Oxidative modification by low levels of HOOH can transform myoglobin to an oxidase

Heme d formation in a Shewanella benthica hemoglobin

In our continued investigations of microbial globins, we solved the structure of a truncated hemoglobin from Shewanella benthica, an obligate psychropiezophilic bacterium. The distal side of the heme active site is lined mostly with hydrophobic residues, with the exception of a tyrosine, Tyr34 (CD1) and a histidine, His24 (B13). We found that purified SbHbN, when crystallized in the ferric form with polyethylene glycol as precipitant, turned into a green color over weeks. The electron density obtained from the green crystals accommodated a trans heme d, a chlorin-type derivative featuring a γ-spirolactone and a vicinal hydroxyl group on a pyrroline ring. In solution, exposure of the protein to one equivalent of hydrogen peroxide resulted in a similar green color change, but caused by the formation of multiple products. These were oxidation species released on protein denaturation, likely including heme d, and a species with heme covalently attached to the polypeptide. The Tyr34Phe replacement prevented the formation of both heme d and the covalent linkage. The ready modification of heme b by SbHbN expands the range of chemistries supported by the globin fold and offers a route to a novel heme cofactor.

The More, the Merrier? Multiple Myoglobin Genes in Fish Species, Especially in Gray Bichir (Polypterus senegalus) and Reedfish (Erpetoichthys calabaricus)

The members of the globin superfamily are a classical model system to investigate gene evolution and their fates as well as the diversity of protein function. One of the best-known globins is myoglobin (Mb), which is mainly expressed in heart muscle and transports oxygen from the sarcolemma to the mitochondria. Most vertebrates harbor a single copy of the myoglobin gene, but some fish species have multiple myoglobin genes. Phylogenetic analyses indicate an independent emergence of multiple myoglobin genes, whereby the origin is mostly the last common ancestor of each order. By analyzing different transcriptome data sets, we found at least 15 multiple myoglobin genes in the polypterid gray bichir (Polypterus senegalus) and reedfish (Erpetoichthys calabaricus). In reedfish, the myoglobin genes are expressed in a broad range of tissues but show very different expression values. In contrast, the Mb genes of the gray bichir show a rather scattered expression pattern; only a few Mb genes were found expressed in the analyzed tissues. Both, gray bichir and reedfish possess lungs which enable them to inhabit shallow and swampy waters throughout tropical Africa with frequently fluctuating and low oxygen concentrations. The myoglobin repertoire probably reflects the molecular adaptation to these conditions. The sequence divergence, the substitution rate, and the different expression pattern of multiple myoglobin genes in gray bichir and reedfish imply different functions, probably through sub- and neofunctionalization during evolution.

Reversible Oxidative Modifications in Myoglobin and Functional Implications

Myoglobin (Mb), an oxygen-binding heme protein highly expressed in heart and skeletal muscle, has been shown to undergo oxidative modifications on both an inter- and intramolecular level when exposed to hydrogen peroxide (H₂O₂) in vitro. Here, we show that exposure to H₂O₂ increases the peroxidase activity of Mb. Reaction of Mb with H₂O₂ causes covalent binding of heme to the Mb protein (Mb-X), corresponding to an increase in peroxidase activity when ascorbic acid is the reducing co-substrate. Treatment of H₂O₂-reacted Mb with ascorbic acid reverses the Mb-X crosslink. Reaction with H₂O₂ causes Mb to form dimers, trimers, and larger molecular weight Mb aggregates, and treatment with ascorbic acid regenerates Mb monomers. Reaction of Mb with H₂O₂ causes formation of dityrosine crosslinks, though the labile nature of the crosslinks broken by treatment with ascorbic acid suggests that the reversible aggregation of Mb is mediated by crosslinks other than dityrosine. Disappearance of a peptide containing a tryptophan residue when Mb is treated with H₂O₂ and the peptide’s reappearance after subsequent treatment with ascorbic acid suggest that tryptophan side chains might participate in the labile crosslinking. Taken together, these data suggest that while exposure to H₂O₂ causes Mb-X formation, increases Mb peroxidase activity, and causes Mb aggregation, these oxidative modifications are reversible by treatment with ascorbic acid. A caveat is that future studies should demonstrate that these and other in vitro findings regarding properties of Mb have relevance in the intracellular milieu, especially in regard to actual concentrations of metMb, H₂O₂, and ascorbate that would be found in vivo.

Genetic and functional diversity of the multiple lungfish myoglobins

It is known that the West African lungfish (Protopterus annectens) harbours multiple myoglobin (Mb) genes that are differentially expressed in various tissues and that the Mbs differ in their abilities to confer tolerance towards hypoxia. Here, we show that other lungfish species (Protopterus dolloi, Protopterus aethiopicus and Lepidosiren paradoxa) display a similar diversity of Mb genes and have orthologous Mb genes. To investigate the functional diversification of these genes, we studied the structures, O₂ binding properties and nitrite reductase enzymatic activities of recombinantly expressed P. annectens Mbs (PanMbs). CD spectroscopy and small-angle X-ray scattering revealed the typical globin-fold in all investigated recombinant Mbs, indicating a conserved structure. The highest O₂ affinity was measured for PanMb2 (P₅₀  = 0.88 Torr at 20 °C), which is mainly expressed in the brain, whereas the muscle-specific PanMb1 has the lowest O₂ affinity (P₅₀  = 3.78 Torr at 20 °C), suggesting that tissue-specific O₂ requirements have resulted in the emergence of distinct Mb types. Two of the mainly neuronally expressed Mbs (PanMb3 and PanMb4b) have the highest nitrite reductase rates. These data show different O₂ binding and enzymatic properties of lungfish Mbs, reflecting multiple subfunctionalisation and neofunctionalisation events that occurred early in the evolution of lungfish. Some Mbs may have also taken over the functions of neuroglobin and cytoglobin, which are widely expressed in vertebrates but appear to be missing in lungfish.

Thiol-induced hemoglobin oxidation

Addition of cysteine in the mM range to purified oxyhemoglobin, red blood cell lysate or red blood cell suspensions leads to oxidation of the hemoprotein. The rate and extent of the process depend on the initial hemoglobin and cysteine concentrations, and the reaction is limited by the total destruction of the sulfhydryl groups. Similar results are obtained employing glutathione, but the rate of the process is considerably slower. Oxidation of the purified hemoprotein is faster than in the red blood cell lysate. This difference is mainly due to the inhibitory effect of catalase present in the lysate. Addition of sodium azide increases the rate of oxyhemoglobin oxidation in the lysate, while addition of catalase reduces the rate of oxidation of the purified hemoprotein. The results are interpreted in terms of a mechanism comprising the oxidation of the oxyhemoglobin by the -SH group, with concomitant formation of superoxide anion and hydrogen peroxide. These species further contribute to the oxyhemoglobin oxidation. A chain oxidation of the thiol, catalyzed by the hemoprotein, explains the extensive cysteine destruction.

