An EPR study of the peroxyl radicals induced by hydrogen peroxide in the haem proteins

New insights into controlling radical migration pathways in heme enzymes gained from the study of a dye-decolorising peroxidase

In heme enzymes, such as members of the dye-decolorising peroxidase (DyP) family, the formation of the highly oxidising catalytic Fe(iv)-oxo intermediates following reaction with hydrogen peroxide can lead to free radical migration (hole hopping) from the heme to form cationic tyrosine and/or tryptophan radicals. These species are highly oxidising (∼1 V vs. NHE) and under certain circumstances can catalyse the oxidation of organic substrates. Factors that govern which specific tyrosine or tryptophan the free radical migrates to in heme enzymes are not well understood, although in the case of tyrosyl radical formation the nearby proximity of a proton acceptor is a recognised facilitating factor. By using an A-type member of the DyP family (DtpAa) as an exemplar, we combine protein engineering, X-ray crystallography, hole-hopping calculations, EPR spectroscopy and kinetic modelling to provide compelling new insights into the control of radical migration pathways following reaction of the heme with hydrogen peroxide. We demonstrate that the presence of a tryptophan/tyrosine dyad motif displaying a T-shaped orientation of aromatic rings on the proximal side of the heme dominates the radical migration landscape in wild-type DtpAa and continues to do so following the rational engineering into DtpAa of a previously identified radical migration pathway in an A-type homolog on the distal side of the heme. Only on disrupting the proximal dyad, through removal of an oxygen atom, does the radical migration pathway then switch to the engineered distal pathway to form the desired tyrosyl radical. Implications for protein design and biocatalysis are discussed.

Course of Plasmodium infection studied using 2D-COS on human erythrocytes

BACKGROUND

The threat of malaria is still present in the world. Recognizing the type of parasite is important in determining a treatment plan. The golden routine involves microscopic diagnostics of Giemsa-stained thin blood smears, however, alternative methods are also constantly being sought, in order to gain an additional insight into the course of the disease. Spectroscopic methods, e.g., Raman spectroscopy, are becoming increasingly popular, due to the non-destructive nature of these techniques.

METHODS

The study included patients hospitalized for malaria caused by Plasmodium falciparum or Plasmodium vivax, in the Department of Infectious Diseases at the University Hospital in Krakow, Poland, as well as healthy volunteers. The aim of this study was to assess the possibility of using Raman spectroscopy and 2D correlation (2D-COS) spectroscopy in understanding the structural changes in erythrocytes depending on the type of attacking parasite. EPR spectroscopy and two-trace two-dimensional (2T2D) correlation was also used to examine the specificity of paramagnetic centres found in the infected human blood.

RESULTS

Two-dimensional (2D) correlation spectroscopy facilitates the identification of the hidden relationship, allowing for the discrimination of Raman spectra obtained during the course of disease in human red blood cells, infected by P. falciparum or P. vivax. Synchronous cross-peaks indicate the processes taking place inside the erythrocyte during the export of the parasite protein towards the cell membrane. In contrast, moieties that generate asynchronous 2D cross-peaks are characteristic of the respective ligand-receptor domains. These changes observed during the course of the infection, have different dynamics for P. falciparum and P. vivax, as indicated by the asynchronous correlation cross-peaks. Two-trace two-dimensional (2T2D) spectroscopy, applied to EPR spectra of blood at the beginning of the infection, showed differences between P. falciparum and P. vivax.

CONCLUSIONS

A unique feature of 2D-COS is the ability to discriminate the collected Raman and EPR spectra. The changes observed during the course of a malaria infection have different dynamics for P. falciparum and P. vivax, indicated by the reverse sequence of events. For each type of parasite, a specific recycling process for iron was observed in the infected blood.

Chitosan-Based Gastric Dressing Materials Loaded with Pomegranate Peel as Bioactive Agents: Pharmacokinetics and Effects on Experimentally Induced Gastric Ulcers in Rabbits

This study reported the fabrication and characterization of gastric dressing, composed of gelatine (GEL), chitosan (CH), and pomegranate peel (PP) extract. The structural changes occurring after γ-irradiation of GEL−CH−PP dressing were reported. The results showed that the electron paramagnetic resonance (EPR) spectroscopy of un-irradiated GEL−CH−PP showed two paramagnetic centers, which corresponded to g = 2.19 and g = 2.002. After irradiation, a new active centre appeared at g = 2.0035 at 10 kGy. The Fourier transform infrared spectroscopy (FTIR) analyses revealed an increase in peak intensity at C−H chains, as well as the C=O carboxyl groups at 10 kGy, due to the cross-linking phenomenon. The X-ray diffraction analysis showed a low change of crystallinity between the range of 2θ (15−30°). Moreover, γ-rays enhanced scavenging DPPH radical activity (51±%) and chelating power activities 79.12%. A significant inhibition of antibacterial and anti-biofilm activities (p < 0.01) was noticed. The hemolysis rates showed 0.42%, suggesting a high hemocompatibility, and exhibited significant anti-inflammatory activity in vitro (48%). In vivo, the healing effects of GEL−CH−PP dressing showed that the incidence and severity of gastric histopathological lesions decreased, compared with the ulcerated group, which could explain the bioavailability and the pharmacokinetic findings. The results highlight the loading of bioactive agents into polymer-based gastric dressings, with promising pharmacokinetics properties and effects on the induced ulcera in rabbits.

Radical in the Peroxide-Produced F-Type Ferryl Form of Bovine Cytochrome c Oxidase

The reduction of O₂ in respiratory cytochrome c oxidases (CcO) is associated with the generation of the transmembrane proton gradient by two mechanisms. In one of them, the proton pumping, two different types of the ferryl intermediates of the catalytic heme a₃-Cu_B center P and F forms, participate. Equivalent ferryl states can be also formed by the reaction of the oxidized CcO (O) with H₂O₂. Interestingly, in acidic solutions a single molecule of H₂O₂ can generate from the O an additional F-type ferryl form (F^•) that should contain, in contrast to the catalytic F intermediate, a free radical at the heme a₃-Cu_B center. In this work, the formation and the endogenous decay of both the ferryl iron of heme a₃ and the radical in F^• intermediate were examined by the combination of four experimental approaches, isothermal titration calorimetry, electron paramagnetic resonance, and electronic absorption spectroscopy together with the reduction of this form by the defined number of electrons. The results are consistent with the generation of radicals in F^• form. However, the radical at the catalytic center is more rapidly quenched than the accompanying ferryl state of heme a₃, very likely by the intrinsic oxidation of the enzyme itself.

