Focusing of nitric oxide mediated nitrosation and oxidative nitrosylation as a consequence of reaction with superoxide

Nitric oxide and thiols: Chemical biology, signalling paradigms and vascular therapeutic potential

Nitric oxide (• NO) interactions with biological thiols play crucial, but incompletely determined, roles in vascular signalling and other biological processes. Here, we highlight two recently proposed signalling paradigms: (1) the formation of a vasodilating labile nitrosyl ferrous haem (NO-ferrohaem) facilitated by thiols via thiyl radical generation and (2) polysulfides/persulfides and their interaction with • NO. We also describe the specific (bio)chemical routes in which • NO and thiols react to form S-nitrosothiols, a broad class of small molecules, and protein post-translational modifications that can influence protein function through catalytic site or allosteric structural changes. S-Nitrosothiol formation depends upon cellular conditions, but critically, an appropriate oxidant for either the thiol (yielding a thiyl radical) or • NO (yielding a nitrosonium [NO+ ]-donating species) is required. We examine the roles of these collective • NO/thiol species in vascular signalling and their cardiovascular therapeutic potential.

Nitric oxide augments signaling for contraction in hypoxic pulmonary arterial smooth muscle—Implications for hypoxic pulmonary hypertension

Introduction: Hypoxic persistent pulmonary hypertension in the newborn (PPHN) is usually treated with oxygen and inhaled nitric oxide (NO), both pulmonary arterial relaxants. But treatment failure with NO occurs in 25% of cases. We previously demonstrated that 72 h exposure to hypoxia, modeling PPHN, sensitized pulmonary artery smooth muscle cells (PASMC) to the contractile agonist thromboxane and inhibited relaxant adenylyl cyclase (AC) activity.

Methods: In this study, we examined the effects of sodium nitroprusside (SNP), as NO donor, on the thromboxane-mediated contraction and NO-independent relaxation pathways and on reactive oxygen species (ROS) accumulation in PASMC. In addition, we examined the effect of the peroxynitrite scavenger 5,10,15,20-Tetrakis (4-sulfonatophenyl)porphyrinato Iron (III) (FeTPPS) on these processes.

Results: Exposure of PASMC to 72 h hypoxia increased total intracellular ROS compared to normoxic control cells and this was mitigated by treatment of cells with either SNP or FeTPPS. Total protein nitrosylation was increased in hypoxic PASMC compared to controls. Both normoxic and hypoxic cells treated with SNP exhibited increased total protein nitrosylation and intracellular nitrite; this was reduced by treatment with FeTPPS. While cell viability and mitochondrial number were unchanged by hypoxia, mitochondrial activity was decreased compared to controls; addition of FeTPPS did not alter this. Basal and maximal mitochondrial metabolism and ATP turnover were reduced in hypoxic PASMC compared to controls. Hypoxic PASMC had higher basal Ca2+, and a heightened peak Ca2+ response to thromboxane challenge compared to controls. Addition of SNP further elevated the peak Ca2+ response, while addition of FeTPPS brought peak Ca2+ response down to control levels. AC mediated relaxation was impaired in hypoxic PASMC compared to controls but was normalized following treatment with FeTPPS. Addition of SNP inhibited adenylyl cyclase activity in both normoxic and hypoxic PASMC. Moreover, addition of the Ca2+ chelator BAPTA improved AC activity, but the effect was minimal.

Discussion: We conclude that NO independently augments contraction and inhibits relaxation pathways in hypoxic PASMC, in part by a mechanism involving nitrogen radical formation and protein nitrosylation. These observations may partially explain impaired effectiveness of NO when treating hypoxic pulmonary hypertension.

Recovery of reduced thiol groups by superoxide-mediated denitrosation of nitrosothiols

Nitrosation of critical thiols has been elaborated as reversible posttranslational modification with regulatory function in multiple disorders. Reversibility of S-nitrosation is generally associated with enzyme-mediated one-electron reductions, catalyzed by the thioredoxin system, or by nitrosoglutathione reductase. In the present study, we confirm previous evidence for a non-enzymatic de-nitrosation of nitrosoglutathione (GSNO) by superoxide. The interaction leads to the release of nitric oxide that subsequently interacts with a second molecule of superoxide (O2•-) to form peroxynitrite. Despite the formation of peroxynitrite, approximately 40-70% of GSNO yielded reduced glutathione (GSH), depending on the applied analytical assay. The concept of O2•- dependent denitrosation was then applied to S-nitrosated enzymes. S-nitrosation of isocitrate dehydrogenase (ICDH; NADP+-dependent) was accompanied by an inhibition of the enzyme and could be reversed by dithiothreitol. Treatment of nitrosated ICDH with O2•- indicated ca. 50% recovery of enzyme activity. Remaining inhibition was largely consequence of oxidative modifications evoked either by O2•- or by peroxynitrite. Recovery of activity in S-nitrosated enzymes by O2•- appears relevant only for selected examples. In contrast, recovery of reduced glutathione from the interaction of GSNO with O2•- could represent a mechanism to regain reducing equivalents in situations of excess O2•- formation, e.g. in the reperfusion phase after ischemia.

A physiologically relevant role for NO stored in vascular smooth muscle cells: A novel theory of vascular NO signaling

S-nitrosothiols (SNO), dinitrosyl iron complexes (DNIC), and nitroglycerine (NTG) dilate vessels via activation of soluble guanylyl cyclase (sGC) in vascular smooth muscle cells. Although these compounds are often considered to be nitric oxide (NO) donors, attempts to ascribe their vasodilatory activity to NO-donating properties have failed. Even more puzzling, many of these compounds have vasodilatory potency comparable to or even greater than that of NO itself, despite low membrane permeability. This raises the question: How do these NO adducts activate cytosolic sGC when their NO moiety is still outside the cell? In this review, we classify these compounds as ‘nitrodilators’, defined by their potent NO-mimetic vasoactivities despite not releasing requisite amounts of free NO. We propose that nitrodilators activate sGC via a preformed nitrodilator-activated NO store (NANOS) found within the vascular smooth muscle cell. We reinterpret vascular NO handling in the framework of this NANOS paradigm, and describe the knowledge gaps and perspectives of this novel model.

ROS Pleiotropy in Melanoma and Local Therapy with Physical Modalities

Metabolic energy production naturally generates unwanted products such as reactive oxygen species (ROS), causing oxidative damage. Oxidative damage has been linked to several pathologies, including diabetes, premature aging, neurodegenerative diseases, and cancer. ROS were therefore originally anticipated as an imperative evil, a product of an imperfect system. More recently, however, the role of ROS in signaling and tumor treatment is increasingly acknowledged. This review addresses the main types, sources, and pathways of ROS in melanoma by linking their pleiotropic roles in antioxidant and oxidant regulation, hypoxia, metabolism, and cell death. In addition, the implications of ROS in various physical therapy modalities targeting melanoma, such as radiotherapy, electrochemotherapy, hyperthermia, photodynamic therapy, and medical gas plasma, are also discussed. By including ROS in the main picture of melanoma skin cancer and as an integral part of cancer therapies, a greater understanding of melanoma cell biology is presented, which ultimately may elucidate additional clues on targeting therapy resistance of this most deadly form of skin cancer.

The Labile Iron Pool Reacts Rapidly and Catalytically with Peroxynitrite

While investigating peroxynitrite-dependent oxidation in murine RAW 264.7 macrophage cells, we observed that removal of the Labile Iron Pool (LIP) by chelation increases the intracellular oxidation of the fluorescent indicator H2DCF, so we concluded that the LIP reacts with peroxynitrite and decreases the yield of peroxynitrite-derived oxidants. This was a paradigm-shifting finding in LIP biochemistry and raised many questions. In this follow-up study, we address fundamental properties of the interaction between the LIP and peroxynitrite by using the same cellular model and fluorescence methodology. We have identified that the reaction between the LIP and peroxynitrite has catalytic characteristics, and we have estimated that the rate constant of the reaction is in the range of 106 to 107 M-1s-1. Together, these observations suggest that the LIP represents a constitutive peroxynitrite reductase system in RAW 264.7 cells.

