Dinitrosyl iron complexes with thiolate ligands: Physico-chemistry, biochemistry and physiology

DNIC is a Structure Providing Specificity of Physiological Effects of NO

Metabolism of nitric oxide (NO) donors: dinitrosyl iron complexes (DNIC), nitrosothiols (RSNO), and nitroprusside was studied on a chick embryo model. The obtained results give reason to assume that DNIC constituting the main pool of nitroso compounds in the vast majority of tissues are NO donors immediately interacting with the physiological target of NO, and other NO donors can perform this function after their transformation into DNIC. NO is released from DNIC not spontaneously, but under a joint influence of a factor destroying the complex and a target having chemical affinity for NO. A similar mechanism is apparently implicated in NO passage through the cell membrane.

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.

Unraveling signatures of chicken genetic diversity and divergent selection in breed-specific patterns of early myogenesis, nitric oxide metabolism and post-hatch growth

Introduction: Due to long-term domestication, breeding and divergent selection, a vast genetic diversity in poultry currently exists, with various breeds being characterized by unique phenotypic and genetic features. Assuming that differences between chicken breeds divergently selected for economically and culturally important traits manifest as early as possible in development and growth stages, we aimed to explore breed-specific patterns and interrelations of embryo myogenesis, nitric oxide (NO) metabolism and post-hatch growth rate (GR). Methods: These characteristics were explored in eight breeds of different utility types (meat-type, dual purpose, egg-type, game, and fancy) by incubating 70 fertile eggs per breed. To screen the differential expression of seven key myogenesis associated genes (MSTN, GHR, MEF2C, MYOD1, MYOG, MYH1, and MYF5), quantitative real-time PCR was used. Results: We found that myogenesis associated genes expressed in the breast and thigh muscles in a coordinated manner showing breed specificity as a genetic diversity signature among the breeds studied. Notably, coordinated ("accord") expression patterns of MSTN, GHR, and MEFC2 were observed both in the breast and thigh muscles. Also, associated expression vectors were identified for MYOG and MYOD1 in the breast muscles and for MYOG and MYF5 genes in the thigh muscles. Indices of NO oxidation and post-hatch growth were generally concordant with utility types of breeds, with meat-types breeds demonstrating higher NO oxidation levels and greater GR values as compared to egg-type, dual purpose, game and fancy breeds. Discussion: The results of this study suggest that differences in early myogenesis, NO metabolism and post-hatch growth are breed-specific; they appropriately reflect genetic diversity and accurately capture the evolutionary history of divergently selected chicken breeds.

Some Beneficial Effects of Inert Gases on Blood Oxidative Metabolism: In Vivo Study

The aim of the study was to assess the effect of external use of inert gases (helium and argon) on the state of free radical processes in vivo. The experiment was performed on 30 male Wistar stock rats (age-3 months, weight-200-220 g.), randomly distributed into 3 equal groups. The first group of animals was intact (n = 10). The animals of the second and third groups were treated with argon and helium streams, respectively. Our research has allowed us to establish that the studied inert gases have a modulating effect on the state of oxidative metabolism of rat blood, and the nature of this effect is directly determined by the type of gas. The results of this study allowed us to establish the potential antioxidant effect of the helium stream, mainly realized due to the activation of the catalytic properties of the enzymatic link of the antioxidant system of rat blood plasma. At the same time, the revealed features of shifts in oxidative metabolism during treatment with argon flow include not only stimulation of the antioxidant system but also the pronounced induction of free radical oxidation. Thus, the conducted studies made it possible to verify the specificity of the response of the oxidative metabolism of blood plasma to the use of inert gases, depending on their type.

The Influence of Cationic Nitrosyl Iron Complex with Penicillamine Ligands on Model Membranes, Membrane-Bound Enzymes and Lipid Peroxidation

This paper shows the biological effects of cationic binuclear tetranitrosyl iron complex with penicillamine ligands (TNIC-PA). Interaction with a model membrane was assessed using a fluorescent probes technique. Antioxidant activity was studied using a thiobarbituric acid reactive species assay (TBARS) and a chemiluminescence assay. The catalytic activity of monoamine oxidase (MAO) was determined by measuring liberation of ammonia. Antiglycation activity was determined fluometrically by thermal glycation of albumine by D-glucose. The higher values of Stern-Volmer constants (K_SV) obtained for the pyrene located in hydrophobic regions (3.9 × 10⁴ M^-1) compared to K_SV obtained for eosin Y located in the polar headgroup region (0.9 × 10⁴ M^-1) confirms that TNIC-PA molecules prefer to be located in the hydrophobic acyl chain region, close to the glycerol group of lipid molecules. TNIC-PA effectively inhibited the process of spontaneous lipid peroxidation, due to additive contributions from releasing NO and penicillamine ligand (IC50 = 21.4 µM) and quenched luminol chemiluminescence (IC50 = 3.6 μM). High activity of TNIC-PA in both tests allows us to assume a significant role of its radical-scavenging activity in the realization of antioxidant activity. It was shown that TNIC-PA (50-1000 μM) selectively inhibits the membrane-bound enzyme MAO-A, a major source of ROS in the heart. In addition, TNIC-PA is an effective inhibitor of non-enzymatic protein glycation. Thus, the evaluated biological effects of TNIC-PA open up the possibility of its practical application in chemotherapy for socially significant diseases, especially cardiovascular diseases.

