Formation of novel bioactive metabolites from the reactions of pro-inflammatory oxidants with polyphenolics

Dietary polyphenolics such as those in soy or red wine can have beneficial effects on the development of chronic human diseases. The mechanisms of action of isoflavones have been diverse and include their roles as weak estrogens, inhibitors of tyrosine kinase-dependent signal transduction processes and as antioxidants. Recent insights into the oxidative stress model of atherosclerosis suggest an interesting synthesis of these concepts. Sites of inflammation are associated with the formation of complex mixtures of reactive oxygen, nitrogen and halogenating species capable of modifying both endogenous biomolecules and polyphenolics. Of particular significance are the halogenation reactions mediated by myeloperoxidase that can modify key amino acids such as arginine and polyphenolics such as genistein. Hypochlorite, the reaction product of myeloperoxidase can halogenate polyphenolics to form stable derivatives with modified biological activity. Thus the in situ metabolism at sites of inflammation is unique and generates novel pharmacophores with potentially distinct modes of action from the parent compounds.

Reaction of S-nitrosoglutathione with the heme group of deoxyhemoglobin

The mechanism of interaction between S-nitrosoglutathione (GSNO) and hemoglobin is a crucial component of hypotheses concerning the role played by S-nitrosohemoglobin in vivo. We previously demonstrated (Patel, R. P., Hogg, N., Spencer, N. Y., Kalyanaraman, B., Matalon, S., and Darley-Usmar, V. M. (1999) J. Biol. Chem. 274, 15487-15492) that transnitrosation between oxygenated hemoglobin and GSNO is a slow, reversible process, and that the reaction between GSNO and deoxygenated hemoglobin (deoxyHb) did not conform to second order reversible kinetics. In this study we have reinvestigated this reaction and show that GSNO reacts with deoxyHb to form glutathione, nitric oxide, and ferric hemoglobin. Nitric oxide formed from this reaction is immediately autocaptured to form nitrosylated hemoglobin. GSNO reduction by deoxyHb is essentially irreversible. The kinetics of this reaction depended upon the conformation of the protein, with more rapid kinetics occurring in the high oxygen affinity state (i.e. modification of the Cysbeta-93) than in the low oxygen affinity state (i.e. treatment with inositol hexaphosphate). A more rapid reaction occurred when deoxymyoglobin was used, further supporting the observation that the kinetics of reduction are directly proportional to oxygen affinity. This observation provides a mechanism for how deoxygenation of hemoglobin/myoglobin could facilitate nitric oxide release from S-nitrosothiols and represents a potential physiological mechanism of S-nitrosothiol metabolism.

Mechanisms of the pro- and anti-oxidant actions of nitric oxide in atherosclerosis

The association of nitric oxide (NO) with cardiovascular disease has long been recognized and the extensive research on this topic has revealed both pro- and anti-atherosclerotic effects. While these contradictory findings were initially perplexing recent studies offer molecular mechanisms for the integration of these data in the context of our current understanding of the biochemistry of NO. The essential findings are that the biochemical properties of NO allow its exploitation as both a cell signaling molecule, through its interaction with redox centers in heme proteins, and an extremely rapid reaction with other biologically relevant free radicals. The direct reaction of NO with free radicals can have either pro- or antioxidant effects. In the cell, antioxidant properties of NO can be greatly amplified by the activation of signal transduction pathways that lead to the increased synthesis of endogenous antioxidants or down regulate responses to pro-inflammatory stimuli. These findings will be discussed in the context of atherosclerosis.

Cell signaling by reactive nitrogen and oxygen species in atherosclerosis

The production of reactive oxygen and nitrogen species has been implicated in atherosclerosis principally as means of damaging low-density lipoprotein that in turn initiates the accumulation of cholesterol in macrophages. The diversity of novel oxidative modifications to lipids and proteins recently identified in atherosclerotic lesions has revealed surprising complexity in the mechanisms of oxidative damage and their potential role in atherosclerosis. Oxidative or nitrosative stress does not completely consume intracellular antioxidants leading to cell death as previously thought. Rather, oxidative and nitrosative stress have a more subtle impact on the atherogenic process by modulating intracellular signaling pathways in vascular tissues to affect inflammatory cell adhesion, migration, proliferation, and differentiation. Furthermore, cellular responses can affect the production of nitric oxide, which in turn can strongly influence the nature of oxidative modifications occurring in atherosclerosis. The dynamic interactions between endogenous low concentrations of oxidants or reactive nitrogen species with intracellular signaling pathways may have a general role in processes affecting wound healing to apoptosis, which can provide novel insights into the pathogenesis of atherosclerosis.

