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

Photocatalyzed C3-H Nitrosylation of Imidazo[1,2-a]pyridine under Continuous Flow and External Photocatalyst-, Oxidant-, and Additive-Free Conditions

This study reports a protocol for the highly regioselective photocatalyzed C-H nitrosylation of imidazo[1,2-a]pyridine scaffolds at the C3 position under a combination of visible-light irradiation and continuous flow without any external photocatalyst. This protocol involves mild and safe conditions and shows good tolerance to air and water along with excellent functional group compatibility and site selectivity, generating various 3-nitrosoimidazo[1,2-a]pyridines in excellent yields under photocatalyst-, oxidant-, and additive-free conditions.Notably, the proposed nitrosylation reaction, which introduces the chromophore NO into imidazo[1,2-a]pyridine scaffolds, occurs efficiently under visible-light irradiation without any additional photocatalyst owing to the intense light-absorption characteristics of the nitrosylation products. This study could guide future studies on the development of green organic-synthesis strategies with a wide variety of potential applications.

Effects of Maillard reaction products on myoglobin-mediated lipid oxidation during refrigerated storage of carp

Lipid oxidation is the major cause of quality deterioration in freshwater fish, especially mediated by myoglobin (Mb). This study aimed to investigate the antioxidant mechanism of Maillard reaction products (MRPs) in Mb-mediated lipid oxidation in common carp (Cyprinus carpio). MRPs exhibited promising antioxidant and antimicrobial capacities based on the reduced content of peroxide and thiobarbituric acid-reactive substances and inhibited microbial growth. MRPs inhibited the oxidation of Mb by lowering the transfer from oxymyoglobin to metmyoglobin and improving the stability of heme iron. The correlation analysis showed that MRPs regulated the formation of free radicals by maintaining the reduced structure of Mb and the integrity of heme iron, and also directly inhibited the formation of oxidation products in a chain radical reaction. The texture and electronic nose analysis indicated that MRPs could delay the structural disruption and flavor deterioration of surimi. Therefore, MRPs could effectively inhibit Mb-induced lipid oxidation and further control the resulting changes in the flavor and texture of surimi.

Hidden Complexity in the Mechanism of the Autoreduction of Myoglobin Compound II

The non-native oxidation of horse heart myoglobin with hydrogen peroxide produces compound II which autoreduces by utilizing an internal oxidation site. Here, we utilize full UV-visible time-dependent kinetics with global kinetic singular value decomposition analysis to explore the mechanism and uncover more detail about the high-valent heme spectral features. By varying the hydrogen peroxide and myoglobin concentration, we were able to uncover more detailed spectra of myoglobin compound II and the autoreduction rate under several different pH conditions. The compound II spectra demonstrate pH-dependent features with an inflection point around pH 5.7 ± 0.1. The rate of autoreduction of compound II, k 2, increases with lower pH with a half-power proton dependence and no indication of a pK a > 3.9 ± 0.2, indicating that the autoreduction is still dependent on the protonation of the ferryl oxo species. The k 2 also demonstrates both hydrogen peroxide and myoglobin dependency. At myoglobin concentrations greater than 6.6 μM, the k 2 is myoglobin-independent, but for lower concentrations, a pH-sensitive concentration dependence is seen.

Amelioration of Cadmium-Induced Oxidative Damage in Wistar Rats by Vitamin C, Zinc and N-Acetylcysteine

