Kinetics and Mechanism of•NO2Reacting with Various Oxidation States of Myoglobin

Reactivity of nitrogen species with inorganic and organic compounds in water

Many studies on the reactive nitrogen species (RNS, ^●NO₂, ^●NO and ^●NH₂) with pollutants in water have been performed to understand the abatement of inorganic and organic compounds by these species, and the mechanisms of the formation of oxidative transformation products, especially nitrogenous oxidized byproducts. In this review, approaches to generate RNS in aqueous solution is first presented, followed by a summary of their reactivity with a wide range of compounds. The second-order rate constants (k, M^-1 s^-1) for the reactivity of ^●NO₂ and ^●NO with a wide range of inorganic radical and nonradical species were correlated with thermodynamic one-electron oxidation potentials (E⁰). The positive correlation between log(k) versus E⁰ suggests one-electron transfer reactions. The Hammett-type correlations were developed for the reactions of ^●NO₂ and ^●NH₂ with organic compounds, using the unsubstituted benzene as a reference molecule (i.e., Σσ_o,p,m = 0) to calculate Σσ_o,p,m = σ_o + σ_p + σ_m for each organic molecule. Linear negative correlations of log(k) with Σσ_o,p,m were obtained for both ^●NO₂ and ^●NH₂, suggesting electrophilic substitution mechanism. The correlations presented herein may assist in eliminating organic micropollutants in water treatment and reuse processes.

Ordered Motions in the Nitric-Oxide Dioxygenase Mechanism of Flavohemoglobin and Assorted Globins with Tightly Coupled Reductases

Nitric-oxide dioxygenases (NODs) activate and combine O₂ with NO to form nitrate. A variety of oxygen-binding hemoglobins with associated partner reductases or electron donors function as enzymatic NODs. Kinetic and structural investigations of the archetypal two-domain microbial flavohemoglobin-NOD have illuminated an allosteric mechanism that employs selective tunnels for O₂ and NO, gates for NO and nitrate, transient O₂ association with ferric heme, and an O₂ and NO-triggered, ferric heme spin crossover-driven, motion-controlled, and dipole-regulated electron-transfer switch. The proposed mechanism facilitates radical-radical coupling of ferric-superoxide with NO to form nitrate while preventing suicidal ferrous-NO formation. Diverse globins display the structural and functional motifs necessary for a similar allosteric NOD mechanism. In silico docking simulations reveal monomeric erythrocyte hemoglobin alpha-chain and beta-chain intrinsically matched and tightly coupled with NADH-cytochrome b₅ oxidoreductase and NADPH-cytochrome P450 oxidoreductase, respectively, forming membrane-bound flavohemoglobin-like mammalian NODs. The neuroprotective neuroglobin manifests a potential NOD role in a close-fitting ternary complex with membrane-bound NADH-cytochrome b₅ oxidoreductase and cytochrome b₅. Cytoglobin interfaces weakly with cytochrome b₅ for O₂ and NO-regulated electron-transfer and coupled NOD activity. The mechanistic model also provides insight into the evolution of O₂ binding cooperativity in hemoglobin and a basis for the discovery of allosteric NOD inhibitors.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Allostery in the nitric oxide dioxygenase mechanism of flavohemoglobin

The substrates O₂ and NO cooperatively activate the NO dioxygenase function of Escherichia coli flavohemoglobin. Steady-state and transient kinetic measurements support a structure-based mechanistic model in which O₂ and NO movements and conserved amino acids at the E11, G8, E2, E7, B10, and F7 positions within the globin domain control activation. In the cooperative and allosteric mechanism, O₂ migrates to the catalytic heme site via a long hydrophobic tunnel and displaces LeuE11 away from the ferric iron, which forces open a short tunnel to the catalytic site gated by the ValG8/IleE15 pair and LeuE11. NO permeates this tunnel and leverages upon the gating side chains triggering the CD loop to furl, which moves the E and F-helices and switches an electron transfer gate formed by LysF7, GlnE7, and water. This allows FADH₂ to reduce the ferric iron, which forms the stable ferric–superoxide–TyrB10/GlnE7 complex. This complex reacts with internalized NO with a bimolecular rate constant of 10¹⁰ M⁻¹ s⁻¹ forming nitrate, which migrates to the CD loop and unfurls the spring-like structure. To restart the cycle, LeuE11 toggles back to the ferric iron. Actuating electron transfer with O₂ and NO movements averts irreversible NO poisoning and reductive inactivation of the enzyme. Together, structure snapshots and kinetic constants provide glimpses of intermediate conformational states, time scales for motion, and associated energies.

Nitric Oxide Dioxygenase Activity of a Nitrosyl Complex of Cobalt(II) Porphyrinate in the Presence of Hydrogen Peroxide via Putative Peroxynitrite Intermediate

The reaction of a cobalt porphyrin complex, [(F₈TPP)Co], 1 {F₈TPP = 5,10,15,20- tetrakis(2,6-difluorophenyl)porphyrinate dianion} in dichloromethane with nitric oxide (NO) led to the nitrosyl complex, [(F₈TPP)Co(NO)], 2. Spectroscopic studies and structural characterization revealed it as a bent nitrosyl of {CoNO}⁸ description. It was stable in the presence of dioxygen. However, it reacts with H₂O₂ in acetonitrile (or THF) solution at -40 °C (or -80 °C) to result in the corresponding Co(III)-nitrate complex, [(F₈TPP)Co(NO₃)], 3. The reaction presumably proceeds via the formation of a Co-peroxynitrite intermediate. X-Band electron paramagnetic resonance and electrospray ionization-mass spectroscopic studies suggest the intermediate formation of the [(porphyrin)Co(III)-O^•] radical, which in turn supports the generation of the corresponding Co(IV)-oxo species during the reaction. This is in accord with the homolytic cleavage of the O-O bond in heme-peroxynitrite proposed in the nitric oxide dioxygenases activity. In addition, the characteristic peroxynitrite-induced phenol ring reaction was also observed.