Peroxide stimulated transition between the ferryl intermediates of bovine cytochrome c oxidase

During catalysis of cytochrome c oxidases (CcO) several ferryl intermediates of the catalytic heme a₃-Cu_B center are observed. In the P_M ferryl state, produced by the reaction of two-electron reduced CcO with O₂, the ferryl iron of heme a₃ and a free radical are present at the catalytic center. The radical reduction stimulates the transition of the P_M into another ferryl F state. Similar ferryl states can be also generated from the oxidized CcO (O) in the reaction with H₂O₂. The P_M, the product of the reaction of the O with one molecule of peroxide, is transformed into the F state by the second molecule of H₂O₂. However, the chemical nature of this transition has not been unambiguously elucidated yet. Here, we examined the redox state of the peroxide-produced P_M and F states by the one-electron reduction. The F form and interestingly also the major fraction of the P_M sample, likely another P-type ferryl form (P_R), were found to be the one oxidizing equivalent above the O state. However, the both P-type forms are transformed into the F state by additional molecule of H₂O₂. It is suggested that the P_R-to-F transition is due to the binding of H₂O₂ to Cu_B triggering a structural change together with the uptake of H⁺ at the catalytic center. In the P_M-to-F conversion, these two events are complemented with the annihilation of radical by the intrinsic oxidation of the enzyme.

Biochemical basis and metabolic interplay of redox regulation

Accumulated evidence strongly indicates that oxidative stress, characterized by an imbalance between reactive oxygen species (ROS) production and antioxidants in favor of oxidants, plays an important role in disease pathogenesis. However, ROS can act as signaling molecules and fulfill essential physiological functions at basal levels. Each ROS would be different in the extent to stimulate and contribute to different pathophysiological effects. Importantly, multiple ROS generators can be activated either concomitantly or sequentially by relevant signaling molecules for redox biological functions. Here, we summarized the current knowledge related to chemical and biochemical features of primary ROS species and corresponding antioxidants. Metabolic pathways of five major ROS generators and five ROS clearance systems were described, including their ROS products, specific ROS enriched tissue, cell and organelle, and relevant functional implications. We provided an overview of ROS generation and induction at different levels of metabolism. We classified 11 ROS species into three types based on their reactivity and target selectivity and presented ROS homeostasis and functional implications in pathological and physiological status. This article intensively reviewed and refined biochemical basis, metabolic signaling and regulation, functional insights, and provided guidance for the identification of novel therapeutic targets.

Radical Formation and Migration in Myoglobins

Three EPR signals from individual free radical species have been identified in the EPR spectra of horse heart metmyoglobin (HH metMb) mixed with hydrogen peroxide (H2O2). The peroxyl radical EPR signal was assigned to the Trp14-OO• radical, the seven component signal – to the Tyr103• radical and the singlet EPR signal was assigned to the Tyr146• radical. Apo-Mb (haem free HH Mb) added in various concentrations to the native metMb prior to H2O2 addition affected the yields of the three types of radicals. As the concentrations of metMb and H2O2 were kept constant, the yield of the primary radical formed is the same in all experiments, H2O2 being unable to produce any radical in the reaction with a haem free protein. Nevertheless, the addition of apo-Mb resulted in an increase of the Tyr146• radical concentration and in a quantitatively similar decrease of the Tyr103• radical concentration. These changes were dependent on the concentration of the added apo-Mb. Thus we show that a radical transfer Tyr103• → Tyr146• occurs and that this reaction is protein concentration dependent. The question whether this radical transfer is inter- or intra-molecular is discussed. A similarity is drawn between the system studied and the sperm whale metMb/H2O2 system, for which the radical transfer Tyr103• → Tyr151• has been previously suggested.

Interaction of Glossoscolex paulistus extracellular hemoglobin with hydrogen peroxide: Formation and decay of ferryl-HbGp

The giant extracellular hemoglobin from earthworm Glossoscolex paulistus (HbGp) reacts with hydrogen peroxide, displaying peroxidase activity in the presence of guaiacol. The formation of ferryl-HbGp (compound II) from the peroxidase cycle was studied in the present work. The hypervalent ferryl-HbGp species was formed directly by the reaction of oxy-HbGp and hydrogen peroxide. The oxy-HbGp heme groups (144) under different excess of H₂O₂, relative to heme, showed an influence in the total amount of ferryl-HbGp at the end of the reaction. The ferryl-HbGp was formed with second order rate constant of 27.1±0.5M^-1s^-1, at pH7.0 and 25°C. The increase of the pH value to 8.0 induces both faster formation and decay of ferryl-HbGp, together with oligomeric dissociation induced by the presence of H₂O₂, as observed by DLS. This effect of dissociation increases the heme exposure and decreases the ferryl-HbGp stability, affecting the rate constant as a parallel reaction. At pH7.0, high excess of H₂O₂, above 1:5 oxy-HbGp heme: H₂O₂, produces the aggregation of the protein. Our results show for the first time, for an extracellular giant hemoglobin, the possible effects of oxidative stress induced by hydrogen peroxide.

Oxidation Reactions in Kink Banded Regions of UHMMPE Fiber-Based Laminates Used in Body Armor: A Mechanistic Study

This work demonstrates the synergy between the thermo-mechanical and humidity induced degradation as well as the oxidation reactions in the kink-banded areas of ultra-high molar mass polyethylene (UHMMPE) fiber-based laminates used in body armor. For aged materials, the energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) results reveal high concentrations of oxygen containing products, and the EPR results demonstrate the presence of the peroxyl radicals (RO₂ ^• ) in the kink-banded areas. After one year of dark ambient storage, very long-lived RO₂ ^• radicals were observed primarily in the samples exposed to ageing conditions of elevated temperatures, humidity, and mechanical stress. The total percentage of crystallinity, as measured by differential scanning calorimetry, of the kinkbanded fibers was unchanged, indicating that the degradation occurs primarily in the amorphous region, and may also involve recrystallization processes of the degraded chains. However, the most abundant orthorhombic crystalline phase decreases from 77 % to 70 %. This decrease in the orthorhombic structure leads to more diffusion of oxygen into the kink-banded region, enhancing the oxidation processes. No changes are observed in the monoclinic phase of the kinked fibers, which remained constant and constituted ~2 % of the total crystallinity.