Nitric oxide and heme-NO stimulate superoxide production by NADPH oxidase 5

Nitric oxide (NO) is a ubiquitous cell signaling molecule which mediates widespread and diverse processes in the cell. These NO dependent effects often involve activation (e.g. NO binding to the heme group of soluble guanylyl cyclase for cGMP production) or inactivation (e.g. S-nitrosation) of protein targets. We studied the effect of NO and heme-NO on the transmembrane signaling enzyme NADPH oxidase 5 (NOX5), a heme protein which produces superoxide in response to increases in intracellular calcium. We found that treatment with NO donors increases NOX5 activity through heme-dependent effects, and that this effect could be recapitulated by the addition of heme-NO. This work adds to our understanding of NOX5 regulation in the cell but also provides a framework for understanding how NO could cause widespread changes in hemeprotein activity based on different affinities for heme v. heme-NO, and helps explain the opposing roles NO plays in activation and inactivation of hemeprotein targets.

Redox-Neutral S-nitrosation Mediated by a Dicopper Center

A redox-neutral S-nitrosation of thiol has been achieved at a dicopper(I,I) center. Treatment of dicopper (I,I) complex with excess NO^. and thiol generates a dicopper (I,I) di-S-nitrosothiol complex [Cu^I Cu^I (RSNO)₂ ]²⁺ or dicopper (I,I) mono-S-nitrosothiol complex [Cu^I Cu^I (RSNO)]²⁺ , which readily release RSNO in 88-94 % yield. The S-nitrosation proceeds by a mixed-valence [Cu^II Cu^III (μ-O)(μ-NO)]²⁺ species, which deprotonates RS-H at the basic μ-O site and nitrosates RS^- at the μ-NO site. The [Cu^II Cu^III (μ-O)(μ-NO)]²⁺ complex is also competent for O-nitrosation of MeOH. A rare [Cu^II Cu^II (μ-NO)(OMe)]²⁺ intermediate was isolated and fully characterized, suggesting the S-nitrosation may proceed through the intermediary of analogous [Cu^II Cu^II (μ-NO)(SR)]²⁺ species. This redox- and proton-neutral S-nitrosation process is the first functional model of ceruloplasmin in mediating S-nitrosation of external thiols, with implications for biological copper sites in the interconversion of NO^. /RSNO.

Differential oxidative stress responses and tobacco-specific nitrosamine accumulation in two burley varieties

Tobacco-specific nitrosamines (TSNAs) are carcinogens that accumulate in tobacco leaves during curing, storage, and processing, and their amounts in processed tobacco vary dependent on several intrinsic and extrinsic factors. Here, we assessed the hypothesis that there is a link between reactive oxygen species levels in leaves and TSNA formation during curing. First, we show that burley varieties KT 204LC and NCBH 129LC accumulate TSNAs to different levels but not as a result of a variety-specific abundance of TSNA precursors. Next, we measured the levels of reactive oxygen species, and we show that the variety that accumulates more TSNAs, NCBH 129LC, had significantly higher levels of hydrogen peroxide than KT 204LC. The NCBH 129LC also has more oxidatively damaged and glutathionylated proteins. Finally, we analyzed the antioxidant levels in KT 204LC and NCBH 129LC and their tolerance to oxidative stress. NCBH 129LC contained more of the essential antioxidant glutathione and was more tolerant to the oxidative stress-generating compound paraquat. Collectively, our data suggest that there is indeed a link between foliar oxidative stress parameters and the extent to which TSNAs accumulate in cured tobacco leaves.

The uncoupling of respiration in plant mitochondria: keeping reactive oxygen and nitrogen species under control

Plant mitochondrial respiration involves the operation of various alternative pathways. These pathways participate, both directly and indirectly, in the maintenance of mitochondrial functions though they do not contribute to energy production, being uncoupled from the generation of an electrochemical gradient across the mitochondrial membrane and thus from ATP production. Recent findings suggest that uncoupled respiration is involved in reactive oxygen species (ROS) and nitric oxide (NO) scavenging, regulation, and homeostasis. Here we discuss specific roles and possible functions of uncoupled mitochondrial respiration in ROS and NO metabolism. The mechanisms of expression and regulation of the NDA-, NDB- and NDC-type non-coupled NADH and NADPH dehydrogenases, the alternative oxidase (AOX), and the uncoupling protein (UCP) are examined in relation to their involvement in the establishment of the stable far-from-equilibrium state of plant metabolism. The role of uncoupled respiration in controlling the levels of ROS and NO as well as inducing signaling events is considered. Secondary functions of uncoupled respiration include its role in protection from stress factors and roles in biosynthesis and catabolism. It is concluded that uncoupled mitochondrial respiration plays an important role in providing rapid adaptation of plants to changing environmental factors via regulation of ROS and NO.

Tracing the path of inhaled nitric oxide: Biological consequences of protein nitrosylation

Nitric oxide (NO) is a comprehensive regulator of vascular and airway tone. Endogenous NO produced by nitric oxide synthases regulates multiple signaling cascades, including activation of soluble guanylate cyclase to generate cGMP, relaxing smooth muscle cells. Inhaled NO is an established therapy for pulmonary hypertension in neonates, and has been recently proposed for the treatment of hypoxic respiratory failure and acute respiratory distress syndrome due to COVID-19. In this review, we summarize the effects of endogenous and exogenous NO on protein S-nitrosylation, which is the selective and reversible covalent attachment of a nitrogen monoxide group to the thiol side chain of cysteine. This posttranslational modification targets specific cysteines based on the acid/base sequence of surrounding residues, with significant impacts on protein interactions and function. S-nitrosothiol (SNO) formation is tightly compartmentalized and enzymatically controlled, but also propagated by nonenzymatic transnitrosylation of downstream protein targets. Redox-based nitrosylation and denitrosylation pathways dynamically regulate the equilibrium of SNO-proteins. We review the physiological roles of SNO proteins, including nitrosohemoglobin and autoregulation of blood flow through hypoxic vasodilation, and pathological effects of nitrosylation including inhibition of critical vasodilator enzymes; and discuss the intersection of NO source and dose with redox environment, in determining the effects of protein nitrosylation.

The role of mitochondrial reactive oxygen species, NO and H2 S in ischaemia/reperfusion injury and cardioprotection

Redox signalling in mitochondria plays an important role in myocardial ischaemia/reperfusion (I/R) injury and in cardioprotection. Reactive oxygen and nitrogen species (ROS/RNS) modify cellular structures and functions by means of covalent changes in proteins including among others S‐nitros(yl)ation by nitric oxide (NO) and its derivatives, and S‐sulphydration by hydrogen sulphide (H₂S). Many enzymes are involved in the mitochondrial formation and handling of ROS, NO and H₂S under physiological and pathological conditions. In particular, the balance between formation and removal of reactive species is impaired during I/R favouring their accumulation. Therefore, various interventions aimed at decreasing mitochondrial ROS accumulation have been developed and have shown cardioprotective effects in experimental settings. However, ROS, NO and H₂S play also a role in endogenous cardioprotection, as in the case of ischaemic pre‐conditioning, so that preventing their increase might hamper self‐defence mechanisms. The aim of the present review was to provide a critical analysis of formation and role of reactive species, NO and H₂S in mitochondria, with a special emphasis on mechanisms of injury and protection that determine the fate of hearts subjected to I/R. The elucidation of the signalling pathways of ROS, NO and H₂S is likely to reveal novel molecular targets for cardioprotection that could be modulated by pharmacological agents to prevent I/R injury.