The Reaction of Oxy Hemoglobin with Nitrite: Mechanism, Antioxidant-Modulated Effect, and Implications for Blood Substitute Evaluation

The autocatalytic reaction between nitrite and the oxy form of globins involves free radicals. For myoglobin (Mb), an initial binding of nitrite to the iron-coordinated oxygen molecule was proposed; the resulting ferrous-peroxynitrate species was not detected, but its decay product, the high-valent ferryl form, was demonstrated in stopped-flow experiments. Reported here are the stopped flow spectra recorded upon mixing oxy Hb (native, as well as chemically-derivatized in the form of several candidates of blood substitutes) with a supraphysiological concentration of nitrite. The data may be fitted to a simple kinetic model involving a transient met-aqua form, in contrast to the ferryl detected in the case of Mb in a similar reaction sequence. These data are in line with a previous observation of a transient accumulation of ferryl Hb under auto-catalytic conditions at much lower concentrations of nitrite (Grubina, R. et al. J. Biol. Chem. 2007, 282, 12916). The simple model for fitting the stopped-flow data leaves a small part of the absorbance changes unaccounted for, unless a fourth species is invoked displaying features similar to the oxy and tentatively assigned as ferrous-peroxynitrate. Density functional theory (DFT) calculations support this latter assignment. The reaction allows for differentiating between the reactivities of various chemically modified hemoglobins, including candidates for blood substitutes. Polymerization of hemoglobin slows the nitrite-induced oxidation, in sharp contrast to oxidative-stress type reactions which are generally accelerated, not inhibited. Sheep hemoglobin is found to be distinctly more resistant to reaction with nitrite compared to bovine Hb, at large nitrite concentrations (stopped-flow experiments directly observing the oxy + nitrite reaction) as well as under auto-catalytic conditions. Copolymerization of Hb with bovine serum albumin (BSA) using glutaraldehyde leads to a distinct increase of the lag time compared to native Hb as well as to any other form of derivatization examined in the present study. The Hb-BSA copolymer also displays a slower initial reaction with nitrite under stopped-flow conditions, compared to native Hb.

A Six-Coordinate Peroxynitrite Low-Spin Iron(III) Porphyrinate Complex-The Product of the Reaction of Nitrogen Monoxide (·NO(g)) with a Ferric-Superoxide Species

Peroxynitrite (-OON═O, PN) is a reactive nitrogen species (RNS) which can effect deleterious nitrative or oxidative (bio)chemistry. It may derive from reaction of superoxide anion (O2•-) with nitric oxide (·NO) and has been suggested to form an as-yet unobserved bound heme-iron-PN intermediate in the catalytic cycle of nitric oxide dioxygenase (NOD) enzymes, which facilitate a ·NO homeostatic process, i.e., its oxidation to the nitrate anion. Here, a discrete six-coordinate low-spin porphyrinate-FeIII complex [(PIm)FeIII(-OON═O)] (3) (PIm; a porphyrin moiety with a covalently tethered imidazole axial "base" donor ligand) has been identified and characterized by various spectroscopies (UV-vis, NMR, EPR, XAS, resonance Raman) and DFT calculations, following its formation at -80 °C by addition of ·NO(g) to the heme-superoxo species, [(PIm)FeIII(O2•-)] (2). DFT calculations confirm that 3 is a six-coordinate low-spin species with the PN ligand coordinated to iron via its terminal peroxidic anionic O atom with the overall geometry being in a cis-configuration. Complex 3 thermally transforms to its isomeric low-spin nitrato form [(PIm)FeIII(NO3-)] (4a). While previous (bio)chemical studies show that phenolic substrates undergo nitration in the presence of PN or PN-metal complexes, in the present system, addition of 2,4-di-tert-butylphenol (2,4DTBP) to complex 3 does not lead to nitrated phenol; the nitrate complex 4a still forms. DFT calculations reveal that the phenolic H atom approaches the terminal PN O atom (farthest from the metal center and ring core), effecting O-O cleavage, giving nitrogen dioxide (·NO2) plus a ferryl compound [(PIm)FeIV═O] (7); this rebounds to give [(PIm)FeIII(NO3-)] (4a).The generation and characterization of the long sought after ferriheme peroxynitrite complex has been accomplished.

Mechanism of HRP-catalyzed nitrite oxidation by H2O2 revisited: Effect of nitroxides on enzyme inactivation and its catalytic activity

The peroxidative activity of horseradish peroxidase (HRP) undergoes progressive inactivation while catalyzing the oxidation of nitrite by H₂O₂. The extent of inactivation increases as the pH increases, [nitrite] decreases or [H₂O₂] increases, and is accompanied by a loss of the Soret peak of HRP along with yellow-greenish coloration of the solution. HRP-catalyzed nitrite oxidation by H₂O₂ involves not only the formation of compounds I and II as transient heme species, but also compound III, all of which in turn, oxidize nitrite yielding ^•NO₂. The rate constant of nitrite oxidation by compound III is at least 10-fold higher than that by compound II, which is also reducible by ^•NO₂ where its reduction by nitrite is the rate-determining step of the catalytic cycle. The extent of the loss of the Soret peak of HRP is lower than the loss of its peroxidative activity implying that deterioration of the heme moiety leading to iron release only partially contributes toward heme inactivation. Cyclic stable nitroxide radicals, such as 2,2,6,6-tetramethyl-piperidine-N-oxyl (TPO), 4-OH-TPO and 4-NH₂-TPO at µM concentrations detoxify ^•NO₂ thus protecting HRP against inactivation mediated by this radical. Hence, HRP inactivation proceeds via nitration of the porphyrin ring most probably through compound I reaction with ^•NO₂, which partially leads to deterioration of the heme moiety. The nitroxide acts catalytically since its oxidation by ^•NO₂ yields the respective oxoammonium cation, which is readily reduced back to the nitroxide by H₂O₂, superoxide ion radical, and nitrite. In addition, the nitroxide catalytically inhibits tyrosine nitration mediated by HRP/H₂O₂/nitrite reactions system as it efficiently competes with tyrosyl radical for ^•NO₂. The inhibition by nitroxides of tyrosine nitration is demonstrated also in the case of microperoxidase (MP-11) and cytochrome c revealing an additional role played by nitroxide antioxidants.

