Oxidation of oxymyoglobin by nitric oxide through dissociation from cobalt nitrosyls

A nitrosyl iron complex with 3.4-dichlorothiophenolyl ligands: synthesis, structures and its reactions with targets - carriers of nitrogen oxide (NO) in vivo

In this work, a new binuclear nitrosyl complex with 3.4-dichlorothiophenolyl ligands [Fe₂(SC₆H₃Cl₂)₂(NO)₄] has been synthesized. Nitrosyl iron complexes (NICs) are systems for the storage and delivery of NO in the body. There is a dynamic equilibrium between dinitrosyl iron units bound to low molecular weight ligands and high molecular weight (protein) ligands in vivo. From this point of view, the transformation of the studied complex in DMSO and buffer, as well as in biological systems, has been analyzed. In DMSO, it decomposes into mononuclear NICs, which quickly decay in buffer solutions with NO release. The high molecular weight product is formed as a result of the binding of the complex to bovine serum albumin (the Stern-Volmer constant is 2.1 × 10⁵ M^-1). In this case, the complex becomes a prolonged NO-donor. Such a long-term effect has been observed for the first time. Similarly, in a system with oxyhemoglobin, NO generation is slower; the UV-vis spectra show a gradual formation of methemoglobin. On the other hand, reduced glutathione has little effect on the NO-donor properties of the complex despite the fact that ligand substitution is observed in the system and a binuclear product is formed. Mucin binds the complex, and the decomposition mechanism is different from that for buffer solutions. Thus, these proteins and glutathione are able to participate in the transformation of the complex and modulate its properties as a potential drug.

The Chemistry of HNO: Mechanisms and Reaction Kinetics

Azanone (HNO, also known as nitroxyl) is the protonated form of the product of one-electron reduction of nitric oxide (^•NO), and an elusive electrophilic reactive nitrogen species of increasing pharmacological significance. Over the past 20 years, the interest in the biological chemistry of HNO has increased significantly due to the numerous beneficial pharmacological effects of its donors. Increased availability of various HNO donors was accompanied by great progress in the understanding of HNO chemistry and chemical biology. This review is focused on the chemistry of HNO, with emphasis on reaction kinetics and mechanisms in aqueous solutions.

Sodium borohydride and thiol mediated nitrite release from nitroaromatic antibiotics

Nitroaromatic antibiotics are used to treat a variety of bacterial and parasitic infections. These prodrugs require reductive bioactivation for activity, which provides a pathway for the release of nitrogen oxide species such as nitric oxide, nitrite, and/or nitroxyl. Using sodium borohydride and 2-aminoethanol as model reductants, this work examines release of nitrogen oxide species from various nitroaromatic compounds through several characterization methods. Specifically, 4- and 5-nitroimidazoles reproducibly generate higher amounts of nitrite (not nitric oxide or nitroxyl) than 2-nitroimidazoles during the reaction of model hydride donors or thiols. Mass spectrometric analysis shows clean formation of products resulting from nucleophile addition and nitro group loss. 2-Nitrofurans generate nitrite upon addition of sodium borohydride or 2-aminoethanethiol, but these complex reactions do not produce clean organic products. A mechanism that includes nucleophile addition to the carbon βto the nitro group to generate a nitronate anion followed by protonation and nitrous acid elimination explains the observed products and labeling studies. These systematic studies give a better understanding of the release mechanisms of nitrogen oxide species from these compounds allowing for the design of more efficient therapeutics.

Biological signaling by small inorganic molecules

Small redox active molecules such as reactive nitrogen and oxygen species and hydrogen sulfide have emerged as important biological mediators that are involved in various physiological and pathophysiological processes. Advancement in understanding of cellular mechanisms that tightly regulate both generation and reactivity of these molecules is central to improved management of various disease states including cancer and cardiovascular dysfunction. Imbalance in the production of redox active molecules can lead to damage of critical cellular components such as cell membranes, proteins and DNA and thus may trigger the onset of disease. These small inorganic molecules react independently as well as in a concerted manner to mediate physiological responses. This review provides a general overview of the redox biology of these key molecules, their diverse chemistry relevant to physiological processes and their interrelated nature in cellular signaling.

