Isolation and oxygenation reactions of nitrosylmyoglobins

Effects of Surface Treatment with Thymol on the Lipid Oxidation Processes, Fatty Acid Profile and Color of Sliced Salami during Refrigerated Storage

The oxidation of unsaturated fatty acids and the adverse transformation of pigments from meat and spices are the primary causes of chemical degradation in processed meat products. Thymol is found in a variety of plant extracts that have been proven to effectively inhibit or slow down oxidative processes. The objective of our study was to determine whether thymol treatment of the surface of sliced paprika salami could be applied to inhibit lipid oxidation and color change during refrigerated storage. During eight weeks of storage, the malondialdehyde (MDA) levels and the ratios of saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), polyunsaturated fatty acids (PUFAs), and n6/n3 in thymol-treated salami remained unchanged (p ≥ 0.05), whereas in the controls, the MDA levels increased by approximately twelvefold and the ratio of SFAs in the lipid fraction increased (p < 0.001), while the ratio of PUFAs decreased (p < 0.001). The application of thymol prevented decrease in yellowness (b*) of the slices and reduced decreases in redness (a*) and brightness (chroma).

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.

Oxygen-mediated oxidation of ferrous nitrosylated nitrobindins

The O₂-mediated oxidation of all-β-barrel ferrous nitrosylated nitrobindin from Arabidopsis thaliana (At-Nb(II)-NO), Mycobacterium tuberculosis (Mt-Nb(II)-NO), and Homo sapiens (Hs-Nb(II)-NO) to ferric derivative (At-Nb(III), Mt-Nb(III), and Hs-Nb(III), respectively) has been investigated at pH 7.0 and 20.0 °C. Unlike ferrous nitrosylated horse myoglobin, human serum heme-albumin and human hemoglobin, the process in Nb(II)-NO is mono-exponential and linearly dependent on the O₂ concentration, displaying a bimolecular behavior, characterized by k_on = (6.3 ± 0.8) × 10³ M^-1 s^-1, (1.4 ± 0.2) × 10³ M^-1 s^-1, and (3.9 ± 0.5) × 10³ M^-1 s^-1 for At-Nb(II)-NO, Mt-Nb(II)-NO, and Hs-Nb(II)-NO, respectively. No intermediate is detected, indicating that the O₂ reaction with Nb(II)-NO is the rate-limiting step and that the subsequent conversion of the heme-Fe(III)-N(O)OO^- species (i.e., N-bound peroxynitrite to heme-Fe(III)) to heme-Fe(III) and NO₃^- is much faster. A similar mechanism can be invoked for ferrous nitrosylated human neuroglobin and rabbit hemopexin, in which the heme-Fe(III)-N(O)OO^- species is formed as well, although the rate-limiting step seems represented by the reshaping of the six-coordinated heme-Fe(III) complex. Although At-Nb(II)-NO and Mt-Nb(II)-NO are partially (while Hs-Nb(II)-NO is almost completely) penta-coordinated, density functional theory (DFT) calculations rule out that the cleavage of the proximal heme-Fe-His bond in Nb(II)-NO is responsible for the more stable heme-Fe(III)-N(O)OO^- species. Moreover, the oxidation of the penta-coordinated heme-Fe(II)-NO adduct does not depend on O₂ binding at the proximal side of the metal center. These features may instead reflect the peculiarity of Nb folding and of the heme environment, with a reduced steric constraint for the formation of the heme-Fe(III)-N(O)OO^- complex.

Mechanism and regulation of ferrous heme-nitric oxide (NO) oxidation in NO synthases

Nitric oxide (NO) synthases (NOSs) catalyze the formation of NO from l-arginine. We have shown previously that the NOS enzyme catalytic cycle involves a large number of reactions but can be characterized by a global model with three main rate-limiting steps. These are the rate of heme reduction by the flavin domain (k_r ), of dissociation of NO from the ferric heme-NO complex (k_d ), and of oxidation of the ferrous heme-NO complex (k _ox). The reaction of oxygen with the ferrous heme-NO species is part of a futile cycle that does not directly contribute to NO synthesis but allows a population of inactive enzyme molecules to return to the catalytic cycle, and thus, enables a steady-state NO synthesis rate. Previously, we have reported that this reaction does involve the reaction of oxygen with the NO-bound ferrous heme complex, but the mechanistic details of the reaction, that could proceed via either an inner-sphere or an outer-sphere mechanism, remained unclear. Here, we present additional experiments with neuronal NOS (nNOS) and inducible NOS (iNOS) variants (nNOS W409F and iNOS K82A and V346I) and computational methods to study how changes in heme access and electronics affect the reaction. Our results support an inner-sphere mechanism and indicate that the particular heme-thiolate environment of the NOS enzymes can stabilize an N-bound Fe^III-N(O)OO^- intermediate species and thereby catalyze this reaction, which otherwise is not observed or favorable in proteins like globins that contain a histidine-coordinated heme.