Nitroxide radicals as research tools: Elucidating the kinetics and mechanisms of catalase-like and "suicide inactivation" of metmyoglobin

BACKGROUND

Metmyoglobin (MbFe(III)) reaction with H(2)O(2) has been a subject of study over many years. H(2)O(2) alone promotes heme destruction frequently denoted "suicide inactivation," yet the mechanism underlying H(2)O(2) dismutation associated with MbFe(III) inactivation remains obscure.

METHODS

MbFe(III) reaction with excess H(2)O(2) in the absence and presence of the nitroxide was studied at pH 5.3-8.1 and 25°C by direct determination of reaction rate constants using rapid-mixing stopped-flow technique, by following H(2)O(2) depletion, O(2) evolution, spectral changes of the heme protein, and the fate of the nitroxide by EPR spectroscopy.

RESULTS

The rates of both H(2)O(2) dismutation and heme inactivation processes depend on [MbFe(III)], [H(2)O(2)] and pH. Yet the inactivation stoichiometry is independent of these variables and each MbFe(III) molecule catalyzes the dismutation of 50±10 H(2)O(2) molecules until it is inactivated. The nitroxide catalytically enhances the catalase-like activity of MbFe(III) while protecting the heme against inactivation. The rate-determining step in the absence and presence of the nitroxide is the reduction of MbFe(IV)O by H(2)O(2) and by nitroxide, respectively.

CONCLUSIONS

The nitroxide effects on H(2)O(2) dismutation catalyzed by MbFe(III) demonstrate that MbFe(IV)O reduction by H(2)O(2) is the rate-determining step of this process. The proposed mechanism, which adequately fits the pro-catalytic and protective effects of the nitroxide, implies the intermediacy of a compound I-H(2)O(2) adduct, which decomposes to a MbFe(IV)O and an inactivated heme at a ratio of 25:1.

GENERAL SIGNIFICANCE

The effects of nitroxides are instrumental in elucidating the mechanism underlying the catalysis and inactivation routes of heme proteins.

Differential thermal stability and oxidative vulnerability of the hemoglobin variants, HbA2 and HbE

Apart from few early biophysical studies, the relative thermal instability of HbE has been only shown by clinical investigations. We have compared in vitro thermal stability of HbE with HbA2 and HbA using optical spectroscopy. From absorption measurements in the soret region, synchronous fluorescence spectroscopy and dynamic light scattering experiments, we have found thermal stability of the three hemoglobin variants following the order HbE<HbA<HbA2 in terms of structural unfolding and aggregation pattern. We have found formation of intermolecular dityrosine fluorophores with characteristic fluorescence signature, at pH >11.0 in all the three variants. Under oxidative stress conditions in presence of hydrogen peroxide, HbE has been found to be more vulnerable to aggregation compared to HbA and HbA2. Taken together, these studies have shown thermal and oxidative instability of HbE and points towards the role of HbE in the upregulation of redox regulators and chaperone proteins in erythrocyte proteome of patients suffering from HbEbeta thalassemia.

Reactive intermediates: molecular and MS-based approaches to assess the functional significance of chemical-protein adducts

Biologically reactive intermediates formed as endogenous products of various metabolic processes are considered important factors in a variety of human diseases, including Parkinson's disease and other neurological disorders, diabetes and complications thereof, and other inflammatory-associated diseases. Chemical-induced toxicities are also frequently mediated via the bioactivation of relatively stable organic molecules to reactive electrophilic metabolites. Indeed, chemical-induced toxicities have long been known to be associated with the ability of electrophilic metabolites to react with a variety of targets within the cell, including their covalent adduction to nucleophilic residues in proteins, and nucleotides within DNA. Although we possess considerable knowledge of the various biochemical mechanisms by which chemicals undergo metabolic bioactivation, we understand far less about the processes that couple bioactivation to toxicity. Identifying specific sites within a protein, which are targets for adduction, can provide the initial information necessary to determine whether such adventitious posttranslational modifications significantly alter either protein structure and/or function. To address this problem, we have developed mass spectrometry (MS)-based approaches to identify specific amino acid targets of electrophile adduction (electrophile-binding motifs), coupled with molecular modeling of such adducts, to determine the potential structural and functional consequences. Where appropriate, functional assays are subsequently conducted to assess protein function.

Oxidation of myofibrillar proteins and impaired functionality: underlying mechanisms of the carbonylation pathway

The potential impact of protein oxidation on the functional properties of myofibrillar proteins (MP) was investigated in the present study. To accomplish this purpose, myofibrillar proteins were oxidized in vitro for 12 days at 37 °C in the presence of Cu(2+), Fe(3+), and Mb in combination with H(2)O(2) and analyzed at sampling times for chemical changes induced by oxidative reactions and functional properties. The oxidation measurements included specific protein carbonyls (α-aminoadipic semialdehyde, AAS), advanced lysine oxidation products (α-aminoadipic acid, AAA, and Schiff bases), and thiobarbituric acid-reactive substances (TBARS). The factors and mechanisms involved in the oxidative degradation of lysine residues through the carbonylation pathway are precisely described. According to the present results, intense lipid and protein carbonylation, principally induced by Cu(2+)/systems, leads to a fast and severe loss of MP functionality, including impaired water-holding, foaming, and gelling capacities. The implication of Mb in the oxidation events enhances the production of AAA and Schiff bases, compromising to a larger extent the solubility of MP and worsening the aggregation and the gelling capacity. The connection between the oxidation-induced chemical changes and the loss of protein functionality is thoroughly discussed.

The redox activity of hemoglobins: from physiologic functions to pathologic mechanisms

Pentacoordinate respiratory hemoproteins such as hemoglobin and myoglobin have evolved to supply cells with oxygen. However, these respiratory heme proteins are also known to function as redox enzymes, reacting with compounds such as nitric oxide and peroxides. The recent discoveries of hexacoordinate hemoglobins in vertebrates and nonsymbiotic plants suggest that the redox activity of globins is inherent to the molecule. The uncontrolled formation of radical species resulting from such redox chemistry on respiratory hemoproteins can lead to oxidative damage and cellular toxicity. In this review, we examine the functions of various globins and the mechanisms by which these globins act as redox enzymes under physiologic conditions. Evidence that redox reactions also occur under disease conditions, leading to pathologic complications, also is examined, focusing on recent discoveries showing that the ferryl oxidation state of these hemoproteins is present in these disease states in vivo. In addition, we review the latest advances in the understanding of globin redox mechanisms and how they might affect cellular signaling pathways and how they might be controlled therapeutically or, in the case of hemoglobin-based blood substitutes, through rational design.