Biochemical aspects of the reaction of hemoglobin and NO: implications for Hb-based blood substitutes

The role of Hemoglobin (Hb) on nitric oxide (NO) biology has received much attention. Until recently, the reaction between erythrocytic Hb and NO was generally considered in the context of mechanisms that safely detoxify NO. However, recent insights suggest that properties associated with the red blood cell limit NO-Hb interactions under physiological conditions, and provide some resolution to the question of how NO functions in the presence of blood. Furthermore, Hb-dependent mechanisms that preserve, not destroy NO bioactivity in vivo have also been proposed. The emerging picture suggests that the interplay between NO and erythrocytic Hb is important in regulating the functions of both these molecules in vivo. However, Hb-dependent scavenging and loss of NO function is significant when this heme protein is present outside the red blood cell. This can occur during hemolysis or administration of Hb-based blood substitutes. Scavenging of NO is a significant problem that limits the use of Hb-based blood substitutes in the clinic, and development of Hb molecules that do not efficiently react with NO remains an important area of investigation. In this article, the reactions between NO and erythrocytic Hb or cell-free Hb are described and the effects on NO and Hb function in vivo and development of blood substitutes discussed.

Evidence for peroxynitrite as a signaling molecule in flow-dependent activation of c-Jun NH(2)-terminal kinase

The c-Jun NH(2)-terminal kinase (JNK), also known as stress-activated protein kinase, is a mitogen-activated protein kinase that determines cell survival in response to environmental stress. Activation of JNK involves redox-sensitive mechanisms and physiological stimuli such as shear stress, the dragging force generated by blood flow over the endothelium. Laminar shear stress has antiatherogenic properties and controls structure and function of endothelial cells by mechanisms including production of nitric oxide (NO) and superoxide (O(-)(2)). Here we show that both NO and O(-)(2) are required for activation of JNK by shear stress in endothelial cells. The present study also demonstrates that exposure of endothelial cells to shear stress increases tyrosine nitration, a marker of reactive nitrogen species formation. Furthermore, inhibitors or scavengers of NO, O(-)(2), or reactive nitrogen species prevented shear-dependent increase in tyrosine nitration and activation of JNK. Peroxynitrite alone, added to cells as a bolus or generated over 60 min by 3-morpholinosydnonimine, also activates JNK. These results suggest that reactive nitrogen species, in this case most likely peroxynitrite, act as signaling molecules in the mechanoactivation of JNK.

Chlorination and nitration of soy isoflavones

Diets enriched in soy foods containing a high concentration of isoflavonoids are associated with a decrease in the incidence of several chronic inflammatory diseases. Studies with experimental models of diseases, such as atherosclerosis, suggest that these effects can be ascribed to the biological properties of the isoflavones. Since the isoflavones and tyrosine have structural similarities and modifications to tyrosine by inflammatory oxidants such as hypochlorous acid (HOCl) and peroxynitrite (ONOO(-)) have been recently recognized, we hypothesized that the isoflavones also react with HOCl and ONOO(-). Using an in vitro approach, we demonstrate in the present study that the isoflavones genistein, daidzein, and biochanin-A can be chlorinated and nitrated by these oxidants. These reactions were investigated using high-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance. In the reaction with HOCl, both mono- and dichlorinated derivatives of genistein and biochanin-A are formed, whereas with daidzein only a monochlorinated derivative was detected. The reaction between genistein or daidzein and ONOO(-) yielded a mononitrated product. However, no nitrated product was detected with biochanin-A. Furthermore, the reaction between genistein and sodium nitrite and HOCl yielded a chloronitrogenistein derivative, as well as a dichloronitrogenistein derivative. These results indicate that the ability of the isoflavones to react with these oxidant species depends on their structure and suggest that they could be formed under conditions where these reactive species are generated under pathological conditions.