Structure and function of haemoglobins

Haemoglobin (Hb) is widely known as the iron-containing protein in blood that is essential for O₂ transport in mammals. Less widely recognised is that erythrocyte Hb belongs to a large family of Hb proteins with members distributed across all three domains of life-bacteria, archaea and eukaryotes. This review, aimed chiefly at researchers new to the field, attempts a broad overview of the diversity, and common features, in Hb structure and function. Topics include structural and functional classification of Hbs; principles of O₂ binding affinity and selectivity between O₂/NO/CO and other small ligands; hexacoordinate (containing bis-imidazole coordinated haem) Hbs; bacterial truncated Hbs; flavohaemoglobins; enzymatic reactions of Hbs with bioactive gases, particularly NO, and protection from nitrosative stress; and, sensor Hbs. A final section sketches the evolution of work on the structural basis for allosteric O₂ binding by mammalian RBC Hb, including the development of newer kinetic models. Where possible, reference to historical works is included, in order to provide context for current advances in Hb research.

Heme-based catalytic properties of human serum albumin

Human serum albumin (HSA): (i) controls the plasma oncotic pressure, (ii) modulates fluid distribution between the body compartments, (iii) represents the depot and carrier of endogenous and exogenous compounds, (iv) increases the apparent solubility and lifetime of hydrophobic compounds, (v) affects pharmacokinetics of many drugs, (vi) inactivates toxic compounds, (vii) induces chemical modifications of some ligands, (viii) displays antioxidant properties, and (ix) shows enzymatic properties. Under physiological and pathological conditions, HSA has a pivotal role in heme scavenging transferring the metal-macrocycle from high- and low-density lipoproteins to hemopexin, thus acquiring globin-like reactivity. Here, the heme-based catalytic properties of HSA are reviewed and the structural bases of drug-dependent allosteric regulation are highlighted.

Phenol nitration induced by an {Fe(NO)2}(10) dinitrosyl iron complex

Cellular dinitrosyl iron complexes (DNICs) have long been considered NO carriers. Although other physiological roles of DNICs have been postulated, their chemical functionality outside of NO transfer has not been demonstrated thus far. Here we report the unprecedented dioxygen reactivity of a N-bound {Fe(NO)(2)}(10) DNIC, [Fe(TMEDA)(NO)(2)] (1). In the presence of O(2), 1 becomes a nitrating agent that converts 2,4,-di-tert-butylphenol to 2,4-di-tert-butyl-6-nitrophenol via formation of a putative iron-peroxynitrite [Fe(TMEDA)(NO)(ONOO)] (2) that is stable below -80 °C. Iron K-edge X-ray absorption spectroscopy on 2 supports a five-coordinated metal center with a bound peroxynitrite in a cyclic bidentate fashion. The peroxynitrite ligand of 2 readily decays at increased temperature or under illumination. These results suggest that DNICs could have multiple physiological or deleterious roles, including that of cellular nitrating agents.

Reactions of HNO with heme proteins: new routes to HNO-heme complexes and insight into physiological effects

The formation and interconversion of nitrogen oxides has been of interest in numerous contexts for decades. Early studies focused on gas-phase reactions, particularly with regard to industrial and atmospheric environments, and on nitrogen fixation. Additionally, investigation of the coordination chemistry of nitric oxide (NO) with hemoglobin dates back nearly a century. With the discovery in the early 1980s that NO is biosynthesized as a molecular signaling agent, the literature has been focused on the biological effects of nitrogen oxides, but the original concerns remain relevant. For instance, hemoglobin has long been known to react with nitrite, but this reductase activity has recently been considered to be important to produce NO under hypoxic conditions. The association of nitrosyl hydride (HNO; also commonly referred to as nitroxyl) with heme proteins can also produce NO by reductive nitrosylation. Furthermore, HNO is considered to be an intermediate in bacterial denitrification, but conclusive identification has been elusive. The authors of this article have approached the bioinorganic chemistry of HNO from different perspectives, which have converged because heme proteins are important biological targets of HNO.

The molecular mechanism of mammalian NO-synthases: a story of electrons and protons

Since its discovery, nitric oxide synthase (NOS), the enzyme responsible for NO biosynthesis in mammals, has been the subject of extensive investigations regarding its catalytic and molecular mechanisms. These studies reveal the high degree of sophistication of NOS functioning and regulation. However, the precise description of the NOS molecular mechanism and in particular of the oxygen activation chemistry is still lacking. The reaction intermediates implicated in NOS catalysis continue to elude identification and the current working paradigm is increasingly contested. Consequently, the last three years has seen the emergence of several competing models. All these models propose the same global reaction scheme consisting of two successive oxidation reactions but they diverge in the details of their reaction sequence. The major discrepancies concern the number, source and characteristics of proton and electron transfer processes. As a result each model proposes distinct reaction pathways with different implied oxidative species. This review aims to examine the different experimental evidence concerning NOS proton and electron transfer events and the role played by the substrates and cofactors in these processes. The resulting discussion should provide a comparative picture of all potential models for the NOS molecular mechanism.