Mechanism of the biomolecular synthesis of PEDOT:PSS: importance of heme degradation by hydrogen peroxide

The use of biomolecules as oxidants for the synthesis of conducting polymers provides an important tool for the control of polymer properties.

ESR and X-ray Structure Investigations on the Binding and Mechanism of Inhibition of the Native State of Myeloperoxidase with Low Molecular Weight Fragments

As an early visitor to the injured loci, neutrophil-derived human Myeloperoxidase (hMPO) offers an attractive protein target to modulate the inflammation of the host tissue through suitable inhibitors. We describe a novel methodology of using low temperature ESR spectroscopy (6 K) and FAST™ technology to screen a diverse series of small molecules that inhibit the peroxidase function through reversible binding to the native state of MPO. Our initial efforts to profile molecules on the inhibition of MPO-initiated nitration of the Apo-A1 peptide (AEYHAKATEHL) assay showed several potent (with sub-micro molar IC50s) but spurious inhibitors that either do not bind to the heme pocket in the enzyme or retain high (>50 %) anti oxidant potential. Such molecules when taken forward for X-ray did not yield inhibitor-bound co-crystals. We then used ESR to confirm direct binding to the native state enzyme, by measuring the binding-induced shift in the electronic parameter g to rank order the molecules. Molecules with a higher rank order-those with g-shift Rrelative ≥15-yielded well-formed protein-bound crystals (n = 33 structures). The co-crystal structure with the LSN217331 inhibitor reveals that the chlorophenyl group projects away from the heme along the edges of the Phe366 and Phe407 side chain phenyl rings thereby sterically restricting the access to the heme by the substrates like H2O2. Both ESR and antioxidant screens were used to derive the mechanism of action (reversibility, competitive substrate inhibition, and percent antioxidant potential). In conclusion, our results point to a viable path forward to target the native state of MPO to tame local inflammation.

Hemoglobin-mediated synthesis of PEDOT:PSS: enhancing conductivity through biological oxidants

Hemoglobin is used as an oxidant to generate highly conductive PEDOT:PSS with bipolarons, while catalase generates a less conductive polymer that possesses polarons.

[Myeloperoxidase-induced biodegradation of single-walled carbon nanotubes is mediated by hypochlorite]

Broad prospects for the use of single-walled carbon nanotubes (SWNTs) in medicine and biotechnology raise the concerns about both their toxicity, and the mechanisms of biodegradation and excretion from the body. SWNTs biodegradation as a result of catalytic activity of myeloperoxidase (MPO) was shown in the isolated MPO system as well as in the suspension of neutrophils [Kagan V.E., et al., 2010]. In the present study we analyzed the ability of different MPO-produced oxidants to participate in the modification and degradation of SWNTs. The comparison of the ability of various peroxidases to degrade SWNTs in vitro revealed that myeloperoxidase, due to its ability to produce hypochlorite, and lactoperoxidase, due to its ability to produce hypobromite, are extremely efficient in the degradation of carbon nanotubes. The biodegradation of SWNTs in the model system can also be caused by free radicals generated as a result of heme degradation and, to a lesser extent, by active oxoferryl intermediates of peroxidases. Our experiments showed that in the presence of blood plasma, peroxidase intermediates or free radical products of heme degradation were unable to initiate biodegradation of carbon nanotubes, only the generation of hypochlorite by MPO can cause the biodegradation of carbon nanotubes in vivo. Titration of SWNTs suspension containing plasma with hypochlorite at high concentrations resulted in the decrease in the optical absorbance of the suspension indicating the degradation of nanotubes. Our results clearly indicate that hypochlorite can serve as a main oxidizing agent which is able to modify and degrade nanotubes in the sites of inflammation and in the phagosomes.

Radical formation in cytochrome c oxidase

The formation of radicals in bovine cytochrome c oxidase (bCcO), during the O(2) redox chemistry and proton translocation, is an unresolved controversial issue. To determine if radicals are formed in the catalytic reaction of bCcO under single turnover conditions, the reaction of O(2) with the enzyme, reduced by either ascorbate or dithionite, was initiated in a custom-built rapid freeze quenching (RFQ) device and the products were trapped at 77K at reaction times ranging from 50μs to 6ms. Additional samples were hand mixed to attain multiple turnover conditions and quenched with a reaction time of minutes. X-band (9GHz) continuous wave electron paramagnetic resonance (CW-EPR) spectra of the reaction products revealed the formation of a narrow radical with both reductants. D-band (130GHz) pulsed EPR spectra allowed for the determination of the g-tensor principal values and revealed that when ascorbate was used as the reductant the dominant radical species was localized on the ascorbyl moiety, and when dithionite was used as the reductant the radical was the SO(2)(-) ion. When the contributions from the reductants are subtracted from the spectra, no evidence for a protein-based radical could be found in the reaction of O(2) with reduced bCcO. As a surrogate for radicals formed on reaction intermediates, the reaction of hydrogen peroxide (H(2)O(2)) with oxidized bCcO was studied at pH 6 and pH 8 by trapping the products at 50μs with the RFQ device to determine the initial reaction events. For comparison, radicals formed after several minutes of incubation were also examined, and X-band and D-band analysis led to the identification of radicals on Tyr-244 and Tyr-129. In the RFQ measurements, a peroxyl (ROO) species was formed, presumably by the reaction between O(2) and an amino acid-based radical. It is postulated that Tyr-129 may play a central role as a proton loading site during proton translocation by ejecting a proton upon formation of the radical species and then becoming reprotonated during its reduction via a chain of three water molecules originating from the region of the propionate groups of heme a(3). This article is part of a Special Issue entitled: "Allosteric cooperativity in respiratory proteins".

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.