Biochemical characterization of human S-nitrosohemoglobin. Effects on oxygen binding and transnitrosation

S-Nitrosation of cysteine beta93 in hemoglobin (S-nitrosohemoglobin (SNO-Hb)) occurs in vivo, and transnitrosation reactions of deoxygenated SNO-Hb are proposed as a mechanism leading to release of NO and control of blood flow. However, little is known of the oxygen binding properties of SNO-Hb or the effects of oxygen on transnitrosation between SNO-Hb and the dominant low molecular weight thiol in the red blood cell, GSH. These data are important as they would provide a biochemical framework to assess the physiological function of SNO-Hb. Our results demonstrate that SNO-Hb has a higher affinity for oxygen than native Hb. This implies that NO transfer from SNO-Hb in vivo would be limited to regions of extremely low oxygen tension if this were to occur from deoxygenated SNO-Hb. Furthermore, the kinetics of the transnitrosation reactions between GSH and SNO-Hb are relatively slow, making transfer of NO+ from SNO-Hb to GSH less likely as a mechanism to elicit vessel relaxation under conditions of low oxygen tension and over the circulatory lifetime of a given red blood cell. These data suggest that the reported oxygen-dependent promotion of S-nitrosation from SNO-Hb involves biochemical mechanisms that are not intrinsic to the Hb molecule.

Biological aspects of reactive nitrogen species

Nitric oxide (NO) plays an important role as a cell-signalling molecule, anti-infective agent and, as most recently recognised, an antioxidant. The metabolic fate of NO gives rise to a further series of compounds, collectively known as the reactive nitrogen species (RNS), which possess their own unique characteristics. In this review we discuss this emerging aspect of the NO field in the context of the formation of the RNS and what is known about their effects on biological systems. While much of the insight into the RNS has been gained from the extensive chemical characterisation of these species, to reveal biological consequences this approach must be complemented by direct measures of physiological function. Although we do not know the consequences of many of the dominant chemical reactions of RNS an intriguing aspect is now emerging. This review will illustrate how, when specificity and amplification through cell signalling mechanisms are taken into account, the less significant reactions, in terms of yield or rates, can explain many of the biological responses of exposure of cells or physiological systems to RNS.

The induction of GSH synthesis by nanomolar concentrations of NO in endothelial cells: a role for gamma-glutamylcysteine synthetase and gamma-glutamyl transpeptidase

Nitric oxide protects cells from oxidative stress through a number of direct scavenging reactions with free radicals but the effects of nitric oxide on the regulation of antioxidant enzymes are only now emerging. Using bovine aortic endothelial cells as a model, we show that nitric oxide, at physiological rates of production (1-3 nM/s), is capable of inducing the synthesis of glutathione through a mechanism involving gamma-glutamylcysteine synthetase and gamma-glutamyl transpeptidase. This novel nitric oxide signalling pathway is cGMP-independent and we hypothesize that it makes an important contribution to the anti-atherosclerotic and antioxidant properties of nitric oxide.

Molecular mechanisms of the copper dependent oxidation of low-density lipoprotein

There is little doubt that oxidative modification of low-density lipoprotein (LDL) is an important process during atherogenesis. This conclusion has been derived in a relatively short period of time since the initial descriptions of LDL oxidation with a significant contribution from Professor Esterbauer and colleagues. In this short overview, we have described the mechanisms by which copper promotes LDL oxidation focussing on the importance of lipid hydroperoxides in this process. These mechanisms are discussed in the context of the ongoing debate as to relevance of metal dependent LDL oxidation in vivo and as a model reaction for assessing antioxidants.

A causative role for redox cycling of myoglobin and its inhibition by alkalinization in the pathogenesis and treatment of rhabdomyolysis-induced renal failure

Muscle injury (rhabdomyolysis) and subsequent deposition of myoglobin in the kidney causes renal vasoconstriction and renal failure. We tested the hypothesis that myoglobin induces oxidant injury to the kidney and the formation of F2-isoprostanes, potent renal vasoconstrictors formed during lipid peroxidation. In low density lipoprotein (LDL), myoglobin induced a 30-fold increase in the formation of F2-isoprostanes by a mechanism involving redox cycling between ferric and ferryl forms of myoglobin. In an animal model of rhabdomyolysis, urinary excretion of F2-isoprostanes increased by 7.3-fold compared with controls. Administration of alkali, a treatment for rhabdomyolysis, improved renal function and significantly reduced the urinary excretion of F2-isoprostanes by approximately 80%. EPR and UV spectroscopy demonstrated that myoglobin was deposited in the kidneys as the redox competent ferric myoglobin and that it's concentration was not decreased by alkalinization. Kinetic studies demonstrated that the reactivity of ferryl myoglobin, which is responsible for inducing lipid peroxidation, is markedly attenuated at alkaline pH. This was further supported by demonstrating that myoglobin-induced oxidation of LDL was inhibited at alkaline pH. These data strongly support a causative role for oxidative injury in the renal failure of rhabdomyolysis and suggest that the protective effect of alkalinization may be attributed to inhibition of myoglobin-induced lipid peroxidation.