Effect of solvents and glutathione on the decomposition of the nitrosyl iron complex with N-ethylthiourea ligands: An experimental and theoretical study

Dinitrosyl iron complexes (DNICs) are a depot and potential source of free NO in organisms. Their synthetic analog, N-ethylthiourea DNIC [Fe(SC(NH₂)(NHC₂H₅))₂(NO)₂]⁺Cl^-∙[Fe(SC(NH₂)(NHC₂H₅))Cl(NO)₂]⁰ (complex 1), as cardioprotective and cytostatic agent is a promising prodrug for the treatment of socially relevant diseases. In this work, transformation mechanism of complex 1 has been studied in anaerobic aqueous solution (pH = 7.0), DMSO, and ethanol. It was shown that the solvent has a significant effect on the decomposition of complex. According to EPR-spectroscopy, only cationic part of complex is found upon its dissolution in water; only neutral part is retained in DMSO, and both fragments are present in ethanol. Effective generation of NO occurs in an aqueous solution. The structures of the decomposition products were proposed for all solvents, their UV-spectra and rate constants were calculated. From the experimental and theoretical data obtained, it follows that complex 1 is most stable in DMSO. Solutions of complex in a DMSO-water mixture can be used to improve its bioavailability in further in vitro and in vivo studies. Also, we have analyzed its interaction with glutathione (GSH), which can participate in the metabolism of this compound. This study shows that complex 1 reacts with GSH to form a new binuclear DNIC with two GS^--ligands. It was found that the resulting complex is a more prolonged NO-donor than the initial one: k = 6.1∙10^-3·s^-1 in buffer, k = 6.4∙10^-5 s^-1 with GSH. This reaction may prevent S-glutathionylation of the essential enzyme systems and is important for metabolism of complex, associated with its antitumor activity.

Cold Argon Athmospheric Plasma for Biomedicine: Biological Effects, Applications and Possibilities

Currently, plasma medicine is a synthetic direction that unites the efforts of specialists of various profiles. For the successful formation of plasma medicine, it is necessary to solve a large complex of problems, including creating equipment for generating cold plasma, revealing the biological effects of this effect, as well as identifying and justifying the most promising areas of its application. It is known that these biological effects include antibacterial and antiviral activity, the ability to stimulate hemocoagulation, pro-regenerative properties, etc. The possibility of using the factor in tissue engineering and implantology is also shown. Based on this, the purpose of this review was to form a unified understanding of the biological effects and biomedical applications of argon cold plasma. The review shows that cold plasma, like any other physical and chemical factors, has dose dependence, and the variable parameter in this case is the exposure of its application. One of the significant characteristics determining the specificity of the cold plasma effect is the carrier gas selection. This gas carrier is not just an ionized medium but modulates the response of biosystems to it. Finally, the perception of cold plasma by cellular structures can be carried out by activating a special molecular biosensor, the functioning of which significantly depends on the parameters of the medium (in the field of plasma generation and the cell itself). Further research in this area can open up new prospects for the effective use of cold plasma.

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.

The Di-Iron Protein YtfE Is a Nitric Oxide-Generating Nitrite Reductase Involved in the Management of Nitrosative Stress

Previously characterized nitrite reductases fall into three classes: siroheme-containing enzymes (NirBD), cytochrome c hemoproteins (NrfA and NirS), and copper-containing enzymes (NirK). We show here that the di-iron protein YtfE represents a physiologically relevant new class of nitrite reductases. Several functions have been previously proposed for YtfE, including donating iron for the repair of iron–sulfur clusters that have been damaged by nitrosative stress, releasing nitric oxide (NO) from nitrosylated iron, and reducing NO to nitrous oxide (N₂O). Here, in vivo reporter assays confirmed that Escherichia coli YtfE increased cytoplasmic NO production from nitrite. Spectroscopic and mass spectrometric investigations revealed that the di-iron site of YtfE exists in a mixture of forms, including nitrosylated and nitrite-bound, when isolated from nitrite-supplemented, but not nitrate-supplemented, cultures. Addition of nitrite to di-ferrous YtfE resulted in nitrosylated YtfE and the release of NO. Kinetics of nitrite reduction were dependent on the nature of the reductant; the lowest K_m, measured for the di-ferrous form, was ∼90 μM, well within the intracellular nitrite concentration range. The vicinal di-cysteine motif, located in the N-terminal domain of YtfE, was shown to function in the delivery of electrons to the di-iron center. Notably, YtfE exhibited very low NO reductase activity and was only able to act as an iron donor for reconstitution of apo-ferredoxin under conditions that damaged its di-iron center. Thus, YtfE is a high-affinity, low-capacity nitrite reductase that we propose functions to relieve nitrosative stress by acting in combination with the co-regulated NO-consuming enzymes Hmp and Hcp.

Iron in leaves: chemical forms, signalling, and in-cell distribution

Iron (Fe) is an essential transition metal. Based on its redox-active nature under biological conditions, various Fe compounds serve as cofactors in redox enzymes. In plants, the photosynthetic machinery has the highest demand for Fe. In consequence, the delivery and incorporation of Fe into cofactors of the photosynthetic apparatus is the focus of Fe metabolism in leaves. Disturbance of foliar Fe homeostasis leads to impaired biosynthesis of chlorophylls and composition of the photosynthetic machinery. Nevertheless, mitochondrial function also has a significant demand for Fe. The proper incorporation of Fe into proteins and cofactors as well as a balanced intracellular Fe status in leaf cells require the ability to sense Fe, but may also rely on indirect signals that report on the physiological processes connected to Fe homeostasis. Although multiple pieces of information have been gained on Fe signalling in roots, the regulation of Fe status in leaves has not yet been clarified in detail. In this review, we give an overview on current knowledge of foliar Fe homeostasis, from the chemical forms to the allocation and sensing of Fe in leaves.