Antibiotic resistance in Mycobacterium tuberculosis: peroxidase intermediate bypass causes poor isoniazid activation by the S315G mutant of M. tuberculosis catalase-peroxidase (KatG)

KatG (catalase-peroxidase) in Mycobacterium tuberculosis is responsible for activation of isoniazid (INH), a pro-drug used to treat tuberculosis infections. Resistance to INH is a global health problem most often associated with mutations in the katG gene. The origin of INH resistance caused by the KatG[S315G] mutant enzyme is examined here. Overexpressed KatG[S315G] was characterized by optical, EPR, and resonance Raman spectroscopy and by studies of the INH activation mechanism in vitro. Catalase activity and peroxidase activity with artificial substrates were moderately reduced (50 and 35%, respectively), whereas the rates of formation of oxyferryl heme:porphyrin pi-cation radical and the decay of heme intermediates were approximately 2-fold faster in KatG[S315G] compared with WT enzyme. The INH binding affinity for the resting enzyme was unchanged, whereas INH activation, measured by the rate of formation of an acyl-nicotinamide adenine dinucleotide adduct considered to be a bactericidal molecule, was reduced by 30% compared with WT KatG. INH resistance is suggested to arise from a redirection of catalytic intermediates into nonproductive reactions that interfere with oxidation of INH. In the resting mutant enzyme, a rapid evolution of 5-c heme to 6-c species occurred in contrast with the behavior of WT KatG and KatG[S315T] and consistent with greater flexibility at the heme edge in the absence of the hydroxyl of residue 315. Insights into the effects of mutations at residue 315 on enzyme structure, peroxidation kinetics, and specific interactions with INH are presented.

The influence of ferrylhemoglobin and methemoglobin on the human erythrocyte membrane

The aim of the study was to examine and compare the effects of methemoglobin (metHb) and ferrylhemoglobin (ferrylHb) on the erythrocyte membrane. Kinetic studies of the decay of ferrylhemoglobin (*HbFe(IV)=O denotes ferryl derivative of hemoglobin present 5 min after initiation of the reaction of metHb with H(2)O(2); ferrylHb) showed that autoredecay of this derivative is slower than its decay in the presence of whole erythrocytes and erythrocyte membranes. It provides evidence for interactions between ferrylHb and the erythrocyte membrane. Both hemoglobin derivatives induced small changes in the structure and function of the erythrocyte membrane which were more pronounced for ferrylHb. The amount of ferrylHb bound to erythrocyte membranes increased with incubation time and, after 2 h, was twice that of membrane-bound metHb. The incubation of erythrocytes with metHb or ferrylHb did not influence osmotic fragility and did not initiate peroxidation of membrane lipids in whole erythrocytes as well as in isolated erythrocyte membranes. Membrane acetylcholinesterase activity increased by about 10% after treatment of whole erythrocytes with both metHb and ferrylHb. ESR spectra of membrane-bound maleimide spin label demonstrated minor changes in the conformation of label-binding proteins in ferrylHb-treated erythrocyte membranes. The fluidity of the membrane surface layer decreased slightly after incubation of erythrocytes and isolated erythrocyte membranes with ferrylHb and metHb. In whole erythrocytes, these changes were not stable and disappeared during longer incubation.

Radical sites in Mycobacterium tuberculosis KatG identified using electron paramagnetic resonance spectroscopy, the three-dimensional crystal structure, and electron transfer couplings

Catalase-peroxidase (KatG) from Mycobacterium tuberculosis, a Class I peroxidase, exhibits high catalase activity and peroxidase activity with various substrates and is responsible for activation of the commonly used antitubercular drug, isoniazid (INH). KatG readily forms amino acid-based radicals during turnover with alkyl peroxides, and this work focuses on extending the identification and characterization of radicals forming on the millisecond to second time scale. Rapid freeze-quench electron paramagnetic resonance spectroscopy (RFQ-EPR) reveals a change in the structure of the initially formed radical in the presence of INH. Heme pocket binding of the drug and knowledge that KatG[Y229F] lacks this signal provides evidence for radical formation on residue Tyr(229). High field RFQ-EPR spectroscopy confirmed a tryptophanyl radical signal, and new analyses of X-band RFQ-EPR spectra also established its presence. High field EPR spectroscopy also confirmed that the majority radical species is a tyrosyl radical. Site-directed mutagenesis, along with simulations of EPR spectra based on x-ray structural data for particular tyrosine and tryptophan residues, enabled assignments based on predicted hyperfine coupling parameters. KatG mutants W107F, Y229F, and the double mutant W107F/Y229F showed alteration in type and yield of radical species. Results are consistent with formation of a tyrosyl radical reasonably assigned to residue Tyr(229) within the first few milliseconds of turnover. This is followed by a mixture of tyrosyl and tryptophanyl radical species and finally to only a tyrosyl radical on residue Tyr(353), which lies more distant from the heme. The radical processing of enzyme lacking the Trp(107)-Tyr(229)-Met(255) adduct (found as a unique structural feature of catalase-peroxidases) is suggested to be a reasonable assignment of the phenomena.

Reaction of Aplysia limacina metmyoglobin with hydrogen peroxide

Myoglobin (Mb) from gastropod mollusc Aplysia limacina shows only 20% sequence homology to the 'prototype' sperm whale Mb but exhibits a typical Mb fold and can reversibly bind oxygen. An intriguing feature of aplysia Mb is that it lacks the distal histidine and displays a ligand stabilisation based on an arginine. Here we report the reaction of aplysia metMb with hydrogen peroxide studied by optical and electron paramagnetic resonance (EPR) spectroscopies. Two electron oxidation of the protein by H2O2 results in formation of two intermediates typical for this class of reactions, the oxoferryl haem state and a globin-bound free radical. An unusual characteristic of the aplysia Mb reaction is formation, prior to haem oxidation, of an optically distinct compound with an EPR spectrum typical of the low spin Fe3+ haem state. This compound is interpreted as the complex between H2O2 and the ferric haem state (Compound), formed prior to cleavage of the dioxygen bond. We conclude that H2O2 is singly deprotonated in Compound which can thus be notated as [Fe3+--OOH]. A new low spin ferric haem state has been observed over the period of Compound decay, and hypotheses have been formulated as to its identity and role. The location of the protein bound radical observed in aplysia Mb is discussed in light of the fact that the protein does not have any tyrosine residues, the most common site of free radical formation in the haem protein/peroxide systems. All intermediates of the reaction are kinetically characterised.