Nitric oxide-dependent induction of glutathione synthesis through increased expression of gamma-glutamylcysteine synthetase

The nitric oxide (NO) donors S-nitrosopenicillamine or DetaNONOate, which release NO at a rate of 0-15 nM sec-1, were exposed to rat aortic vascular smooth muscle cells for a period of 0-24 h. This treatment resulted in an increase in total glutathione levels of two- to threefold under conditions where no cytotoxicity was detected. The signaling pathways do not involve activation of protein kinase G Ialpha nor are they cGMP dependent. Oxidation of reduced glutathione (GSH) was found after exposure to NO for 3-4 h at rates of formation at or above 8 nM sec-1. Increased intracellular GSH was due to enhanced expression of the rate-limiting enzyme for GSH synthesis, gamma-glutamylcysteine synthetase. Since NO has been shown previously to protect cells against oxidative stress, we propose that the increase in GSH by NO is a potential mechanism for enhancing the antioxidant defenses of the cell. This result also has important implications for identifying redox-sensitive cell signaling pathways that can be activated by NO.

Pathophysiology of nitric oxide and related species: free radical reactions and modification of biomolecules

Since its initial discovery as an endogenously produced bioactive mediator, nitric oxide (.NO) has been found to play a critical role in the cellular function of nearly all organ systems. Furthermore, aberrant production of .NO or reactive nitrogen species (RNS) derived from .NO, has been implicated in a number of pathological conditions, such as acute lung disease, atherosclerosis and septic shock. While .NO itself is fairly non-toxic, secondary RNS are oxidants and nitrating agents that can modify both the structure and function of numerous biomolecules both in vitro, and in vivo. The mechanisms by which RNS mediate toxicity are largely dictated by its unique reactivity. The study of how reactive nitrogen species (RNS) derived from .NO interact with biomolecules such as proteins, carbohydrates and lipids, to modify both their structure and function is an area of active research, which is lending major new insights into the mechanisms underlying their pathophysiological role in human disease. In the context of .NO-dependent pathophysiology, these biochemical reactions will play a major role since they: (i) lead to removal of .NO and decreased efficiency of .NO as an endothelial-derived relaxation factor (e.g. in hypertension, atherosclerosis) and (ii) lead to production of other intermediate species and covalently modified biomolecules that cause injury and cellular dysfunction during inflammation. Although the physical and chemical properties of .NO and .NO-derived RNS are well characterised, extrapolating this fundamental knowledge to a complicated biological environment is a current challenge for researchers in the field of .NO and free radical research. In this review, we describe the impact of .NO and .NO-derived RNS on biological processes primarily from a biochemical standpoint. In this way, it is our intention to outline the most pertinent and relevant reactions of RNS, as they apply to a diverse array of pathophysiological states. Since reactions of RNS in vivo are likely to be vast and complex, our aim in this review is threefold: (i) address the major sources and reactions of .NO-derived RNS in biological systems, (ii) describe current knowledge regarding the functional consequences underlying .NO-dependent covalent modification of specific biomolecules, and (iii) to summarise and critically evaluate the available evidence implicating these reactions in human pathology. To this end, three areas of special interest have been chosen for detailed description, namely, formation and role of S-nitrosothiols, modulation of lipid oxidation/nitration by RNS, and tyrosine nitration mechanisms and consequences.

Formation of the NO donors glyceryl mononitrate and glyceryl mononitrite from the reaction of peroxynitrite with glycerol

Peroxynitrite (ONOO-), formed from the rapid reaction of superoxide (O2-.) with NO, is known to generate stable compounds capable of donating NO on reaction with thiols and molecules containing hydroxy groups. Using glycerol as a model compound for the reactions of ONOO- with biomolecules containing hydroxy groups, we separated the products and identified them by HPLC/MS. It was shown that both glyceryl mononitrate and glyceryl mononitrite were formed and released NO on incubation with copper and l-cysteine. The compounds were stable over a period of 4h when shielded from light and kept on ice. Slow spontaneous decomposition occurred in the buffer used for the bioassay, but this was not sufficient to explain the vasorelaxing properties of these NO donors. It is concluded that the stable organic nitrate and nitrite have the capacity to be metabolized by vascular tissues, resulting in vasorelaxation.