Therapeutic Effects of Inhaled Nitric Oxide Therapy in COVID-19 Patients

The global COVID-19 pandemic has become the largest public health challenge of recent years. The incidence of COVID-19-related acute hypoxemic respiratory failure (AHRF) occurs in up to 15% of hospitalized patients. Antiviral drugs currently available to clinicians have little to no effect on mortality, length of in-hospital stay, the need for mechanical ventilation, or long-term effects. Inhaled nitric oxide (iNO) administration is a promising new non-standard approach to directly treat viral burden while enhancing oxygenation. Along with its putative antiviral affect in COVID-19 patients, iNO can reduce inflammatory cell-mediated lung injury by inhibiting neutrophil activation, lowering pulmonary vascular resistance and decreasing edema in the alveolar spaces, collectively enhancing ventilation/perfusion matching. This narrative review article presents recent literature on the iNO therapy use for COVID-19 patients. The authors suggest that early administration of the iNO therapy may be a safe and promising approach for the treatment of COVID-19 patients. The authors also discuss unconventional approaches to treatment, continuous versus intermittent high-dose iNO therapy, timing of initiation of therapy (early versus late), and novel delivery systems. Future laboratory and clinical research is required to define the role of iNO as an adjunct therapy against bacterial, viral, and fungal infections.

[Functioning of pregnan X receptor under conditions of nitrosative stress]

Pregnan X receptor (PXR) is a nuclear receptor that plays an important role in the regulation of the expression of biotransformation and metabolic enzymes. The functioning and possible mechanisms of PXR regulation under conditions of nitrosative stress have not been studied, which served as the purpose of this study. The work was performed on Caco-2 cells. Nitrosative stress (NS) was modeled using S-nitrosoglutathione (GSNO) at concentrations of 1 μM, 10 μM, 50 μM, 100 μM, and 500 μM and incubation during of 3 h, 24 h, and 72 h. The amount of PXR was assessed byWestern blotting. Incubation of Caco-2 cells with all concentrations GSNO for 3 h led to a decrease in the amount of PXR. Incubation with GSNO (1-50 μM) for 24 h was accompanied by an increase in the amount of PXR, while at a concentration of 100 μM this indicator did not significantly differ from the control, at a concentration of 500 μM it was lower. Prolonged incubation (72 h) enhanced NS and led to a normalization (1 μM GSNO) or a decrease of the PXR level (10-500 μM GSNO). The induction of PXR by GSNO was mediated by the effect of the nitrosative stress product bityrosine on the transcription factor. It was shown that bityrosine at concentrations of 0,4 mM and 1 mM increased the amount of PXR.

Dinitrosyl Iron Complexes (DNICs). From Spontaneous Assembly to Biological Roles

Dinitrosyl iron complexes (DNICs) are spontaneously and rapidly generated in cells. Their assembly requires nitric oxide (NO), biothiols, and nonheme iron, either labile iron or iron-sulfur clusters. Despite ubiquitous detection by electron paramagnetic resonance in NO-producing cells, the DNIC's chemical biology remains only partially understood. In this Forum Article, we address the reaction mechanisms for endogenous DNIC formation, with a focus on a labile iron pool as the iron source. The capability of DNICs to promote S-nitrosation is discussed in terms of S-nitrosothiol generation associated with the formation and chemical reactivity of DNICs. We also highlight how elucidation of the chemical reactivity and the dynamics of DNICs combined with the development of detection/quantification methods can provide further information regarding their participation in physiological and pathological processes.

Putative Role of Ligands of DNIC in the Physiological Action of the Complex

In a relatively isolated system of avian embryo, the metabolism of NO, a component of the dinitrosyl iron complexes (DNIC), the main NO donor in most tissues, depends on the ligands that make up the complex. This fact corroborates the earlier hypothesis that these ligands perform a regulatory function in NO metabolism. It is also shown that nitrite injected into the embryo is not oxidized to nitrate like NO in DNIC, but is accumulated outside the amniotic sac. Normally, nitrite is present in an embryo in trace amounts. These facts suggest that NO in the embryo is transferred from the donor molecule to a target in the embryo tissues further transformed with minimum oxidation to nitrite.

Physico-Chemistry of Dinitrosyl Iron Complexes as a Determinant of Their Biological Activity

In this article we minutely discuss the so-called "oxidative" mechanism of mononuclear form of dinitrosyl iron complexes (M-DNICs) formations proposed by the author. M-DNICs are proposed to be formed from their building material-neutral NO molecules, Fe2+ ions and anionic non-thiol (L-) and thiol (RS-) ligands based on the disproportionation reaction of NO molecules binding with divalent ion irons in pairs. Then a protonated form of nitroxyl anion (NO-) appearing in the reaction is released from this group and a neutral NO molecule is included instead. As a result, M-DNICs are produced. Their resonance structure is described as [(L-)2Fe2+(NO)(NO+)], in which nitrosyl ligands are represented by NO molecules and nitrosonium cations in equal proportions. Binding of hydroxyl ions with the latter causes conversion of these cations into nitrite anions at neutral pH values and therefore transformation of DNICs into the corresponding high-spin mononitrosyl iron complexes (MNICs) with the resonance structure described as [(L-)2Fe2+(NO)]. In case of replacing L- by thiol-containing ligands, which are characterized by high π-donor activity, electron density transferred from sulfur atoms to iron-dinitrosyl groups neutralizes the positive charge on nitrosonium cations, which prevents their hydrolysis, ensuring relatively a high stability of the corresponding M-DNICs with the resonance structure [(RS-)2Fe2+ (NO, NO+)]. Therefore, M-DNICs with thiol-containing ligands, as well as their binuclear analogs (B-DNICs, respective resonance structure [(RS-)2Fe2+2 (NO, NO+)2]), can serve donors of both NO and NO+. Experiments with solutions of B-DNICs with glutathione or N-acetyl-L-cysteine (B-DNIC-GSH or B-DNIC-NAC) showed that these complexes release both NO and NO+ in case of decomposition in the presence of acid or after oxidation of thiol-containing ligands in them. The level of released NO was measured via optical absorption intensity of NO in the gaseous phase, while the number of released nitrosonium cations was determined based on their inclusion in S-nitrosothiols or their conversion into nitrite anions. Biomedical research showed the ability of DNICs with thiol-containing ligands to be donors of NO and NO+ and produce various biological effects on living organisms. At the same time, NO molecules released from DNICs usually have a positive and regulatory effect on organisms, while nitrosonium cations have a negative and cytotoxic effect.