Inactivation of alcohol dehydrogenase (ADH) by ferryl derivatives of human hemoglobin

In this paper, inactivation of alcohol dehydrogenase (ADH) by products of reactions of H2O2 with metHb has been studied. Inactivation of the enzyme was studied in two systems corresponding to two kinetic stages of the reaction. In the first system H2O2 was added to the mixture of metHb and ADH [the (metHb+ADH)+H2O2] system (ADH was present in the system since the moment of addition of H2O2 i. e. since the very beginning of the reaction of metHb with H2O2). In the second system ADH was added to the system 5 min after the initiation of the reaction of H2O2 with metHb [the (metHb+H2O2)5 min+ADH] system. In the first case all the products of reaction of H2O2 with metHb (non-peroxyl and peroxyl radicals and non-radical products, viz. hydroperoxides and *HbFe(IV)=O) could react with the enzyme causing its inactivation. In the second system, enzyme reacted almost exclusively with non-radical products (though a small contribution of reactions with peroxyl radicals cannot be excluded). ADH inactivation was observed in both system. Hydrogen peroxide alone did not inactivate ADH at the concentrations employed evidencing that enzyme inactivation was due exclusively to products of reaction of H2O2 with metHb. The rate and extent of ADH inactivation were much higher in the first than in the second system. The dependence of ADH activity on the time of incubation with ferryl derivatives of Hb can be described by a sum of three exponentials in the first system and two exponentials in the second system. Reactions of appropriate forms of the ferryl derivatives of hemoglobin have been tentatively ascribed to these exponentials. The extent of the enzyme inactivation in the second system was dependent on the proton concentration, being at the highest at pH 7.4 and negligible at pH 6.0. The reaction of H2O2 with metHb resulted in the formation of cross-links of Hb subunits (dimers and trimers). The amount of the dimers formed was much lower in the first system i. e. when the radical forms dominated the reaction of inactivation.

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.

Formation of isodialuric acid lesion within DNA oligomers via one-electron oxidation of 5-hydroxyuracil: characterization, stability and excision repair

5-Hydroxyuracil is a major oxidized nucleobase that can be generated by the action of ^•OH radical and one-electron oxidants. The latter modified base that exhibits a low ionization potential is highly susceptible to further degradation upon exposure to various oxidants. Emphasis was placed in thiswork on the formation and characterization of one-electron oxidation products of 5-hydroxyuracil within DNA fragments of defined sequence. For this purpose, 5-hydroxyuracil containing single- and double-stranded oligonucleotides of various lengths were synthesized and then exposed to the oxidizing action of iridium salts. Isodialuric acid was found to be formed almost quantitatively by a one-electron oxidation mechanism for which relevant information was inferred from a freeze-quenched ESR study. Information on the stability of isodialuric acid thus formed and its conversion products in aqueous solutions was also gained from experiments performed at acidic, neutral and alkali pH’s. Moreover, biochemical features dealing with the substrate specificity of several bacterial and yeast base excision repair enzymes to remove isodialuric acid from site-specifically modified DNA fragments were determined.

Characterization of acatalasemic erythrocytes treated with low and high dose hydrogen peroxide. Hemolysis and aggregation

The effects of hydrogen peroxide on normal and acatalasemic erythrocytes were examined. Severe hemolysis of acatalasemic erythrocytes and a small tyrosine radical signal (g = 2.005) associated with the formation of ferryl hemoglobin were observed upon the addition of less than 0.25 mM hydrogen peroxide. However, when the concentration of hydrogen peroxide was increased to 0.5 mM, acatalasemic erythrocytes became insoluble in water and increased the tyrosine radical signal. Polymerization of hemoglobin and aggregation of the erythrocytes were observed. On the other hand, normal erythrocytes exhibited only mild hemolysis by the addition of hydrogen peroxide under similar conditions. From these results, the scavenging of hydrogen peroxide by hemoglobin generates the ferryl hemoglobin species (H-Hb-Fe(IV)=O) plus protein-based radicals (*Hb-Fe(IV)=O). These species induce hemolysis of erythrocytes, polymerization of hemoglobin, and aggregation of the acatalasemic erythrocytes. A mechanism for the onset of Takarara disease is proposed.

Mitochondrial dysfunction in patients with severe sepsis: An EPR interrogation of individual respiratory chain components

Electron paramagnetic resonance (EPR) spectra of complex biological systems contain information about the paramagnetic centres present. Retrieving such information is important since paramagnetic species are common intermediates of all redox reactions in both normal and abnormal metabolism. However, it is often difficult to determine the nature and content of all paramagnetic species present because the EPR signals from individual centres overlap. Here, we apply our deconvolution method based on spectra subtraction with variable coefficient to quantify individual paramagnetic components of human muscle biopsies taken from critically ill patients with severe sepsis. We use low temperature EPR spectroscopy to identify and quantify nine different paramagnetic species in the tissue. These include the majority of the mitochondrial iron-sulfur centres and the first in vivo report of a mitochondrial radical assigned to a spin-coupled pair of semiquinones (SQ*-SQ*). We have previously demonstrated in these same muscle biopsies that biochemical assays of mitochondrial dysfunction correlate with clinical outcomes (D. Brealey, M. Brand, I. Hargreaves, S. Heales, J. Land, R. Smolenski, N.A. Davies, C.E. Cooper, M. Singer, Association between mitochondrial dysfunction and severity and outcome of septic shock. Lancet 360 (2002) 219-223.). Analysis of the paramagnetic centres in the muscle confirms and extends these findings: the (SQ*-SQ*) radical species negatively correlates with the illness severity of the patient (APACHE II score) and a decreased concentration of mitochondrial Complex I iron-sulfur redox centres is linked to mortality.

