Direct ESR detection or peroxynitrite-induced tyrosine-centred protein radicals in human blood plasma

Antioxidant peptides, the guardian of life from oxidative stress

Reactive oxygen species (ROS) are produced during oxidative metabolism in aerobic organisms. Under normal conditions, ROS production and elimination are in a relatively balanced state. However, under internal or external environmental stress, such as high glucose levels or UV radiation, ROS production can increase significantly, leading to oxidative stress. Excess ROS production not only damages biomolecules but is also closely associated with the pathogenesis of many diseases, such as skin photoaging, diabetes, and cancer. Antioxidant peptides (AOPs) are naturally occurring or artificially designed peptides that can reduce the levels of ROS and other pro-oxidants, thus showing great potential in the treatment of oxidative stress-related diseases. In this review, we discussed ROS production and its role in inducing oxidative stress-related diseases in humans. Additionally, we discussed the sources, mechanism of action, and evaluation methods of AOPs and provided directions for future studies on AOPs.

Thiyl radicals and induction of protein degradation

Thiyl radicals are important intermediates in the redox biology and chemistry of thiols. These radicals can react via hydrogen transfer with various C-H bonds in peptides and proteins, leading to the generation of carbon-centered radicals, and, potentially, to irreversible protein damage. This review summarizes quantitative information on reaction kinetics and product formation, and discusses the significance of these reactions for protein degradation induced by thiyl radical formation.

Protein thiyl radical reactions and product formation: a kinetic simulation

Protein thiyl radicals are important intermediates generated in redox processes of thiols and disulfides. Thiyl radicals efficiently react with glutathione and ascorbate, and the common notion is that these reactions serve to eliminate thiyl radicals before they can enter potentially hazardous processes. However, over the past years increasing evidence has been provided for rather efficient intramolecular hydrogen transfer processes of thiyl radicals in proteins and peptides. Based on rate constants published for these processes, we have performed kinetic simulations of protein thiyl radical reactivity. Our simulations suggest that protein thiyl radicals enter intramolecular hydrogen transfer reactions to a significant extent even under physiologic conditions, i.e., in the presence of 30 µM oxygen, 1 mM ascorbate, and 10 mM glutathione. At lower concentrations of ascorbate and glutathione, frequently observed when tissue is exposed to oxidative stress, the extent of irreversible protein thiyl radical-dependent protein modification increases.

Cobalt phthalocyanine tetracarboxylic acid modified reduced graphene oxide: a sensitive matrix for the electrocatalytic detection of peroxynitrite and hydrogen peroxide

The electrocatalytic properties of cobalt phthalocyanine modified reduced graphene oxide for peroxynitrite and hydrogen peroxide are investigated.

Sequence conservation of apolipoprotein A-I affords novel insights into HDL structure-function

We performed alignment of apolipoprotein A-I (apoA-I) sequences from 31 species of animals. We found there is specific conservation of salt bridge-forming residues in the first 30 residues of apoA-I and general conservation of a variety of residue types in the central domain, helix 2/3 to helix 7/8. In the lipid-associating domain, helix 7 and helix 10 are the most and least conserved helixes, respectively. Furthermore, eight residues are completely conserved: P66, R83, P121, E191, and P220, and three of seven Tyr residues in human apoA-I, Y18, Y115, and Y192, are conserved. Residue Y18 appears to be important for assembly of HDL. E191-Y192 represents the only completely conserved pair of adjacent residues in apoA-I; Y192 is a preferred target for site-specific oxidative modification within atheroma, and molecular dynamic simulations suggest that the conserved pair E191-Y192 is in a solvent-exposed loop-helix-loop. Molecular dynamics testing of human apoA-I showed that M112 and M148 interact with Y115, a microenvironment unique to human apoA-I. Finally, conservation of Arg residues in the α11/3 helical wheel position 7 supports several possibilities: interactions with adjacent phospholipid molecules and/or oxidized lipids and/or binding of antioxidant enzymes through cation-π orbital interactions. We conclude that sequence alignment of apoA-I provides unique insights into apoA-I structure-function relationship.