Dinitrosyl iron complexes (DNICs) as inhibitors of the SARS-CoV-2 main protease

By repurposing DNICs designed for other medicinal purposes, the possibility of protease inhibition was investigated in silico using AutoDock 4.2.6 (AD4) and in vitro via a FRET protease assay. AD4 was validated as a predictive computational tool for coordinatively unsaturated DNIC binding using the only known crystal structure of a protein-bound DNIC, PDB- (calculation RMSD = 1.77). From the in silico data the dimeric DNICs TGTA-RRE, [(μ-S-TGTA)Fe(NO)₂]₂ (TGTA = 1-thio-β-d-glucose tetraacetate) and TG-RRE, [(μ-S-TG)Fe(NO)₂]₂ (TG = 1-thio-β-d-glucose) were identified as promising leads for inhibition via coordinative inhibition at Cys-145 of the SARS-CoV-2 Main Protease (SC2M^pro). In vitro studies indicate inhibition of protease activity upon DNIC treatment, with an IC₅₀ of 38 ± 2 μM for TGTA-RRE and 33 ± 2 μM for TG-RRE. This study presents a simple computational method for predicting DNIC-protein interactions; the in vitro study is consistent with in silico leads.

Biological Mechanisms of S-Nitrosothiol Formation and Degradation: How Is Specificity of S-Nitrosylation Achieved?

The modification of protein cysteine residues underlies some of the diverse biological functions of nitric oxide (NO) in physiology and disease. The formation of stable nitrosothiols occurs under biologically relevant conditions and time scales. However, the factors that determine the selective nature of this modification remain poorly understood, making it difficult to predict thiol targets and thus construct informatics networks. In this review, the biological chemistry of NO will be considered within the context of nitrosothiol formation and degradation whilst considering how specificity is achieved in this important post-translational modification. Since nitrosothiol formation requires a formal one-electron oxidation, a classification of reaction mechanisms is proposed regarding which species undergoes electron abstraction: NO, thiol or S-NO radical intermediate. Relevant kinetic, thermodynamic and mechanistic considerations will be examined and the impact of sources of NO and the chemical nature of potential reaction targets is also discussed.

Stepwise nitrosylation of the nonheme iron site in an engineered azurin and a molecular basis for nitric oxide signaling mediated by nonheme iron proteins

Mononitrosyl and dinitrosyl iron species, such as {FeNO}⁷, {FeNO}⁸ and {Fe(NO)₂}⁹, have been proposed to play pivotal roles in the nitrosylation processes of nonheme iron centers in biological systems. Despite their importance, it has been difficult to capture and characterize them in the same scaffold of either native enzymes or their synthetic analogs due to the distinct structural requirements of the three species, using redox reagents compatible with biomolecules under physiological conditions. Here, we report the realization of stepwise nitrosylation of a mononuclear nonheme iron site in an engineered azurin under such conditions. Through tuning the number of nitric oxide equivalents and reaction time, controlled formation of {FeNO}⁷ and {Fe(NO)₂}⁹ species was achieved, and the elusive {FeNO}⁸ species was inferred by EPR spectroscopy and observed by Mössbauer spectroscopy, with complemental evidence for the conversion of {FeNO}⁷ to {Fe(NO)₂}⁹ species by UV-Vis, resonance Raman and FT-IR spectroscopies. The entire pathway of the nitrosylation process, Fe(ii) → {FeNO}⁷ → {FeNO}⁸ → {Fe(NO)₂}⁹, has been elucidated within the same protein scaffold based on spectroscopic characterization and DFT calculations. These results not only enhance the understanding of the dinitrosyl iron complex formation process, but also shed light on the physiological roles of nitric oxide signaling mediated by nonheme iron proteins.

Stepwise nitrosylation from Fe(ii) to {FeNO}⁷, {FeNO}⁸ and then to {Fe(NO)₂}⁹ is reported for the first time in the same protein scaffold, providing deeper understanding of the detailed mechanism of dinitrosyl iron complex formation.

Hypothetical mechanism of light action on nitric oxide physiological effects

Nitric oxide is a universal cellular mediator. It is involved in many physiological processes, including those induced by light. The disability for complete analysis of the nitric oxide metabolites in tissues prevents the exact understanding of the role of NO in a particular process. The sensitivity and selectivity of an enzymatic sensor developed in our lab is based on the detection of all NO groups that carry a positive charge or acquire it in the chemical processes. Using this sensor, we have shown that dinitrosyl iron complexes (DNIC), being principal nitric oxide donors in most of living tissues, undergo transformations under light irradiation in the wavelength range of 400-700 nm. These changes are not associated with nitric oxide release to the environment. But a nitrosyl iron complex without thiol ligands (Fe(NO)n) is produced. Moreover, in the moment of the complex reorganization, the chemical bond between the NO group and other components apparently weakens and, in the presence of a substance possessing chemical affinity to the NO group, the latter acquires the ability of transition from the complex to this substance. Therefore, the efficiency of NO donors first of all depends on the existence of the NO target and its status including that under the action of light. The activation of a donor compound by light can facilitate the transfer of NO to the target. Transfer of NO from the donor to the target occurs without releasing NO, or with a minimum time of its stay in the unbound state.