Nitroxides catalytically inhibit nitrite oxidation and heme inactivation induced by H2O2, nitrite and metmyoglobin or methemoglobin

Stable nitroxide radicals have multiple biological effects, although the mechanisms underlying them are not fully understood. Their protective effect against oxidative damage has been mainly attributed to scavenging deleterious radicals, oxidizing reduced metal ions and reducing oxyferryl centers of heme proteins. Yet, the potential of nitroxides to protect heme proteins against inactivation while suppressing or enhancing their catalytic activities has been largely overlooked. We have studied the effect of nitroxides, including TPO (2,2,6,6-tetramethylpiperidin-N-oxyl), 4-OH-TPO, 4-oxo-TPO and 3-carbamoyl proxyl, on the peroxidase-like activity of metmyoglobin (MbFe^III) and methemoglobin (HbFe^III) using nitrite as an electron donor by following heme absorption, H₂O₂ consumption, O₂ evolution and nitrite oxidation. The results demonstrate that the peroxidase-like activity is accompanied by a progressive heme inactivation where MbFe^III is far more resistant than HbFe^III. Nitroxides convert the peroxidase-like activity into catalase-like activity while inhibiting heme inactivation and nitrite oxidation in a dose-dependent manner. The nitroxide facilitates H₂O₂ dismutation, yet none of its reactions with any of the intermediates formed in these systems is rate-determining, and therefore its effect on the rate of the catalysis is hardly dependent on the kind of the nitroxide derivative and its concentration. The nitroxide at µM concentrations range catalytically inhibits nitrite oxidation, and consequently prevents tyrosine nitration induced by heme protein/H₂O₂/nitrite due to its fast oxidation by ^•NO₂ forming the respective oxoammonium cation, which is reduced back to the nitroxide by H₂O₂ and by superoxide radical. The nitroxides are superior over common antioxidants, which their reaction with ^•NO₂ always yields secondary radicals leading eventually to consumption of the antioxidant. A mechanism is proposed, and the kinetic simulations fit very well the experimental data in the case of MbFe^III where most of the rate constants of the reactions involved are independently known.

Six-coordinate nitrito and nitrato complexes of manganese porphyrin

Reaction of small increments of NO2 gas with sublimed amorphous layers of Mn(II)(TPP) (TPP = meso-tetra-phenylporphyrinato dianion) in a vacuum cryostat leads to formation of the 5-coordinate monodentate nitrato complex Mn(III)(TPP)(η(1)-ONO2) (II). This transformation proceeds through the two distinct steps with initial formation of the five coordinate O-nitrito complex Mn(III)(TPP)(η(1)-ONO) (I) as demonstrated by the electronic absorption spectra and by FTIR spectra using differently labeled nitrogen dioxide. A plausible mechanism for the second stage of reaction is offered based on the spectral changes observed upon subsequent interaction of (15)NO2 and NO2 with the layered Mn(TPP). Low-temperature interaction of I and II with the vapors of various ligands L (L = O-, S-, and N-donors) leads to formation of the 6-coordinate O-nitrito Mn(III)(TPP)(L)(η(1)-ONO) and monodentate nitrato Mn(III)(TPP)(L)(η(1)-ONO2) complexes, respectively. Formation of the 6-coordinate O-nitrito complex is accompanied by the shifts of the ν(N═O) band to lower frequency and of the ν(N-O) band to higher frequency. The frequency difference between these bands Δν = ν(N═O) - ν(N-O) is a function of L and is smaller for the stronger bases. Reaction of excess NH3 with I leads to formation of Mn(TPP)(NH3)(η(1)-ONO) and of the cation [Mn(TPP)(NH3)2](+) plus ionic nitrite. The nitrito complexes are relatively unstable, but several of the nitrato species can be observed in the solid state at room temperature. For example, the tetrahydrofuran complex Mn(TPP)(THF)(η(1)-ONO2) is stable in the presence of THF vapors (∼5 mm), but it loses this ligand upon high vacuum pumping at RT. When L = dimethylsulfide (DMS), the nitrato complex is stable only to ∼-30 °C. Reactions of II with the N-donor ligands NH3, pyridine, or 1-methylimidazole are more complex. With these ligands, the nitrato complexes Mn(III)(TPP)(L)(η(1)-ONO2) and the cationic complexes [Mn(TPP)(L)2](+) coexist in the layer at room temperature, the latter formed as a result of NO3(-) displacement when L is in excess.

Involvement of ferryl in the reaction between nitrite and the oxy forms of globins

The reaction between nitrite and the oxy forms of globins has complex autocatalytic kinetics with several branching steps and evolves through chain reactions mediated by reactive species (including radicals) such as hydrogen peroxide, ferryl and nitrogen dioxide, starting with a lag phase, after which it proceeds onto an autocatalytic phase. Reported here are UV-Vis spectra collected upon stopped-flow mixing of myoglobin with a supraphysiological excess of nitrite. The best fit to the experimental data follows an A → B → C reaction scheme involving the formation of a short-lived intermediate identified as ferryl. This is consistent with a mechanism where nitrite binds to oxy myoglobin to generate an undetectable ferrous-peroxynitrate intermediate, whose decay leads to nitrate and ferryl. The ferryl is then reduced to met by the excess nitrite. DFT calculations reveal an essentially barrierless reaction between nitrite and the oxy heme, with a notable outer-sphere component; the resulting metastable ferrous-peroxynitrate adduct is found to feature a very low barrier towards nitrate liberation, with ferryl as a final product-in good agreement with experiment.