Peroxynitrite-An ugly biofactor?

Cellular damage occurring under oxidative conditions has been ascribed mainly to the formation of peroxynitrite (ONOOH/ONOO(-)) that originates from the reaction of NO(*) with O(2) (*-). The detrimental effects of peroxynitrite are exacerbated by the reaction with CO(2) that leads to ONOOC(O)O(-), which further decays to the strong oxidant radicals NO(2) (*) and CO(3) (*-). The reaction with CO(2), however, may redirect peroxynitrite specificity. An excessive formation of peroxynitrite represents an important mechanism contributing to the DNA damage, the inactivation of metabolic enzymes, ionic pumps, and structural proteins, and the disruption of cell membranes. Because of its ability to oxidize biomolecules, peroxynitrite is implicated in an increasing list of diseases, including neurodegenerative and cardiovascular disorders, inflammation, pain, autoimmunity, cancer, and aging. However, peroxynitrite displays also protective activities: (i) at high concentrations, it shows anti-viral, anti-microbial, and anti-parasitic actions; and (ii) at low concentrations, it stimulates protective mechanisms in the cardiovascular, nervous, and respiratory systems. The detrimental effects of peroxynitrite and related reactive species are impaired by (pseudo-) enzymatic systems, mainly represented by heme-proteins (e.g., hemoglobin and myoglobin). Here, we report biochemical aspects of peroxynitrite actions being at the root of its biomedical effects.

Peroxynitrite and nitrosoperoxycarbonate, a tightly connected oxidizing-nitrating couple in the reactive nitrogen-oxygen species family: new perspectives for protection from radical-promoted injury by flavonoids

Peroxynitrite is the product of the reaction of nitric oxide with superoxide radical and is implicated in the pathogenesis of a wide variety of human diseases, being responsible for in-vivo oxidation/nitration events. Nitrosoperoxycarbonate anion, formed by the interaction of peroxynitrite with CO2/bicarbonate at physiological concentrations, provides a new interpretation of oxidative/nitrative processes formerly attributed to peroxynitrite. The aim of this review is to summarize the chemistry and biology of peroxynitrite and radical species related to nitrosoperoxycarbonate anion, as well as the information available regarding the molecular mechanisms that determine and regulate radical-promoted injury by the two tightly connected species at physiological concentrations. Interception of carbonate and nitro radicals produced by interaction of peroxynitrite with CO2/bicarbonate, as in-vivo prevention of pathological events, creates new perspectives for the evaluation of safe scavengers of oxidative/nitrative stress at the physiological level. In this respect, natural products such as flavonoids hold a preeminent position among the vast array of compounds endowed with such properties.

Two different pathways are involved in peroxynitrite-induced senescence and apoptosis of human erythrocytes

CO(2) changes the biochemistry of peroxynitrite basically in two ways: (i) nitrating species is the CO(3)(-) / ()NO(2) radical pair, and (ii) peroxynitrite diffusion distance is significantly reduced. For peroxynitrite generated extracellularly this last effect is particularly dramatic at low cell density because CO(3)(-) and ()NO(2) are short-lived and decay mostly in the extracellular space or at the cell surface/membrane. This study was aimed to distinguish between peroxynitrite-induced extra- and intracellular modifications of red blood cells (RBC). Our results show that at low cell density and in the presence of CO(2) peroxynitrite induced the oxidation of surface thiols, the formation of 3-nitrotyrosine and DMPO-RBC adducts, and the down-regulation of glycophorins A and C (biomarkers of senescence). Reactivation of glycolysis reversed only the oxidation of surface thiols. Without CO(2) peroxynitrite also induced the oxidation of hemoglobin and glutathione, the accumulation of lactate, a decrease in ATP, the clustering of band 3, the externalization of phosphatidylserine, and the activation of caspases 8 and 3 (biomarkers of apoptosis). The latter biomarkers were all reversed by reactivation of glycolysis. We hypothesize that cell senescence could (generally) be derived by irreversible radical-mediated oxidation of membrane targets, while the appearance of apoptotic biomarkers could be bolstered by oxidation of intracellular targets. These results suggest that, depending on extracellular homolysis or diffusion to the intracellular space, peroxynitrite prompts RBCs toward either senescence or apoptosis through different oxidation mechanisms.