How is Nitric Oxide (NO) Converted into Nitrosonium Cations (NO+) in Living Organisms? (Based on the Results of Optical and EPR Analyses of Dinitrosyl Iron Complexes with Thiol-Containing Ligands)

The present work provides theoretical and experimental foundations for the ability of dinitrosyl iron complexes (DNICs) with thiol-containing ligands to be not only the donors of neutral NO molecules, but also the donors of nitrosonium cations (NO+) in living organisms ensuring S-nitrosation of various proteins and low-molecular-weight compounds. It is proposed that the emergence of those cations in DNICs is related to disproportionation reaction of NO molecules, initiated by their binding with Fe2+ ions (two NO molecules per one ion). At the same time, possible hydrolysis of iron-bound nitrosonium cations is prevented by the electron density transition to nitrosonium cations from sulfur atoms of thiol-containing ligands, which are included in the coordination sphere of iron. It allows supposing that iron in iron-nitrosyl complexes of DNICs has a d 7 electronic configuration. This supposition is underpinned by experimental data revealing that a half of nitrosyl ligands are converted into S-nitrosothiols (RSNOs) when those complexes decompose, with the other half of those ligands released in the form of neutral NO molecules.

Role of GSH and Iron-Sulfur Glutaredoxins in Iron Metabolism-Review

Glutathione (GSH) was initially identified and characterized for its redox properties and later for its contributions to detoxification reactions. Over the past decade, however, the essential contributions of glutathione to cellular iron metabolism have come more and more into focus. GSH is indispensable in mitochondrial iron-sulfur (FeS) cluster biosynthesis, primarily by co-ligating FeS clusters as a cofactor of the CGFS-type (class II) glutaredoxins (Grxs). GSH is required for the export of the yet to be defined FeS precursor from the mitochondria to the cytosol. In the cytosol, it is an essential cofactor, again of the multi-domain CGFS-type Grxs, master players in cellular iron and FeS trafficking. In this review, we summarize the recent advances and progress in this field. The most urgent open questions are discussed, such as the role of GSH in the export of FeS precursors from mitochondria, the physiological roles of the CGFS-type Grx interactions with BolA-like proteins and the cluster transfer between Grxs and recipient proteins.

Modulating the Bioactivity of Nitric Oxide as a Therapeutic Strategy in Cardiac Surgery

Cardiac surgery, including cardioplegic arrest and extracorporeal circulation, causes endothelial dysfunction, which can lead to no-reflow phenomenon and reduction of myocardial pump function. Nitric oxide (NO) deficiency is involved in this pathologic process, thereby providing a fundamental basis for the use of NO replacement therapy. Presently used drugs and additives to cardioplegic and heart preservation solutions are not able to reliably protect endothelial cells and cardiomyocytes from ischemia-reperfusion injury. This review discusses promising NO-releasing compounds of various chemical classes for cardioplegia and reperfusion, which effectively maintain NO homeostasis under experimental conditions, and presents the mechanisms of their action on the cardiovascular system. Incomplete preclinical studies and a lack of toxicity assessment, however, hinder translation of these drug candidates into the clinic. Perspectives for modulation of endothelial function using NO-mediated mechanisms are discussed. They are based on the cardioprotective potential of targeting vascular gap junctions and endothelial ion channels, intracoronary administration of progenitor cells, and endothelial-specific microRNAs. Some of these strategies may provide important therapeutic benefits for human cardiovascular interventions.

Synthesis and Metabolism of Nitric Oxide (NO) in Chicken Embryos and in the Blood of Adult Chicken

In chicken embryos, nitric oxide (NO) is accumulated in the pool of NO donors: S-nitrosothiols, nitrosyl-iron complexes, high-molecular-weight nitro-compounds. Oxidation of NO to nitrate occurs with different intensity in the embryos of different chicken breeds. In some embryos, NO donors accumulate almost without oxidation. Stable concentration of NO donors and nitrate in the blood of adult chicken is a result of dynamic equilibrium between NO synthesis and elimination (oxidation, consumption by other tissues, and excretion). As NO oxidation occurs mainly not in the blood, but in other tissues, decomposition of NO donors and NO oxidation are determined the properties of these tissues, in particular, the presence of physiological targets of NO, rather than spontaneous processes. Hence, evaluation of the intensity of NO metabolism is important for prediction of the efficiency of preparations containing NO donors and stimulators of its synthesis.

Modification of Biochemical Properties of Nitrosothiol by Fe3+ Cation: A Presumable Physiological Role

In the presence of Fe³⁺ cation, S-nitrosoglutathione (GSNO) loses the potency to inhibit catalase in the system containing hemoglobin (an NO trap) with iron chelator or -SH inhibitor (a "sulfhydric poison" Hg²⁺). In the absence of hemoglobin, the inhibitory potency is retained in both cases. These properties are characteristic of dinitrosyl-iron complexes containing ferrous iron and thiols (DNIC/RSH). Since the potency to inhibit catalase results from the presence of -NO group, its loss in the presence of hemoglobin relates probably to transfer of this group to hemoglobin. The nitrosothiols are relatively stable compounds, so their ability to release NO under the action of iron chelators, which is characteristic of DNIC/RSH, can have important physiological implications, because the role of such chelators can be played by some endogenous agents as well. Thus, release of NO from the donor compounds can be controlled and regulated. Probably, the agents such as nitrosothiol+Fe³⁺ are the major constituents in the pool of nitroso compounds.

What is the Mechanism of Nitric Oxide Conversion into Nitrosonium Ions Ensuring S-Nitrosating Processes in Living Organisms

Here, I present the data testifying that the conversion of free radical NO molecules to nitrosonium ions (NO⁺), which are necessary for the realization of one of NO biological effects (S-nitrosation), may occur in living organisms after binding NO molecules to loosely bound iron (Fe²⁺ ions) with the subsequent mutual one-electron oxidation-reduction of NO molecules (their disproportionation). Inclusion of thiol-containing substances as iron ligands into this process prevents hydrolysis of NO⁺ ions bound to iron thus providing the formation of stable dinitrosyl iron complexes (DNIC) with thiol ligands. Such complexes act in living organisms as donors of NO and NO⁺, providing stabilization and transfer of these agents via the autocrine and paracrine pathways. Without loosely bound iron (labile iron pool) and thiols participating in the DNIC formation, NO functioning as one of universal regulators of diverse metabolic processes would be impossible.

