine unit, in which 2-hydroxyethyl luciferin as a novel substrate of firefly luciferase was characterized by stronger and more sustained bioluminescent signals than the most popular substrates of d-luciferin and 6'-aminoluciferin. In vitro experiments showed that HNO-8 could selectively respond to HNO generated from Angeli's salt(AS) in the range 1-50 μM, with a limit of detection of 0.196 μM. The probe was successfully applied for visualizing HNO in luciferase-transfected Huh7 cancer cells. We envision that HNO-8 could be used as a powerful bioluminescent sensor for researching HNO biological roles.

A nitroxyl-responsive near-infrared fluorescent chemosensor for visualizing H2S/NO crosstalk in biological systems

We present a near-infrared (NIR) fluorescent probe, NR-HNO, which was successfully applied to visualizing H2S/NO "crosstalk" by the fluorescence detection of nitroxyl with a fast response time (5 min) and a large Stokes shift (131 nm) in living cells and tissue; it was also used to image nitroxyl in live mice.

Dansyl-appended CuII-complex-based nitroxyl (HNO) sensing with living cell imaging application and DFT studies

We introduce herein, a novel copper complex-based fluorescent probe[Cu^II(DQ₄₆₈)Cl]⁺that exhibits a significant fluorescence turn-on response towards nitroxyl with high selectivity over other biological reactive oxygen, nitrogen and sulfur species, including nitric oxide.

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.

Non-heme High-Spin {FeNO}6-8 Complexes: One Ligand Platform Can Do It All

Heme and non-heme iron-nitrosyl complexes are important intermediates in biology. While there are numerous examples of low-spin heme iron-nitrosyl complexes in different oxidation states, much less is known about high-spin (hs) non-heme iron-nitrosyls in oxidation states other than the formally ferrous NO adducts ({FeNO}⁷ in the Enemark-Feltham notation). In this study, we present a complete series of hs-{FeNO}^6-8 complexes using the TMG₃tren coligand. Redox transformations from the hs-{FeNO}⁷ complex [Fe(TMG₃tren)(NO)]²⁺ to its {FeNO}⁶ and {FeNO}⁸ analogs do not alter the coordination environment of the iron center, allowing for detailed comparisons between these species. Here, we present new MCD, NRVS, XANES/EXAFS, and Mössbauer data, demonstrating that these redox transformations are metal based, which allows us to access hs-Fe(II)-NO^-, Fe(III)-NO^-, and Fe(IV)-NO^- complexes. Vibrational data, analyzed by NCA, directly quantify changes in Fe-NO bonding along this series. Optical data allow for the identification of a "spectator" charge-transfer transition that, together with Mössbauer and XAS data, directly monitors the electronic changes of the Fe center. Using EXAFS, we are also able to provide structural data for all complexes. The magnetic properties of the complexes are further analyzed (from magnetic Mössbauer). The properties of our hs-{FeNO}^6-8 complexes are then contrasted to corresponding, low-spin iron-nitrosyl complexes where redox transformations are generally NO centered. The hs-{FeNO}⁸ complex can further be protonated by weak acids, and the product of this reaction is characterized. Taken together, these results provide unprecedented insight into the properties of biologically relevant non-heme iron-nitrosyl complexes in three relevant oxidation states.

Structure and function of haemoglobins

Haemoglobin (Hb) is widely known as the iron-containing protein in blood that is essential for O₂ transport in mammals. Less widely recognised is that erythrocyte Hb belongs to a large family of Hb proteins with members distributed across all three domains of life-bacteria, archaea and eukaryotes. This review, aimed chiefly at researchers new to the field, attempts a broad overview of the diversity, and common features, in Hb structure and function. Topics include structural and functional classification of Hbs; principles of O₂ binding affinity and selectivity between O₂/NO/CO and other small ligands; hexacoordinate (containing bis-imidazole coordinated haem) Hbs; bacterial truncated Hbs; flavohaemoglobins; enzymatic reactions of Hbs with bioactive gases, particularly NO, and protection from nitrosative stress; and, sensor Hbs. A final section sketches the evolution of work on the structural basis for allosteric O₂ binding by mammalian RBC Hb, including the development of newer kinetic models. Where possible, reference to historical works is included, in order to provide context for current advances in Hb research.

Synthesis of Nitric Oxide Donors Derived from Piloty's Acid and Study of Their Effects on Dopamine Secretion from PC12 Cells

This study investigated the mechanisms and kinetics of nitric oxide (NO) generation by derivatives of Piloty's acid (NO-donors) under physiological conditions. In order to qualitatively and quantitatively measure NO release, electron paramagnetic resonance (EPR) was carried out with NO spin trapping. In addition, voltammetric techniques, including cyclic voltammetry and constant potential amperometry, were used to confirm NO release from Piloty's acid and its derivatives. The resulting data showed that Piloty's acid derivatives are able to release NO under physiological conditions. In particular, electron-withdrawing substituents favoured NO generation, while electron-donor groups reduced NO generation. In vitro microdialysis, performed on PC12 cell cultures, was used to evaluate the dynamical secretion of dopamine induced by the Piloty's acid derivatives. Although all the studied molecules were able to induce DA secretion from PC12, only those with a slow release of NO have not determined an autoxidation of DA itself. These results confirm that the time-course of NO-donors decomposition and the amount of NO released play a key role in dopamine secretion and auto-oxidation. This information could drive the synthesis or the selection of compounds to use as potential drugs for the therapy of Parkinson's disease (PD).