Hypoxia-inducible factor 1α from a high-altitude fish enhances cytoprotection and elevates nitric oxide production in hypoxic environment

Hypoxia-inducible factors (HIFs) are master transcription factor regulating hypoxic responses in vertebrates. Species of Schizothoracine, a sub-family of cyprinidae, are highly endemic to the hypoxic Qinghai-Tibetan Plateau (QTP). What roles the HIFs play in hypoxic adaptation in the Schizothoracine fish is little known. In this study, the HIF-1α/B gene from Gymnocypris dobula (Gd) was characterized. The predicted protein for Gd-HIF-1α/B contains the main domains (bHLH, PAS, PAC, ODD, N-TAD, and C-TAD). Moreover, a specific mutation that the proline hydroxylation motif (LXXLAP) mutated into PxxLAP was observed in Gd-HIF-1α/B CODD domain, which may lead to changes in the function. To clarify whether HIF-1α/B of G. dobula possesses hypoxic adaptive features, Gd-HIF1α/B and Schizothorax prenanti-HIF1α/B (Sp-HIF1α/B) were cloned into an expression vector and transfected into 293T cells. Cell viability was found to be significantly higher in cells transfected with Gd-HIF-1α/B than those transfected with Sp-HIF-1α/B under hypoxic conditions. In addition, G. dobula HIF-1α/B showed stronger activity in transactivating the expression of nitric oxide (NO)-synthesizing enzyme, NOS2B under hypoxia stresses than the orthologous gene from S. prenanti, which were accompanied with upregulated expressions of NOS2B in heart of G. dobula, which may attribute to elevated NO levels detected in G. dobula than the lower land species. These results indicated that the HIF-1α plays an important role in mediating the iNOS signaling system in the process of evolutionary adaptation of the Schizothoracine to the highland environment.

[The role of ATP-dependent potassium channels and nitric oxide system in the neuroprotective effect of preconditioning]

AIM

To study a role of ATP-dependent potassium channels (K+ATP) in the neuroprotective effect of ischemic (IP) and pharmacological (PP) preconditioning and evaluate the dynamics of blood nitric oxide (NO) metabolites in cerebral ischemia.

MATERIAL AND METHODS

A model of ischemic stroke induced by the electrocoagulation of a middle cerebral artery (MCA) branch was used in male rats (n=86). Glibenclamide, a selective inhibitor of ATP-sensitive K+ channels, and diazoxide, a potassium channel activator, were used. IP and PP were performed 24 h before MCA occlusion. Blood concentrations of NO, NO3- and NO2-were measured 5, 24 and 72 h after occlusion.

RESULTS

IP decreased a lesion area by 37% (p<0/05) and the preliminary introduction ofglibenclamide levelled the effect of IP. A protective effect of PP was similar to that of IP. A decrease in oxygenated R-conformers of Hb-NO and a reverse increase in non-oxygenated T-conformers as well as NO3- и NO2-were noted 5h after MCA occlusion. In the first 24 h after MCA occlusion, contents of NO3- and NO2- returned to normal values. There were changes in the concentrations of Hb-NO complexes as well, with the predominance of R-conformers and minimal contents of T-conformers. Moreover, the correlations between K+ATP channel blockade and the decrease in serum NO3- and NO2 were found (p<0/03).

CONCLUSION

The neuroprotective effect of preconditioning is caused by the activation of K+ATP channels. An analysis of NO metabolite concentrations in the blood of rats with IP suggests that Hb-NO complexes belonging to R-conformers deposit and carry NO in tissues releasing NO accumulated via R→T transfer in conditions of ischemia.

The Plasma Membrane: A Platform for Intra- and Intercellular Redox Signaling

Membranes are of outmost importance to allow for specific signal transduction due to their ability to localize, amplify, and direct signals. However, due to the double-edged nature of reactive oxygen species (ROS)—toxic at high concentrations but essential signal molecules—subcellular localization of ROS-producing systems to the plasma membrane has been traditionally regarded as a protective strategy to defend cells from unwanted side-effects. Nevertheless, specialized regions, such as lipid rafts and caveolae, house and regulate the activated/inhibited states of important ROS-producing systems and concentrate redox targets, demonstrating that plasma membrane functions may go beyond acting as a securing lipid barrier. This is nicely evinced by nicotinamide adenine dinucleotide phosphate (NADPH)-oxidases (NOX), enzymes whose primary function is to generate ROS and which have been shown to reside in specific lipid compartments. In addition, membrane-inserted bidirectional H₂O₂-transporters modulate their conductance precisely during the passage of the molecules through the lipid bilayer, ensuring time-scaled delivery of the signal. This review aims to summarize current evidence supporting the role of the plasma membrane as an organizing center that serves as a platform for redox signal transmission, particularly NOX-driven, providing specificity at the same time that limits undesirable oxidative damage in case of malfunction. As an example of malfunction, we explore several pathological situations in which an inflammatory component is present, such as inflammatory bowel disease and neurodegenerative disorders, to illustrate how dysregulation of plasma-membrane-localized redox signaling impacts normal cell physiology.

The labile iron pool attenuates peroxynitrite-dependent damage and can no longer be considered solely a pro-oxidative cellular iron source

The ubiquitous cellular labile iron pool (LIP) is often associated with the production of the highly reactive hydroxyl radical, which forms through a redox reaction with hydrogen peroxide. Peroxynitrite is a biologically relevant peroxide produced by the recombination of nitric oxide and superoxide. It is a strong oxidant that may be involved in multiple pathological conditions, but whether and how it interacts with the LIP are unclear. Here, using fluorescence spectroscopy, we investigated the interaction between the LIP and peroxynitrite by monitoring peroxynitrite-dependent accumulation of nitrosated and oxidized fluorescent intracellular indicators. We found that, in murine macrophages, removal of the LIP with membrane-permeable iron chelators sustainably accelerates the peroxynitrite-dependent oxidation and nitrosation of these indicators. These observations could not be reproduced in cell-free assays, indicating that the chelator-enhancing effect on peroxynitrite-dependent modifications of the indicators depended on cell constituents, presumably including LIP, that react with these chelators. Moreover, neither free nor ferrous-complexed chelators stimulated intracellular or extracellular oxidative and nitrosative chemistries. On the basis of these results, LIP appears to be a relevant and competitive cellular target of peroxynitrite or its derived oxidants, and thereby it reduces oxidative processes, an observation that may change the conventional notion that the LIP is simply a cellular source of pro-oxidant iron.

Taking up the cudgels for the traditional reactive oxygen and nitrogen species detection assays and their use in the cardiovascular system

Reactive oxygen and nitrogen species (RONS such as H2O2, nitric oxide) confer redox regulation of essential cellular functions (e.g. differentiation, proliferation, migration, apoptosis), initiate and catalyze adaptive stress responses. In contrast, excessive formation of RONS caused by impaired break-down by cellular antioxidant systems and/or insufficient repair of the resulting oxidative damage of biomolecules may lead to appreciable impairment of cellular function and in the worst case to cell death, organ dysfunction and severe disease phenotypes of the entire organism. Therefore, the knowledge of the severity of oxidative stress and tissue specific localization is of great biological and clinical importance. However, at this level of investigation quantitative information may be enough. For the development of specific drugs, the cellular and subcellular localization of the sources of RONS or even the nature of the reactive species may be of great importance, and accordingly, more qualitative information is required. These two different philosophies currently compete with each other and their different needs (also reflected by different detection assays) often lead to controversial discussions within the redox research community. With the present review we want to shed some light on these different philosophies and needs (based on our personal views), but also to defend some of the traditional assays for the detection of RONS that work very well in our hands and to provide some guidelines how to use and interpret the results of these assays. We will also provide an overview on the "new assays" with a brief discussion on their strengths but also weaknesses and limitations.