The Inhibiting Effect of Dinitrosyl Iron Complexes with Thiol-containing Ligands on the Growth of Endometrioid Tumours in Rats with Experimental Endometriosis

The possibility that binuclear dinitrosyl iron complexes with glutathione and cysteine (DNIC-GSН and B-DNIC-Cys) have a strong cytotoxic effect on the growth of endometrioid tumours (EMT) in rats with surgically induced experimental endometriosis established in our previous studies has been supported with experimental data. The increase in the DNIC-GSН or B-DNIC-Cys dose from 10 (in our previous studies) to 20 μmol/kg (after i/p administration to experimental rats) fully suppressed the growth of uterine tissues implanted onto the inner surface of the abdominal wall. At 2 μmol/kg DNIC-GSН, the median value of EMT volume increased from 0 to 15 mm³, while the mean size of EMT-from 55 to 77 mm³ (data from EMT measurements in 10 experimental rats). After treatment of animals with B-DNIC with N-acetyl-L-cysteine (10 μmol/kg) known for its ability to penetrate easily through the cell membrane, the inhibiting effect on EMT growth diminished as could be evidenced from the transformation of ~30% of the implants into large-size EMT. Possible reasons for this phenomenon are discussed.

Protective efficacy of dinitrosyl iron complexes with reduced glutathione in cardioplegia and reperfusion

Disturbed homeostasis of nitric oxide (NO) is one of the causes of myocardial ischemia/reperfusion (I/R) injury during open-heart surgery. This study was designed to explore mechanisms of action of dinitrosyl iron complexes with reduced glutathione ({(GS^-)₂Fe⁺(NO⁺)₂}⁺, DNIC-GS) added to crystalloid cardioplegia or reperfusion solution in isolated working rat hearts. Hearts of male Wistar rats were subjected to cardioplegic arrest by St. Thomas' Hospital cardioplegic solution (STH) and normothermic global ischemia followed by reperfusion. DNIC-GS were used with STH or during early reperfusion. Lactate dehydrogenase (LDH) activity in the coronary effluent and myocardial contents of adenine nucleotides, phosphocreatine, and lactate were determined spectrophotometrically. Reactive oxygen species (ROS) formation in the coronary effluent and myocardial DNIC content was assessed by EPR technique. Cardioplegia or reperfusion with DNIC-GS significantly improved recovery of coronary flow and cardiac function compared with control. Carboxy-[2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidozoline-1-oxy-3-oxide] (C-PTIO), a selective NO scavenger, reduced/abolished protective action of DNIC-GS. Enhanced recovery of cardiac function with DNIC-GS reduced LDH release in the coronary effluent, augmented recovery of myocardial energy state, and decreased formation of ROS-generating systems at reperfusion. Beneficial effects of DNIC-GS were related to the transfer of [Fe(NO)₂] cores to thiol groups of myocardial proteins to form intracellular DNIC pools. The study concluded that DNIC-GS is a promising adjunct agent for metabolic and antioxidant protection of the heart during cardioplegic arrest and reperfusion.

Ascorbate attenuates red light mediated vasodilation: Potential role of S-nitrosothiols

There is significant therapeutic advantage of nitric oxide synthase (NOS) independent nitric oxide (NO) production in maladies where endothelium, and thereby NOS, is dysfunctional. Electromagnetic radiation in the red and near infrared region has been shown to stimulate NOS-independent but NO-dependent vasodilation, and thereby has significant therapeutic potential. We have recently shown that red light induces acute vasodilatation in the pre-constricted murine facial artery via the release of an endothelium derived substance. In this study we have investigated the mechanism of vasodilatation and conclude that 670 nm light stimulates vasodilator release from an endothelial store, and that this vasodilator has the characteristics of an S-nitrosothiol (RSNO). This study shows that 670 nm irradiation can be used as a targeted and non-invasive means to release biologically relevant amounts of vasodilator from endothelial stores. This raises the possibility that these stores can be pharmacologically built-up in pathological situations to improve the efficacy of red light treatment. This strategy may overcome eNOS dysfunction in peripheral vascular pathologies for the improvement of vascular health.

Insight into chalcogenolate-bound {Fe(NO)2}9 dinitrosyl iron complexes (DNICs): covalent character versus ionic character

The synthesis, characterization and transformation of the thermally unstable {Fe(NO)₂}⁹ dinitrosyl iron complex (DNIC) [(OMe)₂Fe(NO)₂]⁻ (2) were investigated.

Redox-Sensing Iron-Sulfur Cluster Regulators

SIGNIFICANCE

Iron-sulfur cluster proteins carry out multiple functions, including as regulators of gene transcription/translation in response to environmental stimuli. In all known cases, the cluster acts as the sensory module, where the inherent reactivity/fragility of iron-sulfur clusters with small/redox-active molecules is exploited to effect conformational changes that modulate binding to DNA regulatory sequences. This promotes an often substantial reprogramming of the cellular proteome that enables the organism or cell to adapt to, or counteract, its changing circumstances. Recent Advances: Significant progress has been made recently in the structural and mechanistic characterization of iron-sulfur cluster regulators and, in particular, the O₂ and NO sensor FNR, the NO sensor NsrR, and WhiB-like proteins of Actinobacteria. These are the main focus of this review.

CRITICAL ISSUES

Striking examples of how the local environment controls the cluster sensitivity and reactivity are now emerging, but the basis for this is not yet fully understood for any regulatory family.

FUTURE DIRECTIONS

Characterization of iron-sulfur cluster regulators has long been hampered by a lack of high-resolution structural data. Although this still presents a major future challenge, recent advances now provide a firm foundation for detailed understanding of how a signal is transduced to effect gene regulation. This requires the identification of often unstable intermediate species, which are difficult to detect and may be hard to distinguish using traditional techniques. Novel approaches will be required to solve these problems.