CO2 impairs peroxynitrite-mediated inhibition of human caspase-3

Peroxynitrite (ONOO-) is a transient powerful oxidant produced in vivo as the reaction of nitrogen monoxide (.NO) with superoxide (O2.-). The peroxynitrite reactivity is modulated by carbon dioxide (CO2) which enhances the peroxynitrite-mediated nitration of aromatics and partially impairs the oxidation of thiols. Here, the effect of CO2 on the peroxynitrite-mediated inhibition of human caspase-3, the execution enzyme of the apoptotic cascade, is reported. Peroxynitrite inhibits the catalytic activity of human caspase-3 by oxidizing the Sgamma atom of the Cys catalytic residue. In the absence of CO2, 1.0 equivalent of peroxynitrite inactivates 1.0 equivalent of human caspase-3. In the presence of the physiological concentration of CO2 (=1.3x10(-3) M), 1.0 equivalent of peroxynitrite inactivates only 0.38 equivalents of human caspase-3. Peroxynitrite affects the kcat value of the human caspase-3 catalyzed hydrolysis of N-acetyl-Asp-Glu-Val-Asp-7-amido-4-methylcoumarin, without altering Km. Both in the absence and presence of CO2, the reducing agent dithiothreitol does not prevent human caspase-3 inhibition by peroxynitrite and does not reverse the peroxynitrite-induced inactivation of human caspase-3. These results represent the first evidence for modulation of peroxynitrite-mediated inhibition of cysteine proteinase action by CO2, supporting the role of CO2 in fine tuning of cell processes (e.g., apoptosis).

ESR spin trapping of a carbon-centered free radical from agonist-stimulated human platelets

Several free radical intermediates formed during synthesis of prostaglandin H synthase (PGHS) catalyze the biosynthesis of prostaglandins from arachidonic acid (AA). We attempted to directly detect free radical intermediates of PGHS in cells. Studies were carried out using human platelets, which possess significant PGHS activity. Electron spin resonance (ESR) spectra showed a g = 2.005 signal radical, which was formed by the incubation of collagen, thrombin, AA, and a variety of peroxides with human platelets. The ESR spectra obtained using 5,5-dimethyl-1 pyrroline N-oxide (DMPO) and alpha-phenyl N-tert.-butylnitron (PBN) were typical of an immobilized nitroxide. Extensive Pronase digestion of both the DMPO and PBN adducts allowed us to deduce that it was a carbon-centered radical. The formation of this radical was inhibited by potassium cyanide and by desferroxamine. Peroxides stimulated formation of the g = 2.005 signal radical and inhibited platelet aggregation induced by AA. PGHS cosubstrates increased the intensity of the radical signal but inhibited platelet aggregation induced by AA. Both S-nitro-L-glutathione and reduced glutathione quenched the g = 2.005 radical but could not restore platelet aggregatory activity. These results suggest that the carbon-centered radical is a self-destructing free radical formed during peroxide-mediated deactivation of PGHS in human platelets.

Phenolic antioxidants and the protection of low density lipoprotein from peroxynitrite-mediated oxidations at physiologic CO2