Electrochemical reverse engineering: A systems-level tool to probe the redox-based molecular communication of biology

The intestine is the site of digestion and forms a critical interface between the host and the outside world. This interface is composed of host epithelium and a complex microbiota which is "connected" through an extensive web of chemical and biological interactions that determine the balance between health and disease for the host. This biology and the associated chemical dialogues occur within a context of a steep oxygen gradient that provides the driving force for a variety of reduction and oxidation (redox) reactions. While some redox couples (e.g., catecholics) can spontaneously exchange electrons, many others are kinetically "insulated" (e.g., biothiols) allowing the biology to set and control their redox states far from equilibrium. It is well known that within cells, such non-equilibrated redox couples are poised to transfer electrons to perform reactions essential to immune defense (e.g., transfer from NADH to O₂ for reactive oxygen species, ROS, generation) and protection from such oxidative stresses (e.g., glutathione-based reduction of ROS). More recently, it has been recognized that some of these redox-active species (e.g., H₂O₂) cross membranes and diffuse into the extracellular environment including lumen to transmit redox information that is received by atomically-specific receptors (e.g., cysteine-based sulfur switches) that regulate biological functions. Thus, redox has emerged as an important modality in the chemical signaling that occurs in the intestine and there have been emerging efforts to develop the experimental tools needed to probe this modality. We suggest that electrochemistry provides a unique tool to experimentally probe redox interactions at a systems level. Importantly, electrochemistry offers the potential to enlist the extensive theories established in signal processing in an effort to "reverse engineer" the molecular communication occurring in this complex biological system. Here, we review our efforts to develop this electrochemical tool for in vitro redox-probing.

Nitrosothiol formation and S-nitrosation signaling through nitric oxide synthases

Nitric oxide (NO) is a gaseous signaling molecule impacting many biological pathways. NO is produced in mammals by three nitric oxide synthase (NOS) isoforms: neuronal (nNOS), endothelial (eNOS), and inducible (iNOS). nNOS and eNOS produce low concentrations of NO for paracrine signaling; NO produced and released from one cell diffuses to a neighboring cell where it binds and activates soluble guanylyl cyclase (sGC). iNOS produces high concentrations of NO using NO toxicity to amplify the innate immune response. Recent work has also defined protein cysteine S-nitrosation as a pathway of sGC-independent NO signaling. Though many studies have shown that S-nitrosation regulates the activity of NOS isoforms and other proteins in vivo, many issues need to be resolved to establish S-nitrosation as a viable signaling mechanism. Several chemical mechanisms result in S-nitrosation including transition metal-catalyzed pathways, NO oxidation followed by thiolate reaction, and thiyl radical recombination with NO. Once formed, nitrosothiols can be transferred between cellular cysteine residues via transnitrosation reactions. However, it is largely unclear how these chemical processes result in selective S-nitrosation of specific cellular cysteine residues. S-nitrosation site selectivity may be imparted via direct interactions or colocalization with NOS isoforms that focus chemical or transnitrosation mechanisms of nitrosothiol formation or transfer. Here, we discuss chemical mechanisms of nitrosothiol formation, S-nitrosation of NOS isoforms, and potential S-nitrosation signaling cascades resulting from NOS S-nitrosation.

Nitrosative Stress, Hypernitrosylation, and Autoimmune Responses to Nitrosylated Proteins: New Pathways in Neuroprogressive Disorders Including Depression and Chronic Fatigue Syndrome

Nitric oxide plays an indispensable role in modulating cellular signaling and redox pathways. This role is mainly effected by the readily reversible nitrosylation of selective protein cysteine thiols. The reversibility and sophistication of this signaling system is enabled and regulated by a number of enzymes which form part of the thioredoxin, glutathione, and pyridoxine antioxidant systems. Increases in nitric oxide levels initially lead to a defensive increase in the number of nitrosylated proteins in an effort to preserve their function. However, in an environment of chronic oxidative and nitrosative stress (O&NS), nitrosylation of crucial cysteine groups within key enzymes of the thioredoxin, glutathione, and pyridoxine systems leads to their inactivation thereby disabling denitrosylation and transnitrosylation and subsequently a state described as "hypernitrosylation." This state leads to the development of pathology in multiple domains such as the inhibition of enzymes of the electron transport chain, decreased mitochondrial function, and altered conformation of proteins and amino acids leading to loss of immune tolerance and development of autoimmunity. Hypernitrosylation also leads to altered function or inactivation of proteins involved in the regulation of apoptosis, autophagy, proteomic degradation, transcription factor activity, immune-inflammatory pathways, energy production, and neural function and survival. Hypernitrosylation, as a consequence of chronically elevated O&NS and activated immune-inflammatory pathways, can explain many characteristic abnormalities observed in neuroprogressive disease including major depression and chronic fatigue syndrome/myalgic encephalomyelitis. In those disorders, increased bacterial translocation may drive hypernitrosylation and autoimmune responses against nitrosylated proteins.

High oxidative stress adversely affects NFκB mediated induction of inducible nitric oxide synthase in human neutrophils: Implications in chronic myeloid leukemia

Increasing evidence support bimodal action of nitric oxide (NO) both as a promoter and as an impeder of oxygen free radicals in neutrophils (PMNs), however impact of high oxidative stress on NO generation is less explored. In the present study, we comprehensively investigated the effect of high oxidative stress on inducible nitric oxide synthase (iNOS) expression and NO generation in human PMNs. Our findings suggest that PMA or diamide induced oxidative stress in PMNs from healthy volunteers, and high endogenous ROS in PMNs of chronic myeloid leukemia (CML) patients attenuate basal as well as LPS/cytokines induced NO generation and iNOS expression in human PMNs. Mechanistically, we found that under high oxidative stress condition, S-glutathionylation of NFκB (p50 and p65 subunits) severely limits iNOS expression due to its reduced binding to iNOS promoter, which was reversed in presence of DTT. Furthermore, by using pharmacological inhibitors, scavengers and molecular approaches, we identified that enhanced ROS generation via NOX2 and mitochondria, reduced Grx1/2 expression and GSH level associated with NFκB S-glutathionylation in PMNs from CML patients. Altogether data obtained suggest that oxidative status act as an important regulator of NO generation/iNOS expression, and under enhanced oxidative stress condition, NOX2-mtROS-NFκB S-glutathionylation is a feed forward loop, which attenuate NO generation and iNOS expression in human PMNs.

Mitochondria-targeted esculetin alleviates mitochondrial dysfunction by AMPK-mediated nitric oxide and SIRT3 regulation in endothelial cells: potential implications in atherosclerosis

Mitochondria-targeted compounds are emerging as a new class of drugs that can potentially alter the pathophysiology of those diseases where mitochondrial dysfunction plays a critical role. We have synthesized a novel mitochondria-targeted esculetin (Mito-Esc) with an aim to investigate its effect during oxidative stress-induced endothelial cell death and angiotensin (Ang)-II-induced atherosclerosis in ApoE^−/− mice. Mito-Esc but not natural esculetin treatment significantly inhibited H2O2- and Ang-II-induced cell death in human aortic endothelial cells by enhancing NO production via AMPK-mediated eNOS phosphorylation. While L-NAME (NOS inhibitor) significantly abrogated Mito-Esc-mediated protective effects, Compound c (inhibitor of AMPK) significantly decreased Mito-Esc-mediated increase in NO production. Notably, Mito-Esc promoted mitochondrial biogenesis by enhancing SIRT3 expression through AMPK activation; and restored H2O2-induced inhibition of mitochondrial respiration. siSIRT3 treatment not only completely reversed Mito-Esc-mediated mitochondrial biogenetic marker expressions but also caused endothelial cell death. Furthermore, Mito-Esc administration to ApoE^−/− mice greatly alleviated Ang-II-induced atheromatous plaque formation, monocyte infiltration and serum pro-inflammatory cytokines levels. We conclude that Mito-Esc is preferentially taken up by the mitochondria and preserves endothelial cell survival during oxidative stress by modulating NO generation via AMPK. Also, Mito-Esc-induced SIRT3 plays a pivotal role in mediating mitochondrial biogenesis and perhaps contributes to its anti-atherogenic effects.