Cytochrome P450 regulation: the interplay between its heme and apoprotein moieties in synthesis, assembly, repair, and disposal

Heme is vital to our aerobic universe. Heme cellular content is finely tuned through an exquisite control of synthesis and degradation. Heme deficiency is deleterious to cells, whereas excess heme is toxic. Most of the cellular heme serves as the prosthetic moiety of functionally diverse hemoproteins, including cytochromes P450 (P450s). In the liver, P450s are its major consumers, with >50% of hepatic heme committed to their synthesis. Prosthetic heme is the sine qua non of P450 catalytic biotransformation of both endo- and xenobiotics. This well-recognized functional role notwithstanding, heme also regulates P450 protein synthesis, assembly, repair, and disposal. These less well-appreciated aspects are reviewed herein.

Boron stabilizes peroxide mediated changes in the structure of heme proteins

Boron is reported in this study to stabilize the structure of heme proteins exposed to peroxides. The oxidized heme protein (15 microM) was treated with H(2)O(2) (10mM) in 1M glycine-NaOH buffer (pH 9.2) at 25 degrees C in absence/presence of boron, and characterized by visible absorption spectroscopy, gel exclusion chromatography, native PAGE, HPLC and DLS. Spectral analysis of exposed heme proteins revealed a decrease in absorbance in the Soret region, which was stabilized by boron. The native PAGE analysis of exposed heme proteins showed high molecular weight products; the band intensity was lesser in presence of boron. Further, elution profile of the exposed heme proteins on Sephadex G-200 column and HPLC revealed more than one peak (aggregate formation) when compared to the respective untreated proteins. DLS, which measures the hydrodynamic radius (R(H)), was used to ascertain whether the peaks correspond to monomer, dimer or aggregate forms. The R(H) of boron pretreated heme proteins was close to R(H) of the respective heme protein. Non-heme protein RNase did not show any change when exposed to peroxide. Taken together, results conclude that boron stabilizes the structure of heme proteins, which might be due to specific sites on heme proteins that can bind to borate ions.

Acetaminophen inhibits hemoprotein-catalyzed lipid peroxidation and attenuates rhabdomyolysis-induced renal failure

Hemoproteins, hemoglobin and myoglobin, once released from cells can cause severe oxidative damage as a consequence of heme redox cycling between ferric and ferryl states that generates radical species that induce lipid peroxidation. We demonstrate in vitro that acetaminophen inhibits hemoprotein-induced lipid peroxidation by reducing ferryl heme to its ferric state and quenching globin radicals. Severe muscle injury (rhabdomyolysis) is accompanied by the release of myoglobin that becomes deposited in the kidney, causing renal injury. We previously showed in a rat model of rhabdomyolysis that redox cycling between ferric and ferryl myoglobin yields radical species that cause severe oxidative damage to the kidney. In this model, acetaminophen at therapeutic plasma concentrations significantly decreased oxidant injury in the kidney, improved renal function, and reduced renal damage. These findings also provide a hypothesis for potential therapeutic applications for acetaminophen in diseases involving hemoprotein-mediated oxidative injury.

Tyrosine residues as redox cofactors in human hemoglobin: implications for engineering nontoxic blood substitutes

Respiratory proteins such as myoglobin and hemoglobin can, under oxidative conditions, form ferryl heme iron and protein-based free radicals. Ferryl myoglobin can safely be returned to the ferric oxidation state by electron donation from exogenous reductants via a mechanism that involves two distinct pathways. In addition to direct transfer between the electron donor and ferryl heme edge, there is a second pathway that involves "through-protein" electron transfer via a tyrosine residue (tyrosine 103, sperm whale myoglobin). Here we show that the heterogeneous subunits of human hemoglobin, the alpha and beta chains, display significantly different kinetics for ferryl reduction by exogenous reductants. By using selected hemoglobin mutants, we show that the alpha chain possesses two electron transfer pathways, similar to myoglobin. Furthermore, tyrosine 42 is shown to be a critical component of the high affinity, through-protein electron transfer pathway. We also show that the beta chain of hemoglobin, lacking the homologous tyrosine, does not possess this through-protein electron transfer pathway. However, such a pathway can be engineered into the protein by mutation of a specific phenylalanine residue to a tyrosine. High affinity through-protein electron transfer pathways, whether native or engineered, enhance the kinetics of ferryl removal by reductants, particularly at low reductant concentrations. Ferryl iron has been suggested to be a major cause of the oxidative toxicity of hemoglobin-based blood substitutes. Engineering hemoglobin with enhanced rates of ferryl removal, as we show here, is therefore likely to result in molecules better suited for in vivo oxygen delivery.

Iron chelators can protect against oxidative stress through ferryl heme reduction

Iron chelators such as desferrioxamine have been shown to ameliorate oxidative damage in vivo. The mechanism of this therapeutic action under non-iron-overload conditions is, however, complex, as desferrioxamine has properties that can impact on oxidative damage independent of its capacity to act as an iron chelator. Desferrioxamine can act as a reducing agent to remove cytotoxic ferryl myoglobin and hemoglobin and has recently been shown to prevent the formation of a highly cytotoxic heme-to-protein cross-linked derivative of myoglobin. In this study we have examined the effects of a wide range of iron chelators, including the clinically used hydroxypyridinone CP20 (deferriprone), on the stability of ferryl myoglobin and on the formation of heme-to-protein cross-linking. We show that all hydroxypyridinones, as well as many other iron chelators, are efficient reducing agents of ferryl myoglobin. These compounds are also effective at preventing the formation of cytotoxic derivatives of myoglobin such as heme-to-protein cross-linking. These results show that the use of iron chelators in vivo may ameliorate oxidative damage under conditions of non-iron overload by at least two mechanisms. The antioxidant effects of chelators in vivo cannot, therefore, be attributed solely to iron chelation.