A reductant-resistant ratiometric, colorimetric and far-red fluorescent probe for rapid and ultrasensitive detection of nitroxyl

A reductant-resistant ratiometric, colorimetric and far-red fluorescent probe for rapid and ultrasensitive detection of nitroxyl was developed.

The opposing roles of NO and oxidative stress in cardiovascular disease

Nitric oxide (NO) plays a pivotal role in the maintenance of cardiovascular homeostasis. A reduction in the bioavailability of endogenous NO, manifest as a decrease in the production and/or impaired signaling, is associated with many cardiovascular diseases including hypertension, atherosclerosis, stroke and heart failure. There is substantial evidence that reactive oxygen species (ROS), generated predominantly from NADPH oxidases (Nox), are responsible for the reduced NO bioavailability in vascular and cardiac pathologies. ROS can compromise NO function via a direct inactivation of NO, together with a reduction in NO synthesis and oxidation of its receptor, soluble guanylyl cyclase. Whilst nitrovasodilators are administered to compensate for the ROS-mediated loss in NO bioactivity, their clinical utility is limited due to the development of tolerance and resistance and systemic hypotension. Moreover, efforts to directly scavenge ROS with antioxidants has had limited clinical efficacy. This review outlines the therapeutic utility of NO-based therapeutics in cardiovascular diseases and describes the source and impact of ROS in these pathologies, with particular focus on the interaction with NO. Future therapeutic approaches in the treatment of cardiovascular diseases are highlighted with a focus on nitroxyl (HNO) donors as an alternative to traditional NO donors and the development of novel Nox inhibitors.

Nitrosodisulfide [S2NO]- (perthionitrite) is a true intermediate during the "cross-talk" of nitrosyl and sulfide

Nitrosodisulfide S₂NO^- is a controversial intermediate in the reactions of S-nitrosothiols with HS^- that produce NO and HNO. QM-MM molecular dynamics simulations combined with TD-DFT analysis contribute to a clear identification of S₂NO^- in water, acetone and acetonitrile, accounting for the UV-Vis signatures and broadening the mechanistic picture of N/S signaling in biochemistry.

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.

Metalloneurochemistry and the Pierian Spring: 'Shallow Draughts Intoxicate the Brain'

Metal ions perform critical and diverse functions in nervous system physiology and pathology. The field of metalloneurochemistry aims to understand the mechanistic bases for these varied roles at the molecular level. Here, we review several areas of research that illustrate progress toward achieving this ambitious goal and identify key challenges for the future. We examine the use of lithium as a mood stabilizer, the roles of mobile zinc and copper in the synapse, the interplay of nitric oxide and metals in retrograde signaling, and the regulation of iron homeostasis in the brain. These topics were chosen to demonstrate not only the breadth of the field, but also to highlight opportunities for discovery by studying such complex systems in greater detail. We are beginning to uncover the principles by which receptors and transmitters utilize metal ions to modulate neurotransmission. These advances have revealed exciting new insights into the intricate mechanisms that give rise to learning, memory, and sensory perception, while opening many new avenues for further exploration.

Photoinitiated Reactivity of a Thiolate-Ligated, Spin-Crossover Nonheme {FeNO}(7) Complex with Dioxygen

The nonheme iron complex, [Fe(NO)(N3PyS)]BF4, is a rare example of an {FeNO}(7) species that exhibits spin-crossover behavior. The comparison of X-ray crystallographic studies at low and high temperatures and variable-temperature magnetic susceptibility measurements show that a low-spin S = 1/2 ground state is populated at 0-150 K, while both low-spin S = 1/2 and high-spin S = 3/2 states are populated at T > 150 K. These results explain the observation of two N-O vibrational modes at 1737 and 1649 cm(-1) in CD3CN for [Fe(NO)(N3PyS)]BF4 at room temperature. This {FeNO}(7) complex reacts with dioxygen upon photoirradiation with visible light in acetonitrile to generate a thiolate-ligated, nonheme iron(III)-nitro complex, [Fe(III)(NO2)(N3PyS)](+), which was characterized by EPR, FTIR, UV-vis, and CSI-MS. Isotope labeling studies, coupled with FTIR and CSI-MS, show that one O atom from O2 is incorporated in the Fe(III)-NO2 product. The O2 reactivity of [Fe(NO)(N3PyS)]BF4 in methanol is dramatically different from CH3CN, leading exclusively to sulfur-based oxidation, as opposed to NO· oxidation. A mechanism is proposed for the NO· oxidation reaction that involves formation of both Fe(III)-superoxo and Fe(III)-peroxynitrite intermediates and takes into account the experimental observations. The stability of the Fe(III)-nitrite complex is limited, and decay of [Fe(III)(NO2)(N3PyS)](+) leads to {FeNO}(7) species and sulfur oxygenated products. This work demonstrates that a single mononuclear, thiolate-ligated nonheme {FeNO}(7) complex can exhibit reactivity related to both nitric oxide dioxygenase (NOD) and nitrite reductase (NiR) activity. The presence of the thiolate donor is critical to both pathways, and mechanistic insights into these biologically relevant processes are presented.