The interplay of nitric oxide and peroxynitrite with signal transduction pathways: implications for disease

Since the discovery that at least one form of endothelium derived relaxing factor is nitric oxide (NO), numerous studies have uncovered diverse roles for this free radical in a variety of physiological and pathophysiological processes. NO production, a process mediated by a family of enzymes termed NO synthases, has been detected in most cell types. Many of the effects of NO are thought to be mediated through its direct interaction with specific and defined cell signaling pathways. The nature of such interactions are highly dependent on the concentration of NO and cell type. Furthermore, specific NO derived reaction products, such as peroxynitrite, also have the potential to effect cell signal transduction events. As with NO, this can occur through diverse mechanisms and depends on concentration and cell type. It is perhaps not surprising that the reported effects of NO in different disease states are often conflicting. In this brief overview, a framework for placing these apparently disparate properties of NO will be described and will focus on the effects of NO and peroxynitrite on signaling pathways.

The globin-based free radical of ferryl hemoglobin is detected in normal human blood

Normal human venous blood was studied by electron paramagnetic resonance (EPR) spectroscopy at -196 degrees C. The EPR signal of free radicals in frozen blood is shown to have the same radiospectroscopic parameters and properties as the signal of the globin based free radical, .Hb(Fe(IV)=O), formed in the reaction of purified methemoglobin (metHb) with H2O2 and therefore has been assigned as such. The globin-based radicals and metHb exhibited significant variation (fluctuations) in different frozen samples taken from the same liquid blood sample. In any given sample a high concentration of free radicals was associated with a low concentration of metHb and vice versa, i.e. the fluctuations were always of opposite sense. No such fluctuations were observed in the concentration of two other paramagnetic components of blood, transferrin and ceruloplasmin. The time course of free radical formation and decay upon the addition of H2O2 to purified metHb was studied at three different molar ratios H2O2/metHb. This kinetic study together with the results of an annealing experiment allow us to propose a mechanism for the formation and decay of the globin-based radical in blood. Within this mechanism, the source of H2O2 in blood is considered to be dismutation of O-2 radicals produced via autoxidation of Hb. We postulate that the dismutation is intensified on the phase separation surfaces during cooling and freezing of a blood sample. The fluctuations are explained within this hypothesis.

Reduction of Cu(II) by lipid hydroperoxides: implications for the copper-dependent oxidation of low-density lipoprotein

The Cu(II)-promoted oxidation of lipids is a lipid hydroperoxide (LOOH)-dependent process that has been used routinely to assess the oxidizability of low-density lipoprotein (LDL) in human subjects. Metal-dependent redox reactions, including those mediated by copper, have been implicated in the pathogenesis ofatherosclerosis. Despite its widespread use and possible biological significance, key elements of the mechanism are not clear. For example, although it is evident that copper acts as a catalyst, which implies a redox cycle between the Cu(II) and Cu(I) redox states, the reductants remain uncertain. In LDL these could include alpha-tocopherol, amino acid residues on the protein and LOOH. However, both alpha-tocopherol and amino acid residues are probably consumed before the most rapid phase of lipid peroxidation occurs, suggesting that another reductant must be donating electrons to Cu(II), the most likely candidate being LOOH. This role has been disputed, since LDLs nominally devoid of LOOH are still capable of reducing Cu(II) to Cu(I) and thermodynamic calculations for this reaction are not favourable. Direct investigation of the role of LOOH as reductant has not been reported and in the present study, using simple lipid systems and LDL, we have re-examined this issue using the Cu(I) chelator bathocuproine. We have shown that Cu(II) may promote lipid peroxidation in liposomes, which do not contain either protein or alpha-tocopherol, and that this is associated with reduction to Cu(I). The data also indicate that an equilibrium between free Cu(II) and LOOH exists, which only in the presence of an oxidizable substrate, i.e. unsaturated fatty acids, is shifted towards formation of Cu(I) and lipid-derived peroxyl radicals. We propose that reduction of Cu(II) by LOOH is a necessary component in sustaining the propagation of lipid peroxidation and that the formation of peroxyl radicals and their products in a lipid environment is sufficient to overcome thermodynamic barriers to the reaction.