Reactive Nitrogen Species in Mitochondria and Their Implications in Plant Energy Status and Hypoxic Stress Tolerance

Hypoxic and anoxic conditions result in the energy crisis that leads to cell damage. Since mitochondria are the primary organelles for energy production, the support of these organelles in a functional state is an important task during oxygen deprivation. Plant mitochondria adapted the strategy to survive under hypoxia by keeping electron transport operative even without oxygen via the use of nitrite as a terminal electrons acceptor. The process of nitrite reduction to nitric oxide (NO) in the mitochondrial electron transport chain recycles NADH and leads to a limited rate of ATP production. The produced ATP alongside with the ATP generated by fermentation supports the processes of transcription and translation required for hypoxic survival and recovery of plants. Non-symbiotic hemoglobins (called phytoglobins in plants) scavenge NO and thus contribute to regeneration of NAD(+) and nitrate required for the operation of anaerobic energy metabolism. This overall operation represents an important strategy of biochemical adaptation that results in the improvement of energy status and thereby in protection of plants in the conditions of hypoxic stress.

L-Plastin S-glutathionylation promotes reduced binding to β-actin and affects neutrophil functions

Posttranslational modifications (PTMs) of cytoskeleton proteins due to oxidative stress associated with several pathological conditions often lead to alterations in cell function. The current study evaluates the effect of nitric oxide (DETA-NO)-induced oxidative stress-related S-glutathionylation of cytoskeleton proteins in human PMNs. By using in vitro and genetic approaches, we showed that S-glutathionylation of L-plastin (LPL) and β-actin promotes reduced chemotaxis, polarization, bactericidal activity, and phagocytosis. We identified Cys-206, Cys-283, and Cys-460as S-thiolated residues in the β-actin-binding domain of LPL, where cys-460 had the maximum score. Site-directed mutagenesis of LPL Cys-460 further confirmed the role in the redox regulation of LPL. S-Thiolation diminished binding as well as the bundling activity of LPL. The presence of S-thiolated LPL was detected in neutrophils from both diabetic patients and db/db mice with impaired PMN functions. Thus, enhanced nitroxidative stress may results in LPL S-glutathionylation leading to impaired chemotaxis, polarization, and bactericidal activity of human PMNs, providing a mechanistic basis for their impaired functions in diabetes mellitus.

Nitrite-mediated renal vasodilatation is increased during ischemic conditions via cGMP-independent signaling

The kidney is vulnerable to hypoxia, and substantial efforts have been made to ameliorate renal ischemic injury secondary to pathological conditions. Stimulation of the nitrate-nitrite-nitric oxide pathway is associated with renal and cardiovascular protection in disease models, but less is known about the vascular effects during renal ischemia. This study was aimed at investigating the vascular effects of nitrite in the kidney during normoxic and ischemic conditions. Using a multiwire myograph system, we assessed nitrite-mediated relaxation (10(-9)-10(-4)mol/L) in isolated and preconstricted renal interlobar arteries from C57BL/6 mice under normal conditions (pO2 13kPa; pH 7.4) and with low oxygen tension and low pH to mimic ischemia (pO2 3kPa; pH 6.6). Xanthine oxidoreductase expression was analyzed by quantitative PCR, and production of reactive nitrogen species was measured by DAF-FM DA fluorescence. During normoxia significant vasodilatation (15±3%) was observed only at the highest concentration of nitrite, which was dependent on NO-sGC-cGMP signaling. The vasodilatory responses to nitrite were greatly sensitized and enhanced during hypoxia with low pH, demonstrating significant dilatation (11±1%) already in the physiological range (10(-8)mol/L), with a maximum response of 27±2% at 10(-4) mol/L. In contrast to normoxia, and to that observed with a classical NO donor (DEA NONOate), this sensitization was independent of sGC-cGMP signaling. Moreover, inhibition of various enzymatic systems reported to reduce nitrite in other vascular beds, i.e., aldehyde oxidase (raloxifene), aldehyde dehydrogenase (cyanamide), and NO synthase (L-NAME), had no effect on the nitrite response. However, inhibition of xanthine oxidoreductase (XOR; febuxostat or allopurinol) abolished the sensitized response to nitrite during hypoxia and acidosis. In conclusion, in contrast to normoxia, nitrite exerted potent vasorelaxation during ischemic conditions already at physiological concentrations. This effect was dependent on functional XOR but independent of classical downstream signaling by sGC-cGMP.

Insights into the diverse effects of nitric oxide on tumor biology

Among its many roles in cellular biology, nitric oxide (·NO) has long been associated with cancers both as a protumorigenic and as an antitumorigenic agent. The dual nature of this signaling molecule in varied settings is attributable to its temporal and concentration-dependent effects that produce different phenotypes. The steady-state ·NO concentration within the cell is a balance between its rate of enzymatic synthesis from the three nitric oxide synthase (NOS) isoforms and consumption via numerous metabolic pathways and demonstrates strong dependence on the tissue oxygen concentration. NOS expression and ·NO production are often deregulated and associated with numerous types of cancers with dissimilar prognostic outcomes. ·NO influences several facets of tumor initiation and progression including DNA damage, chronic inflammation, angiogenesis, epithelial-mesenchymal transition, and metastasis, to name a few. The role of ·NO as an epigenetic modulator has also recently emerged and has potentially important mechanistic implications in regulating transcription of oncogenes and tumor-suppressor genes. ·NO-derived cellular adducts such as dinitrosyliron complexes and the formation of higher nitrogen oxides further alter its cellular behavior. Among anticancer strategies, the use of NOS as a prognostic biomarker and modulation of ·NO production for therapeutic benefit have gained importance over the past decade. Numerous ·NO-releasing drugs and NOS inhibitors have been evaluated in preclinical and clinical settings to arrest tumor growth. Taken together, ·NO affects various arms of cancer signaling networks. An overview of this complex interplay is provided in this chapter.

Mechanisms and kinetic profiles of superoxide-stimulated nitrosative processes in cells using a diaminofluorescein probe

In this study, we examined the mechanisms and kinetic profiles of intracellular nitrosative processes using diaminofluorescein (DAF-2) as a target in RAW 264.7 cells. The intracellular formation of the fluorescent, nitrosated product diaminofluorescein triazol (DAFT) from both endogenous and exogenous nitric oxide (NO) was prevented by deoxygenation and by cell membrane-permeable superoxide (O2(-)) scavengers but not by extracellular bovine Cu,Zn-SOD. In addition, the DAFT formation rate decreased in the presence of cell membrane-permeable Mn porphyrins that are known to scavenge peroxynitrite (ONOO(-)) but was enhanced by HCO3(-)/CO2. Together, these results indicate that nitrosative processes in RAW 264.7 cells depend on endogenous intracellular O2(-) and are stimulated by ONOO(-)/CO2-derived radical oxidants. The N2O3 scavenger sodium azide (NaN3) only partially attenuated the DAFT formation rate and only with high NO (>120 nM), suggesting that DAFT formation occurs by nitrosation (azide-susceptible DAFT formation) and predominantly by oxidative nitrosylation (azide-resistant DAFT formation). Interestingly, the DAFT formation rate increased linearly with NO concentrations of up to 120-140 nM but thereafter underwent a sharp transition and became insensitive to NO. This behavior indicates the sudden exhaustion of an endogenous cell substrate that reacts rapidly with NO and induces nitrosative processes, consistent with the involvement of intracellular O2(-). On the other hand, intracellular DAFT formation stimulated by a fixed flux of xanthine oxidase-derived extracellular O2(-) that also occurs by nitrosation and oxidative nitrosylation increased, peaked, and then decreased with increasing NO, as previously observed. Thus, our findings complementarily show that intra- and extracellular O2(-)-dependent nitrosative processes occurring by the same chemical mechanisms do not necessarily depend on NO concentration and exhibit different unusual kinetic profiles with NO dynamics, depending on the biological compartment in which NO and O2(-) interact.

