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

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.

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.

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.

Inhibition of nitric oxide production enhances the activity of facial nerve tubulin polymerization and the ability of tau to promote microtubule assembly after neurorrhaphy

We previously reported that inhibition of nitric oxide (NO) production promotes rat reconnected facial nerve regeneration. However, the underlying mechanism is obscure. Microtubule assembly is known to be essential to axon regeneration; nevertheless, tubulins and microtubule-associated proteins (MAPs) have been demonstrated as targets for NO and peroxynitrite. Thus, we hypothesized that NO and/or peroxynitrite may affect facial nerve regeneration via influencing on microtubule assembly. First, tubulins and tau (a MAP) were extracted from facial nerves of normal rats, treated with NO donor or peroxynitrite, and processed for microtubule assembly assay. We found that peroxynitrite, DEA NONOate, and Angeli's salt reduced the tubulin polymerization activity to a greater extent than GSNO, SIN-1, and SNAP. Additionally, SIN-1, peroxynitrite, and Angeli's salt impaired the ability of tau to promote microtubule assembly. Next, nitrosative stress biomarkers 3-nitrotyrosine (3-NT) and S-nitrosylated cysteine (SNO-Cys) were immunolabeled in facial nerves. Both biomarkers were highly upregulated in proximal and distal stumps of reconnected facial nerves at 3 days and 1 week after neurorrhaphy. Notably, the expression of 3-NT was greatly reduced at 2 weeks, whereas that of SNO-Cys was maintained. Conversely, inhibition of NO production with L-NAME prevented the upregulation of SNO-Cys. Further, we used tubulins and tau extracted from facial nerves of sham-operated, nerve suture + vehicle treatment, and nerve suture + L-NAME treatment rats to perform microtubule assembly assay. We found that L-NAME treatment enhanced polymerization activity of tubulins and ability of tau to promote microtubule assembly. It is noteworthy that α-tubulin plays a more important role than β-tubulin since the activity of microtubule assembly using α-tubulin extracted from L-NAME-treated rats was greatly elevated, whereas that using β-tubulin extracted from L-NAME-treated rats was not. Overall, our findings support that inhibition of NO production reduces nitrosative stress, and may thus facilitate microtubule assembly and facial nerve regeneration.

Endothelial pulsatile shear stress is a backstop for COVID-19

There has not been any means to inhibit replication of the SARS-CoV-2 virus responsible for the rapid, deadly spread of the COVID-19 pandemic and an effective, safe, tested across diverse populations vaccine still requires extensive investigation. This review deals with the repurpose of a wellness technology initially fabricated for combating physical inactivity by increasing muscular activity. Its action increases pulsatile shear stress (PSS) to the endothelium such that the bioavailability of nitric oxide (NO) and other mediators are increased throughout the body. In vitro evidence indicates that NO inhibits SARS-CoV-2 virus replication but there are no publications of NO delivery to the virus in vivo. It will be shown that increased PSS has potential in vivo to exert anti-viral properties of NO as well as to benefit endothelial manifestations of COVID-19 thereby serving as a safe and effective backstop.

Regulation of the Proteolytic Activity of Cysteine Cathepsins by Oxidants

Besides their primary involvement in the recycling and degradation of proteins in endo-lysosomal compartments and also in specialized biological functions, cysteine cathepsins are pivotal proteolytic contributors of various deleterious diseases. While the molecular mechanisms of regulation via their natural inhibitors have been exhaustively studied, less is currently known about how their enzymatic activity is modulated during the redox imbalance associated with oxidative stress and their exposure resistance to oxidants. More specifically, there is only patchy information on the regulation of lung cysteine cathepsins, while the respiratory system is directly exposed to countless exogenous oxidants contained in dust, tobacco, combustion fumes, and industrial or domestic particles. Papain-like enzymes (clan CA, family C1, subfamily C1A) encompass a conserved catalytic thiolate-imidazolium pair (Cys25-His159) in their active site. Although the sulfhydryl group (with a low acidic pKa) is a potent nucleophile highly susceptible to chemical modifications, some cysteine cathepsins reveal an unanticipated resistance to oxidative stress. Besides an introductory chapter and peculiar attention to lung cysteine cathepsins, the purpose of this review is to afford a concise update of the current knowledge on molecular mechanisms associated with the regulation of cysteine cathepsins by redox balance and by oxidants (e.g., Michael acceptors, reactive oxygen, and nitrogen species).

Changes in Activity of Cysteine Cathepsins B and L in Brain Structures of Mice with Aggressive and Depressive-Like Behavior Formed under Conditions of Social Stress

We studied activity of lysosomal cysteine proteases, cathepsins B and L, in brain structures (frontal cortex, caudate nucleus, hippocampus, and hypothalamus) of C57Bl/6J mice with aggressive and depressive-like behavior formed under conditions of chronic social stress (repeated experience of victories and defeats within 20 days). Mice with depressive-like behavior showed increased activity of cathepsin В in the hypothalamus and nucleus caudatus and increased activity of cathepsin L in the hippocampus compared to control animals not subjected to agonistic confrontations. In mice with aggressive behavior, protease activity in the studied brain structures was not changed. In 4 h after immune system activation with LPS (250 μg/kg), cathepsin L activity in the hippocampus of control mice increased in comparison with mice receiving saline. In contrast to control animals, LPS caused a decrease in activity of the enzyme in the caudate nucleus and frontal cortex of aggressive mice and in the hippocampus of mice with depressive-like behavior.