The present study was performed to determine the protective effects of vitamin C, zinc, and N-acetylcysteine, individually or in combination with Cd, to monitor their amelioration capability against Cd-induced oxidative damage in Wistar rats. We investigated and demonstrated that cadmium is a toxic element that damages rat liver and kidney tissues. Vitamin C, zinc, and NAC have been proven to have protective roles against Cd toxic effects. Nine groups of rats were studied as the experimental group. The present experiment was conducted for 45 days. Liver and kidneys were excised for biochemical evaluation by assaying antioxidant enzymes and lipid oxidation products to assess the impact of Cd toxicity and its amelioration by co-administration of vitamin C, zinc, and NAC along with Cd. Basal metabolic rates and tissue respiration rates of liver and kidney were significantly decreased (p < 0.05) during Cd toxicity. Serum biochemical parameters were also found to be significantly altered to cope with Cd toxicity. All the antioxidant enzymes and products were significant inhibited (p < 0.05) or elevated in rat liver and kidney tissues during Cd-induced toxicity. Our results suggest that co-administration of vitamin C, zinc, and NAC to rats ameliorates oxidative damage induced by Cd toxicity. From the results obtained in the present study, all the agents tested had protective effects against Cd-induced oxidative damage.

ortho-Substituent effect on the crystal packing and solid state speciation of aromaticC-nitroso compounds

The solid state behaviour of a short series aromaticC-nitroso compounds has been studied as a function of the electronic and steric nature of theortho-substituent on the ring.

Immuno-spin trapping of macromolecules free radicals in vitro and in vivo - One stop shopping for free radical detection

The only general technique that allows the unambiguous detection of free radicals is electron spin resonance (ESR). However, ESR spin trapping has severe limitations especially in biological systems. The greatest limitation of ESR is poor sensitivity relative to the low steady-state concentration of free radical adducts, which in cells and in vivo is much lower than the best sensitivity of ESR. Limitations of ESR have led to an almost desperate search for alternatives to investigate free radicals in biological systems. Here we explore the use of the immuno-spin trapping technique, which combine the specificity of the spin trapping to the high sensitivity and universal use of immunological techniques. All of the immunological techniques based on antibody binding have become available for free radical detection in a wide variety of biological systems.

Calcium, zinc and vitamin E ameliorate cadmium-induced renal oxidative damage in albino Wistar rats

This study was aimed to examine the protective effects of supplementation with calcium + zinc (Ca + Zn) or vitamin E (Vit-E) on Cd-induced renal oxidative damage. Young albino Wistar rats (180 ± 10 g) (n = 6) control rats, Cd, Cd + Ca + Zn, and Cd + Vit-E experimental groups and the experimental period was 30 days. Rats were exposed to Cd (20 mg/kg body weight) alone treated as Cd treated group and the absence or presence of Ca + Zn (2 mg/kg each) or Vit-E (20 mg/kg body weight) supplementation treated as two separate groups. The activities of the stress marker enzymes superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), glutathione peroxidase (GPx), glutathione-S-transferase (GST) and lipid peroxidase (LPx) were determined in renal mitochondrial fractions of experimental rats. We observed quantitative changes in SOD isoenzymatic patterns by non-denaturing PAGE analysis, and quantified band densities. These results showed that Cd exposure leads to decreases in SOD, CAT, GR, and GPx activities and a concomitant increase in LPx and GST activities. Ca + Zn and Vit-E administration with Cd significantly reversed Cd-induced perturbations in oxidative stress marker enzymes. However, Vit-E showed more inhibitory activity against Cd than did Ca + Zn, and it protected against Cd-induced nephrotoxicity.

Oxidation of thiamine on reaction with nitrogen dioxide generated by ferric myoglobin and hemoglobin in the presence of nitrite and hydrogen peroxide