Nitric oxide synthases, S-nitrosylation and cardiovascular health: from molecular mechanisms to therapeutic opportunities (review)

The understanding of nitric oxide (NO) signaling has grown substantially since the identification of endothelial derived relaxing factor (EDRF). NO has emerged as a ubiquitous signaling molecule involved in diverse physiological and pathological processes. Perhaps the most significant function, independent of EDRF, is that of NO signaling mediated locally in signaling modules rather than relying upon diffusion. In this context, NO modulates protein function via direct post-translational modification of cysteine residues. This review explores NO signaling and related reactive nitrogen species involved in the regulation of the cardiovascular system. A critical concept in the understanding of NO signaling is that of the nitroso-redox balance. Reactive nitrogen species bioactivity is fundamentally linked to the production of reactive oxygen species. This interaction occurs at the chemical, enzymatic and signaling effector levels. Furthermore, the nitroso-redox equilibrium is in a delicate balance, involving the cross-talk between NO and oxygen-derived species signaling systems, including NADPH oxidases and xanthine oxidase.

Reaction of ruthenium(II) complexes with 2,2-diphenyl-1-picrylhydrazyl (DPPH•) and hydroxyl radicals

The reaction of the complexes trans-[RuII(NO+)(NH3)4L] and [RuII(NO+)HEDTA] with 2,2-diphenyl-1-picrylhydrazyl (DPPH•) and hydroxyl (OH•) radicals has been investigated at 25.0 ± 0.1 °C using spectroscopic (UV-vis and electron paramagnetic resonance) and electrochemical techniques (differential pulse voltammetry and cyclic voltammetry). The redox potential of RuIII/RuII for the ruthenium nitrosyl complexes was determined and is in the range of +2.2 V (L = HEDTA) to +2.6 V (L = isn) versus the normal hydrogen electrode . The trans-[RuII(NO+)(NH3)4L]3+ and [RuII(NO+)HEDTA] complexes do not react with the DPPH• radical in aqueous solution at pH = 3.0. However, the corresponding aquo species, trans-[RuII(H2O)(NH3)4L]2+ and [RuII(H2O)HEDTA]+, respectively, react quantitatively, resulting in metal center oxidation. The trans-[RuII(NO+)(NH3)4L]3+ and [RuII(NO+)HEDTA] complexes react with OH• through a complicated pathway that leads to metal center oxidation followed by a series of secondary reactions. In addition to acting as NO donors, after their reduction, these ruthenium(II) nitrosyl complexes exhibit, through their generated aquo species, a radical scavenging ability, which is important for better understanding the already proven biological activity of these ruthenium nitrosyls in vivo.

Nitroxidative chemistry interferes with fluorescent probe chemistry: implications for nitric oxide detection using 2,3-diaminonaphthalene

Simultaneous production of nitric oxide (NO) and superoxide generates peroxynitrite and causes nitroxidative stress. The fluorometric method for NO detection is based on the formation of a fluorescent product from the reaction of a nonfluorescent probe molecule with NO-derived nitrosating species. Here, we present an example of how nitroxidative chemistry could interact with fluorescent probe chemistry. 2,3-Naphthotriazole (NAT) is the NO-derived fluorescent product of 2,3-diaminonaphthalene (DAN), a commonly used NO-detecting molecule. We show that NO/superoxide cogeneration, and particularly peroxynitrite, mediates the chemical decomposition of NAT. Moreover, the extent of NAT decomposition depends on the relative fluxes of NO and superoxide; the maximum effect being reached at almost equivalent generation rates for both radicals. The rate constant for the reaction of NAT with peroxynitrite was determined to be 2.2×10(3)M(-1)s(-1). Further, various peroxynitrite scavengers were shown to effectively inhibit NO/superoxide- and peroxynitrite-mediated decomposition of NAT. Taken together, the present study suggests that the interference of a fluorometric NO assay can be originated from the interaction between the final fluorescent product and the formed reactive nitrogen and oxygen species.

Unexpected NO transfer reaction between trans-[Ru(II)(NO+)(NH3)4(L)]3+ and Fe(III) species: observation of a heterobimetallic NO-bridged intermediate

The reaction between trans-[Ru(II)(NO(+))(NH3)4(L)](3+), L = ImN, IsN, Nic, P(OMe)3, P(OEt)3, and P(OH)(OEt)2, and the Fe(III) species [Fe(III)(TPPS)], metmyoglobin, and hemoglobin was monitored by UV-vis, EPR, and electrochemical techniques (DPV, CV). No reaction was observed when L = ImN, IsN, Nic, and P(OH)(OEt)2. However, when L = P(OMe)3 and P(OEt)3, the reaction was quantitative and the products were trans-[Ru(III)(H2O)(NH3)4(P(OR)3)](3+) and [Fe(II)(NO(+))] species. Reaction kinetics data and DFT calculations suggest a two-step reaction mechanism with the initial formation of a bridged [Ru-(μNO)-Fe] intermediate, which was confirmed through electrochemical techniques (E(0)' = -0.47 V vs NHE). The calculated specific rate constant values for the reaction were in the ranges k1 = 1.1 to 7.7 L mol(-1) s(-1) and k2 = 2.4 × 10(-3) to 11.4 × 10(-3) s(-1) for L = P(OMe)3 and P(OEt)3. The oxidation of the ruthenium center (Ru(II) to Ru(III)) containing the nitrosonium ligand suggests that NO can act as an electron transfer bridge between the two metal centers.

Sensing hypoxia by mitochondria: a unifying hypothesis involving S-nitrosation

SIGNIFICANCE

Sudden hypoxia requires a rapid response in tissues with high energy demand. Mitochondria are rapid sensors for a lack of oxygen, but no consistent mechanism for the sensing process and the subsequent counter-regulation has been described.

RECENT ADVANCES

In the present hypothesis review, we suggest an oxygen-sensing mechanism by mitochondria that is initiated at low oxygen tension by electrons from the respiratory chain, leading to the reduction of intracellular nitrite to nitric oxide ((•)NO) that would subsequently compete with oxygen for binding to cytochrome c oxidase. This allows superoxide ((•)O2(-)) formation in hypoxic areas, leading to S-nitrosation and the inhibition of mitochondrial Krebs cycle enzymes. With more formation of (•)O2(-), peroxynitrite is generated and known to damage the connection between the mitochondrial matrix and the outer membrane.

CRITICAL ISSUES

A fundamental question on a regulatory mechanism is its reversibility. Readmission of oxygen and opening of the mitochondrial KATP-channel would allow electrons from glycerol-3-phosphate to selectively reduce the ubiquinone pool to generate (•)O2(-) at both sides of the inner mitochondrial membrane. On the cytosolic side, superoxide is dismutated and will support H2O2/Fe(2+)-dependent transcription processes and on the mitochondrial matrix side, it could lead to the one-electron reduction and reactivation of S-nitrosated proteins.