The distal pocket histidine residue in horse heart myoglobin directs the O-binding mode of nitrite to the heme iron

It is now well-established that mammalian heme proteins are reactive with various nitrogen oxide species and that these reactions may play significant roles in mammalian physiology. For example, the ferrous heme protein myoglobin (Mb) has been shown to reduce nitrite (NO(2)(-)) to nitric oxide (NO) under hypoxic conditions. We demonstrate here that the distal pocket histidine residue (His64) of horse heart metMb(III) (i.e., ferric Mb(III)) has marked effects on the mode of nitrite ion coordination to the iron center. X-ray crystal structures were determined for the mutant proteins metMb(III) H64V (2.0 A resolution) and its nitrite ion adduct metMb(III) H64V-nitrite (1.95 A resolution), and metMb(III) H64V/V67R (1.9 A resolution) and its nitrite ion adduct metMb(III) H64V/V67R-nitrite (2.0 A resolution). These are compared to the known structures of wild-type (wt) hh metMb(III) and its nitrite ion adduct hh metMb(III)-nitrite, which binds NO(2)(-) via an O-atom in a trans-FeONO configuration. Unlike wt metMb(III), no axial H(2)O is evident in either of the metMb(III) mutant structures. In the ferric H64V-nitrite structure, replacement of the distal His residue with Val alters the binding mode of nitrite from the nitrito (O-binding) form in the wild-type protein to a weakly bound nitro (N-binding) form. Reintroducing a H-bonding residue in the H64V/V67R double mutant restores the O-binding mode of nitrite. We have also examined the effects of these mutations on reactivities of the metMb(III)s with cysteine as a reducing agent and of the (ferrous) Mb(II)s with nitrite ion under anaerobic conditions. The Mb(II)s were generated by reduction of the Mb(III) precursors in a second-order reaction with cysteine, the rate constants for this step following the order H64V/V67R > H64V >> wt. The rate constants for the oxidation of the Mb(II)s by nitrite (giving NO as the other product) follow the order wt > H64V/V67R >> H64V and suggest a significant role of the distal pocket H-bonding residue in nitrite reduction.

Fast ferrous heme-NO oxidation in nitric oxide synthases

During catalysis, the heme in nitric oxide synthase (NOS) binds NO before releasing it to the environment. Oxidation of the NOS ferrous heme-NO complex by O2 is key for catalytic cycling, but the mechanism is unclear. We utilized stopped-flow methods to study the reaction of O2 with ferrous heme-NO complexes of inducible and neuronal NOS enzymes. We found that the reaction does not involve heme-NO dissociation, but instead proceeds by a rapid direct reaction of O2 with the ferrous heme-NO complex. This behavior is novel and may distinguish heme-thiolate enzymes, such as NOS, from related heme proteins.

The potential of Angeli's salt to decrease nitric oxide scavenging by plasma hemoglobin

Release of hemoglobin from the erythrocyte during intravascular hemolysis contributes to the pathology of a variety of diseased states. This effect is partially due to the enhanced ability of cell-free plasma hemoglobin, which is primarily found in the ferrous, oxygenated state, to scavenge nitric oxide. Oxidation of the cell-free hemoglobin to methemoglobin, which does not effectively scavenge nitric oxide, using inhaled nitric oxide has been shown to be effective in limiting pulmonary and systemic vasoconstriction. However, the ferric heme species may be reduced back to ferrous hemoglobin in plasma and has the potential to drive injurious redox chemistry. We propose that compounds that selectively convert cell-free hemoglobin to ferric, and ideally iron-nitrosylated heme species that do not actively scavenge nitric oxide, would effectively treat intravascular hemolysis. We show here that nitroxyl generated by Angeli's salt (sodium alpha-oxyhyponitrite, Na2N2O3) preferentially reacts with cell-free hemoglobin compared to that encapsulated in the red blood cell under physiologically relevant conditions. Nitroxyl oxidizes oxygenated ferrous hemoglobin to methemoglobin and can convert the methemoglobin to a more stable, less toxic species, iron-nitrosyl hemoglobin. These results support the notion that Angeli's salt or a similar compound could be used to effectively treat conditions associated with intravascular hemolysis.

NO dissociation represents the rate limiting step for O2-mediated oxidation of ferrous nitrosylated Mycobacterium leprae truncated hemoglobin O

Mycobacterium leprae truncated hemoglobin O (trHbO) protects from nitrosative stress and sustains mycobacterial respiration. Here, kinetics of M. leprae trHbO(II)-NO denitrosylation and of O(2)-mediated oxidation of M. leprae trHbO(II)-NO are reported. Values of the first-order rate constant for *NO dissociation from M. leprae trHbO(II)-NO (k(off)) and of the first-order rate constant for O(2)-mediated oxidation of M. leprae trHbO(II)-NO (h) are 1.3 x 10(-4) s(-1) and 1.2 x 10(-4) s(-1), respectively. The coincidence of values of k(off) and h suggests that O(2)-mediated oxidation of M. leprae trHbO(II)-NO occurs with a reaction mechanism in which *NO, that is initially bound to heme(II), is displaced by O(2) but may stay trapped in a protein cavity(ies) close to heme(II). Next, M. leprae trHbO(II)-O(2) reacts with *NO giving the transient Fe(III)-OONO species preceding the formation of the final product M. leprae trHbO(III). *NO dissociation from heme(II)-NO represents the rate limiting step for O(2)-mediated oxidation of M. leprae trHbO(II)-NO.