Reactivities of oxo and peroxo intermediates studied by hemoprotein mutants

A series of myoglobin mutants, in which distal sites are modified by site-directed mutagenesis, are able to catalyze peroxidase, catalase, and P450 reactions even though their proximal histidine ligands are intact. More importantly, reactions of P450, catalase, and peroxidase substrates and compound I of myoglobin mutants can be observed spectroscopically. Thus, detailed oxidation mechanisms were examined. On the basis of these results, we suggest that the different reactivities of P450, catalase, and peroxidase are mainly caused by their active site structures, but not the axial ligand. We have also prepared compound 0 under physiological conditions by employing a mutant of cytochrome c 552. Compound 0 is not able to oxidize ascorbic acid.

Structural basis of peroxide-mediated changes in human hemoglobin: a novel oxidative pathway

Hydrogen peroxide (H(2)O(2)) triggers a redox cycle between ferric and ferryl hemoglobin (Hb) leading to the formation of a transient protein radical and a covalent hemeprotein cross-link. Addition of H(2)O(2) to highly purified human hemoglobin (HbA(0)) induced structural changes that primarily resided within beta subunits followed by the internalization of the heme moiety within alpha subunits. These modifications were observed when an equal molar concentration of H(2)O(2) was added to HbA(0) yet became more abundant with greater concentrations of H(2)O(2). Mass spectrometric and amino acid analysis revealed for the first time that betaCys-93 and betaCys-112 were oxidized extensively and irreversibly to cysteic acid when HbA(0) was treated with H(2)O(2). Oxidation of further amino acids in HbA(0) exclusive to the beta-globin chain included modification of betaTrp-15 to oxyindolyl and kynureninyl products as well as betaMet-55 to methionine sulfoxide. These findings may therefore explain the premature collapse of the beta subunits as a result of the H(2)O(2) attack. Analysis of a tryptic digest of the main reversed phase-high pressure liquid chromatography fraction revealed two alpha-peptide fragments (alpha128-alpha139) and a heme moiety with the loss of iron, cross-linked between alphaSer-138 and the porphyrin ring. The novel oxidative pathway of HbA(0) modification detailed here may explain the diverse oxidative, toxic, and potentially immunogenic effects associated with the release of hemoglobin from red blood cells during hemolytic diseases and/or when cell-free Hb is used as a blood substitute.

Ascorbate removes key precursors to oxidative damage by cell-free haemoglobin in vitro and in vivo

Haemoglobin initiates free radical chemistry. In particular, the interactions of peroxides with the ferric (met) species of haemoglobin generate two strong oxidants: ferryl iron and a protein-bound free radical. We have studied the endogenous defences to this reactive chemistry in a rabbit model following 20% exchange transfusion with cell-free haemoglobin stabilized in tetrameric form [via cross-linking with bis-(3,5-dibromosalicyl)fumarate]. The transfusate contained 95% oxyhaemoglobin, 5% methaemoglobin and 25 microM free iron. EPR spectroscopy revealed that the free iron in the transfusate was rendered redox inactive by rapid binding to transferrin. Methaemoglobin was reduced to oxyhaemoglobin by a slower process (t(1/2) = 1 h). No globin-bound free radicals were detected in the plasma. These redox defences could be fully attributed to a novel multifunctional role of plasma ascorbate in removing key precursors of oxidative damage. Ascorbate is able to effectively reduce plasma methaemoglobin, ferryl haemoglobin and globin radicals. The ascorbyl free radicals formed are efficiently re-reduced by the erythrocyte membrane-bound reductase (which itself uses intra-erythrocyte ascorbate as an electron donor). As well as relating to the toxicity of haemoglobin-based oxygen carriers, these findings have implications for situations where haem proteins exist outside the protective cell environment, e.g. haemolytic anaemias, subarachnoid haemorrhage, rhabdomyolysis.

A new sensitive assay reveals that hemoglobin is oxidatively modified in vivo

Free radical formation in heme proteins is recognised as a factor in mediating the toxicity of peroxides in oxidative stress. As well as initiating free radical damage, heme proteins damage themselves. Under extreme conditions, where oxidative stress and low pH coincide (e.g., myoglobin in the kidney following rhabdomyolysis and hemoglobin in the CSF subsequent to subarachnoid hemorrhage), peroxide can induce covalent heme to protein cross-linking. In this paper we show that, even at neutral pH, the heme in hemoglobin is covalently modified by oxidation. The product, which we term OxHm, is a "green heme" iron chlorin with a distinct optical spectrum. OxHm formation can be quantitatively prevented by reductants of ferryl iron, e.g., ascorbate. We have developed a simple, robust, and reproducible HPLC assay to study the extent of OxHm formation in the red cell in vivo. We show that hemoglobin is oxidatively damaged even in normal blood; approximately 1 in 2,000 heme groups exist as OxHm in the steady state. We used a simple model (physical exercise) to demonstrate that OxHm increases significantly during acute oxidative stress. The exercise-induced increase is short-lived, suggesting the existence of an active mechanism for repairing or removing the damaged heme proteins.

Endonuclease-like activity of heme proteins

Heme proteins, metmyoglobin, methemoglobin, and metcytochrome c showed unusual affinity for double-stranded DNA. Calorimetric studies show that binding of methemoglobin to calf thymus DNA (CTDNA) is weakly endothermic, and the binding constant is 4.9+/-0.7x10(5) M(-1). The Soret absorption bands of the heme proteins remained unchanged, in the presence of excess CTDNA, but a new circular dichroic band appeared at 210 nm. Helix melting studies indicated that the protein-DNA mixture denatures at a lower temperature than the individual components. Thermograms obtained by differential scanning calorimetry of the mixture indicated two distinct transitions, which are comparable to the thermograms obtained for individual components, but there was a reduction in the excess heat capacity. Activation of heme proteins by hydrogen peroxide resulted in the formation of high valent Fe(IV) oxo intermediates, and CTDNA reacted rapidly under these conditions. The rate was first-order in DNA concentration, and this reactivity resulted in DNA strand cleavage. Upon activation with hydrogen peroxide, for example, the heme proteins converted the supercoiled pUC18 DNA into nicked circular and linear DNA. No reaction occurred in the absence of the heme protein, or hydrogen peroxide. These data clearly indicate a novel property of several heme proteins, and this is first report of the endonuclease-like activity of the heme proteins.

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.