The kinetics of the reaction of nitrogen dioxide with iron(II)- and iron(III) cytochrome c

The reactions of NO2 with both oxidized and reduced cytochrome c at pH 7.2 and 7.4, respectively, and with N-acetyltyrosine amide and N-acetyltryptophan amide at pH 7.3 were studied by pulse radiolysis at 23 °C. NO2 oxidizes N-acetyltyrosine amide and N-acetyltryptophan amide with rate constants of (3.1±0.3)×10(5) and (1.1±0.1)×10(6) M(-1) s(-1), respectively. With iron(III)cytochrome c, the reaction involves only its amino acids, because no changes in the visible spectrum of cytochrome c are observed. The second-order rate constant is (5.8±0.7)×10(6) M(-1) s(-1) at pH 7.2. NO2 oxidizes iron(II)cytochrome c with a second-order rate constant of (6.6±0.5)×10(7) M(-1) s(-1) at pH 7.4; formation of iron(III)cytochrome c is quantitative. Based on these rate constants, we propose that the reaction with iron(II)cytochrome c proceeds via a mechanism in which 90% of NO2 oxidizes the iron center directly-most probably via reaction at the solvent-accessible heme edge-whereas 10% oxidizes the amino acid residues to the corresponding radicals, which, in turn, oxidize iron(II). Iron(II)cytochrome c is also oxidized by peroxynitrite in the presence of CO2 to iron(III)cytochrome c, with a yield of ~60% relative to peroxynitrite. Our results indicate that, in vivo, NO2 will attack preferentially the reduced form of cytochrome c; protein damage is expected to be marginal, the consequence of formation of amino acid radicals on iron(III)cytochrome c.

Nitric oxide interaction with oxy-coboglobin models containing trans-pyridine ligand: two reaction pathways

The oxy-cobolglobin models of the general formula (Py)Co(Por)(O2) (Por = meso-tetraphenyl- and meso-tetra-p-tolylporphyrinato dianions) were constructed by sequential low-temperature interaction of Py and dioxygen with microporous layers of Co-porphyrins. At cryogenic temperatures small increments of NO were introduced into the cryostat and the following reactions were monitored by the FTIR and UV-visible spectroscopy during slow warming. Similar to the recently studied (NH3)Co(Por)(O2) system (Kurtikyan et al. J. Am. Chem. Soc., 2012, 134, 13671-13680), this interaction leads to the nitric oxide dioxygenation reaction with the formation of thermally unstable nitrato complexes (Py)Co(Por)(η(1)-ONO2). The reaction proceeds through the formation of the six-coordinate peroxynitrite adducts (Py)Co(Por)(OONO), as was demonstrated by FTIR measurements with the use of isotopically labeled (18)O2, (15)NO, N(18)O, and (15)N(18)O species and DFT calculations. In contrast to the ammonia system, however, the binding of dioxygen in (Py)Co(Por)(O2) is weaker and the second reaction pathway takes place due to autoxidation of NO by rebound O2 that in NO excess gives N2O3 and N2O4 species adsorbed in the layer. This leads eventually to partial formation of (Py)Co(Por)(NO) and (Py)Co(Por)(NO2) as a result of NO and NO2 reactions with five-coordinate Co(Por)(Py) complexes that are present in the layer after the O2 has been released. The former is thermally unstable and at room temperature passes to the five-coordinate nitrosyl complex, while the latter is a stable compound. In these experiments at 210 K, the layer consists mostly of six-coordinate nitrato complexes and some minor quantities of six-coordinate nitro and nitrosyl species. Their relative quantities depend on the experimental conditions, and the yield of nitrato species is proportional to the relative quantity of peroxynitrite intermediate. Using differently labeled nitrogen oxide isotopomers in different stages of the process the formation of the caged radical pair after homolytic disruption of the O-O bond in peroxynitrite moiety is clearly shown. The composition of the layers upon farther warming to room temperature depends on the experimental conditions. In vacuo the six-coordinate nitrato complexes decompose to give nitrate anion and oxidized cationic complex Co(III)(Por)(Py)2. In the presence of NO excess, however, the nitro-pyridine complexes (Py)Co(Por)(NO2) are predominantly formed formally indicating the oxo-transfer reactivity of (Py)Co(Por)(η(1)-ONO2) with regard to NO. Using differently labeled nitrogen in nitric oxide and coordinated nitrate a plausible mechanism of this reaction is suggested based on the isotope distribution in the nitro complexes formed.

Nitroxyl accelerates the oxidation of oxyhemoglobin by nitrite

Angeli's salt (Na₂N₂O₃) decomposes into nitroxyl (HNO) and nitrite (NO₂(-)), compounds of physiological and therapeutic interest for their impact on biological signaling both through nitric oxide and nitric oxide independent pathways. Both nitrite and HNO oxidize oxygenated hemoglobin to methemoglobin. Earlier work has shown that HNO catalyzes the reduction of nitrite by deoxygenated hemoglobin. In this work, we have shown that HNO accelerates the oxidation of oxygenated hemoglobin by NO₂(-). We have demonstrated this HNO mediated acceleration of the nitrite/oxygenated hemoglobin reaction with oxygenated hemoglobin being in excess to HNO and nitrite (as would be found under physiological conditions) by monitoring the formation of methemoglobin in the presence of Angeli's salt with and without added NO₂(-). In addition, this acceleration has been demonstrated using the HNO donor 4-nitrosotetrahydro-2H-pyran-4-yl pivalate, a water-soluble acyloxy nitroso compound that does not release NO₂(-) but generates HNO in the presence of esterase. This HNO donor was used both with and without NO₂(-) and acceleration of the NO₂(-) induced formation of methemoglobin was observed. We found that the acceleration was not substantially affected by catalase, superoxide dismutase, c-PTIO, or IHP, suggesting that it is not due to formation of extramolecular peroxide, NO₂ or H₂O₂, or to modulation of allosteric properties. In addition, we found that the acceleration is not likely to be related to HNO binding to free reduced hemoglobin, as we found HNO binding to reduced hemoglobin to be much weaker than has previously been proposed. We suggest that the mechanism of the acceleration involves local propagation of autocatalysis in the nitrite-oxygenated Hb reaction. This acceleration of the nitrite oxyhemoglobin reaction could affect studies aimed at understanding physiological roles of HNO and perhaps nitrite and use of these agents in therapeutics such as hemolytic anemias, heart failure, and ischemia reperfusion injury.