Activation of Peroxynitrite by Inducible Nitric-oxide Synthase

In mammals, nitric oxide (NO) is an essential biological mediator that is exclusively synthesized by nitric-oxide synthases (NOSs). However, NOSs are also directly or indirectly responsible for the production of peroxynitrite, a well known cytotoxic agent involved in numerous pathophysiological processes. Peroxynitrite reactivity is extremely intricate and highly depends on activators such as hemoproteins. NOSs present, therefore, the unique ability to both produce and activate peroxynitrite, which confers upon them a major role in the control of peroxynitrite bioactivity. We report here the first kinetic analysis of the interaction between peroxynitrite and the oxygenase domain of inducible NOS (iNOSoxy). iNOSoxy binds peroxynitrite and accelerates its decomposition with a second order rate constant of 22 x 10(4) m(-1)s(-1) at pH 7.4. This reaction is pH-dependent and is abolished by the binding of substrate or product. Peroxynitrite activation is correlated with the observation of a new iNOS heme intermediate with specific absorption at 445 nm. iNOSoxy modifies peroxynitrite reactivity and directs it toward one-electron processes such as nitration or one-electron oxidation. Taken together our results suggest that, upon binding to iNOSoxy, peroxynitrite undergoes homolytic cleavage with build-up of an oxo-ferryl intermediate and concomitant release of a NO(2)(.) radical. Successive cycles of peroxynitrite activation were shown to lead to iNOSoxy autocatalytic nitration and inhibition. The balance between peroxynitrite activation and self-inhibition of iNOSoxy may determine the contribution of NOSs to cellular oxidative stress.

O2-mediated oxidation of hemopexin-heme(II)-NO

Hemopexin (HPX), serving as scavenger and transporter of toxic plasma heme, has been postulated to play a key role in the homeostasis of NO. Here, kinetics of HPX-heme(II) nitrosylation and O2-mediated oxidation of HPX-heme(II)-NO are reported. NO reacts reversibly with HPX-heme(II) yielding HPX-heme(II)-NO, according to the minimum reaction scheme: HPX-heme(II)+NO kon<-->koff HPX-heme(II)-NO values of kon, koff, and K (=kon/koff) are (6.3+/-0.3)x10(3)M-1s-1, (9.1+/-0.4)x10(-4)s-1, and (6.9+/-0.6)x10(6)M-1, respectively, at pH 7.0 and 10.0 degrees C. O2 reacts with HPX-heme(II)-NO yielding HPX-heme(III) and NO3-, by means of the ferric heme-bound peroxynitrite intermediate (HPX-heme(III)-N(O)OO), according to the minimum reaction scheme: HPX-heme(II)-NO+O2 hon<--> HPX-heme(III)-N(O)OO l-->HPX-heme(III)+NO3- the backward reaction rate is negligible. Values of hon and l are (2.4+/-0.3)x10(1)M-1s-1 and (1.4+/-0.2)x10(-3)s-1, respectively, at pH 7.0 and 10.0 degrees C. The decay of HPX-heme(III)-N(O)OO (i.e., l) is rate limiting. The HPX-heme(III)-N(O)OO intermediate has been characterized by optical absorption spectroscopy in the Soret region (lambdamax=409 nm and epsilon409=1.51x10(5)M-1cm-1). These results, representing the first kinetic evidence for HPX-heme(II) nitrosylation and O2-mediated oxidation of HPX-heme(II)-NO, might be predictive of transient (pseudo-enzymatic) function(s) of heme carriers.

Sites and Mechanisms of Aconitase Inactivation by Peroxynitrite: Modulation by Citrate and Glutathione

Aconitases are iron-sulfur cluster-containing proteins present both in mitochondria and cytosol of cells; the cubane iron-sulfur (Fe-S) cluster in the active site is essential for catalytic activity, but it also renders aconitase highly vulnerable to reactive oxygen and nitrogen species. This study examined the sites and mechanisms of aconitase inactivation by peroxynitrite (ONOO-), a strong oxidant and nitrating agent readily formed from superoxide anion and nitric oxide generated by mitochondria. ONOO- inactivated aconitase in a dose-dependent manner (half-maximal inhibition was observed with approximately 3 microM ONOO-). Low levels of ONOO- caused the conversion of the Fe-S cluster from the [4Fe-4S]2+ form to the inactive [3Fe-4S]1+ form with the loss of labile iron, as confirmed by low-temperature EPR analysis. In the presence of the substrate, citrate, 66-fold higher concentrations of ONOO- were required for half-maximal inhibition. The protective effects of citrate corresponded to its binding to the active site. The inactivation of aconitase in the presence of citrate was due to ONOO--mediated cysteine thiol loss and tyrosine nitration in the enzyme as shown by Western blot analyses. LC/MS/MS analyses revealed that ONOO- treatment to aconitase resulted in nitration of tyrosines 151 and 472 and oxidation to sulfonic acid of cysteines 126 and 385. The latter is one of the three cysteine residues in aconitase that binds to the Fe-S cluster. All other modified tyrosine and cysteine residues were adjacent to the binding site, thus suggesting that these modifications caused conformational changes leading to active-site disruption. Aconitase cysteine thiol modifications other than oxidation to sulfonic acid, such as S-glutathionylation, also decreased aconitase activity, thus indicating that glutathionylation may be an important means of modulating aconitase activity under oxidative and nitrative stress. Taken together, these results demonstrate that the Fe-S cluster in the active site, cysteine 385 bound to the Fe-S cluster, and tyrosine and cysteine residues in the vicinity of the active site are important targets of oxidative and/or nitrative attack, which is selectively controlled by the mitochondrial matrix citrate levels. The mechanisms inherent in aconitase inactivation by ONOO- are discussed in terms of the mitochondrial matrix metabolic and thiol redox state.