It is shown that nitrogen dioxide oxidizes thiamine to thiamine disulfide, thiochrome, and oxodihydrothiochrome (ODTch). The latter is formed during oxidation of thiochrome by nitrogen dioxide. Nitrogen dioxide was produced by incubation of nitrite with horse ferric myoglobin and human hemoglobin in the presence of hydrogen peroxide. After addition of tyrosine or phenol to aqueous solutions containing oxoferryl forms of the hemoproteins, thiamine, and nitrite, the yield of thiochrome greatly increased, whereas the yield of ODTch decreased. In the presence of high concentrations of tyrosine or phenol compounds ODTch was not formed at all. The neutral form of thiamine with the closed thiazole cycle and minor tricyclic form of thiamine do not enter the heme pocket of the protein and do not interact with the oxoferryl heme complex Fe(IV=O) or porphyrin radical. The tricyclic form of thiamine is oxidized to thiochrome by tyrosyl radicals located on the surface of the hemoprotein. The thiol form of thiamine is oxidized to thiamine disulfide by both hemoprotein tyrosyl radicals and oxoferryl heme complexes. Nitrite and also tyrosine, tyramine, and phenol readily penetrate into the heme pocket of the protein and reduce the oxyferryl complex to ferric cation. These reactions yield nitrogen dioxide as well as tyrosyl and phenoxyl radicals of tyrosine molecules and phenol compounds, respectively. Tyrosyl and phenoxyl radicals of low molecular weight compounds oxidize thiamine only to thiochrome and thiamine disulfide. The effect of oxoferryl forms of myoglobin and hemoglobin, nitrogen dioxide, and phenol on thiamine oxidative transformation as well as antioxidant properties of the hydrophobic thiamine metabolites thiochrome and ODTch are discussed.

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".

Determination of separate inhibitor and substrate binding sites in the dehaloperoxidase-hemoglobin from Amphitrite ornata

Dehaloperoxidase-hemoglobin (DHP A) is a dual function protein found in the terrebellid polychaete Amphitrite ornata. A. ornata is an annelid, which inhabits estuary mudflats with other polychaetes that secrete a range of toxic brominated phenols. DHP A is capable of binding and oxidatively dehalogenating some of these compounds. DHP A possesses the ability to bind halophenols in a distinct, internal distal binding pocket. Since its discovery, the distal binding pocket has been reported as the sole binding location for halophenols; however, data herein suggest a distinction between inhibitor (monohalogenated phenol) and substrate (trihalogenated phenol) binding locations. Backbone (13)Calpha, (13)Cbeta, carbonyl (13)C, amide (1)H, and amide (15)N resonance assignments have been made, and various halophenols were titrated into the protein. (1)H-(15)N HSQC experiments were collected at stoichiometric intervals during each titration, and binding locations specific for mono- and trihalogenated phenols have been identified. Titration of monohalogenated phenol induced primary changes around the distal binding pocket, while introduction of trihalogenated phenols created alterations of the distal histidine and the local area surrounding W120, a structural region that corresponds to a possible dimer interface region recently observed in X-ray crystal structures of DHP A.

Dinitrosyl iron complexes bind with hemoglobin as markers of oxidative stress

Prooxidant and antioxidant properties of nitric oxide (NO) during oxidative stress are mostly dependent on its interaction with reactive oxygen species, Fe ions, and hemoproteins. One form of NO storage and transportation in cells and tissues is dinitrosyl iron complexes (DNIC), which can bind with both low-molecular-weight thiols and proteins, including hemoglobin. It was shown that dinitrosyl iron complexes bound with hemoglobin (Hb-DNIC) were formed in rabbit erythrocytes after bringing low-molecular-weight DNIC with thiosulfate into blood. It was ascertained that Hb-DNIC intercepted free radicals reacting with hemoglobin SH-groups and prevented oxidative modification of this protein caused by hydrogen peroxide. Destruction of Hb-DNIC can take place in the presence of both hydrogen peroxide and tert-butyl hydroperoxide. Hb-DNIC can also be destroyed at the enzymatic generation of superoxide-anion radical in the xanthine-xanthine oxidase system. If aeration in this system was absent, formation of the nitrosyl R-form of hemoglobin could be seen during the process of Hb-DNIC destruction. Study of Hb-DNIC interaction with reactive oxygen metabolites is important for understanding NO and Hb roles in pathological processes that could result from oxidative stress.

