Decomposition of Piloty's acid derivatives – Toward the understanding of factors controlling HNO release

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.

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.

Discovery of endogenous nitroxyl as a new redox player in Arabidopsis thaliana

Nitroxyl (HNO) is the one-electron reduced and protonated congener of nitric oxide (•NO), owning a distinct chemical profile. Based on real-time detection, we demonstrate that HNO is endogenously formed in Arabidopsis. Senescence and hypoxia induce shifts in the redox balance, triggering HNO decay or formation mediated by non-enzymatic •NO/HNO interconversion with cellular reductants. The stimuli-dependent HNO generation supports or competes with •NO signalling, depending on the local redox environment.

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.

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.

Kinetic Study on the Reactivity of Azanone (HNO) toward Cyclic C-Nucleophiles

Azanone (HNO) is an elusive electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present a comparative kinetic study of HNO reactivity toward selected cyclic C-nucleophiles under aqueous conditions at pH 7.4. We applied the competition kinetics method, which is based on the use of a fluorescein-derived boronate probe FlBA and two parallel HNO reactions: with the studied scavenger or with O₂ (k = 1.8 × 10⁴ M⁻¹s⁻¹). We determined the second-order rate constants of HNO reactions with 13 structurally diverse C-nucleophiles (k = 33–20,000 M⁻¹s⁻¹). The results show that the reactivity of HNO toward C-nucleophiles depends strongly on the structure of the scavenger. The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with C-nucleophiles is provided.

Solid-gas reactions for nitroxyl (HNO) generation in the gas phase

We present a novel nitroxyl (HNO) generation method, which avoids the need of using a liquid system or extreme experimental conditions. This method consists of the reaction between a gaseous base and an HNO donor (Piloty's acid) in the solid phase, allowing the formation of gaseous HNO in a fast and economical way. Detection of HNO was carried out indirectly, measuring the nitrous oxide (N₂O) byproduct of HNO dimerization using infrared spectroscopy, and directly, using mass spectrometry techniques and an electrochemical HNO sensor.

Boronate-Based Probes for Biological Oxidants: A Novel Class of Molecular Tools for Redox Biology

Boronate-based molecular probes are emerging as one of the most effective tools for detection and quantitation of peroxynitrite and hydroperoxides. This review discusses the chemical reactivity of boronate compounds in the context of their use for detection of biological oxidants, and presents examples of the practical use of those probes in selected chemical, enzymatic, and biological systems. The particular reactivity of boronates toward nucleophilic oxidants makes them a distinct class of probes for redox biology studies. We focus on the recent progress in the design and application of boronate-based probes in redox studies and perspectives for further developments.

Induction of caveolin-3/eNOS complex by nitroxyl (HNO) ameliorates diabetic cardiomyopathy

Nitroxyl (HNO), one-electron reduced and protonated sibling of nitric oxide (NO), is a potential regulator of cardiovascular functions. It produces positive inotropic, lusitropic, myocardial anti-hypertrophic and vasodilator properties. Despite of these favorable actions, the significance and the possible mechanisms of HNO in diabetic hearts have yet to be fully elucidated. H9c2 cells or primary neonatal mouse cardiomyocytes were incubated with normal glucose (NG) or high glucose (HG). Male C57BL/6 mice received intraperitoneal injection of streptozotocin (STZ) to induce diabetes. Here, we demonstrated that the baseline fluorescence signals of HNO in H9c2 cells were reinforced by both HNO donor Angeli's salt (AS), and the mixture of hydrogen sulfide (H₂S) donor sodium hydrogen sulfide (NaHS) and NO donor sodium nitroprusside (SNP), but decreased by HG. Pretreatment with AS significantly reduced HG-induced cell vitality injury, apoptosis, reactive oxygen species (ROS) generation, and hypertrophy in H9c2 cells. This effect was mediated by induction of caveolin-3 (Cav-3)/endothelial nitric oxide (NO) synthase (eNOS) complex. Disruption of Cav-3/eNOS by pharmacological manipulation or small interfering RNA (siRNA) abolished the protective effects of AS in HG-incubated H9c2 cells. In STZ-induced diabetic mice, administration of AS ameliorated the development of diabetic cardiomyopathy, as evidenced by improved cardiac function and reduced cardiac hypertrophy, apoptosis, oxidative stress and myocardial fibrosis without affecting hyperglycemia. This study shed light on how interaction of NO and H₂S regulates cardiac pathology and provide new route to treat diabetic cardiomyopathy with HNO.