[Possibilities for the diagnosis of inflammatory reaction in ischemic stroke]

AIM

To study diagnostic possibilities for determining the content of nitrite and N-nitroso compounds (NO₂^-+RNNO) in blood plasma and cerebrospinal fluid (CSF) in patients with acute ischemic stroke (IS).

MATERIAL AND METHODS

Twenty-four patients with IS were examined. The content of NO oxidation products was determined in venous blood and CSF by using an enzyme sensor based on the unique property of nitrite (NO₂^-), N-nitroso compounds (RNNO), S-nitrosothiols and dinitrosyl iron complexes to inhibit the enzyme catalase in the presence of halide ions. The study was conducted on the 1st day of IS.

RESULTS AND CONCLUSION

CSF in patients with IS contained nitrite and N-nitroso compounds (NO₂^-+RNNO) in concentrations ranging from 0.4 to 2.0 µm. The relationship between the size of IS and the concentration of NO₂^-+RNNO in CSF was shown. It was 1.01±0.13 µm in patients with medium IS and 0.71±0.07 µm in patients with small IS (U-criterion 16.5; p<0.05). There was no correlation between the severity of neurological deficit at the time of hospitalization and discharge from the hospital and the content of NO₂^-+RNNO in CSF (r=0.134; p>0.5; r=0.155; p>0.5, respectively). Plasma NO₂^-+RNNO levels were not associated with the presence and size of IS though they were elevated in patients with inflammatory complications. In conclusion, NO₂^-+RNNO can be considered as a marker of inflammation in patients with IS. Their presence in CSF reflects the extent of brain damage, but not the presence of concomitant inflammatory diseases.

NO-Donor Nitrosyl Iron Complex with 2-Aminophenolyl Ligand Induces Apoptosis and Inhibits NF-κB Function in HeLa Cells

NO donating iron nitrosyl complex with 2-aminothiophenyl ligand (2-AmPh complex) was studied for its ability to cause cell death and affect nuclear factor kappa B (NF-κB) signaling. The complex inhibited viability of HeLa cells and induced cell death that was accompanied by loss of mitochondrial membrane potential and characteristic for apoptosis phosphatidylserine externalization. At IC50, 2-AmPh caused decrease in nuclear content of NF-κB p65 polypeptide and mRNA expression of NF-κB target genes encoding interleukin-8 and anti-apoptotic protein BIRC3. mRNA levels of interleukin-6 and anti-apoptotic protein BIRC2 encoding genes were not affected. Our data demonstrate that NO donating iron nitrosyl complex 2-AmPh can inhibit tumor cell viability and induce apoptosis that is preceded by impairment of NF-κB function and suppression of a subset of NF-κB target genes.

Mechanisms of Specific Embryonic Effects of Nitrogen Oxide

The study of NO metabolism in chicken embryos showed that the intensity of oxidation of both endogenous and exogenous for the embryo NO donors to nitrate is determined by the presence or state of NO targets, rather than donor concentration. The mechanism of this oxidation and its physiological role are discussed. It was also shown that oxidation product nitrate is actively eliminated from the amnionic sac.

Mass spectrometric detection of iron nitrosyls, sulfide oxidation and mycothiolation during nitrosylation of the NO sensor [4Fe-4S] NsrR

Identification of RRE-type iron-nitrosyl species formed upon nitrosylation of [4Fe–4S] NsrR.

The bacterial nitric oxide (NO)-sensing transcriptional regulator NsrR binds a [4Fe–4S] cluster that enables DNA-binding and thus repression of the cell's NO stress response. Upon exposure to NO, the cluster undergoes a complex nitrosylation reaction resulting in a mixture of iron-nitrosyl species, which spectroscopic studies have indicated are similar to well characterized low molecular weight dinitrosyl iron complex (DNIC), Roussin's Red Ester (RRE) and Roussin's Black Salt (RBS). Here we report mass spectrometric studies that enable the unambiguous identification of NsrR-bound RRE-type species, including a persulfide bound form that results from the oxidation of cluster sulfide. In the presence of the low molecular weight thiols glutathione and mycothiol, glutathionylated and mycothiolated forms of NsrR were readily formed.

Wavelength-dependence of vasodilation and NO release from S-nitrosothiols and dinitrosyl iron complexes by far red/near infrared light

Far red/near infrared (R/NIR) energy is a novel therapy, but its mechanism of action is poorly characterized. Cytochrome c oxidase (Cco) of the mitochondrial electron transport chain is considered the primary photoacceptor for R/NIR to photolyze a putative heme nitrosyl in Cco to liberate free nitric oxide (NO). We previously observed R/NIR light directly liberates NO from nitrosylated hemoglobin and myoglobin, and recently suggested S-nitrosothiols (RSNO) and dinitrosyl iron complexes (DNIC) may be primary sources of R/NIR-mediated NO. Here we indicate R/NIR light exposure induces wavelength dependent dilation of murine facial artery, with longer wavelengths (740, and 830 nm) exhibiting reduced potency when compared to 670 nm. R/NIR also stimulated NO release from pure solutions of low molecular weight RSNO (GSNO and SNAP) and glutathione dinitrosyl iron complex (GSH-DNIC) in a power- and wavelength-dependent manner, with the greatest effect at 670 nm. NO release from SNAP using 670 was nearly ten-fold more than GSNO or GSH-DNIC, with no substantial difference in NO production at 740 nm and 830 nm. Thermal effects of irradiation on vasodilation or NO release from S-nitrosothiols and DNIC was minimal. Our results suggest 670 nm is the optimal wavelength for R/NIR treatment of certain vascular-related diseases.