Synthesis and chemical and biological comparison of nitroxyl- and nitric oxide-releasing diazeniumdiolate-based aspirin derivatives

Structural modifications of nonsteroidal anti-inflammatory drugs (NSAIDs) have successfully reduced the side effect of gastrointestinal ulceration without affecting anti-inflammatory activity, but they may increase the risk of myocardial infarction with chronic use. The fact that nitroxyl (HNO) reduces platelet aggregation, preconditions against myocardial infarction, and enhances contractility led us to synthesize a diazeniumdiolate-based HNO-releasing aspirin and to compare it to an NO-releasing analogue. Here, the decomposition mechanisms are described for these compounds. In addition to protection against stomach ulceration, these prodrugs exhibited significantly enhanced cytotoxcity compared to either aspirin or the parent diazeniumdiolate toward nonsmall cell lung carcinoma cells (A549), but they were not appreciably toxic toward endothelial cells (HUVECs). The HNO-NSAID prodrug inhibited cylcooxgenase-2 and glyceraldehyde 3-phosphate dehydrogenase activity and triggered significant sarcomere shortening on murine ventricular myocytes compared to control. Together, these anti-inflammatory, antineoplasic, and contractile properties suggest the potential of HNO-NSAIDs in the treatment of inflammation, cancer, or heart failure.

Differences in oxygen-dependent nitric oxide metabolism by cytoglobin and myoglobin account for their differing functional roles

The endogenous vasodilator nitric oxide (NO) is metabolized in tissues in an oxygen-dependent manner. In skeletal and cardiac muscle, high concentrations of myoglobin (Mb) function as a potent NO scavenger. However, the Mb concentration is very low in vascular smooth muscle, where low concentrations of cytoglobin (Cygb) may play a major role in metabolizing NO. Questions remain regarding how low concentrations of Cygb and Mb differ in terms of NO metabolism, and the basis for their different cellular roles and functions. In this study, electrode techniques were used to perform comparative measurements of the kinetics of NO consumption by Mb and Cygb. UV/Vis spectroscopic methods and computer simulations were performed to study the reaction of Mb and Cygb with ascorbate (Asc) and the underlying mechanism. It was observed that the initial rate of Cygb(3+) reduction by Asc was 415-fold greater than that of Mb(3+). In the low [O2] range (0-50 μM), the Cygb-mediated NO consumption rate is ~ 500 times more sensitive to changes in O2 concentration than that of Mb. The reduction of Cygb(3+) by Asc follows a reversible kinetic model, but that of Mb(3+) is irreversible. A reaction mechanism for Cygb(3+) reduction by Asc is proposed, and the reaction equilibrium constants are determined. Our results suggest that the rapid reduction of Cygb by cellular reductants enables Cygb to efficiently regulate NO metabolism in the vascular wall in an oxygen-dependent manner, but the slow rate of Mb reduction does not show this oxygen dependence.

Characterization of the function of cytoglobin as an oxygen-dependent regulator of nitric oxide concentration

The endogenous vasodilator nitric oxide (NO) is metabolized in tissues in an O(2)-dependent manner. This regulates NO levels in the vascular wall; however, the underlying molecular basis of this O(2)-dependent NO consumption remains unclear. While cytoglobin (Cygb) was discovered a decade ago, its physiological function remains uncertain. Cygb is expressed in the vascular wall and can consume NO in an O(2)-dependent manner. Therefore, we characterize the process of the O(2)-dependent consumption of NO by Cygb in the presence of the cellular reductants and reducing systems ascorbate (Asc) and cytochrome P(450) reductase (CPR), measure rate constants of Cygb reduction by Asc and CPR, and propose a reaction mechanism and derive a related kinetic model for this O(2)-dependent NO consumption involving Cygb(Fe(3+)) as the main intermediate reduced back to ferrous Cygb by cellular reductants. This kinetic model expresses the relationship between the rate of O(2)-dependent consumption of NO by Cygb and rate constants of the molecular reactions involved. The predicted rate of O(2)-dependent consumption of NO by Cygb is consistent with experimental results supporting the validity of the kinetic model. Simulations based on this kinetic model suggest that the high efficiency of Cygb in regulating the NO consumption rate is due to the rapid reduction of Cygb by cellular reductants, which greatly increases the rate of consumption of NO at higher O(2) concentrations, and binding of NO to Cygb, which reduces the rate of consumption of NO at lower O(2) concentrations. Thus, the coexistence of Cygb with efficient reductants in tissues allows Cygb to function as an O(2)-dependent regulator of NO decay.