Cytochrome c acts as a cardiolipin oxygenase required for release of proapoptotic factors

Programmed death (apoptosis) is turned on in damaged or unwanted cells to secure their clean and safe self-elimination. The initial apoptotic events are coordinated in mitochondria, whereby several proapoptotic factors, including cytochrome c, are released into the cytosol to trigger caspase cascades. The release mechanisms include interactions of B-cell/lymphoma 2 family proteins with a mitochondria-specific phospholipid, cardiolipin, to cause permeabilization of the outer mitochondrial membrane. Using oxidative lipidomics, we showed that cardiolipin is the only phospholipid in mitochondria that undergoes early oxidation during apoptosis. The oxidation is catalyzed by a cardiolipin-specific peroxidase activity of cardiolipin-bound cytochrome c. In a previously undescribed step in apoptosis, we showed that oxidized cardiolipin is required for the release of proapoptotic factors. These results provide insight into the role of reactive oxygen species in triggering the cell-death pathway and describe an early role for cytochrome c before caspase activation.

Reaction of haem containing proteins and enzymes with hydroperoxides: The radical view

The reaction between hydroperoxides and the haem group of proteins and enzymes is important for the function of many enzymes but has also been implicated in a number of pathological conditions where oxygen binding proteins interact with hydrogen peroxide or other peroxides. The haem group in the oxidized Fe3+ (ferric) state reacts with hydroperoxides with a formation of the Fe4+=O (oxoferryl) haem state and a free radical primarily located on the pi-system of the haem. The radical is then transferred to an amino acid residue of the protein and undergoes further transfer and transformation processes. The free radicals formed in this reaction are reviewed for a number of proteins and enzymes. Their previously published EPR spectra are analysed in a comparative way. The radicals directly detected in most systems are tyrosyl radicals and the peroxyl radicals formed on tryptophan and possibly cysteine. The locations of the radicals in the proteins have been reported as follows: Tyr133 in soybean leghaemoglobin; alphaTyr42, alphaTrp14, betaTrp15, betaCys93, (alphaTyr24-alphaHis20), all in the alpha- and beta-subunits of human haemoglobin; Tyr103, Tyr151 and Trp14 in sperm whale myoglobin; Tyr103, Tyr146 and Trp14 in horse myoglobin; Trp14, Tyr103 and Cys110 in human Mb. The sequence of events leading to radical formation, transformation and transfer, both intra- and intermolecularly, is considered. The free radicals induced by peroxides in the enzymes are reviewed. Those include: lignin peroxidase, cytochrome c peroxidase, cytochrome c oxidase, turnip isoperoxidase 7, bovine catalase, two isoforms of prostaglandin H synthase, Mycobacterium tuberculosis and Synechocystis PCC6803 catalase-peroxidases.

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.

EPR spectroscopic characterization of the manganese center and a free radical in the oxalate decarboxylase reaction: identification of a tyrosyl radical during turnover

Several molecular mechanisms for cleavage of the oxalate carbon-carbon bond by manganese-dependent oxalate decarboxylase have recently been proposed involving high oxidation states of manganese. We have examined the oxalate decarboxylase from Bacillus subtilis by electron paramagnetic resonance in perpendicular and parallel polarization configurations to test for the presence of such species in the resting state and during enzymatic turnover. Simulation and the position of the half-field Mn(II) line suggest a nearly octahedral metal geometry in the resting state. No spectroscopic signature for Mn(III) or Mn(IV) is seen in parallel mode EPR for samples frozen during turnover, consistent either with a large zero-field splitting in the oxidized metal center or undetectable levels of these putative high-valent intermediates in the steady state. A narrow, featureless g = 2.0 species was also observed in perpendicular mode in the presence of substrate, enzyme, and dioxygen. Additional splittings in the signal envelope became apparent when spectra were taken at higher temperatures. Isotopic editing resulted in an altered line shape only when tyrosine residues of the enzyme were specifically deuterated. Spectral processing confirmed multiple splittings with isotopically neutral enzyme that collapsed to a single prominent splitting in the deuterated enzyme. These results are consistent with formation of an enzyme-based tyrosyl radical upon oxalate exposure. Modestly enhanced relaxation relative to abiological tyrosyl radicals was observed, but site-directed mutagenesis indicated that conserved tyrosine residues in the active site do not host the unpaired spin. Potential roles for manganese and a peripheral tyrosyl radical during steady-state turnover are discussed.

Three different oxygen-induced radical species in endothelial nitric-oxide synthase oxygenase domain under regulation by L-arginine and tetrahydrobiopterin

Endothelial nitric-oxide synthase (eNOS) plays important roles in vascular physiology and homeostasis. Whether eNOS catalyzes nitric oxide biosynthesis or the synthesis of reactive oxygen species such as superoxide, hydrogen peroxide, and peroxynitrite is dictated by the bioavailability of tetrahydrobiopterin (BH(4)) and L-arginine during eNOS catalysis. The effect of BH(4) and L-arginine on oxygen-induced radical intermediates has been investigated by single turnover rapid-freeze quench and EPR spectroscopy using the isolated eNOS oxygenase domain (eNOS(ox)). Three distinct radical intermediates corresponding to >50% of the heme were observed during the reaction between ferrous eNOS(ox) and oxygen. BH(4)-free eNOS(ox) produced the superoxide radical very efficiently in the absence of L-arginine. L-Arginine decreased the formation rate of superoxide by an order of magnitude but not its final level or EPR line shape. For BH(4)-containing eNOS(ox), only a stoichiometric amount of BH(4) radical was produced in the presence of L-arginine, but in its absence a new radical was obtained. This new radical could be either a peroxyl radical of BH(4) or an amino acid radical was in the vicinity of the heme. Formation of this new radical is very rapid, >150 s(-1), and it was subsequently converted to a BH(4) radical. The trapping of the superoxide radical by cytochrome c in the reaction of BH(4)(-) eNOS(ox) exhibited a limiting rate of approximately 15 s(-1), the time for the superoxide radical to leave the heme pocket and reach the protein surface; this reveals a general problem of the regular spin-trapping method in determining radical formation kinetics. Cytochrome c failed to trap the new radical species. Together with other EPR characteristics, our data strongly support the conclusion that this new radical is not a superoxide radical or a mixture of superoxide and biopterin radicals. Our study points out distinct roles of BH(4) and L-arginine in regulating eNOS radical intermediates. BH(4) prevented superoxide formation by chemical conversions of the Fe(II)O(2) intermediate, and l-arginine delayed superoxide formation by electronic interaction with the heme-bound oxygen.