Reactivity Studies of the Fe(III) and Fe(II)NO Forms of Human Neuroglobin Reveal a Potential Role against Oxidative Stress

Neuroglobin, recently discovered in the brain and in the retina of vertebrates, belongs to the class of hexacoordinate globins, in which the distal histidine coordinates the iron center in both the Fe(II) and Fe(III) forms. As for most other hexacoordinate globins, the physiological function of neuroglobin is still unclear, but seems to be related to neuronal survival following acute hypoxia. In this study, we have addressed the question whether human neuroglobin could act as a scavenger of toxic species, such as nitrogen monoxide, peroxynitrite, and hydrogen peroxide, which are generated at high levels in the brain during hypoxia; we have also investigated the kinetics of the reactions of its Fe(III) (metNGB) and Fe(II)NO forms with several reagents. Binding of cyanide or NO* to metNGB follows bi-exponential kinetics, showing the existence of two different protein conformations. In the presence of excess NO*, metNGB is converted into NGBFe(II)NO by reductive nitrosylation, in analogy to the reactions of NO* with metmyoglobin and methemoglobin. The Fe(II)NO form of neuroglobin is oxidized to metNGB by peroxynitrite and dioxygen, two reactions that also take place in hemoglobin, albeit at lower rates. In contrast to myoglobin and hemoglobin, metNGB unexpectedly does not generate the cytotoxic ferryl form of the protein upon addition of either peroxynitrite or hydrogen peroxide. Taken together, our data indicate that human neuroglobin may be an efficient scavenger of reactive oxidizing species and thus may play a role in the cellular defense against oxidative stress.

Hemoglobin-nitric oxide cooperativity: is NO the third respiratory ligand?

Through its cooperative binding mechanism, hemoglobin is an effective transporter of oxygen and carbon dioxide. Although data have recently been presented suggesting otherwise, the rate at which nitric oxide binds to hemoglobin is not cooperative. On the other hand, the rate at which nitric oxide dissociates from hemoglobin is cooperative so that, similar to the case of oxygen, the cooperativity in equilibrium ligand binding is manifested in the dissociation rate rather than the association rate. Two general factors that diminish the likelihood that hemoglobin transports nitric oxide are the slow dissociation rate of nitric oxide from hemoglobin and the very fast hemoglobin oxidation reaction, which converts nitric oxide to the inert molecule nitrate. Despite these factors the possibility that NO is delivered by hemoglobin under certain conditions or through more complicated mechanisms needs further study.

Crystal structures of ferrous horse heart myoglobin complexed with nitric oxide and nitrosoethane

The interactions of nitric oxide (NO) and organic nitroso compounds with heme proteins are biologically important, and adduct formation between NO-containing compounds and myoglobin (Mb) have served as prototypical systems for studies of these interactions. We have prepared crystals of horse heart (hh) MbNO from nitrosylation of aqua-metMb crystals, and we have determined the crystal structure of hh MbNO at a resolution of 1.9 A. The Fe-N-O angle of 147 degrees in hh MbNO is larger than the corresponding 112 degrees angle previously determined from the crystal structure of sperm whale MbNO (Brucker et al., Proteins 1998;30:352-356) but is similar to the 150 degrees angle determined from a MS XAFS study of a frozen solution of hh MbNO (Rich et al., J Am Chem Soc 1998;120:10827-10836). The Fe-N(O) bond length of 2.0 A (this work) is longer than the 1.75 A distance determined from the XAFS study and suggests distal pocket influences on FeNO geometry. The nitrosyl N atom is located 3.0 A from the imidazole N(epsilon) atom of the distal His64 residue, suggesting electrostatic stabilization of the FeNO moiety by His64. The crystal structure of the nitrosoethane adduct of ferrous hh Mb was determined at a resolution of 1.7 A. The nitroso O atom of the EtNO ligand is located 2.7 A from the imidazole N(epsilon) atom of His64, suggesting a hydrogen bond interaction between these groups. To the best of our knowledge, the crystal structure of hh Mb(EtNO) is the first such determination of a nitrosoalkane adduct of a heme protein.