Nitroxyl (HNO): A Reduced Form of Nitric Oxide with Distinct Chemical, Pharmacological, and Therapeutic Properties

Nitroxyl (HNO), the one-electron reduced form of nitric oxide (NO), shows a distinct chemical and biological profile from that of NO. HNO is currently being viewed as a vasodilator and positive inotropic agent that can be used as a potential treatment for heart failure. The ability of HNO to react with thiols and thiol containing proteins is largely used to explain the possible biological actions of HNO. Herein, we summarize different aspects related to HNO including HNO donors, chemistry, biology, and methods used for its detection.

Glutathione sulfinamide serves as a selective, endogenous biomarker for nitroxyl after exposure to therapeutic levels of donors

Nitroxyl (HNO) donors exhibit promising pharmacological characteristics for treatment of cardiovascular disorders, cancer, and alcoholism. However, whether HNO also serves as an endogenous signaling agent is currently unknown, largely because of the inability to selectively and sensitively detect HNO in a cellular environment. Although a number of methods to detect HNO have been developed recently, sensitivity and selectivity against other nitrogen oxides or biological reductants remain problematic. To improve selectivity, the electrophilic nature of HNO has been harnessed to generate modifications of thiols and phosphines that are unique to HNO, especially compared to nitric oxide (NO). Given high bioavailability, glutathione (GSH) is expected to be a major target of HNO. As a result, the putative selective product glutathione sulfinamide (GS(O)NH2) may serve as a high-yield biomarker of HNO production. In this work, the formation of GS(O)NH2 after exposure to HNO donors was investigated. Fluorescent labeling followed by separation and detection using capillary zone electrophoresis with laser-induced fluorescence allowed quantitation of GS(O)NH2 with nanomolar sensitivity, even in the presence of GSH and derivatives. Formation of GS(O)NH2 was found to occur exclusively upon exposure of GSH to HNO donors, thus confirming selectivity. GS(O)NH2 was detected in the lysate of cells treated with low-micromolar concentrations of HNO donors, verifying that this species has sufficient stability to server as a biomarker of HNO. Additionally, the concentration-dependent formation of GS(O)NH2 in cells treated with an HNO donor suggests that the concentration of GS(O)NH2 can be correlated to intracellular levels of HNO.

Quantitative detection of nitroxyl upon trapping with glutathione and labeling with a specific fluorogenic reagent

Donors of nitroxyl (HNO) have shown promise for treatment of stroke, heart failure, alcoholism and cancer. However, comparing the pharmacological capacities of various donors is difficult without first quantifying the amount of HNO released from each donor. Detection and quantitation of HNO has been complicated by the rapid self-consumption of HNO through irreversible dimerization, poor selectivity of trapping agents against other nitrogen oxides, and/or low sensitivity towards HNO. Here, an assay is described for the trapping of HNO by glutathione (GSH) followed by labeling of GSH with the fluorogenic agent, naphthalene-2,3-dicarboxaldehyde (NDA), and subsequent quantitation by fluorescence difference. The newly developed assay was used to validate the pH-dependence of HNO release from isopropylamine NONOate (IPA/NO), which is a dual donor of HNO and NO at physiological pH. Furthermore, varied assay conditions were utilized to suggest the ratios of the products of the reaction of GSH with HNO. At intracellular concentrations of GSH, the disulfide (GSSG) was the major product, but significant concentrations of glutathione sulfinamide (GS(O)NH₂) were also detected. This suggests that GS(O)NH₂, which is a selective biomarker of HNO, may be produced in concentrations that are amenable to in vivo analysis.

Hydroxylamine acutely activates glucose uptake in L929 fibroblast cells

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

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

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

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.

Nitroxyl in the central nervous system

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

Nitroxyl (HNO) signaling

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

The effects of nitroxyl (HNO) on soluble guanylate cyclase activity: interactions at ferrous heme and cysteine thiols

It has been previously proposed that nitric oxide (NO) is the only biologically relevant nitrogen oxide capable of activating the enzyme soluble guanylate cyclase (sGC). However, recent reports implicate HNO as another possible activator of sGC. Herein, we examine the affect of HNO donors on the activity of purified bovine lung sGC and find that, indeed, HNO is capable of activating this enzyme. Like NO, HNO activation appears to occur via interaction with the regulatory ferrous heme on sGC. Somewhat unexpectedly, HNO does not activate the ferric form of the enzyme. Finally, HNO-mediated cysteine thiol modification appears to also affect enzyme activity leading to inhibition. Thus, sGC activity can be regulated by HNO via interactions at both the regulatory heme and cysteine thiols.

The emergence of nitroxyl (HNO) as a pharmacological agent

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