Redox reactions of hemoglobin

Redox reactions of hemoglobin have gained importance because of the general interest of the role of oxidative stress in diseases and the possible role of red blood cells in oxidative stress. Although electron paramagnetic resonance (EPR) is extremely valuable in studying hemoglobin redox reactions it has not been adequately used. We have focused in this review on the important contributions of EPR to our understanding of hemoglobin redox reactions. We have limited our discussion to the redox reactions thought to occur under physiological conditions. This includes autoxidation as well as the reactions of hydrogen peroxide generated by superoxide dismutation. We have also discussed redox reactions associated with nitric oxide produced in the circulation. We have pinpointed the value of using EPR to detect and study the paramagnetic species and free radicals formed during these reactions. We have shown how EPR not only identifies the paramagnetic species formed but can also be used to provide insights into the mechanism involved in the redox reactions.

Probing the free radicals formed in the metmyoglobin-hydrogen peroxide reaction

The reaction between metmyoglobin and hydrogen peroxide results in the two-electron reduction of H2O2 by the protein, with concomitant formation of a ferryl-oxo heme and a protein-centered free radical. Sperm whale metmyoglobin, which contains three tyrosine residues (Tyr-103, Tyr-146, and Tyr-151) and two tryptophan residues (Trp-7 and Trp-14), forms a tryptophanyl radical at residue 14 that reacts with O2 to form a peroxyl radical and also forms distinct tyrosyl radicals at Tyr-103 and Tyr-151. Horse metmyoglobin, which lacks Tyr-151 of the sperm whale protein, forms an oxygen-reactive tryptophanyl radical and also a phenoxyl radical at Tyr-103. Human metmyoglobin, in addition to the tyrosine and tryptophan radicals formed on horse metmyoglobin, also forms a Cys-110-centered thiyl radical that can also form a peroxyl radical. The tryptophanyl radicals react both with molecular oxygen and with the spin trap 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS). The spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) traps the Tyr-103 radicals and the Cys-110 thiyl radical of human myoglobin, and 2-methyl-2-nitrosopropane (MNP) traps all of the tyrosyl radicals. When excess H2O2 is used, DBNBS traps only a tyrosyl radical on horse myoglobin, but the detection of peroxyl radicals and the loss of tryptophan fluorescence support tryptophan oxidation under those conditions. Kinetic analysis of the formation of the various free radicals suggests that tryptophanyl radical and tyrosyl radical formation are independent events, and that formation of the Cys-110 thiyl radical on human myoglobin occurs via oxidation of the thiol group by the Tyr-103 phenoxyl radical. Peptide mapping studies of the radical adducts and direct EPR studies at low temperature and room temperature support the conclusions of the EPR spin trapping studies.

UVA-ketoprofen-induced hemoglobin radicals detected by immuno-spin trapping

Ketoprofen (3-benzoyl-alpha-methylbenzeneacetic acid, KP) is a widely used nonsteroidal anti-inflammatory drug (NSAID) that causes both phototoxicity and photoallergy. Here, we investigated the formation of hemoglobin radicals, in both purified hemoglobin and red blood cells (RBC), induced by ultraviolet A (UVA)-KP by using "immuno-spin trapping," a novel approach that combines the specificity of spin trapping with the sensitivity of antigen-antibody interactions. The methemoglobin (metHb) radicals react covalently with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to form nitroxyl radical adducts that are oxidized to the corresponding nitrone adducts, which in turn are specifically recognized by antiserum against DMPO nitrone. We found that the formation of nitrone adducts in metHb depended on the UVA dose, the KP concentration and the presence of DMPO, as determined by enzyme-linked immunosorbent assay and Western blotting. Adduct formation decreased when irradiation was carried out in the presence of catalase or nitrogen, suggesting that H2O2 plays a key role in KP-UVA-induced metHb radical formation. KP in the dark did not generate metHb radical-derived nitrone adducts, whereas UVA alone resulted in the formation of metHb radical-derived nitrone adducts that increased with UVA dose from 4 to 10 J/cm2. However, KP (25 and 200 microM) plus UVA (4 and 10 J/cm2) resulted in a significant increase in the formation of metHb radical-derived nitrone adducts as compared with UVA or KP alone, indicating that KP photosensitized the production of the metHb radicals in the presence of UVA. In contrast, no metHb radical-derived nitrone adduct was detected in the absence of DMPO, even though KP and UVA were present. We also detected the hemoglobin radical formation in RBC as well as in hemolysates. The endogenous antioxidants and exogenous reduced glutathione inhibited the protein radical formation. These studies have shown that the immuno-spin-trapping technique can be used to detect radical damage in proteins as a result of photosensitizing reactions. The successful detection of protein radical formation caused by KP photosensitization could help further understand the photoallergic effect of this NSAID.

Immunochemical detection of hemoglobin-derived radicals formed by reaction with hydrogen peroxide: involvement of a protein-tyrosyl radical

To investigate the involvement of a hemoglobin radical in the human oxyhemoglobin (oxyHb) or metHb/H2O2 system, we have used a new approach called "immuno-spin trapping," which combines the specificity and sensitivity of both spin trapping and antigen:antibody interactions. Previously, a novel rabbit polyclonal anti-DMPO nitrone adduct antiserum, which specifically recognizes protein radical-derived nitrone adducts, was developed and validated in our laboratory. In the present study, the formation of nitrone adducts on hemoglobin was shown to depend on the oxidation state of the iron heme, the concentrations of H2O2 and DMPO, and time as determined by enzyme-linked immunosorbent assay (ELISA) and by Western blotting. The presence of reduced glutathione or L-ascorbate significantly decreased the level of nitrone adducts on metHb in a dose-dependent manner. To confirm the ELISA results, Western blotting analysis showed that only the complete system (oxy- or metHb/DMPO/H2O2) generates epitopes recognized by the antiserum. The specific modification of tyrosine residues on metHb by iodination nearly abolished antibody binding, while the thiylation of cysteine residues caused a small but reproducible decrease in the amount of nitrone adducts. These findings strongly suggest that tyrosine residues are the site of formation of the immunochemically detectable hemoglobin radical-derived nitrone adducts. In addition, we were able to demonstrate the presence of hemoglobin radical-derived nitrone adducts inside red blood cells exposed to H2O2 and DMPO. In conclusion, our new approach showed several advantages over EPR spin trapping with the anti-DMPO nitrone adduct antiserum by demonstrating the formation of tyrosyl radical-derived nitrone adduct(s) in human oxyHb/metHb at much lower concentrations than was possible with EPR and detecting radicals inside RBC exposed to H2O2.