Hemodynamic Effects of Glutathione-Liganded Binuclear Dinitrosyl Iron Complex: Evidence for Nitroxyl Generation and Modulation by Plasma Albumin

Glutathione-liganded binuclear dinitrosyl iron complex (glut-BDNIC) has been proposed to be a donor of nitric oxide (NO). This study was undertaken to investigate the mechanisms of vasoactivity, systemic hemodynamic effects, and pharmacokinetics of glut-BDNIC. To test the hypothesis that glut-BDNICs vasodilate by releasing NO in its reduced [nitroxyl (HNO)] state, a bioassay method of isolated, preconstricted ovine mesenteric arterial rings was used in the presence of selective scavengers of HNO or NO free radical (NO•); the vasodilatory effects of glut-BDNIC were found to have characteristics similar to those of an HNO donor and markedly different than an NO• donor. In addition, products of the reaction of glut-BDNIC with CPTIO [2-(4-carboxyphenyl)-4,4,5-tetramethyl imidazoline-1-oxyl-3-oxide] were found to have electron paramagnetic characteristics similar to those of an HNO donor compared with an NO• donor. In contrast to S-nitroso-glutathione, which was vasodilative both in vitro and in vivo, the potency of glut-BDNIC-mediated vasodilation was markedly diminished in both rats and sheep. Wire myography showed that plasma albumin contributed to this loss of hypotensive effects, an effect abolished by modification of the cysteine-thiol residue of albumin. High doses of glut-BDNIC caused long-lasting hypotension in rats that can be at least partially attributed to its long circulating half-life of ∼44 minutes. This study suggests that glut-BDNIC is an HNO donor, and that its vasoactive effects are modulated by binding to the cysteine residue of plasma proteins, such as albumin.

Protective effects of a modified apelin-12 and dinitrosyl iron complexes in experimental cardioplegic ischemia and reperfusion

The maintenance of nitric oxide (NO) bioavailability has been recognized as an important component of myocardial protection during cardiac surgery. This study was designed to evaluate the efficacy of using two NO-donating compounds in cardioplegia and reperfusion: (i) a modified peptide apelin-12 (MA12) that activates endothelial NO synthase (eNOS) and (ii) dinitrosyl iron complexes with reduced glutathione (DNIC-GS), a natural NO vehicle. Isolated perfused working rat hearts were subjected to normothermic global ischemia and reperfusion. St. Thomas' Hospital cardioplegic solution (STH) containing 140 μM MA12 or 100 μM DNIC-GS was used. In separate series, 140 μM MA12 or 100 μM DNIC-GS was administered at early reperfusion. Metabolic state of the hearts was evaluated by myocardial content of high-energy phosphates and lactate. Lactate dehydrogenase (LDH) activity in myocardial effluent was used as an index of cell membrane damage. Cardioplegia with MA12 or DNIC-GS improved recovery of coronary flow and cardiac function, and reduced LDH leakage in perfusate compared with STH without additives. Cardioplegic arrest with MA12 significantly enhanced preservation of high-energy phosphates and decreased accumulation of lactate in reperfused hearts. The overall protective effect of cardioplegia with MA12 was significantly greater than with DNIC-GS. The administration of MA12 or DNIC-GS at early reperfusion also increased metabolic and functional recovery of reperfused hearts. In this case, recovery of cardiac contractile and pump function indices was significantly higher if reperfusion was performed with DNIC-GS. The results show that MA12 and DNIC-GS are promising adjunct agents for protection of the heart during cardioplegic arrest and reperfusion.

Glutathione, Glutaredoxins, and Iron

SIGNIFICANCE

Glutathione (GSH) is the most abundant cellular low-molecular-weight thiol in the majority of organisms in all kingdoms of life. Therefore, functions of GSH and disturbed regulation of its concentration are associated with numerous physiological and pathological situations. Recent Advances: The function of GSH as redox buffer or antioxidant is increasingly being questioned. New functions, especially functions connected to the cellular iron homeostasis, were elucidated. Via the formation of iron complexes, GSH is an important player in all aspects of iron metabolism: sensing and regulation of iron levels, iron trafficking, and biosynthesis of iron cofactors. The variety of GSH coordinated iron complexes and their functions with a special focus on FeS-glutaredoxins are summarized in this review. Interestingly, GSH analogues that function as major low-molecular-weight thiols in organisms lacking GSH resemble the functions in iron homeostasis.

CRITICAL ISSUES

Since these iron-related functions are most likely also connected to thiol redox chemistry, it is difficult to distinguish between mechanisms related to either redox or iron metabolisms.

FUTURE DIRECTIONS

The ability of GSH to coordinate iron in different complexes with or without proteins needs further investigation. The discovery of new Fe-GSH complexes and their physiological functions will significantly advance our understanding of cellular iron homeostasis. Antioxid. Redox Signal. 27, 1235-1251.

Selectivity in Physiological Action of Nitric Oxide: A Hypothetical Mechanism

The study showed that dinitrosyl iron complex (NO)₂Fe(RS)₂ containing the thiolate ligands, which is the basic physiological donor of NO, can transfer NO to other molecule only at the moment of rearrangement. This rearrangement can occur during interaction of the complex with more effective iron chelators than the thiolate ligands. In the absence of NO trap, a new complex is formed with a new ligand. NO transfer to a trap can also occur under the action of the agents such as mercury salts or ROS, which interact with the thiolate ligands. Probably, the ligands in the dinitrosyl iron complexes are the structures responsible for interaction of these complexes with physiological targets and for specificity and effectiveness of this interaction.

Effects of Physiological Nitric Oxide Donor on Oxidative Metabolism in Rat Blood

The effects of exogenous dinitrosyl iron complexes with glutathione ligands on the parameters of oxidative metabolism in the rat blood are studied. The intensity of LPO, total antioxidant activity, and MDA level in the plasma are compared in rats receiving intraperitoneal injections of saline without and with dinitrosyl iron complexes in various concentrations (0.15, 0.30, 0.45, and 0.60 mM). Glutathione-containing dinitrosyl iron complexes are characterized by antioxidant effect that non-linearly depended of the dose with optimum at 0.30-0.45 mM.