Achieving Reversible Sensing of Nitroxyl by Tuning the Ligand Environment of Azamacrocyclic Copper(II) Complexes

To elucidate the factors that impart selectivity for nitroxyl (HNO) over nitric oxide (NO), thiols, and H2S in metal-based fluorescent probes, we investigated five Cu(II)-cyclam (14-N4) derivatives. Upon exposure to NO gas at pH 7, no changes occur in the UV-vis spectra of any of the complexes. Addition of Angeli's salt to generate HNO promotes reduction of Cu(II) only in the case of [Cu(II)(14-N4-Ts)(OTf)2], which has the most positive reduction potential of the series. To gain insight into the observed reactivity, we prepared the Cu(II) complex of the mixed thia/aza 14-N2S2 ligand. [Cu(II)(14-N2S2)(OTf)2] reacts reversibly with HNO at pH 7, although nonselectively over thiols and H2S. The recurrent sensing of HNO uncovered with the study of Cu(II) azamacrocyclic complexes is a remarkable feature that opens the door for the design of a new generation of metal-based probes.

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.

Development of green to near-infrared turn-on fluorescent probes for the multicolour imaging of nitroxyl in living systems

The green to near-infrared turn-on fluorescent probes were developed for the multicolour imaging of nitroxyl in living systems.

Hypoxic pulmonary vasoconstriction in isolated rat pulmonary arteries is not inhibited by antagonists of H2 S-synthesizing pathways

An increase in the H₂S (hydrogen sulphide, hereafter sulphide) concentration in pulmonary artery smooth muscle cells (PASMCs) has been proposed to mediate hypoxic pulmonary vasoconstriction (HPV). We evaluated this hypothesis in isolated rat intrapulmonary arteries (IPAs) by examining the effects of the sulphide precursor cysteine and sulphide-synthesis blockers on HPV and also on normoxic pulmonary vasoconstriction (NPV) stimulated by prostaglandin F2α (PGF_2α) and by the drug LY83583, which causes contraction in IPAs by increasing cellular reactive oxygen species levels. Experiments with several blockers of cystathionine γ-lyase (CSE), the enzyme responsible for sulphide synthesis in the vasculature, demonstrated that propargylglycine (PAG, 1 mm) had little or no effect on the NPV caused by PGF_2α or LY83583. Conversely, other CSE antagonists tested, aminooxyacetic acid (AOAA, 100 μm), β-cyanoalanine (BCA, 500 μm) and hydroxylamine (HA, 100 μm), altered the NPV to PGF_2α (BCA increased, HA inhibited) and/or LY83583 (BCA increased, AOAA and HA inhibited). Preincubating IPAs in physiological saline solution (PSS) containing 1 mm cysteine increased the amplitude of the NPV to PGF2^α by ∼50%, and had a similar effect on HPV elicited by hypoxic challenge with 0% O₂. The enhancement of both responses by cysteine was abolished by pretreatment with 1 mm PAG. Measurements carried out with an amperometric electrode demonstrated that incubation with 1 mm cysteine under anoxic conditions (to minimize sulphide oxidation) greatly potentiated the release of sulphide from pieces of rat liver and that this release was strongly antagonized by PAG, indicating that at this concentration PAG could enter cells intact and antagonize CSE. PAG at 1 mm had no effect on HPV recorded in control PSS, or in PSS supplemented with physiological concentrations of cysteine (10 μm), cystine (50 μm) and glutamate (100 μm) in order to prevent the possible depletion of intracellular cysteine during experiments. Application of a combination of 1 mm cysteine and 1 mm α-ketoglutarate to promote sulphide synthesis via the cysteine aminotransferase/mercaptopyruvate sulphurtransferase (CAT/MST) pathway caused an increase in HPV similar to that observed for cysteine. This was partially blocked by the CAT antagonist aspartate (1 mm) and also by PAG. However, HPV was not increased by 1 mm α-ketoglutarate alone, and HPV in the absence of α-ketoglutarate and cysteine was not attenuated by aspartate. Pretreatment of IPAs with dithiothreitol (DTT, 1 mm), proposed to promote the conversion of mitochondrial thiosulphate to sulphide, did not increase the release of sulphide from pieces of rat liver in either the presence or the absence of 1 mm cysteine, and virtually abolished HPV. The results provide evidence that the sulphide precursor cysteine can promote both NPV and HPV in rat IPA by generating sulphide via a PAG-sensitive pathway, presumably CSE. However, HPV evoked under control conditions was unaffected by the blockade of CSE. Moreover, HPV was not affected by the CAT antagonist aspartate and was blocked rather than enhanced by DTT. The data therefore indicate that sulphide generated by CSE or CAT/MST or from thiosulphate is unlikely to contribute to O₂ sensing during HPV in these arteries.