Measurements of nitric oxide on the heme iron and beta-93 thiol of human hemoglobin during cycles of oxygenation and deoxygenation

Nitric oxide has been proposed to be transported by hemoglobin as a third respiratory gas and to elicit vasodilation by an oxygen-linked (allosteric) mechanism. For hemoglobin to transport nitric oxide bioactivity it must capture nitric oxide as iron nitrosyl hemoglobin rather than destroy it by dioxygenation. Once bound to the heme iron, nitric oxide has been reported to migrate reversibly from the heme group of hemoglobin to the beta-93 cysteinyl residue, in response to an oxygen saturation-dependent conformational change, to form an S-nitrosothiol. However, such a transfer requires redox chemistry with oxidation of the nitric oxide or beta-93 cysteinyl residue. In this article, we examine the ability of nitric oxide to undergo this intramolecular transfer by cycling human hemoglobin between oxygenated and deoxygenated states. Under various conditions, we found no evidence for intramolecular transfer of nitric oxide from either cysteine to heme or heme to cysteine. In addition, we observed that contaminating nitrite can lead to formation of iron nitrosyl hemoglobin in deoxygenated hemoglobin preparations and a radical in oxygenated hemoglobin preparations. Using 15N-labeled nitrite, we clearly demonstrate that nitrite chemistry could explain previously reported results that suggested apparent nitric oxide cycling from heme to thiol. Consistent with our results from these experiments conducted in vitro, we found no arterial/venous gradient of iron nitrosyl hemoglobin detectable by electron paramagnetic resonance spectroscopy. Our results do not support a role for allosterically controlled intramolecular transfer of nitric oxide in hemoglobin as a function of oxygen saturation.

Urease enhances the formation of iron nitrosyl hemoglobin in the presence of hydroxyurea

Although it has been shown that hydroxyurea (HU) therapy produces measurable amounts of nitric oxide (NO) metabolites, including iron nitrosyl hemoglobin (HbNO) in patients with sickle cell disease, the in vivo mechanism for formation of these is not known. Much in vitro data and some in vivo data indicates that HU is the NO donor, but other studies suggest a role for nitric oxide synthase (NOS). In this study, we confirm that the NO-forming reactions of HU with hemoglobin (Hb) or other blood constituents is too slow to account for NO production measured in vivo. We hypothesize that, in vivo, HU is partially metabolized to hydroxylamine (HA), which quickly reacts with Hb to form methemoglobin (metHb) and HbNO. We show that addition of urease, which converts HU to HA, to a mixture of blood and HU, greatly enhances HbNO formation.

Interaction of nitrogen monoxide with hemoglobin and the artefactual production of S-nitroso-hemoglobin

Hemoglobin (Hb) is probably the most thoroughly studied protein in the human body. However, it has recently been proposed that in addition to the well known function of dioxygen and carbon dioxide transporter, one of the main roles of hemoglobin is to store and transport nitrogen monoxide. This hypothesis is highly disputed and is in contrast to the proposal that hemoglobin serves as an NO. scavenger in the blood. In this short review, I have presented the current status of research on the much-debated mechanism of the reaction between circulating hemoglobin and NO.. Despite the fact that oxyHb is extremely rapidly oxidized by NO., under basal physiological conditions the biological activity of NO. in the blood vessels is not completely lost. It has been shown that three factors reduce the efficiency of hemoglobin to scavenge NO.: a so-called red blood cell-free zone created close to the vessel wall by intravascular flow, an undisturbed layer around the red blood cells--where the NO. concentration is much smaller than the bulk concentration--and/or the red blood cell membrane. Alternatively, it has been proposed that NO. binds to Cys beta 93 of oxyHb, is liberated after deoxygenation of Hb, and consequently allows for a more effective delivery of O2 to peripheral tissues. However, because of the extremely fast rate of the reaction between NO. and oxyHb, experiments in vitro lead to artefactual production of large amounts of S-nitroso-hemoglobin. These results, together with other data, which challenge most steps of the NO.-transporter hypothesis, are discussed.

Reactions of deoxy-, oxy-, and methemoglobin with nitrogen monoxide. Mechanistic studies of the S-nitrosothiol formation under different mixing conditions

The reaction between hemoglobin (Hb) and NO* has been investigated thoroughly in recent years, but its mechanism is still a matter of substantial controversy. We have carried out a systematic study of the influence of the following factors on the yield of S-nitrosohemoglobin (SNO-Hb) generated from the reaction of NO* with oxy-, deoxy-, and metHb: 1) the volumetric ratio of the protein and the NO* solutions; 2) the rate of addition of the NO* solution to the protein solution; 3) the amount of NO* added; and 4) the concentration of the phosphate buffer. Our results suggest that the highest SNO-Hb yields are mostly obtained by very slow addition of substoichiometric amounts of NO* from a diluted solution. Possible pathways of SNO-Hb formation from the reaction of NO* with oxy-, deoxy-, and metHb are described. Our data strongly suggest that, because of mixing artifacts, care should be taken to use results from in vitro experiments to draw conclusion on the mechanism of the reaction in vivo.