The radical and redox chemistry of myoglobin and hemoglobin: from in vitro studies to human pathology

Recent research has shown that myoglobin and hemoglobin play important roles in the pathology of certain disease states, such as renal dysfunction following rhabdomyolysis and vasospasm following subarachnoid hemorrhages. These pathologies are linked to the interaction of peroxides with heme proteins to initiate oxidative reactions, including generation of powerful vasoactive molecules (the isoprostanes) from free and membrane- bound lipids. This review focuses on the peroxide-induced formation of radicals, their assignment to specific protein residues, and the pseudoperoxidase and prooxidant activities of the heme proteins. The discovery of heme to protein cross-linked forms of myoglobin and hemoglobin in vivo, definitive markers of the participation of these heme proteins in oxidative reactions, and the recent results from heme oxygenase knockout/knockin animal model studies, indicate that higher oxidation states (ferryl) of heme proteins and their associated radicals play a major role in the mechanisms of pathology.

Identification of a Heme-sensing Domain in Iron Regulatory Protein 2

Iron regulatory protein 2 coordinates the cellular regulation of iron metabolism by binding to iron-responsive elements in mRNA. The protein is synthesized constitutively but is rapidly degraded when iron stores are replete. The mechanisms that prevent degradation during iron deficiency or promote degradation during iron sufficiency are not delineated. Iron regulatory protein 2 contains a domain not present in the closely related iron regulatory protein 1, and we found that this domain binds heme with high affinity. A cysteine within the domain is axially liganded to the heme, as occurs in cytochrome P450. The protein-bound heme reacts with molecular oxygen to mediate the oxidation of cysteine, including beta-elimination of the sulfur to yield alanine. This covalent modification may thus mark the protein molecule for degradation by the proteasome system, providing another mechanism by which heme can regulate the level of iron regulatory protein 2.

Role of myoglobin in the antioxidant defense of the heart

Although the primary function of myoglobin (Mb) has been considered to be cellular O2 storage and supply, recent studies have shown that Mb in addition can act as NO oxidase. Here we report that Mb also significantly contributes to the attenuation of oxidative stress in cardiac muscle. In support of this hypothesis, we found that in isolated perfused hearts of Mb-deficient (myo-/-) mice oxidative challenge by intracoronary infused H2O2 (1-300 microM) or superoxide formed by 2,3-dimethoxy-1,4-naphtoquinone (0.1-30 microM), respectively, depressed cardiac contractility to a greater extent than in wild-type (WT) hearts, e.g., up to [H2O2] = 10 microM there was a significant left ventricular developed pressure (LVDP) decrease in myo-/- hearts only (90.4+/-4.2 vs. 98.1+/-0.7% of control, n=6, P<0.05). Likewise in an ischemia/reperfusion protocol, myo-/- hearts showed a delayed recovery of postischemic function as compared with WT controls (e.g., LVDP was 35.6+/-7.5 vs. 22.4+/-5.3 mmHg, respectively, after 10 min of reperfusion, P<0.05, n=8), which correlated well with an enhanced release of reactive oxygen species in myo-/- hearts as measured by online lucigenin-enhanced chemiluminescence [e.g. 465+/-87 relative light units (RLU) in myo-/- vs. 287+/-73 RLU in WT after 2.5 min of reperfusion, P<0.05, n=8]. (31)P NMR spectroscopy revealed concomitantly a more pronounced phosphocreatine overshoot during reperfusion in the knockout but only minute alterations in ATP and pHi. Our data show that lack of Mb leads to increased vulnerability of cardiac function to oxidative challenge either pharmacologically induced or endogenously generated. We propose that Mb is a key element influencing redox pathways in cardiac muscle to functionally and metabolically protect the heart from oxidative damage.

Oxidation of low-density lipoprotein by hemoglobin-hemichrome

Hemoglobin and myoglobin are inducers of low-density lipoprotein oxidation in the presence of H(2)O(2). The reaction of these hemoproteins with H(2)O(2) result in a mixture of protein products known as hemichromes. The oxygen-binding hemoproteins function as peroxidases but as compared to classic heme-peroxidases have a much lower activity on small sized and a higher one on large sized substrates. A heme-globin covalent adduct, a component identified in myoglobin-hemichrome, was reported to be the cause of myoglobin peroxidase activity on low-density lipoprotein. In this study, we analyzed the function of hemoglobin-hemichrome in low-density lipoprotein oxidation. Oxidation of lipids was analyzed by formation of conjugated diene and malondialdehyde; and oxidation of Apo-B protein was analyzed by development of bityrosine fluorescence and covalently cross-linked protein. Hemoglobin-hemichrome has indeed triggered oxidation of both lipids and protein, but unlike myoglobin, hemichrome has required the presence of H(2)O(2). In correlation to this, we found that unlike myoglobin, hemichrome formed by hemoglobin/H(2)O(2) does not contain a globin-heme covalent adduct. Nevertheless, hemoglobin-hemichrome remains oxidatively active towards LDL, indicating that other components of the oxidatively denatured hemoglobin should be considered responsible for its hazardous activity in vascular pathology.

Proteolytic degradation of nitric oxide synthase: effect of inhibitors and role of hsp90-based chaperones

Nitric oxide synthase (NOS) is a highly regulated enzyme that produces nitric oxide, a critical messenger in many physiological processes. In this perspective, we explore the role of proteolytic degradation of NOS, in particular the inducible and neuronal isoforms of NOS, as a mechanism of regulation of the enzyme. The ubiquitin-proteasome and calpain pathways are the major proteolytic systems identified to date that are responsible for this regulated degradation. The degradation of NOS is affected by diverse agents, including glucocorticoids, caveolin, neurotoxic compounds, and certain NOS inhibitors. Some irreversible inactivators of NOS enhance the proteolytic degradation of the enzyme, and this property may be of great importance in understanding the biological effects of these inhibitors, some of which are being developed for clinical use. Analogies with the regulated degradation of liver microsomal cytochromes P450, which are related to NOS, provide a framework for understanding these processes. Finally, a new perspective on the regulation of NOS by hsp90-based chaperones is presented that involves facilitated heme insertion into the enzyme.

Myoglobin-induced lipid oxidation. A review

An overview of myoglobin-initiated lipid oxidation in simple model systems, muscle, and muscle-based foods is presented. The potential role of myoglobin spin and redox states in initiating lipid oxidation is reviewed. Proposed mechanisms for myoglobin-initiated lipid oxidation in muscle tissue (pH 7.4) and meat (pH 5.5) are evaluated with the purpose of putting forward general mechanisms explaining present observations regarding the catalytic events.