Protecting peroxidase activity of multilayer enzyme-polyion films using outer catalase layers

Films constructed layer-by-layer on electrodes with architecture {protein/hyaluronic acid (HA)}n containing myoglobin (Mb) or horseradish peroxidase (HRP) were protected against protein damage by H2O2 by using outer catalase layers. Peroxidase activity for substrate oxidation requires activation by H2O2, but {protein/HA}n films without outer catalase layers are damaged slowly and irreversibly by H2O2. The rate and extent of damage were decreased dramatically by adding outer catalase layers to decompose H2O2. Comparative studies suggest that protection results from catalase decomposing a fraction of the H2O2 as it enters the film, rather than by an in-film diffusion barrier. The outer catalase layers controlled the rate of H2O2 entry into inner regions of the film, and they biased the system to favor electrocatalytic peroxide reduction over enzyme damage. Catalase-protected {protein/HA}n films had an increased linear concentration range for H2O2 detection. This approach offers an effective way to protect biosensors from damage by H2O2.

Reduction of hypervalent states of myoglobin and hemoglobin to their ferrous forms by thymoquinone: the role of GSH, NADH and NADPH

The reactivity of thymoquinone towards different redox states of hemoglobin and myoglobin in the presence of GSH, NADH, and NADPH was evaluated by optical spectral analysis. Thymoquinone reduces the ferryl forms (HbIV/MbIV) of both met-hemoglobin (HbIII) and met-myoglobin (MbIII) to oxy-hemoglobin (HbIIO2) and oxy-myoglobin (MbIIO2) under physiological conditions. The reaction is mediated by the intermediate quinone forms of TQ, that is, glutathionyl-dihydrothymoquinone (DHTQ-GS) and dihydrothymoquinone (DHTQ), formed from direct interaction of TQ with GSH or NADH (NADPH). In vitro incubation of oxidized human erythrocytes with TQ, DHTQ, and the GSH/TQ mixture reduces the intracellular met-Hb at different rates. In the present study, we report that TQ and its reduced derivatives can also prevent lipid peroxidation induced by the MbFeIII/H2O2 system. In this system, lipid peroxidation is induced by MbIV or a putative MbIV/.MbVI composite; it is plausible that the antioxidant function of TQ derivatives is related to their ability to reduce these oxidizing species. This is of particular biological significance, as natural quinones may participate in reducing processes that lead to recovery of hemoglobin and myoglobin during oxidative stress.

Alterations in human red blood cell properties induced by 3-(dimethylamino)phenol (in vitro)

3-(Dimethylamino)phenol (3-DMAP) exists in the environment as a transformation product of ureic herbicides and may also be considered as a derivative of phenoxyherbicides. In this study, the activity of glutathione peroxidase, catalase and superoxide dismutase, as well as the level of free radicals and changes in cell morphology were measured in human erythrocytes exposed (in vitro) to 3-(dimethylamino)phenol. Human erythrocytes were incubated for 1 h in 3-DMAP at concentrations of 10-500 microg per 1 ml erythrocytes of 5% haematocrit. The results show that 3-(dimethylamino)phenol increased the level of free radicals and changed the activity of glutathione peroxidase, catalase, superoxide dismutase and acetylcholinesterase. It also changed cell morphology. All these results corroborated the thesis that 3-DMAP induces oxidative stress in cells. 3-DMAP changed the properties of the cell membrane, caused strong oxidation of haemoglobin, inhibited the levels of enzymatic and non-enzymatic antioxidants, which, in result, lead to generation of free radicals (ROS and semiquinones) that occurred in the exposed cells, predisposing them to oxidative damage.

Crystallographic and Spectroscopic Studies of Peroxide-derived Myoglobin Compound II and Occurrence of Protonated FeIV–O