Effects of iron nitrosylation on sickle cell hemoglobin solubility

One mechanism by which nitric oxide (NO) has been proposed to benefit patients with sickle cell disease is by reducing intracellular polymerization of sickle hemoglobin (HbS). In this study we have examined the ability of nitric oxide to inhibit polymerization by measuring the solubilizing effect of iron nitrosyl sickle hemoglobin (HbS-NO). Electron paramagnetic resonance spectroscopy was used to confirm that, as found in vivo, the primary type of NO ligation produced in our partially saturated NO samples is pentacoordinate alpha-nitrosyl. Linear dichroism spectroscopy and delay time measurements were used to confirm polymerization. Based on sedimentation studies we found that, although fully ligated (100% tetranitrosyl) HbS is very soluble, the physiologically relevant, partially ligated species do not provide a significant solubilizing effect. The average solubilizing effect of 26% NO saturation was 0.045; much less than the 0.15 calculated for the effect of 26% oxygen saturation. Given the small amounts of NO-ligated hemoglobin achievable through any kind of NO therapy, we conclude that NO therapy does not benefit patients through any direct solubilizing effect.

Differences in three kinetic parameters underpin the unique catalytic profiles of nitric-oxide synthases I, II, and III

We previously reported the existence of a special auto-regulation property of neuronal nitric-oxide synthase (NOS) based on NO near-geminate combination and partial trapping of neuronal NOS (nNOS) through a futile regenerating pathway. On this basis, we developed a kinetic simulation model that was proven to predict nNOS catalytic specificities and mutations effects (Santolini, J., Adak, S., Curran, C. M., and Stuehr, D. J. (2001) J. Biol. Chem. 276, 1233-1243; Adak, S., Santolini, J., Tikunova, S., Wang, Q., Johnson, J. D., and Stuehr, D. J. (2001) J. Biol. Chem. 276, 1244-1252). Here we show that the same model simulates and explains the distinct catalytic behaviors of inducible and endothelial NOS (iNOS and eNOS). Their marked differences were linked to variations in three basic parameters (rates of ferric heme reduction, ferric heme.NO dissociation, and ferrous heme.NO oxidation) that together control partitioning between futile and productive pathways and their relative rates. We also incorporated feedback inhibition into the kinetic model to account for potential rebinding of accumulated solution NO. The model accurately simulated the different relative impacts of both NOS.NO interactions (near-geminate combination of NO versus rebinding of solution NO) on catalytic behavior of each NOS isoform, including their speed and extent of heme.NO complex accumulation, K(m) for O(2), and propensity to transform NO into a higher oxide. Thus, individual catalytic behavior of any NOS can be understood through a single unified kinetic model. Because the model defines how different settings of individual kinetic parameters control regulation by two distinct NOS.NO interactions, it sheds light on mechanisms, structural features, and scope of NOS regulation and its physiologic impact.

Does myoglobin protect Trypanosoma cruzi from the antiparasitic effects of nitric oxide?

The hemoflagellate protozoan parasite Trypanosoma cruzi is the causative agent of Chagas disease, a progressive fatal cardiomyopathy widespread in South and Central America. Here, we postulate that the preferential colonization of cardiomyocytes by T. cruzi may reflect the role of myoglobin (Mb) as a nitric oxide (NO) scavenger, protecting the parasite from the trypanocidal effects of NO. The proposal of this novel function of Mb is based on knowledge that ferrous oxygenated Mb reacts rapidly and irreversibly with NO yielding nitrate and ferric oxidized Mb, which is reduced back to the physiologically active form by an intracellular reductase. The postulated protective role of Mb on the viability of T. cruzi is reminiscent of that postulated for hemoglobin in protecting intraerythrocytic Plasmodia from the parasiticidal effect of NO.

Nitric oxide modulates the catalytic activity of myeloperoxidase

Myeloperoxidase (MPO), an abundant protein in neutrophils, monocytes, and subpopulations of tissue macrophages, is believed to play a critical role in host defenses and inflammatory tissue injury. To perform these functions, an array of diffusible radicals and reactive oxidant species may be formed through oxidation reactions catalyzed at the heme center of the enzyme. Myeloperoxidase and inducible nitric-oxide synthase are both stored in and secreted from the primary granules of activated leukocytes, and nitric oxide (nitrogen monoxide; NO) reacts with the iron center of hemeproteins at near diffusion-controlled rates. We now demonstrate that NO modulates the catalytic activity of MPO through distinct mechanisms. NO binds to both ferric (Fe(III), the catalytically active species) and ferrous (Fe(II)) forms of MPO, generating stable low-spin six-coordinate complexes, MPO-Fe(III).NO and MPO-Fe(II).NO, respectively. These nitrosyl complexes were spectrally distinguishable by their Soret absorbance peak and visible spectra. Stopped-flow kinetic analyses indicated that NO binds reversibly to both Fe(III) and Fe(II) forms of MPO through simple one-step mechanisms. The association rate constant for NO binding to MPO-Fe(III) was comparable to that observed with other hemoproteins whose activities are thought to be modulated by NO in vivo. In stark contrast, the association rate constant for NO binding to the reduced form of MPO, MPO-Fe(II), was over an order of magnitude slower. Similarly, a 2-fold decrease was observed in the NO dissociation rate constant of the reduced versus native form of MPO. The lower NO association and dissociation rates observed suggest a remarkable conformational change that alters the affinity and accessibility of NO to the distal heme pocket of the enzyme following heme reduction. Incubation of NO with the active species of MPO (Fe(III) form) influenced peroxidase catalytic activity by dual mechanisms. Low levels of NO enhanced peroxidase activity through an effect on the rate-limiting step in catalysis, reduction of Compound II to the ground-state Fe(III) form. In contrast, higher levels of NO inhibited MPO catalysis through formation of the nitrosyl complex MPO-Fe(III)-NO. NO interaction with MPO may thus serve as a novel mechanism for modulating peroxidase catalytic activity, influencing the regulation of local inflammatory and infectious events in vivo.