FUTURE DIRECTIONS

It remains to be elucidated up to which stage the herein proposed silencing of mitochondria remains reversible and when irreversible changes that ultimately lead to classical reperfusion injury are initiated.

Efficient nitrosation of glutathione by nitric oxide

Nitrosothiols are increasingly regarded as important participants in a range of physiological processes, yet little is known about their biological generation. Nitrosothiols can be formed from the corresponding thiols by nitric oxide in a reaction that requires the presence of oxygen and is mediated by reactive intermediates (NO₂ or N₂O₃) formed in the course of NO autoxidation. Because the autoxidation of NO is second order in NO, it is extremely slow at submicromolar NO concentrations, casting doubt on its physiological relevance. In this paper we present evidence that at submicromolar NO concentrations the aerobic nitrosation of glutathione does not involve NO autoxidation but a reaction that is first order in NO. We show that this reaction produces nitrosoglutathione efficiently in a reaction that is strongly stimulated by physiological concentrations of Mg(2+). These observations suggest that direct aerobic nitrosation may represent a physiologically relevant pathway of nitrosothiol formation.

Bacillus anthracis co-opts nitric oxide and host serum albumin for pathogenicity in hypoxic conditions

Bacillus anthracis is a dangerous pathogen of humans and many animal species. Its virulence has been mainly attributed to the production of Lethal and Edema toxins as well as the antiphagocytic capsule. Recent data indicate that the nitric oxide (NO) synthase (baNOS) plays an important pathogenic role at the early stage of disease by protecting bacteria from the host reactive species and S-nytrosylating the mitochondrial proteins in macrophages. In this study we for the first time present evidence that bacteria-derived NO participates in the generation of highly reactive oxidizing species which could be abolished by the NOS inhibitor L - NAME, free thiols, and superoxide dismutase but not catalase. The formation of toxicants is likely a result of the simultaneous formation of NO and superoxide leading to a labile peroxynitrite and its stable decomposition product, nitrogen dioxide. The toxicity of bacteria could be potentiated in the presence of bovine serum albumin. This effect is consistent with the property of serum albumin to serves as a trap of a volatile NO accelerating its reactions. Our data suggest that during infection in the hypoxic environment of pre-mortal host the accumulated NO is expected to have a broad toxic impact on host cell functions.

Methods for detection and characterization of protein S-nitrosylation

Reversible protein S-nitrosylation, defined as the covalent addition of a nitroso moiety to the reactive thiol group on a cysteine residue, has received increasing recognition as a critical post-translational modification that exerts ubiquitous influence in a wide range of cellular pathways and physiological processes. Due to the lability of the S-NO bond, which is a dynamic modification, and the low abundance of endogenously S-nitrosylated proteins in vivo, unambiguous identification of S-nitrosylated proteins and S-nitrosylation sites remains methodologically challenging. In this review, we summarize recent advancements and the use of state-of-art approaches for the enrichment, systematic identification and quantitation of S-nitrosylation protein targets and their modification sites at the S-nitrosoproteome scale. These advancements have facilitated the global identification of >3000 S-nitrosylated proteins that are associated with wide range of human diseases. These strategies hold promise to site-specifically unravel potential molecular targets and to change S-nitrosylation-based pathophysiology, which may further the understanding of the potential role of S-nitrosylation in diseases.

The contribution of N₂O₃ to the cytotoxicity of the nitric oxide donor DETA/NO: an emerging role for S-nitrosylation

The relationship between the biological activity of NO and its chemistry is complex. The objectives of this study were to investigate the influence of oxygen tension on the cytotoxicity of the NO^• donor DETA/NO and to determine the effects of oxygen tension on the key RNS (reactive nitrogen species) responsible for any subsequent toxicity. The findings presented in this study indicate that the DETA/NO-mediated cytotoxic effects were enhanced under hypoxic conditions. Further investigations revealed that neither ONOO⁻ (peroxynitrite) nor nitroxyl was generated. Fluorimetric analysis in the presence of scavengers suggest for the first time that another RNS, dinitrogen trioxide may be responsible for the cytotoxicity with DETA/NO. Results showed destabilization of HIF (hypoxia inducible factor)-1α and depletion of GSH levels following the treatment with DETA/NO under hypoxia, which renders cells more susceptible to DETA/NO cytotoxicity, and could account for another mechanism of DETA/NO cytotoxicity under hypoxia. In addition, there was significant accumulation of nuclear p53, which showed that p53 itself might be a target for S-nitrosylation following the treatment with DETA/NO. Both the intrinsic apoptotic pathway and the Fas extrinsic apoptotic pathway were also activated. Finally, GAPDH (glyceraldehyde-3-phosphate dehydrogenase) is another important S-nitrosylated protein that may possibly play a key role in DETA/NO-mediated apoptosis and cytotoxicity. Therefore this study elucidates further mechanisms of DETA/NO mediated cytotoxicity with respect to S-nitrosylation that is emerging as a key player in the signalling and detection of DETA/NO-modified proteins in the tumour microenvironment.

Vascular aging: chronic oxidative stress and impairment of redox signaling-consequences for vascular homeostasis and disease

Characteristic morphological and molecular alterations such as vessel wall thickening and reduction of nitric oxide occur in the aging vasculature leading to the gradual loss of vascular homeostasis. Consequently, the risk of developing acute and chronic cardiovascular diseases increases with age. Current research of the underlying molecular mechanisms of endothelial function demonstrates a duality of reactive oxygen and nitrogen species in contributing to vascular homeostasis or leading to detrimental effects when formed in excess. Furthermore, changes in function and redox status of vascular smooth muscle cells contribute to age-related vascular remodeling. The age-dependent increase in free radical formation causes deterioration of the nitric oxide signaling cascade, alters and activates prostaglandin metabolism, and promotes novel oxidative posttranslational protein modifications that interfere with vascular and cell signaling pathways. As a result, vascular dysfunction manifests. Compensatory mechanisms are initially activated to cope with age-induced oxidative stress, but become futile, which results in irreversible oxidative modifications of biological macromolecules. These findings support the 'free radical theory of aging' but also show that reactive oxygen and nitrogen species are essential signaling molecules, regulating vascular homeostasis.

S-nitrosothiols and the S-nitrosoproteome of the cardiovascular system

SIGNIFICANCE

Since their discovery in the early 1990's, S-nitrosylated proteins have been increasingly recognized as important determinants of many biochemical processes. Specifically, S-nitrosothiols in the cardiovascular system exert many actions, including promoting vasodilation, inhibiting platelet aggregation, and regulating Ca(2+) channel function that influences myocyte contractility and electrophysiologic stability.

RECENT ADVANCES

Contemporary developments in liquid chromatography-mass spectrometry methods, the development of biotin- and His-tag switch assays, and the availability of cyanide dye-labeling for S-nitrosothiol detection in vitro have increased significantly the identification of a number of cardiovascular protein targets of S-nitrosylation in vivo.

CRITICAL ISSUES

Recent analyses using modern S-nitrosothiol detection techniques have revealed the mechanistic significance of S-nitrosylation to the pathophysiology of numerous cardiovascular diseases, including essential hypertension, pulmonary hypertension, ischemic heart disease, stroke, and congestive heart failure, among others.

FUTURE DIRECTIONS

Despite enhanced insight into S-nitrosothiol biochemistry, translating these advances into beneficial pharmacotherapies for patients with cardiovascular diseases remains a primary as-yet unmet goal for investigators within the field.