Comparative study of tyrosine radicals in hemoglobin and myoglobins treated with hydrogen peroxide

The reactions of hydrogen peroxide with human methemoglobin, sperm whale metmyoglobin, and horse heart metmyoglobin were studied by electron paramagnetic resonance (EPR) spectroscopy at 10 K and room temperature. The singlet EPR signal, one of the three signals seen in these systems at 10 K, is characterized by a poorly resolved, but still detectable, hyperfine structure that can be used to assign it to a tyrosyl radical. The singlet is detectable as a quintet at room temperature in methemoglobin with identical spectral features to those of the well characterized tyrosyl radical in photosystem II. Hyperfine splitting constants found for Tyr radicals were used to find the rotation angle of the phenoxyl group. Analysis of these angles in the crystal structures suggests that the radical resides on Tyr151 in sperm whale myoglobin, Tyr133 in soybean leghemoglobin, and either alphaTyr42, betaTyr35, or betaTyr130 in hemoglobin. In the sperm whale metmyoglobin Tyr103Phe mutant, there is no detectable tyrosyl radical present. Yet the rotation angle of Tyr103 (134 degrees) is too large to account for the observed EPR spectrum in the wild type. Tyr103 is the closest to the heme. We suggest that Tyr103 is the initial site of the radical, which then rapidly migrates to Tyr151.

A role for the myoglobin redox cycle in the induction of endothelial cell apoptosis

This study investigates the potential role of the ferric/ferryl redox cycle of myoglobin (Mb) in the development of endothelial cell injury. Bovine aortic endothelial cells were incubated with ferric Mb (0.5-100 micro M) in the presence or absence of low steady states of H(2)O(2) (3-4 micro M) generated by glucose oxidase (GOX). The reaction of ferric Mb with H(2)O(2) generated ferryl Mb as monitored spectrophotometrically. Ferryl Mb formation correlated with the induction of apoptosis as indicated by morphological criteria, caspase 3 activation, phosphatidylserine (PS) externalization, and nuclear condensation by Hoechst 33342 staining. The addition of ascorbate or catalase inhibited the formation of ferryl Mb and the onset of apoptosis, whereas apoptosis was enhanced in cells depleted of intracellular glutathione by pretreatment with buthionine sulfoximine. Mb and Mb/GOX suppressed cell cycle progression, but only Mb/GOX produced significant cell loss revealed by the accumulation of sub G1 events. These results suggest a role for the Mb redox cycle in the induction of endothelial cell apoptosis, which may be relevant in the pathophysiology of diseases characterized by the release of Mb from damaged muscle.

Suicide inactivation of peroxidases and the challenge of engineering more robust enzymes

As the number of industrial applications for proteins continues to expand, the exploitation of protein engineering becomes critical. It is predicted that protein engineering can generate enzymes with new catalytic properties and create desirable, high-value, products at lower production costs. Peroxidases are ubiquitous enzymes that catalyze a variety of oxygen-transfer reactions and are thus potentially useful for industrial and biomedical applications. However, peroxidases are unstable and are readily inactivated by their substrate, hydrogen peroxide. Researchers rely on the powerful tools of molecular biology to improve the stability of these enzymes, either by protecting residues sensitive to oxidation or by devising more efficient intramolecular pathways for free-radical allocation. Here, we discuss the catalytic cycle of peroxidases and the mechanism of the suicide inactivation process to establish a broad knowledge base for future rational protein engineering.

Effect of nitrosylmyoglobin and saturated fatty acid anions on metmyoglobin-catalyzed oxidation of aqueous methyl linoleate emulsions

In aqueous methyl linoleate emulsions (pH 7.4, 25 degrees C, air-saturated), nitrosylmyoglobin and saturated fatty acid anions (palmitate and stearate investigated) each showed antioxidant effect on metmyoglobin-induced peroxidation as measured by oxygen depletion rate. For equimolar concentration of nitrosylmyoglobin and metmyoglobin and for metmyoglobin in moderate excess, a reduction in oxygen consumption rate of approximately 70% was observed. Fatty acid anions reduced oxygen consumption rate most significantly for palmitate (up to 60% for a fatty acid:heme protein ratio of 90:1). No further antioxidative effect was seen for fatty acid anions in the presence of nitrosylmyoglobin, whereas nitrosylmyoglobin showed a further antioxidant effect in presence of fatty acid anions in the metmyoglobin-catalyzed process. The antioxidative mechanism of nitrosylmyoglobin and fatty acid anions is different, and while the fatty acid anions seem active in inhibiting initiation of oxidation through protection against metmyoglobin activation into perferrylmyoglobin, as shown by freeze-quench Electron Spin Resonance (ESR) spectroscopy, nitrosylmyoglobin is rather active in the oxygen consuming (propagation) phase.

Characteristics and mechanism of formation of peroxide-induced heme to protein cross-linking in myoglobin

At acidic pH values heme-protein cross-linked myoglobin (Mb-H) forms as a product of a peroxide-induced ferric-ferryl redox cycle. There is evidence that this molecule acts as a marker for heme-protein-induced oxidative stress in vivo and may exacerbate the severity of oxidative damage due to its enhanced prooxidant and pseudoperoxidatic activities. Therefore, an understanding of its properties and mechanism of formation may be important in understanding the association between heme-proteins and oxidative stress. Although the mechanism of formation of heme-protein cross-linked myoglobin is thought to involve a protein radical (possibly a tyrosine) and the ferryl heme, we show that this hypothesis needs revising. We provide evidence that in addition to a protein-based radical the protonated form of the oxoferryl heme, known to be highly reactive and radical-like in nature, is required to initiate cross-linking. This revised mechanism involves radical/radical termination rather than attack of a single radical onto the porphyrin ring. This proposal better explains the pH dependence of cross-linking and may, in part, explain the therapeutic effectiveness of increasing the pH on myoglobin-induced oxidative stress, e.g., therapy for rhabdomyolysis-associated renal dysfunction.