Hemoglobin: a nitric-oxide dioxygenase

Members of the hemoglobin superfamily efficiently catalyze nitric-oxide dioxygenation, and when paired with native electron donors, function as NO dioxygenases (NODs). Indeed, the NOD function has emerged as a more common and ancient function than the well-known role in O2 transport-storage. Novel hemoglobins possessing a NOD function continue to be discovered in diverse life forms. Unique hemoglobin structures evolved, in part, for catalysis with different electron donors. The mechanism of NOD catalysis by representative single domain hemoglobins and multidomain flavohemoglobin occurs through a multistep mechanism involving O2 migration to the heme pocket, O2 binding-reduction, NO migration, radical-radical coupling, O-atom rearrangement, nitrate release, and heme iron re-reduction. Unraveling the physiological functions of multiple NODs with varying expression in organisms and the complexity of NO as both a poison and signaling molecule remain grand challenges for the NO field. NOD knockout organisms and cells expressing recombinant NODs are helping to advance our understanding of NO actions in microbial infection, plant senescence, cancer, mitochondrial function, iron metabolism, and tissue O2 homeostasis. NOD inhibitors are being pursued for therapeutic applications as antibiotics and antitumor agents. Transgenic NOD-expressing plants, fish, algae, and microbes are being developed for agriculture, aquaculture, and industry.

Nitric oxide dioxygenation reaction by oxy-coboglobin models: in-situ low-temperature FTIR characterization of coordinated peroxynitrite

The oxy-cobolglobin models of the general formula (NH(3))Co(Por)(O(2)) (Por = meso-tetra-phenyl and meso-tetra-p-tolylporphyrinato dianions) were constructed by sequential low temperature interaction of NH(3) and dioxygen with microporous layers of Co-porphyrins. At cryogenic temperatures small increments of NO were introduced into the cryostat and the following reactions were monitored by the FTIR and UV-visible spectroscopy during slow warming. Upon warming the layers from 80 to 120 K a set of new IR bands grows with correlating intensities along with the consumption of the ν(O(2)) band. Isotope labeling experiments with (18)O(2), (15)NO and N(18)O along with DFT calculations provides a basis for assigning them to the six-coordinate peroxynitrite complexes (NH(3))Co(Por)(OONO). Over the course of warming the layers from 140 to 170 K these complexes decompose and there are spectral features suggesting the formation of nitrogen dioxide NO(2). Upon keeping the layers at 180-210 K the bands of NO(2) gradually decrease in intensity and the set of new bands grows in the range of 1480, 1270, and 980 cm(-1). These bands have their isotopic counterparts when (15)NO, (18)O(2) and N(18)O are used in the experiments and certainly belong to the 6-coordinate nitrato complexes (NH(3))Co(Por)(η(1)-ONO(2)) demonstrating the ability of oxy coboglobin models to promote the nitric oxide dioxygenation (NOD) reaction similar to oxy-hemes. As in the case of Hb, Mb and model iron-porphyrins, the six-coordinate nitrato complexes are not stable at room temperature and dissociate to give nitrate anion and oxidized cationic complex Co(III)(Por)(NH(3))(1,2).

Iron overload-induced rat liver injury: Involvement of protein tyrosine nitration and the effect of baicalin

Baicalin has been reported to protect against liver injury in iron-overload mice, however, the mechanisms underlying the hepatoprotective properties of baicalin are poorly understood. In this study, we systematically studied the protective effect of baicalin on iron overload induced liver injury, as well as the underlying mechanism based on nitrative stress in rat model. We found that when iron overload rats (500mgiron/kg) were fed baicalin-containing diet (0.3% and 1% w/w) for 45days, baicalin dose dependently protected against iron overload induced liver injury, including alleviation of hepatic pathological damage, decrease of SOD activity, iron content, carbonyl content, and the thiobarbituric acid-reactive substances level in hepatic tissues. It also increased serum iron content, SH content and GPx activity, decreased serum ALT and AST activities. Immunohistochemistry and immunoprecipitation analysis revealed that baicalin could also inhibit iron overload induced protein tyrosine nitration in liver. Moreover, in iron overload rat liver, we found that baicalin decreased the iron overload increased level of glutathione-S-transferases (GSTs) expression, oxidation and nitration. These results suggest that not only oxidative stress, but also nitrative stress, is involved in iron overload induced liver injury, and the underlying mechanism might partially relate to the involvement of GSTs expression and post-translational modification. Baicalin can effectively prevent iron overload caused abnormality and can be a candidate medicine for iron overload diseases.

Mechanisms of peroxynitrite interactions with heme proteins

Oxygenated heme proteins are known to react rapidly with nitric oxide (NO) to produce peroxynitrite (PN) at the heme site. This process could lead either to attenuation of the effects of NO or to nitrosative protein damage. PN is a powerful nitrating and oxidizing agent that has been implicated in a variety of cell injuries. Accordingly, it is important to delineate the nature and variety of reaction mechanisms of PN interactions with heme proteins. In this Forum, we survey the range of reactions of PN with heme proteins, with particular attention to myoglobin and cytochrome c. While these two proteins are textbook paradigms for oxygen binding and electron transfer, respectively, both have recently been shown to have other important functions that involve NO and PN. We have recently described direct evidence that ferrylmyolgobin (ferrylMb) and nitrogen dioxide (NO(2)) are both produced during the reaction of PN and metmyolgobin (metMb) (Su, J.; Groves, J. T. J. Am. Chem. Soc. 2009, 131, 12979-12988). Kinetic evidence indicates that these products evolve from the initial formation of a caged radical intermediate [Fe(IV) horizontal lineO.NO(2)]. This caged pair reacts mainly via internal return with a rate constant k(r) to form metMb and nitrate in an oxygen-rebound scenario. Detectable amounts of ferrylMb are observed by stopped-flow spectrophotometry, appearing at a rate consistent with the rate, k(obs), of heme-mediated PN decomposition. Freely diffusing NO(2), which is liberated concomitantly from the radical pair (k(e)), preferentially nitrates myoglobin Tyr103 and added fluorescein. For cytochrome c, Raman spectroscopy has revealed that a substantial fraction of cytochrome c converts to a beta-sheet structure, at the expense of turns and helices at low pH (Balakrishnan, G.; Hu, Y.; Oyerinde, O. F.; Su, J.; Groves, J. T.; Spiro, T. G. J. Am. Chem. Soc., 2007, 129, 504-505). It is proposed that a short beta-sheet segment, comprising residues 37-39 and 58-61, extends itself into the large 37-61 loop when the latter is destabilized by protonation of H26, which forms an anchoring hydrogen bond to loop residue P44. This conformation change ruptures the Met80-Fe bond, as revealed by changes in ligation-sensitive Raman bands. It also induces peroxidase activity with the same temperature profile. This process is suggested to model the apoptotic peroxidation of cardiolipin by cytochrome c.