Nitric oxide and iron proteins

Nitric oxide interactions with iron are the most important biological reactions in which NO participates. Reversible binding to ferrous haem iron is responsible for the observed activation of guanylate cyclase and inhibition of cytochrome oxidase. Unlike carbon monoxide or oxygen, NO can also bind reversibly to ferric iron. The latter reaction is responsible for the inhibition of catalase by NO. NO reacts with the oxygen adduct of ferrous haem proteins (e.g. oxyhaemoglobin) to generate nitrate and ferric haem; this reaction is responsible for the majority of NO metabolism in the vasculature. NO can also interact with iron-sulphur enzymes (e.g. aconitase, NADH dehydrogenase). This review describes the underlying kinetics, thermodynamics, mechanisms and biological role of the interactions of NO with iron species (protein and non-protein bound). The possible significance of iron reactions with reactive NO metabolites, in particular peroxynitrite and nitroxyl anion, is also discussed.

Reversible and irreversible hemichrome generation by the oxygenation of nitrosylmyoglobin

The repeated oxygenation/reduction/nitrosylation of nitrosylmyoglobin produces low-spin ferric heme hemichromes which have been characterized by electron spin resonance spectroscopy. The predominant myoglobin hemichrome is a chemically reversible dihistidyl complex identified by the g values 1.53, 2.21, and 2.97. Also present is a low-spin ferric hydroxide derivative which is represented by the g values 1.83, 2.18, and 2.59. The formation of these species goes undetected by UV-vis spectroscopy, but the oxygenation of myoglobin to metmyoglobin is correlated with complete conversion of nitric oxide to nitrate which is released following a clear induction period. These results are interpreted in terms of the intermediates generated during the MbNO oxygenation reaction.

(Nitro)Iron(III) Porphyrins. EPR Detection of a Transient Low-Spin Iron(III) Complex and Structural Characterization of an O Atom Transfer Product

The reaction of BF(3).OEt(2) with the bis(nitro) complex of iron(III) picket-fence porphyrin, [K(18C6)(OH(2))][Fe(TpivPP)(NO(2))(2)], leads to the formation of a transient porphyrin intermediate, assigned on the basis of its rhombic low-spin EPR spectrum as the five-coordinate N-bound mono(nitro) iron(III) derivative, [Fe(TpivPP)(NO(2))]. This species is reactive and readily undergoes oxygen atom transfer to form [Fe(III)(TpivPP)(NO(3))] and [Fe(II)(TpivPP)(NO)]. The reactions have been followed by EPR and IR spectroscopy. [Fe(TpivPP)(NO(2))] has a rhombic EPR spectrum (g = 2.60, 2.35, and 1.75) in chlorobenzene and CH(2)Cl(2) and is spectroscopically distinct from the bis(nitro) starting material (g = 2.70, 2.50, and 1.57). Oxidation of the nitrosyl species to [Fe(TpivPP)(NO(3))] proceeds via an intermediate assigned as [Fe(TpivPP)(NO(2))] on the basis of its EPR spectrum. The crystal structure of one of the reaction products, [Fe(TpivPP)(NO(3))], has been determined. The nitrate ion of [Fe(TpivPP)(NO(3))] is bound to the iron(III) ion in a "symmetric" bidentate fashion within the ligand-binding pocket of the porphyrin pickets. Individual Fe-O distances are 2.123(3) and 2.226(3) Å. The dihedral angle between the plane of the nitrate ion and the closest N(p)-Fe-N(p) plane is 10.0 degrees. The Fe-N(p) bonds (and trans N(p)-Fe-N(p) angles) perpendicular and parallel to the plane of the axial ligand average to 2.060(5) Å (154.84(9) degrees ) and 2.083(3) Å (146.14(9) degrees ), respectively. Crystal data for [Fe(TpivPP)(NO(3))]: a = 23.530(2) Å, b = 10.0822(5) Å, c = 48.748(3) Å, beta = 92.145(5) degrees, monoclinic, space group I2/a, V = 11556.4(14) Å(3), Z = 8, FeN(9)O(7)C(64)H(64), 8798 observed data, R(1) = 0.0606, wR(2) = 0.1313, all observations at 127(2) K.

Pathophysiological chemistry of nitric oxide and its oxygenation by-products

Nitric oxide is not only an important intracellular regulator and cytostatic effector but it is also the starting point for a cascade of physiologically important oxygenation reactions, whose by-products and intermediates have a largely unexplored biochemistry. These new biological discoveries are reshaping our appreciation of the remarkably complex chemistry that underlies it.