Nitrosation, thiols, and hemoglobin: energetics and kinetics

Nitrosothiols are powerful vasodilators. Although the mechanism of their formation near neutral pH is an area of intense research, neither the energetics nor the kinetics of this reaction or of subsequent reactions have been addressed. The following considerations may help to guide experiments. (1) The standard Gibbs energy for the homolysis reaction RSNO → RS(•) + NO(•)(aq) is +110 ± 5 kJ mol(-1). (2) The electrode potential of the RSNO, H(+)/RSH, NO(•)(aq) couple is -0.20 ± 0.06 V at pH 7. (3) Thiol nitrosation by NO(2)(-) is favorable by 37 ± 5 kJ mol(-1) at pH 7. (4) N(2)O(3) is not involved in in vivo nitrosation mechanisms for thermodynamic--its formation from NO(2)(-) costs 59 kJ mol(-1)--or kinetic--the reaction being second-order in NO(2)(-)--reasons. (5) Hemoglobin (Hb) cannot catalyze formation of N(2)O(3), be it via the intermediacy of the reaction of Hb[FeNO(2)](2+) with NO(•) (+81 kJ mol(-1)) or reaction of Hb[FeNO](3+) with NO(2)(-) (+88 kJ mol(-1)). (6) Energetically and kinetically viable are nitrosations that involve HNO(2) or NO(•) in the presence of an electron acceptor with an electrode potential higher than -0.20 V. These considerations are derived from existing thermochemical and kinetics data.

The specificity of nitroxyl chemistry is unique among nitrogen oxides in biological systems

The importance of nitric oxide in mammalian physiology has been known for nearly 30 years. Similar attention for other nitrogen oxides such as nitroxyl (HNO) has been more recent. While there has been speculation as to the biosynthesis of HNO, its pharmacological benefits have been demonstrated in several pathophysiological settings such as cardiovascular disorders, cancer, and alcoholism. The chemical biology of HNO has been identified as related to, but unique from, that of its redox congener nitric oxide. A summary of these findings as well as a discussion of possible endogenous sources of HNO is presented in this review.

The new chemical biology of nitrite reactions with hemoglobin: R-state catalysis, oxidative denitrosylation, and nitrite reductase/anhydrase