Oxidative mechanisms of hemoglobin-based blood substitutes

Chemically or genetically altered cell-free hemoglobin (Hb) has been developed as an oxygen carrying therapeutic. Site-directed modifications are introduced and serve to stabilize the protein molecules in a tetrameric and/or a polymeric functional form. Direct cytotoxic effects associated with cell-free Hb have been ascribed to redox reactions (involving either 1 or 2 electron steps) between the heme group and peroxides. These interactions are the basis of the pseudoperoxidase activity of Hb and can be cytotoxic when reactive species are formed at relatively high concentrations during inflammation and typically lead to cell death. Peroxides relevant to biological systems include hydrogen peroxide (H2O2), lipid hydroperoxides (LOOH), and peroxynitrite (ONOO-). Reactions between Hb and peroxides form the ferryl oxidation state of the protein, analogous to compounds I and II formed in the catalytic cycle of many peroxidase enzymes. This higher oxidation state of the protein is a potent oxidant capable of promoting oxidative damage to most classes of biological molecules. Further complications are thought to arise through the disruption of key signaling pathways resulting from alteration of or destruction of important physiological mediators.

The effects of pH on the mechanism of hydrogen peroxide and lipid hydroperoxide consumption by myoglobin: a role for the protonated ferryl species

Myoglobin catalyses the breakdown of lipid hydroperoxides (e.g., HPODE) during which the absorption band of the lipid conjugated diene (234 nm) is partially bleached. The constant for this process is strongly pH-dependent (k = 9.5 x 10(-3)s(-1), pH 7: k = 2.3 x 10(-1)s(-1), pH 5). This rate enhancement is not due to acid-induced changes in protein conformation or the involvement of protein-based radical species, as demonstrated by an almost identical pH dependence of the same reaction catalyzed by ferric haemin. The rate constants for ferryl formation and auto-reduction show different pH dependencies, with a pK of 8.3 for ferryl formation and a projected pK of 3.5 for ferryl auto-reduction. The pH dependence for the auto-reduction of the ferryl species is the same as that of the myoblobin catalyzed breakdown of HPODE. We propose that the protonated form of ferryl myoglobin (Fe(4+) - OH(-)) is the reactive species regulating the peroxidatic activity of myoglobin. The protonated ferryl species abstracts an electron from either the protein or porphyrin, allowing fast regeneration of the ferric species. Alkaline conditions stabilize the ferryl species, making myoglobin considerably less reactive towards lipids and lipid hydroperoxides. These findings are significant for understanding myoglobin-induced oxidative stress in vivo and the development of therapies.

Redox reactions of hemoglobin and myoglobin: biological and toxicological implications

Direct cytotoxic effects associated with hemoglobin (Hb) or myoglobin (Mb) have been ascribed to redox reactions (involving either one- or two-electron steps) between the heme group and peroxides. These interactions are the basis of the pseudoperoxidase activity of these hemoproteins and can be cytotoxic when reactive species are formed at relatively high concentrations during inflammation and typically lead to cell death. Peroxides relevant to biological systems include hydrogen peroxide, lipid hydroperoxides, and peroxynitrite. Reactions between Hb/Mb and peroxides form the ferryl oxidation state of the protein, analogous to compounds I and II formed in the catalytic cycle of many peroxidase enzymes. This higher oxidation state of the protein is a potent oxidant capable of promoting oxidative damage to most classes of biological molecules. Free iron, released from Hb, also has the potential to promote oxidative damage via classical "Fenton" chemistry. It has become increasingly evident that Hb/Mb redox reactions or their by-products play a critical role in the pathophysiology of some disease states. This review briefly discusses the reactions of Hb/Mb with biological peroxides, potential cytotoxicity and the impact of these interactions on modulation of cell signaling pathways regulated by these reactive species. Also discussed in this article is the role of heme-protein chemistry in relation to the toxicity of hemoproteins.

Enhanced lipid oxidation by oxidatively modified myoglobin: role of protein-bound heme

The formation of oxidized low density lipoprotein (LDL) is believed to play a significant role in the pathogenesis of atherosclerosis. Myoglobin in the presence of H(2)O(2) has been shown to catalyze LDL oxidation in vitro. It is established that an oxidatively altered form of myoglobin (Mb-H), which contains a prosthetic heme covalently crosslinked to the apoprotein, is a major product in the reaction of native myoglobin with peroxides. In the current study, we have shown for the first time that Mb-H, in the absence of exogenously added peroxides, oxidizes LDL and purified lipids, as determined by the formation of conjugated dienes, lipid peroxides, and thiobarbituric acid reactive substances. Moreover, the rate of oxidation of pure phosphatidylcholine by Mb-H was found to be at least sevenfold greater than that observed for native myoglobin. The current study strongly suggests a role for Mb-H in the lipid peroxidation observed with myoglobin.

A mechanistic study of self-inactivation of the peroxidase activity in prostaglandin H synthase-1

Prostaglandin H synthase (PGHS) is a self-activating and self-inactivating enzyme. Both the peroxidase and cyclooxygenase activities have a limited number of catalytic turnovers. Sequential stopped-flow measurements were used to analyze the kinetics of PGHS-1 peroxidase self-inactivation during reaction with several different hydroperoxides. The inactivation followed single exponential kinetics, with a first-order rate constant of 0.2-0.5 s-1 at 24 degrees C. This rate was independent of the peroxide species and concentration used, strongly suggesting that the self-inactivation process originates after formation of Compound I and probably with Intermediate II, which contains an oxyferryl heme and a tyrosyl radical. Kinetic scan and rapid scan experiments were used to monitor the heme changes during the inactivation process. The results from both experiments converged to a simple, linear, two-step mechanism in which Intermediate II is first converted in a faster step (0.5-2 s-1) to a new compound, Intermediate III, which undergoes a subsequent slower (0.01-0.05 s-1) transition to a terminal species. Rapid-quench and high pressure liquid chromatography analysis indicated that Intermediate III likely retains an intact heme group that is not covalently linked with the PGHS-1 protein.

Acid-catalysed autoreduction of ferrylmyoblobin in aqueous solution studied by freeze quenching and ESR spectroscopy

Decay of the hypervalent muscle pigment ferrylmyoglobin, formed by activation of metmyoglobin by hydrogen peroxide, was found, when studied by a combination of ESR and UV/VIS spectroscopy in aqueous solution at physiological pH, to proceed by parallel second- and first-order kinetics. At pH below 6.5 a sharp ESR signal (g = 2.003) with an increasing intensity for decreasing pH were observed in solutions frozen in liquid nitrogen, and a broad signal (g = 2.005) was seen throughout the studied pH range also in frozen solutions. The g = 2.005 signal is suggested to arise from an intermediate formed in an intramolecular rate-determining electron-transfer in ferrylmyoglobin, whereas the g 2.003 signal is caused by a radical formed in a proton-assisted electron-transfer initiating the specific acid-catalysed autoreduction.