High resolution crystal structures of myoglobin in the pH range 5.2-8.7 have been used as models for the peroxide-derived compound II intermediates in heme peroxidases and oxygenases. The observed Fe-O bond length (1.86-1.90 A) is consistent with that of a single bond. The compound II state of myoglobin in crystals was controlled by single-crystal microspectrophotometry before and after synchrotron data collection. We observe some radiation-induced changes in both compound II (resulting in intermediate H) and in the resting ferric state of myoglobin. These radiation-induced states are quite unstable, and compound II and ferric myoglobin are immediately regenerated through a short heating above the glass transition temperature (<1 s) of the crystals. It is unclear how this influences our compound II structures compared with the unaffected compound II, but some crystallographic data suggest that the influence on the Fe-O bond distance is minimal. Based on our crystallographic and spectroscopic data we suggest that for myoglobin the compound II intermediate consists of an Fe(IV)-O species with a single bond. The presence of Fe(IV) is indicated by a small isomer shift of delta = 0.07 mm/s from Mössbauer spectroscopy. Earlier quantum refinements (crystallographic refinement where the molecular-mechanics potential is replaced by a quantum chemical calculation) and density functional theory calculations suggest that this intermediate H species is protonated.

Mistletoe alkali inhibits peroxidation in rat liver and kidney

AIM: To explore the antioxidant and free radical scavenger properties of mistletoe alkali (MA).

METHODS: The antioxidant effect of mistletoe alkali on the oxidative stress induced by carbon tetrachloride (CCl₄) in rats was investigated. The rats were divided into four groups (n = 8): CCl₄-treated group (1 mL/kg body weight), MA -treated group (90 mg/kg), CCl₄+MA-treated group and normal control group. After 4 wk of treatment, the level of malondialdehyde (MDA), a lipid peroxidation product (LPO) was measured in serum and homogenates of liver and kidney. Also, the level of glutathione (GSH), and activities of glutathione reductase (GR), glutathione peroxidase (GSPx), superoxide dismutase (SOD), and glutathione-S-transferase (GST) in liver and kidney were determined. Scavenging effects on hydroxyl free radicals produced in vitro by Fenton reaction were studied by ESR methods using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap reagent and H₂O₂/UV as the OH· source. Urinary 8-hydroxydeoxyguanosine (8-OHdG) was determined by competitive ELISA.

RESULTS: In CCl₄-treated group, the level of LPO in serum of liver and kidney was significantly increased compared to controls. The levels of GSH and enzyme activities of SOD, GSPx and GR in liver and kidney were significantly decreased in comparison with controls. In CCl₄+MA-treated group, the changes in the levels of LPO in serum of liver and kidney were not statistically significant compared to controls. The levels of SOD, GSPx and GR in liver and kidney were significantly increased in comparison with controls. There was a significant difference in urinary excretion of 8-OHdG between the CCl₄-treated and MA-treated groups.

CONCLUSION: Oxidative stress may be a major mechanism for the toxicity of CCl₄. MA has a protective effect against CCl₄ toxicity by inhibiting the oxidative damage and stimulating GST activities. Thus, clinical application of MA should be considered in cases with carbon tetrachloride-induced injury.

New approach for the detection of peptide- and protein-based radicals using a pre-fluorescent probe

A novel application for pre-fluorescent probes in the detection of peptide- and protein-based radicals is proposed. Pre-fluorescent probes combine a fluorescent moiety labeled with a paramagnetic nitroxide that acts as a fluorescence quencher. Trapping of a radical by the nitroxide group restores the fluorescence properties. The increase in fluorescence intensity with time reflects the formation and quenching of free radicals and can be employed for the quantitative evaluation of yields and kinetics. In this test system, the pre-fluorescent probe 4-(9-acridinecarbonate)-2,2,6,6-tetramethylpiperidinyl-1-oxyl radical (Ac-Tempo), in which an acridine moiety was labeled with 2,2,6,6-tetramethylpiperidinyl-1-oxy (Tempo), was employed to probe peptide- and protein-based radicals. Peptide-based radicals were generated through the reaction between horseradish peroxidase (HRP)/H(2)O(2) and a derivative of tyrosine. Protein-based radicals were generated through the reaction between myoglobin (Mb) and H(2)O(2). In both cases the Ac-Tempo was found, using a combination of high-performance liquid chromatography (HPLC) and mass spectrometry, to be converted into fluorescent acridine (Ac)-piperidine (4-(9-acridinecarbonate)-2,2,6,6-tetramethylpiperidine).