Because of their critical biological roles, hemoglobin and myoglobin are among the most extensively studied proteins in human history, while nitrite tops the list of most-studied small molecules. And although the reactions between them have been examined for more than 140 years, a series of unusual and critical allosterically modulated reactions have only recently been characterized. In this Account, we review three novel metal- and nitrite-catalyzed reaction pathways in the context of historical studies of nitrite and hemoglobin chemistry and attempt to place them in the biological framework of hypoxic signaling. Haldane first described the reaction between nitrite and deoxymyoglobin, forming iron-nitrosylated myoglobin, in his analysis of the meat-curing process more than a century ago. The reaction of nitrous acid with deoxyhemoglobin to form nitric oxide (NO) and methemoglobin was more fully characterized by Brooks in 1937, while the mechanism and unusual behavior of this reaction were further detailed by Doyle and colleagues in 1981. During the past decade, multiple physiological studies have surprisingly revealed that nitrite represents a biological reservoir of NO that can regulate hypoxic vasodilation, cellular respiration, and signaling. Importantly, chemical analysis of this new biology suggests a vital role for deoxyhemoglobin- and deoxymyoglobin-dependent nitrite reduction in these processes. The use of UV-vis deconvolution and electron paramagnetic resonance (EPR) spectroscopy, in addition to refined gas-phase chemiluminescent NO detection, has led to the discovery of three novel and unexpected chemistries between nitrite and deoxyhemoglobin that may contribute to and facilitate hypoxic NO generation and signaling. First, R-state, or allosteric, autocatalysis of nitrite reduction increases the rate of NO generation by deoxyhemoglobin and results in maximal NO production at approximately 50% hemoglobin oxygen saturation, which is physiologically associated with greatest NO-dependent vasodilation. Second, oxidative denitrosylation of the iron-nitrosyl product formed in the deoxyhemoglobin-nitrite reaction allows for NO formation and release in a partially oxygenated environment. Finally, the deoxyhemoglobin-nitrite reaction participates in a nitrite reductase/anhydrase redox cycle that catalyzes the anaerobic conversion of two molecules of nitrite into dinitrogen trioxide (N(2)O(3)). N(2)O(3) may then nitrosate proteins, diffuse across hydrophobic erythrocyte membrane channels such as aquaphorin or Rh, or reconstitute NO via homolysis to NO and NO(2)(*). Importantly, the nitrite reductase/anhydrase redox pathway also represents a novel mechanism of both anaerobic and metal-catalyzed N(2)O(3) formation and S-nitrosation and may thus play a vital role in NO-dependent signaling in a hypoxic and heme-rich environment. We consider how these reactions may contribute to physiological and pathological hypoxic signaling.

Biological activity of hemoprotein nitrosyl complexes

Chemical and biological functions of hemoprotein nitrosyl complexes as well as their photolysis products are discussed in this review. Chemical properties of nitric oxide are discussed, and major chemical reactions such as interaction with thiols, free radicals, and transition metals are considered. Specific attention is paid to the generation of hemoprotein nitrosyl complexes. The mechanisms of nitric oxide reactions with hemoglobin and cytochrome c and physicochemical properties of their nitrosyl complexes are discussed. A review of photochemical reactions of nitrosyl complexes with various ligands is given. Finally, we observe physiological effects of visible radiation on hemoprotein nitrosyl complexes: smooth muscle relaxation and reactivation of mitochondrial respiration.

Kinetic and mechanistic studies of the NO*-mediated oxidation of oxymyoglobin and oxyhemoglobin

The second-order rate constants for the reactions between nitrogen monoxide and oxymyoglobin or oxyhemoglobin, determined by stopped-flow spectroscopy, increase with increasing pH. At pH 7.0 the rates are (43.6 +/- 0.5) x 10(6) M(-1) x s(-1) for oxymyoglobin and (89 +/- 3) x 10(6) M(-1) x s(-1) for oxyhemoglobin (per heme), whereas at pH 9.5 they are (97 +/- 3) x 10(6) M(-1) x s(-1) and (144 +/- 3) x 10(6) M(-1) x s(-1), respectively. The rate constants for the reaction between oxyhemoglobin and NO* depend neither on the association grade of the protein (dimer/tetramer) nor on the concentration of the phosphate buffer (100-1 mM). The nitrogen monoxide-mediated oxidations of oxymyoglobin and oxyhemoglobin proceed via intermediate peroxynitrito complexes which were characterized by rapid scan UV/vis spectroscopy. The two complexes MbFe(III)OONO and HbFe(III)OONO display very similar spectra with absorption maxima around 500 and 635 nm. These species can be observed at alkaline pH but rapidly decay to the met-form of the proteins under neutral or acidic conditions. The rate of decay of MbFe(III)OONO increases with decreasing pH and is significantly larger than those of the analogous complexes of the two subunits of hemoglobin. No free peroxynitrite is formed during these reactions, and nitrate is formed quantitatively, at both pH 7.0 and 9.0. This result indicates that, as confirmed from protein analysis after reacting the proteins with NO* for 10 times, when peroxynitrite is coordinated to the heme of myoglobin or hemoglobin it rapidly isomerizes to nitrate without nitrating the globins in physiologically significant amounts.