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

Iron coupled with hydroxylamine turns on the "switch" for free radical degradation of organic pollutants under high pH conditions

Reducing iron by hydroxylamine (HA) can promote the generation of reactive oxygen species (ROS) in the Fenton reaction and play a crucial role in the degradation of organic pollutants. However, the performance of this system at wider environmental thresholds is still not sufficiently understood, especially in the highly alkaline environments resulting from human activities. Here, we assessed the impact of solution pH on organic pollutant degradation by goethite with the addition of HA and H2O2. The solid phase variation and ROS generation were analyzed using Mössbauer spectroscopy, X-ray absorption near edge structure spectroscopy, and electron paramagnetic resonance analysis. This study found that under alkaline conditions, the system can continuously scavenge organic pollutants through oxygen-mediated generation of free radicals. At lower pH levels, organic pollutant decomposition, exemplified by the breakdown of bisphenol A (BPA), is primarily driven by the Fenton reaction facilitated by iron. As pH increases, hydroxyl radical (•OH) production decreases, accompanied by decreased BPA removal efficiency. However, the removal efficiency of BPA increased significantly at pH > 9. At pH 12, the removal of BPA exceeded that of the acidic condition after one hour, which is consistent with observations in soil system studies. Unlike the Fenton reaction, which is not sensitive to oxygen content, the removal of BPA under alkaline conditions occurs only under aerobic conditions. H2O2 is hardly involved in the reaction, and the depletion of HA becomes a critical factor in the decomposition of BPA. Importantly, in contrast to acidic conditions, where the dramatic decomposition of BPA occurs mainly in the first 10 min, the decomposition of BPA under alkaline conditions continued to occur over the 2 h of observation until complete removal. For natural systems, the remediation of pollutants depends more on the active time of ROS than on their reactivity. Therefore, this idea can reference pollution remediation strategies in anthropogenically disturbed environments.

A coumarin-naphthalimide-based ratiometric fluorescent probe for nitroxyl (HNO) based on an ICT-FRET mechanism

Nitroxyl (HNO) is an important reactive nitrogen that is associated with various states in physiology and pathology and plays a unique function in living systems. So, it is important to exploit fluorescent probes with high sensitivity and selectivity for sensing HNO. In this paper, a novel ratiometric fluorescent probe for HNO was developed utilizing intramolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET) mechanisms. The probe selected coumarin as energy donor, naphthalimide as energy receptor and 2-(diphenylphosphino)benzoate as the sensing site for detecting HNO. When HNO was not present, the 2-(diphenylphosphino)benzoate unit of the probe restricted electron transfer and the ICT process could not occur, leading to the inhibition of FRET process as well. Thus, in the absence of HNO the probe displayed the intrinsic blue fluorescence of coumarin. When HNO was added, the HNO reacted with the 2-(diphenylphosphino)benzoate unit of the probe to yield a hydroxyl group which resulting in the opening of ICT process and the occurring of FRET process. Thus, after providing HNO the probe displayed yellow fluorescence. In addition, the probe showed good linearity in the ratio of fluorescence intensity at 545 nm and 472 nm (I545 nm/I472 nm) with a concentration of HNO (0.1-20 μM). The probe processed a detection limit of 0.014 μM and a response time of 4 min. The probe also specifically identified HNO over a wide pH scope (pH = 4.00-10.00), including physiological conditions. Cellular experiments had shown that this fluorescent probe was virtually non-cytotoxic and could be applied for ratiometric sensing of HNO in A549 cells.

Quantitative Factors Introduced in the Feasibility Analysis of Reactive Oxygen Species (ROS)-Sensitive Triggers

Reactive oxygen species (ROS) are critical for cellular signaling. Various pathophysiological conditions are also associated with elevated levels of ROS. Hence, ROS-sensitive triggers have been extensively used for selective payload delivery. Such applications are predicated on two key functions: (1) a sufficient magnitude of concentration difference for the interested ROS between normal tissue/cells and intended sites and (2) appropriate reaction kinetics to ensure a sufficient level of selectivity for payload release. Further, ROS refers to a group of species with varying reactivity, which should not be viewed as a uniform group. In this review, we critically analyze data on the concentrations of different ROS species under various pathophysiological conditions and examine how reaction kinetics affect the success of ROS-sensitive linker chemistry. Further, we discuss different ROS linker chemistry in the context of their applications in drug delivery and imaging. This review brings new insights into research in ROS-triggered delivery, highlights factors to consider in maximizing the chance for success and discusses pitfalls to avoid.