Reactions of sperm whale myoglobin with hydrogen peroxide. Effects of distal pocket mutations on the formation and stability of the ferryl intermediate

Distal pocket mutants of sperm whale oxymyoglobin (oxy-Mb) were reacted with a 2.5-fold excess of hydrogen peroxide (HOOH) in phosphate buffer at pH 7.0, 37 degreesC. We describe a mechanism composed of three distinct steps: 1) initial oxidation of oxy- to ferryl-Mb, 2) autoreduction of the ferryl intermediate to ferric metmyoglobin (metMb), and 3) reaction of metMb with an additional HOOH molecule to regenerate the ferryl intermediate creating a pseudoperoxidase catalytic cycle. Mutation of Leu-29(B10) to Phe slows the initial oxidation reaction 3-fold but has little effect on the rate of ferryl reduction to ferric met-aquo-myoglobin. In contrast, the Val-68(E11) to Phe mutation causes a small, 60% increase in the initial oxidation reaction and a much larger 2. 5-fold increase in the rate of autoreduction. Double insertion of Phe at both the B10- and E11-positions (L29F/V68F) produces a mutant with oxidation characteristics of both single mutants, slow initial oxidation, and rapid autoreduction, but an extraordinarily high affinity for O2. Replacing His-64(E7) with Gln produces 3-4-fold increases in both processes. Combining the mutation H64Q with L29F results in a myoglobin with enhanced resistance to metMb formation in the absence of antioxidant enzymes (i.e. catalase and superoxide dismutase) due to its own high pseudoperoxidase activity, which rapidly removes any HOOH produced in the initial stages of autoxidation. This double substitution occurs naturally in the myoglobin of Asian elephants, and similar multiple replacements have been used to reduce selectively the rate of nitric oxide (NO)-induced oxidation of both recombinant MbO2 and HbO2 blood substitute prototypes without altering O2 affinity.

Chemiluminescence assay for oxidatively modified myoglobin

Treatment of myoglobin with H2O2 results in covalent alteration of the heme prosthetic group, in part, to protein-bound adducts. These protein-bound heme adducts are known to be redox active and are suspected to participate in oxidative tissue injury. In the course of our studies on the toxicological role of these heme adducts, we sought to develop a sensitive assay for their detection and quantitation. We have discovered that protein-bound heme adducts, due to their inherent peroxidase activity, can be detected with the use of enhanced chemiluminescence detection reagents, following SDS-PAGE and electroblotting. The assay is specific for protein-bound heme adducts as we have identified conditions where noncovalently bound hemes are completely dissociated from the protein during electrophoresis. Signal intensity was quantified by laser densitometry and found to be linear over a concentration range of 0.44-22 pmol of protein-bound heme adduct, which represented a 20-fold greater sensitivity than the currently available HPLC method. Moreover, we have identified tris(2-carboxyethyl)phosphine as a thiol reducing agent that does not interfere with the detection of the heme-mediated peroxidase activity. The current method may be utilized to identify heme-binding regions of proteins in addition to the detection of oxidatively modified myoglobin.

In vivo endotoxin enhances biliary ethanol-dependent free radical generation

Endotoxemia is associated with alcoholic liver diseases; however, the effect of endotoxin on the oxidation of ethanol is not known. We tested the hypothesis that endotoxin treatment enhances hepatic ethanol radical production. The generation of free radicals by the liver was studied with spin-trapping technique utilizing the primary trap ethanol (0.8 g/kg) and the secondary trap alpha-(4-pyridyl-1-oxide)-N-t-butylnitrone (4-POBN; 500 mg/kg). Electron paramagnetic resonance (EPR) spectra of bile showed six-line signals, which were dependent on ethanol, indicating the trapping of ethanol-dependent radicals. Intravenous injections of Escherichia coli lipopolysaccharide (0.5 mg/kg) 0.5 h before 4-POBN plus ethanol treatment caused threefold increases of biliary radical adducts. EPR analyses of bile from [1-13C]ethanol-treated endotoxic rats showed the presence of species attributable to alpha-hydroxyethyl adduct, carbon-centered adducts, and ascorbate radical. The generation of endotoxin-induced increases of ethanol-dependent radicals was suppressed by 50% on GdCl3 (20 mg/kg i.v.) or desferrioxamine mesylate (1 g/kg i.p.) treatment. Our data show that in vivo endotoxin increases biliary ethanol-dependent free radical formation and that these processes are modulated by Kupffer cell activation and catalytic metals.

Heme protein radicals: formation, fate, and biological consequences

The oxidation of myoglobin by H2O2 yields ferrylmyoglobin, which contains two oxidizing equivalents: the oxoferryl complex and an amino acid radical. This study examines the electron paramagnetic resonance (EPR) properties of the resulting amino acid radicals and their inherent kinetic features at [H2O2]/[protein] ratios close to physiological conditions (i.e., < or = 1). The EPR spectrum obtained with continuous flow at room temperature consisted of a composite of three signals: a low intensity signal and two high intensity signals. The former had a g-value of 2.014, contributed 10-15% to the overall spectrum and was ascribed to a peroxyl radical. Of the two high intensity signals, one consisted of a six-line spectrum (g = 2.0048) that contributed approximately 17-19% to the overall signal; hyperfine splitting constants to ring protons permitted to identify this signal as a tyrosyl radical. The other high intensity signal (with similar g-value and underlying that of the tyrosyl radical) was ascribed to an aromatic amino acid upon comparison with the EPR characteristics for radicals in aromatic amino acid-containing peptides. Analysis of these data in connection with amino acid analysis and the EPR spectra obtained under similar conditions with another hemoprotein, hemoglobin, allowed to suggest a mechanism for the formation of the protein radicals in myoglobin. The aromatic amino acid radical was observed to be relatively long lived in close proximity to the heme iron. Hence, it is likely that this is the first site of protein radical; reduction of the oxoferryl complex by Tyr (FeIV=O + Tyr-OH + H+ --> FeIII + H2O + Tyr-O.)--and alternatively by other amino acids--leads to the subsequent formation of other amino acid radicals within an electron-transfer process throughout the protein. This view suggests that the protein radical(s) is highly delocalized within the globin moiety in a dynamic process encompassing electron tunneling through the backbone chain or H-bonds and leading to the formation of secondary radicals.