Direct detection of the oxygen rebound intermediates, ferryl Mb and NO2, in the reaction of metmyoglobin with peroxynitrite

Oxygenated hemoproteins are known to react rapidly with nitric oxide (NO) to produce peroxynitrite (PN) at the heme site. This process could lead either to attenuation of the effects of NO or to nitrosative protein damage. Peroxynitrite is a powerful nitrating and oxidizing agent that has been implicated in a variety of cell injuries. Accordingly, it is important to delineate the nature and variety of reaction mechanisms of PN reactions with heme proteins. Here, we present direct evidence that ferrylMb and NO(2) are both produced during the reaction of PN and metmyoglobin (metMb). Kinetic evidence indicates that these products evolve from initial formation of a caged radical intermediate [Fe(IV)=O *NO(2)]. This caged pair reacts mainly via internal return with a rate constant k(r) to form metMb and nitrate in an oxygen rebound scenario. Detectable amounts of ferrylMb are observed by stopped-flow spectrophotometry, appearing at a rate consistent with the rate, k(obs), of heme-mediated PN decomposition. Freely diffusing NO(2), which is liberated concomitantly from the radical pair (k(e)), preferentially nitrates Tyr103 in horse heart myoglobin. The ratio of the rates of in-cage rebound and cage escape, k(r)/k(e), was found to be approximately 10 by examining the nitration yields of fluorescein, an external NO(2) trap. This rebound/escape model for the metMb/PN interaction is analogous to the behavior of alkyl hyponitrites and the well-studied geminate recombination processes of deoxymyoglobin with O(2), CO, and NO. The scenario is also similar to the stepwise events of substrate hydroxylation by cytochrome P450 and other oxygenases. It is likely, therefore, that the reaction of metMb with ONOO(-) and that of oxyMb with NO proceed through the same [Fe(IV)=O *NO(2)] caged radical intermediate and lead to similar outcomes. The results indicate that while oxyMb may reduce the concentration of intracellular NO, it would not eliminate the formation of NO(2) as a decomposition product of peroxynitrite.

Role of tissue disorder markers in the evaluation of disease progress and outcome prediction: a prospective cohort study in non-cardiac critically ill patients

BACKGROUND

Acute Physiology and Chronic Health Evaluation (APACHE) has a remarkable association with clinical syndrome, life expectancy, and length of ward stay. But the defects are obvious. It is crucial to detect an effective and convenient evaluation method to monitor disease progress and predict outcome.

OBJECTIVE

To determine whether myoglobin (Mb) and other five tissue disorder biomarkers, troponin-I, creatine-kinase, creatine kinase-muscle brain, aspartate aminotransferase, and lactate dehydrogenase are independent predictors of disease progress and mortality in non-cardiac critical illness.

METHODS

A prospective study with 179 patients admitted to the Intensive Care Unit was conducted. All serum tissue disorder markers and APACHE-II score were measured within 24  hr of admission.

RESULTS

All the six biomarkers were significantly correlated with disease severity stratified by APACHE-II and outcome. Serial blood samples were taken from 17 patients on detection of two new organs failure. The occurrence of organs failure was significantly associated with the elevation of Mb, troponin-I, and APACHE-II. Multivariate analysis demonstrated that elevated Mb was the principal risk factor related to mortality either during hospitalization or 180-day followup. Kaplan-Meier method and log-rank test also showed that patients with elevated Mb levels had significantly shorter survival. The mortality was higher in patients with both Mb>500  ng/ml and APACHE-II>20 than in those with only Mb>500  ng/ml or APACHE-II>20.

CONCLUSION

All the six tissue disorder markers are predictors of disease severity, organ failure, and outcome in non-cardiac critically illness. Among them, Mb plays a pivotable role. The combined use of Mb and APACHE-II suggest an effective method to determine the outcome of critical ill syndrome.

Heme/O2/*NO nitric oxide dioxygenase (NOD) reactivity: phenolic nitration via a putative heme-peroxynitrite intermediate

An oxy-heme complex, the heme-superoxo species (tetrahydrofuran)(F(8))Fe(III)-(O(2)(*-)) (2) (F(8) = an ortho-difluoro substituted tetraarylporphyrinate), reacts with nitrogen monoxide (*NO; nitric oxide) to produce a nitrato-iron(III) compound (F(8))Fe(III)-(NO(3)(-)) (3) (X-ray). The chemistry mimics the action of *NO Dioxygenases (NODs), microbial and mammalian heme proteins which facilitate *NO detoxification/homeostasis. A peroxynitrite intermediate complex is implicated; if 2,4-di-tert-butylphenol is added prior to *NO reaction with 2, o-nitration occurs giving 2,4-di-tert-butyl-6-nitrophenol. The iron product is (F(8))Fe(III)-(OH) (4). The results suggest that heme/O(2)/*NO chemistry may lead to peroxynitrite leakage and/or exogenous substrate oxidative/nitrative reactivity.

Catalytic scavenging of peroxynitrite by catalase

Peroxynitrite (ONOO(-)/ONOOH), the product of the diffusion controlled reaction between nitric oxide (*NO) and superoxide anion (O(2)(-)*), is a strong oxidizing and nitrating agent. Several heme proteins react rapidly with peroxynitrite, some of them catalyze its decomposition. In this work we found, contrary to previous reports, that catalase, a ferriheme enzyme, catalytically scavenges peroxynitrite. The second-order reaction rate constants of peroxynitrite decay catalyzed by catalase increase with decreasing pH and are equal to (2.7+/-0.2) x 10(6), (1.7+/-0.1) x 10(6) and (0.8+/-0.1) x 10(6) M(-1) s(-1) at pH 6.1, 7.1 and 8.0, respectively. This dependence suggests that peroxynitrous acid, ONOOH, is the species that reacts with heme center of catalase. The possible reaction mechanisms of the decay of peroxynitrite catalyzed by catalase and physiological relevance of this reaction are discussed.