The αPROX assay: fluorescence screening of the inhibitory effects of hydrophilic antioxidants on protein oxidation

We report on a new and convenient high-throughput fluorescence technique for determining antioxidant capacities of hydrophilic food samples. The new method is called alphaPROX (anti protein oxidation) and is based on an equimolar complex of diphenylhexatriene propionic acid (DPHPA) and bovine serum albumin (BSA) in aqueous buffer at pH 7.4. DPHPA is a reporter fluorophore that becomes nonfluorescent upon free radical-induced oxidation. In a typical assay, the DPHPA/BSA complex is challenged with peroxyl radicals and shows almost the same susceptibility to oxidation as unlabeled BSA. The progress of protein oxidation and its inhibition by antioxidants at physiological pH is determined from the time-dependent decrease in DPHPA fluorescence intensity. The alphaPROX method was compared to other techniques frequently used to measure antioxidant capacities. In this article, representative results are provided for the inhibitory effects of pure food components, fruit juices, wines, and various polar plant extracts on protein oxidation.

Easy Oxidation and Nitration of Human Myoglobin by Nitrite and Hydrogen Peroxide

The modification of human myoglobin (HMb) by reaction with nitrite and hydrogen peroxide has been investigated. This reaction is important because NO(2) (-) and H(2)O(2) are formed in vivo under conditions of oxidative and nitrative stress, where protein derivatization has been often observed. The abundance of HMb in tissues and in the heart makes it a potential source and target of reactive species generated in the body. The oxidant and nitrating species produced by HMb/H(2)O(2)/NO(2) (-) are nitrogen dioxide and peroxynitrite, which can react with exogenous substrates and endogenous protein residues. Tandem mass analysis of HMb modified by stoichiometric amounts of H(2)O(2) and NO(2) (-) indicated the presence of two endogenous derivatizations: oxidation of C110 to sulfinic acid (76 %) and nitration of Y103 to 3-nitrotyrosine (44 %). When higher concentrations of NO(2) (-) and H(2)O(2) were used, nitration of Y146 and of the heme were also observed. The two-dimensional gel-electrophoretic analysis of the modified HMbs showed spots more acidic than that of wild-type HMb, a result in agreement with the formation of sulfinic acid and nitrotyrosine residues. By contrast, the reaction showed no evidence for the formation of protein homodimers, as observed in the reaction of HMb with H(2)O(2) alone. Both HMb and the modified HMb are active in the H(2)O(2)/NO(2) (-)-dependent nitration of exogenous phenols. Their catalytic activity is quite similar and the endogenous modifications of HMb therefore have little effect on the reactivity of the protein intermediates.

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.

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.

Catalytic activity, stability, unfolding, and degradation pathways of engineered and reconstituted myoglobins

The structural and functional consequences of engineering a positively charged Lys residue and replacing the natural heme with a heme-L-His derivative in the active site of sperm whale myoglobin (Mb) have been investigated. The main structural change caused by the distal T67K mutation appears to be mobilization of the propionate-7 group. Reconstitution of wild-type and T67K Mb with heme-L-His relaxes the protein fragment around the heme because it involves the loss of the interaction of one of the propionate groups which stabilize heme binding to the protein. This modification increases the accessibility of exogenous ligands or substrates to the active site. The catalytic activity of the reconstituted proteins in peroxidase-type reactions is thus significantly increased, particularly with T67K Mb. The T67K mutation slightly reduces the thermodynamic stability and the chemical stability of Mb during catalysis, but somewhat more marked effects are observed by cofactor reconstitution. Hydrogen peroxide, in fact, induces pseudo-peroxidase activity but also promotes oxidative damage of the protein. The mechanism of protein degradation involves two pathways, which depend on the evolution of radical species generated on protein residues by the Mb active species and on the reactivity of phenoxy radicals produced during turnover. Both protein oligomers and heme-protein cross-links have been detected upon inactivation.