Hydrogen peroxide-mediated alteration of the heme prosthetic group of metmyoglobin to an iron chlorin product: evidence for a novel oxidative pathway

Treatment of metmyoglobin with H2O2 is known to lead to the crosslinking of an active site tyrosine residue to the heme [Catalano, C. E., Y. S. Choe, and P. R. Ortiz de Montellano (1989) J. Biol. Chem. 264, 10534-10541]. We have found in this study that this reaction also leads to an altered heme product not covalently bound to the protein. This product was characterized by visible absorption, infrared absorption, and mass and NMR spectrometry as an iron chlorin product formed from the saturation of the double bond between carbon atoms at positions 17 and 18 of pyrrole ring D with concomitant addition of a hydroxyl group on the carbon atom at position 18 and lactonization of the propionic acid to the carbon atom at position 17. Studies with the use of (18)O-labeled H2O2, O2, and H2O clearly indicate that the source of the added oxygen on the heme is water. Evidently, water adds regiospecifically to a cationic site formed on a carbon atom at position 18 after oxidation of the ferric heme prosthetic group with peroxide. Prolonged incubation of the reaction mixture containing the iron hydroxychlorin product led to the formation of an iron dihydroxychlorin product, presumably from a slow addition of water to the initial iron hydroxychlorin. The iron chlorin products characterized in this study are distinct from the meso-oxyheme species, which is thought to be formed during peroxide-mediated degradation of metmyoglobin, cytochrome P450, ferric heme, and model ferric hemes, and give further insight into the mechanism of H2O2-induced heme alterations.

On the molecular mechanism of metmyoglobin-catalyzed reduction of hydrogen peroxide by ascorbate

Hydrogen peroxide induces rapid oxidation of metmyoglobin with an apparent second order rate constant, k1 = 3.4 x 10(4) M-1 min-1. The product of this interaction is ferrylmyoglobin with an unstable free radical on the globin moiety. This activated form of myoglobin is able: (a) to initiate the peroxidation of erythrocyte membranes and (b) to form intra- and intermolecular covalent crosslinkings. The presence of ascorbic acid in amounts stoichiometric to H2O2 efficiently prevents all the above processes. Moreover, in the presence of ascorbic acid a cyclic process is taking place leading to H2O2 reduction, ascorbic acid oxidation, and unmodified metmyoglobin formation (reaction 1).

Formation of free radicals and protein mixed disulfides in rat red cells exposed to dapsone hydroxylamine

The hemolytic activity of dapsone is well known to reside in its N-hydroxylamine metabolites. Addition of dapsone hydroxylamine (DDS-NOH) to red cell suspensions causes damage such that when reintroduced into the circulation of isologous rats, the injured cells are rapidly removed by the spleen. Hemolytic activity is associated with the extensive formation of disulfide-linked hemoglobin adducts on red cell membrane skeletal proteins. To determine if free radicals could be involved in this process, rat red cells were incubated with DDS-NOH in the presence of the spin trap, 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) and subjected to EPR analysis. Addition of DDS-NOH (25-50 microM) to a red cell suspension gave rise to a four-line (1:2:2:1) EPR spectrum with coupling constants identical to those of a DMPO-hydroxyl radical adduct (DMPO-OH) standard. No other radicals were detected; however, preincubation of red cells with cysteamine caused the DDS-NOH-generated DMPO-OH signal to be replaced by a cysteamine thiyl radical adduct signal. DDS-NOH-treated red cells were also found to contain ferrylhemoglobin, indicating the presence of hydrogen peroxide. Furthermore, DDS-NOH was found to stimulate salicylate hydroxylation in red cell suspensions, confirming the presence of oxygen radicals. These data support the hypothesis that oxygen radicals are involved in the mechanism underlying dapsone-induced hemolytic anemia.

Stimulation by nitroxides of catalase-like activity of hemeproteins. Kinetics and mechanism

The ability of stable nitroxide radicals to detoxify hypervalent heme proteins such as ferrylmyoglobin (MbFeIV) produced in the reaction of metmyoglobin (MbFeIII) and H2O2 was evaluated by monitoring O2 evolution, H2O2 depletion, and redox changes of the heme prosthetic group. The rate of H2O2 depletion and O2 evolution catalyzed by MbFeIII was enhanced by stable nitroxides such as 4-OH-2,2,6,6-tetramethyl-piperidinoxyl (TPL) in a catalytic fashion. The reduction of MbFeIV to MbFeIII was the rate-limiting step. Excess TPL over MbFeIII enhanced catalase-like activity more than 4-fold. During dismutation of H2O2, [TPL] and [MbFeIV] remained constant. NADH caused: (a) inhibition of H2O2 decay; (b) progressive reduction of TPL to its respective hydroxylamine TPL-H; and (c) arrest/inhibition of oxygen evolution or elicit consumption of O2. Following depletion of NADH the evolution of O2 resumed, and the initial concentration of TPL was restored. Kinetic analysis showed that two distinct forms of MbFeIV might be involved in the process. In summary, by shuttling between two oxidation states, namely nitroxide and oxoammonium cation, stable nitroxides enhance the catalase mimic activity of MbFeIII, thus facilitating H2O2 dismutation accompanied by O2 evolution and providing protection against hypervalent heme proteins.

Peroxidative interaction of myoglobin and myosin

Met-myoglobin [Fe(III)] was found to induce myosin cross-linking in the presence of H2O2 [Bhoite-Solomon, V. & Shaklai, N. (1992) Biochem. Int. 26, 181-189]. To assess the relevance of these findings to cellular pathology, peroxidation of myosin was performed with physiological divalent iron [Fe(II)] myoglobins in the oxy and deoxy forms. Both myoglobin forms were capable of mediating cross-linking of myosin. Deoxymyoglobin reactivity was similar to that of met-myoglobin, but the reactivity of oxymyoglobin was retarded compared to deoxymyoglobin. Cross-linking of myosin occurred under a low flow rate of H2O2 (3 microM/min) and in the presence of excess oxymyoglobin over H2O2, known to diminish the steady state of the myoglobin active heme [ferryl, Fe(IV)]state. The adenosinetriphosphatase activity of myosin was reduced to about half due to cross-linking. Addition of myoglobin/H2O2 to high myosin concentrations (> = 20 microM) turned the solutions into gels, a phenomenon explained by the further formation of intermolecular super cross-links of soluble myosin. Thus, at cellular ionic strength in which myosin is insoluble, cross-linking of myosin could still be triggered by myoglobin and H2O2. Based on these data, it is suggested that myoglobin-induced cross-linking of myosin and the consequent loss of adenosinetriphosphatase activity may be involved in muscle malfunction under hypoxia when cellular peroxidants increase and the deoxymyoglobin form prevails.