The reaction of cell-free oxyhemoglobin with nitrite under physiologically relevant conditions: Implications for nitrite-based therapies

Nitric oxide (NO*) participates in the regulation of a wide array of biological processes and its deficit contributes to the severity of many diseases. Recently, a role of NO deficiency that occurs as a result of intravascular hemolysis and increases in levels of cell-free hemoglobin in the pathway of chronic anemic pathologies has been suggested. Experimental evidence for deoxyhemoglobin-catalyzed reduction of nitrite to NO* leads to the possibility of nitrite infusion-based therapies to correct NO* deficits. However, the presence of plasma hemoglobin also raises the possibility of deleterious free radical-mediated oxidative damage from the reaction between nitrite and oxyhemoglobin in the vasculature. We show that the conditions required for the reaction between nitrite and oxyhemoglobin to exhibit free radical-mediated autocatalytic kinetics are highly unlikely to occur in the plasma compartment, even during extensive hemolysis and with pharmacological nitrite doses. Although the presence of haptoglobin enhances the rate of the reaction between nitrite and oxyhemoglobin, common plasma antioxidants-ascorbate and urate, as well as catalase-prevent autocatalysis. Our findings suggest that pharmacological doses of nitrite are unlikely to cause free radical or ferrylhemoglobin formation in plasma originating from the reaction of nitrite with cell-free oxyhemoglobin in vivo.

The reaction between nitrite and oxyhemoglobin: a mechanistic study

The nitrite anion (NO(-)(2)) has recently received much attention as an endogenous nitric oxide source that has the potential to be supplemented for therapeutic benefit. One major mechanism of nitrite reduction is the direct reaction between this anion and the ferrous heme group of deoxygenated hemoglobin. However, the reaction of nitrite with oxyhemoglobin (oxyHb) is well established and generates nitrate and methemoglobin (metHb). Several mechanisms have been proposed that involve the intermediacy of protein-free radicals, ferryl heme, nitrogen dioxide (NO(2)), and hydrogen peroxide (H(2)O(2)) in an autocatalytic free radical chain reaction, which could potentially limit the usefulness of nitrite therapy. In this study we show that none of the previously published mechanisms is sufficient to fully explain the kinetics of the reaction of nitrite with oxyHb. Based on experimental data and kinetic simulation, we have modified previous models for this reaction mechanism and show that the new model proposed here is consistent with experimental data. The important feature of this model is that, whereas previously both H(2)O(2) and NO(2) were thought to be integral to both the initiation and propagation steps, H(2)O(2) now only plays a role as an initiator species, and NO(2) only plays a role as an autocatalytic propagatory species. The consequences of uncoupling the roles of H(2)O(2) and NO(2) in the reaction mechanism for the in vivo reactivity of nitrite are discussed.

Interaction of peroxynitrite with myoglobin and hemoglobin

Peroxynitrite, a biological toxin produced in vivo by the nearly diffusion-controlled reaction of nitrogen monoxide with superoxide, can nitrate and oxidize various biomolecules. Modifications caused by peroxynitrite have been linked to many human diseases, in particular, increased levels of free or protein-bound 3-nitrotyrosine, a biomarker for peroxynitrite in vivo, have been detected in a variety of pulmonary and cardiovascular diseases as well as in neurodegenerative and chronic inflammatory disorders. These observations have led to the search for a drug that can scavenge this powerful nitrating and oxidizing agent. Heme proteins, in particular myoglobin and hemoglobin, present in large amounts in muscles and red blood cells, respectively, have been proposed to serve as sinks for peroxynitrite in these cells. This report reviews the current knowledge of the reactions of different forms of myoglobin and hemoglobin with peroxynitrite and discusses their physiological role on the basis of measured rate constants.Key words: myoglobin, hemoglobin, peroxynitrite, tyrosine nitration, iron(III) peroxynitrite complex.

Pulse Radiolysis Studies of the Reactions of CO3•- and NO2• with Nitrosyl(II)myoglobin and Nitrosyl(II)hemoglobin

The reactions of carbonate radical anion [CO3*-, systematic name: trioxidocarbonate*1-] with nitrosyl(II)hemoglobin (HbFe(II)NO) and nitrosyl(II)myoglobin (MbFe(II)NO) were studied by pulse radiolysis in N2O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature. The reactions proceed in two steps: outer-sphere oxidation of the nitrosyliron(II) proteins to their corresponding nitrosyliron(III) forms and subsequent dissociation of NO*. The second-order rate constants measured for the first reaction steps were (4.3 +/- 0.2) x 10(8) and (1.5 +/- 0.3) x 10(8) M(-1) s(-1), for MbFe(II)NO and HbFe(II)NO, respectively. The reactions between nitrogen dioxide and MbFe(II)NO or HbFe(II)NO were studied by pulse radiolysis in N2O-saturated 0.1 M phosphate buffer pH 7.4 containing 5 mM nitrite. Also for the reactions of this oxidant with the nitrosyliron(II) forms of Mb and Hb a two-step reaction was observed: oxidation of the iron was followed by dissociation of NO*. The second-order rate constants measured for the first reaction steps were (2.9 +/- 0.3) x 10(7) and (1.8 +/- 0.3) x 10(7) M(-1) s(-1), for MbFe(II)NO and HbFe(II)NO, respectively. Both radicals appear to be able to oxidize the iron(II) centers of the proteins directly. Only for the reactions with HbFe(II)NO it cannot be excluded that, in a parallel reaction, CO3*- and NO2* first react with amino acid(s) of the globin, which then oxidize the nitrosyliron(II) center.