Kinetic analysis of DAF-FM activation by NO: toward calibration of a NO-sensitive fluorescent dye

Nitric oxide (NO) research in biomedicine has been hampered by the absence of a method that will allow quantitative measurement of NO in biological tissues with high sensitivity and selectivity, and with adequate spatial and temporal resolution. 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) is a NO sensitive fluorescence probe that has been used widely for qualitative assessment of cellular NO production. However, calibration of the fluorescent signal and quantification of NO concentration in cells and tissues using fluorescent probes, have provided significant challenge. In this study we utilize a combination of mathematical modeling and experimentation to elucidate the kinetics of NO/DAF-FM reaction in solution. Modeling and experiments suggest that the slope of fluorescent intensity (FI) can be related to NO concentration according to the equation: ddtFI=2αk(1)NO(2)O(2)DAF-FMkNO+DAF-FM where α is a proportionality coefficient that relates FI to unit concentration of activated DAF-FM, k(1) is the NO oxidation rate constant, and k was estimated to be 4.3±0.6. The FI slope exhibits saturation kinetics with DAF-FM concentration. Interestingly, the effective half-maximum constant (EC(50)) increases proportionally to NO concentration. This result is not in agreement with the proposition that N(2)O(3) is the NO oxidation byproduct that activates DAF-FM. Kinetic analysis suggests that the reactive intermediate should exhibit NO-dependent consumption and thus NO(2)() is a more likely candidate. The derived rate law can be used for the calibration of DAF-FM fluorescence and the quantification of NO concentration in biological tissues.

Nitric oxide reduces NADPH oxidase 5 (Nox5) activity by reversible S-nitrosylation

The NADPH oxidases (Noxs) are a family of transmembrane oxidoreductases that produce superoxide and other reactive oxygen species (ROS). Nox5 was the last of the conventional Nox isoforms to be identified and is a calcium-dependent enzyme that does not depend on accessory subunits for activation. Recently, Nox5 was shown to be expressed in human blood vessels and therefore the goal of this study was to determine whether nitric oxide (NO) can modulate Nox5 activity. Endogenously produced NO potently inhibited basal and stimulated Nox5 activity and this inhibition was reversible with chronic, but not acute, exposure to L-NAME. Nox5 activity was reduced by NO donors, iNOS, and eNOS and in endothelial cells and LPS-stimulated smooth muscle cells in a manner dependent on NO concentration. ROS production was diminished by NO in an isolated enzyme activity assay replete with surplus calcium and NADPH. There was no evidence for NO-dependent changes in tyrosine nitration, glutathiolation, or phosphorylation of Nox5. In contrast, there was evidence for the increased nitrosylation of Nox5 as determined by the biotin-switch assay and mass spectrometry. Four S-nitrosylation sites were identified and of these, mutation of C694 dramatically lowered Nox5 activity, NO sensitivity, and biotin labeling. Furthermore, coexpression of the denitrosylation enzymes thioredoxin 1 and GSNO reductase prevented NO-dependent inhibition of Nox5. The potency of NO against other Nox enzymes was in the order Nox1 ≥ Nox3 > Nox5 > Nox2, whereas Nox4 was refractory. Collectively, these results suggest that endogenously produced NO can directly S-nitrosylate and inhibit the activity of Nox5.

A Conspectus of Cellular Mechanisms of Nitrosothiol Formation from Nitric Oxide

Although chemical mechanisms for the formation of nitrosothiol from •NO have been studied extensively "in the test tube", surprisingly little is known regarding the mechanism(s) of how nitrosothiols are formed in vivo. This lack of understanding has hampered more general acceptance of the concept of cysteine nitrosothiol formation as a generally applicable, regulated, and functionally significant protein posttranslational modification (as opposed to multiple other •NO-induced thiol modifications). Here we provide a brief overview/summary of the cellular formation of nitrosothiols from •NO via two possible mechanisms involving oxygen or transition metals.

An integrated approach to assessing nitroso-redox balance in systemic inflammation

Most studies examining the metabolic fate of NO during systemic inflammation have focused on measuring the quantitatively predominating, stable anions nitrite and nitrate within the circulation. However, these are not necessarily the NO-related products that govern NO metabolism and signaling in tissues. We assessed all major NO derivatives temporally in blood and vital organs during inflammation and explored their relationship to insult severity and redox status. Male rats receiving intraperitoneal endotoxin or vehicle were sacrificed for organ and blood sampling between 0 and 24 h. Endotoxin induced transient and organ-specific changes in a variety of NO metabolites. Nitrite and nitrate increased, peaking at 8 and 12 h, respectively. S- and N-nitrosation and heme-nitrosylation products also peaked at 8 h; these posttranslational protein modifications were associated with decreased myocardial function (echocardiography). Evidence of oxidative stress and systemic inflammation was also obtained. The rise in most NO derivatives was proportional to insult severity. All metabolite levels normalized within 24 h, despite evidence of persisting myocardial dysfunction and clinical unwellness. Our findings point to a complex interplay between NO production, antioxidant defense, and redox status. Although the precise (patho)physiologic roles of specific NO derivatives and their diagnostic/prognostic utility await further investigation, nitroso species in erythrocytes are the most sensitive markers of NO in systemic inflammation, detectable before clinical symptoms manifest.

Endogenous presynaptic nitric oxide supports an anterograde signaling in the central nervous system

The source size and density determine the extent of nitric oxide (NO) diffusion which critically influences NO signaling. In the brain, NO released from postsynaptic somas following NMDA-mediated activation of neuronal nitric oxide synthase (nNOS) retrogradely affects smaller presynaptic targets. By contrast, in guinea pig trigeminal motor nucleus (TMN), NO is produced presynaptically by tiny and disperse nNOS-containing terminals that innervate large nNOS-negative motoneurons expressing the soluble guanylyl-cyclase (sGC); consequently, it is uncertain whether endogenous NO supports an anterograde signaling between pre-motor terminals and postsynaptic trigeminal motoneurons. In retrogradely labeled motoneurons, we indirectly monitored NO using triazolofluorescein (DAF-2T) fluorescence, and evaluated sGC activity by confocal cGMP immunofluorescence. Multiple fibers stimulation enhanced NO content and cGMP immunofluorescence into numerous nNOS-negative motoneurons; NOS inhibitors prevented depolarization-induced effects, whereas NO donors mimicked them. Enhance of cGMP immunofluorescence required extracellular Ca(2+), a nNOS-physiological activator, and was prevented by inhibiting sGC, silencing neuronal activity or impeding NO diffusion. In conclusion, NO released presynaptically from multiple cooperative tiny fibers attains concentrations sufficient to activate sGC in many motoneurons despite of the low source/target size ratio and source dispersion; thus, endogenous NO is an effective anterograde neuromodulator. By adjusting nNOS activation, presynaptic Ca(2+) might modulate the NO diffusion field in the TMN.

Simvastatin re-couples dysfunctional endothelial nitric oxide synthase in experimental subarachnoid hemorrhage

Reduced endothelial nitric oxide synthase (eNOS) function has been linked to secondary complications of subarachnoid hemorrhage (SAH). We previously found that there is increased eNOS function after SAH but that it is uncoupled, leading to secondary complications such as vasospasm, microthromboembolism and neuronal apoptosis. Here we test the hypothesis that recoupling eNOS with simvastatin can prevent these complications. SAH was created in mice that were treated with vehicle or simvastatin starting 2 weeks before or 30 minutes after SAH. SAH increased phosphorylated eNOS which was prevented by pre- or post-treatment with simvastatin. Simvastatin pre-treatment also prevented the increase in eNOS monomer formation that was associated with SAH, decreased superoxide anion radical production and increased NO. These changes were associated with decreased vasospasm, microthromboemboli and neuronal injury. The data suggest that simvastatin re-couples eNOS after SAH, leading to decreased secondary complications such as vasospasm, microthromboemboli and neuronal injury.