Formation of oxysterols during oxidation of low density lipoprotein by peroxynitrite, myoglobin, and copper

Oxidation of low density lipoprotein (LDL) in the artery wall leads to the formation of cholesterol oxidation products that may result in cytotoxicity. Different mechanisms could contribute to LDL oxidation in vivo resulting in characteristic and specific modification of the cholesterol molecule. Alternatively, attack on cholesterol by chain propagating peroxyl radicals could result in the same distribution of oxidation products irrespective of the initial pro-oxidant mechanism. To distinguish between these possibilities we have monitored the formation of nine oxysterols during LDL oxidation, promoted by copper, myoglobin, peroxynitrite, or azo bis amidino propane. Regardless of the oxidant used, the pattern of oxysterol formation was essentially the same. The yields of products identified decreased in the order 7-oxocholesterol > 7 beta-hydroxycholesterol > 7 alpha-hydroxycholesterol > 5,6 beta-epoxycholesterol > 5,6 alpha-epoxycholesterol except in the case of peroxynitrite in which case a higher yield of 5, 6 beta-epoxycholesterol relative to 7-oxocholesterol was found. No formation of cholestane 3 beta, 5 alpha, 6 beta-triol, or the 24-,25-,27-hydroxycholesterols was seen. Concentration of 7-oxocholesterol levels in LDL was positively correlated with the degree of protein modification. Endogenous alpha-tocopherol in LDL or supplementation with butylated hydroxytoluene prevented oxysterol formation. Taken together these data indicate that the oxidation of cholesterol and protein in LDL occur as secondary oxidation events consequent on the attack of fatty acid peroxyl/alkoxyl radicals on the 7-position of cholesterol, and with amino acids on apoB. Furthermore, oxidant processes with atherogenic potential, such as peroxynitrite, copper, and myoglobin are capable of producing oxidized LDL containing cytotoxic mediators.

Redox cycling of human methaemoglobin by H2O2 yields persistent ferryl iron and protein based radicals

The formation and reactivity of ferryl haemoglobin (and myoglobin), which occurs on addition of H2O2, has been proposed as a mechanism contributing to oxidative stress associated with human diseases. However, relatively little is known of the reaction between hydrogen peroxide and human haemoglobin. We have studied the reaction between hydrogen peroxide and purified (catalase free) human metHbA. Addition of H2O2 resulted in production of both ferryl haem iron (detected by optical spectroscopy) and an associated protein radical (detected by EPR spectroscopy). Titrating metHbA with H2O2 showed that maximum ferryl levels could be obtained at a 1:1 stoichiometric ratio of haem to H2O2. No oxygen was evolved during the reaction, indicating that human metHbA does itself not possess catalytic activity. The protein radicals obtained in this reaction reached a steady state concentration, during hydrogen peroxide decomposition, but started to decay once the hydrogen peroxide had been completely exhausted. The presence of catalase, at concentrations around 10(4) fold lower than metHb, increased the apparent stoichiometry of the reaction to 1 mol metHb: approximately 20 mol H2O2 and abolished the protein radical steady state. The biological implications for these results are discussed.

An EPR investigation of human methaemoglobin oxidation by hydrogen peroxide: methods to quantify all paramagnetic species observed in the reaction

The method of Electron Paramagnetic Resonance (EPR) spectroscopy was used to study the reaction of human methaemoglobin (metHb) with hydrogen peroxide. The samples for EPR measurements were rapidly frozen in liquid nitrogen at different times after H2O2 was added at 3- and 10-fold molar excess to 100 microM metHb in 50 mM phosphate buffer, pH 7.4, 37 degrees C. Precautions were taken to remove all catalase from the haemoglobin preparation and no molecular oxygen evolution was detected during the reaction. On addition of H2O2 the EPR signals (-196 degrees C) of both high spin and low spin metHb rapidly decreased and free radicals were formed. The low temperature (-196 degrees C) EPR spectrum of the free radicals formed in the reaction has been deconvoluted into two individual EPR signals, one being an anisotropic signal (g parallel = 2.035 and g perpendicular = 2.0053), and the other an isotropic singlet (g = 2.0042, delta H = 20 G). The former signal was assigned to peroxyl radicals. As the kinetic behaviour of both peroxyl (ROO.) and non-peroxyl (P.) free radicals were similar, we concluded that ROO. radicals are not formed from P. radicals by addition of O2. The time courses for both radicals showed a steady state during the time required for H2O2 to decompose. Once all peroxide was consumed, the radical decayed with a first order rate constant of 1.42 x 10(-3) s-1 (1:3 molar ratio). The level of the steady state was higher and its duration shorter at lower initial concentration of H2O2. The formation of the rhombic Fe(III) non-haem centres with g = 4.35 was found. Their yield was proportional to the H2O2 concentration used and the centres were ascribed to haem degradation products. The reaction was also monitored by EPR spectroscopy at room temperature. The kinetics of the free radicals measured in the reaction mixture at room temperature was similar to that observed when the fast freezing method and EPR measurement at -196 degrees C were used.