Hemoglobins dioxygenate nitric oxide with high fidelity

Distantly related members of the hemoglobin (Hb) superfamily including red blood cell Hb, muscle myoglobin (Mb) and the microbial flavohemoglobin (flavoHb) dioxygenate nitric oxide (.NO). The reaction serves important roles in .NO metabolism and detoxification throughout the aerobic biosphere. Analysis of the stoichiometric product nitrate shows greater than 99% double O-atom incorporation from Hb(18)O(2), Mb(18)O(2) and flavoHb(18)O(2) demonstrating a conserved high fidelity .NO dioxygenation mechanism. Whereas, reactions of .NO with the structurally unrelated Turbo cornutus MbO(2) or free superoxide radical (-O.(2)) yield sub-stoichiometric nitrate showing low fidelity O-atom incorporation. These and other results support a .NO dioxygenation mechanism involving (1) rapid reaction of .NO with a Fe(III-)O.(2) intermediate to form Fe(III-)OONO and (2) rapid isomerization of the Fe(III-)OONO intermediate to form nitrate. A sub-microsecond isomerization event is hypothesized in which the O-O bond homolyzes to form a protein caged [Fe(IV)O .NO(2)] intermediate and ferryl oxygen attacks .NO(2) to form nitrate. Hb functions as a .NO dioxygenase by controlling O(2) binding and electrochemistry, guiding .NO diffusion and reaction, and shielding highly reactive intermediates from solvent water and biomolecules.

Peroxynitrite efficiently mediates the interconversion of redox intermediates of myeloperoxidase

Nitric oxide-derived oxidants (e.g., peroxynitrite) are believed to participate in antimicrobial activities as part of normal host defenses but also in oxidative tissue injury in inflammatory disorders. A similar role is ascribed to the heme enzyme myeloperoxidase (MPO), the most abundant protein of polymorphonuclear leukocytes, which are the terminal phagocytosing effector cells of the innate immune system. Concomitant production of peroxynitrite and release of millimolar MPO are characteristic events during phagocytosis. In order to understand the mode of interaction between MPO and peroxynitrite, we have performed a comprehensive stopped-flow investigation of the reaction between all physiological relevant redox intermediates of MPO and peroxynitrite. Both iron(III) MPO and iron(II) MPO are rapidly converted to compound II by peroxynitrite in monophasic reactions with calculated rate constants of (6.8+/-0.1) x 10(6) M(-1)s(-1) and (1.3+/-0.2) x 10(6) M(-1)s(-1), respectively (pH 7.0 and 25 degrees C). Besides these one- and two-electron reduction reactions of peroxynitrite, which produce nitrogen dioxide and nitrite, a one-electron oxidation to the oxoperoxonitrogen radical must occur in the fast monophasic transition of compound I to compound II mediated by peroxynitrite at pH 7.0 [(7.6+/-0.1) x 10(6) M(-1)s(-1)]. In addition, peroxynitrite induced a steady-state transition from compound III to compound II with a rate of (1.0+/-0.3) x 10(4) M(-1)s(-1). Thus, the interconversion among the various oxidation states of MPO that is prompted by peroxynitrite is remarkable. Reaction mechanisms are proposed and the physiological relevance is discussed.

Apples increase nitric oxide production by human saliva at the acidic pH of the stomach: a new biological function for polyphenols with a catechol group?

Dietary inorganic nitrate is secreted in saliva and reduced to nitrite by bacterial flora. At the acidic pH of the stomach nitrite is present as nitrous acid in equilibrium with nitric oxide (*NO), and other nitrogen oxides with nitrating and nitrosating activity. *NO in the stomach exerts several beneficial effects, but nitrosating/nitrating species have been implicated as a possible cause of epithelial neoplasia at the gastroesophageal junction. We investigated the effects of apple extracts on *NO release by human saliva at pH 2. A water extract obtained from apple homogenate increased *NO release caused by acidification of saliva. Data show that polyphenols were responsible for this activity, with chlorogenic acid and (+)-catechin the most active and concentrated species. However, ferulic acid, a hydroxycinnamic acid with only one aromatic hydroxyl group, did not increase *NO release. Fructose, the most representative sugar in apples, was also inactive. Interestingly, ascorbic acid in saliva induced a SCN(-)-enhanced burst of *NO but, unlike apple, the release was transient. The simultaneous addition of ascorbic acid and apple extract caused a burst of *NO followed by the increased steady-state level characteristic of saliva containing apple extract. Chlorogenic acid and (+)-catechin, but not ferulic acid, formed o-semiquinone radicals and nitrated polyphenols, suggesting the scavenging of *NO(2) by o-semiquinones. Our results propose that some apple polyphenols not only inhibit nitrosation/nitration but also promote *NO bio-availabilty at the gastric level, a previously unappreciated function.

Intra- and intermolecular oxidation of oxymyoglobin and oxyhemoglobin induced by hydroxyl and carbonate radicals

The mechanism of the reactions of myoglobin and hemoglobin with *OH and CO3*- in the presence of oxygen was studied using pulse and gamma-radiolysis. Unlike *NO2, which adds to the porphyrin iron, *OH and CO3*- form globin radicals. These secondary radicals oxidize the Fe(II) center through both intra- and intermolecular processes. The intermolecular pathway was further demonstrated when BSA radicals derived from *OH or CO3*- oxidized oxyhemoglobin and oxymyoglobin to their respective ferric states. The oxidation yields obtained by pulse radiolysis were lower compared to gamma-radiolysis, where the contribution of radical-radical reactions is negligible. Full oxidation yields by *OH-derived globin radicals could be achieved only at relatively high concentrations of the heme protein mainly via an intermolecular pathway. It is suggested that CO3*- reaction with the protein yields Tyr and/or Trp-derived phenoxyl radicals, which solely oxidize the porphyrin iron under gamma-radiolysis conditions. The *OH particularly adds to aromatic residues, which can undergo elimination of H2O forming the phenoxyl radical, and/or react rapidly with O2 yielding peroxyl radicals. The peroxyl radical can oxidize a neighboring porphyrin iron and/or give rise to superoxide, which neither oxidize nor reduce the porphyrin iron. The potential physiological implications of this chemistry are that hemoglobin and myoglobin, being present at relatively high concentrations, can detoxify highly oxidizing radicals yielding the respective ferric states, which are not toxic.