Revealing the role of nitroxyl during hepatic ischemia-reperfusion injury with a NIR-II luminescent nanoprobe

Hepatic ischemia-reperfusion injury (HIRI) can severely impair liver function and has a potential relationship with reactive nitrogen species. Nitroxyl (HNO) has been discovered to be involved in some biological functions and pharmacological activities. However, till now, there has been no knowledge of the role of HNO in the HIRI process, mainly because accurately tracking its fluctuation at the molecular level in vivo is extremely difficult. Herein, we developed a responsive ratiometric near-infrared-II (NIR-II) nanoprobe with rare earth ions-doped nanoparticles (RENPs) as the luminophore and a molecular trigger that can specifically react with HNO to regulate the NIR-II emission of RENPs. With this nanoprobe, we revealed the relationship between HNO and the HIRI process and demonstrated that HNO may be a product of stress reactions during HIRI. This work not only creates a useful tool for visually tracking HNO in vivo but also provides first-hand information about its role in HIRI.

Transformation of hydroxylamine to nitrosated and nitrated products during advanced oxidation process

Recently, hydroxylamine (HAm) was introduced to drive advanced oxidation processes (AOPs) for removing organic contaminants. However, we found that HAm-driven Cu(II)/peroxymonosulfate oxidation of phenol produced p-nitrosophenol, 2-nitrophenol and 4-nitrophenol. The total nitro(so) products accounted for approximately 25.0 % of the phenol transformation at certain condition. SO₄^•- and •OH were identified as the primary and second significant oxidants, respectively. Reactive nitrogen species (RNS) were involved in phenol transformation. The pathway and mechanism of HAm transformation in HAm-driven transition metal ion-catalyzed AOPs were proposed for the first time in this study. The product of HAm via twice single-electron oxidation by Cu(II) is nitroxyl (HNO/NO^-), which is a critical oxidation intermediate of HAm. Further oxidation of HNO by SO₄^•- or •OH is the initial step in propagating radical chain reactions, leading to nitric oxide radical (•NO) and nitrogen dioxide radical (•NO₂) as the primary RNS. HAm is a critical intermediate in natural nitrogen cycle, suggesting that HAm can drive the oxidation processes of pollutants in natural environments. Nitro(so) products will be readily produced when AOPs are applied for ecological remediation. This study highlights the formation of toxic nitrosated and nitrated products in HAm-driven AOPs, and the requirement of risk assessments to evaluate the possible health and ecological impacts.

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.

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.

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 keff 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.

A Case for Hydrogen Sulfide Metabolism as an Oxygen Sensing Mechanism

The ability to detect oxygen availability is a ubiquitous attribute of aerobic organisms. However, the mechanism(s) that transduce oxygen concentration or availability into appropriate physiological responses is less clear and often controversial. This review will make the case for oxygen-dependent metabolism of hydrogen sulfide (H₂S) and polysulfides, collectively referred to as reactive sulfur species (RSS) as a physiologically relevant O₂ sensing mechanism. This hypothesis is based on observations that H₂S and RSS metabolism is inversely correlated with O₂ tension, exogenous H₂S elicits physiological responses identical to those produced by hypoxia, factors that affect H₂S production or catabolism also affect tissue responses to hypoxia, and that RSS efficiently regulate downstream effectors of the hypoxic response in a manner consistent with a decrease in O₂. H₂S-mediated O₂ sensing is then compared to the more generally accepted reactive oxygen species (ROS) mediated O₂ sensing mechanism and a number of reasons are offered to resolve some of the confusion between the two.

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.

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.

Redox and Antioxidant Modulation of Circadian Rhythms: Effects of Nitroxyl, N-Acetylcysteine and Glutathione

The circadian clock at the hypothalamic suprachiasmatic nucleus (SCN) entrains output rhythms to 24-h light cycles. To entrain by phase-advances, light signaling at the end of subjective night (circadian time 18, CT18) requires free radical nitric oxide (NO•) binding to soluble guanylate cyclase (sGC) heme group, activating the cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG). Phase-delays at CT14 seem to be independent of NO•, whose redox-related species were yet to be investigated. Here, the one-electron reduction of NO• nitroxyl was pharmacologically delivered by Angeli’s salt (AS) donor to assess its modulation on phase-resetting of locomotor rhythms in hamsters. Intracerebroventricular AS generated nitroxyl at the SCN, promoting phase-delays at CT14, but potentiated light-induced phase-advances at CT18. Glutathione/glutathione disulfide (GSH/GSSG) couple measured in SCN homogenates showed higher values at CT14 (i.e., more reduced) than at CT18 (oxidized). In addition, administration of antioxidants N-acetylcysteine (NAC) and GSH induced delays per se at CT14 but did not affect light-induced advances at CT18. Thus, the relative of NO• nitroxyl generates phase-delays in a reductive SCN environment, while an oxidative favors photic-advances. These data suggest that circadian phase-locking mechanisms should include redox SCN environment, generating relatives of NO•, as well as coupling with the molecular oscillator.