Dietary phenolic antioxidants have been shown to prevent LDL modifications mediated by several physiologic oxidants including peroxynitrite. However, more recent data demonstrated that CO(2) affected the fate of peroxynitrite in biological fluids and significantly reduced peroxynitrite scavenging by polyphenols, raising doubts concerning their antioxidant activity. We found that the oxidation of LDL lipids mediated by peroxynitrite decreased in the presence of bicarbonate, while Trp oxidation and 3-nitroTyr formation increased, suggesting a redirection of peroxynitrite reactivity toward the protein moiety. We therefore evaluated the protective activity of some phenolic antioxidants (quercetin, oleuropein, resveratrol, (+)-catechin, (-)-epicatechin, tyrosol, alpha- and gamma-tocopherol, ascorbate) on peroxynitrite-mediated oxidation of LDL aromatic residues. Some of these phenols protected LDL Trp from oxidation better than ascorbate or alpha-tocopherol, although protection at 100 microM did not exceed 30-40%. However, the same phenolic antioxidants were more active in inhibiting 3-nitroTyr formation and those with a catechin structure provided significant protection (IC(50%) 40-50 microM). Red wine, a polyphenol-rich beverage, showed a protective effect comparable to that of the most active phenolic antioxidants. Direct EPR studies showed that bicarbonate significantly increased the peroxynitrite-dependent formation of O-semiquinone radicals in red wine, supporting the hypothesis that polyphenols are efficient scavengers of radicals formed by peroxynitrite/CO(2). Ascorbate was a poor inhibitor of peroxynitrite/CO(2)-induced LDL tyrosine nitration, but the simultaneous addition to the most active polyphenols halved their IC(50%). In conclusion, although cooperation with other antioxidants can further decrease the IC(50%) of polyphenolics, as demonstrated for ascorbate, their antioxidant activity appears to occur at concentrations at least 1 order of magnitude higher than their bioavailability.

Peroxynitrite-dependent modifications of tyrosine residues in hemoglobin. Formation of tyrosyl radical(s) and 3-nitrotyrosine

Although peroxynitrite is believed to be one of the most efficient tyrosine-nitrating species of biological relevance so far identified, its nitration efficiency is nevertheless limited. In fact, the nitrating species formed through peroxynitrite decay are caged radicals ((*)OH/(*)NO(2) or, in the presence of carbon dioxide, CO(3)(*-)/(*)NO(2)) and the fraction that escapes from the solvent cage does not exceed 30-35%. One exception may be represented by metal-containing compounds that can enhance the formation of nitrotyrosine through a bimolecular reaction with peroxynitrite. Moreover, if the metal is also regenerated in the reaction, the compound is considered a nitration catalysts and the yield of tyrosine nitration enhanced several fold. Examples of peroxynitrite-dependent nitration catalysts are the Mn-superoxide dismutase, some cytochromes and several metalloporphyrins. On the contrary, it has been claimed that some hemoproteins are scavengers of peroxynitrite and play a role in limiting its biodamaging and bioregulatory activity. In this review, we discuss the case of hemoglobin, which is probably the major target of peroxynitrite in blood. This protein has been reported to protect intracellular and extracellular targets from peroxynitrite-mediated tyrosine nitration. This property is shared with myoglobin and cytochrome c. The possible mechanisms conferring to these proteins a peroxynitrite scavenging role are discussed.

The functions of nitric oxide-mediated signaling and changes in gene expression during the hypersensitive response

Nitric oxide (NO) is a highly reactive molecule that rapidly diffuses and permeates cell membranes. In animals, NO is implicated in a number of diverse physiological processes, such as neurotransmission, vascular smooth muscle relaxation, and platelet inhibition. It may have beneficial effects, e.g., as a messenger in immune responses, but it is also potentially toxic when the antioxidant system is overwhelmed and reactive oxygen intermediates (ROI) accumulate. During the last few years, NO has been detected in several plant species, and an increasing number of reports on its function have implicated NO as an important effector in plant growth, development, and defense. The broad chemistry of NO involves an array of interrelated redox forms with different chemical reactivities and numerous potential biological targets in plants. NO signaling functions depend on its reactivity. ROI are key modulators of NO in triggering cell death, but the nature of the mechanisms by which this occurs in plants is different from those commonly observed in animals. This review focuses on the signaling functions of NO, when channeled through the cell death pathway by ROI.

Peroxynitrite activates kinases of the src family and upregulates tyrosine phosphorylation signaling

The hypothesis that peroxynitrite may act as a signaling molecule able to upregulate protein tyrosine phosphorylation is discussed. This article focuses on the mechanisms for activating kinases of the src family, an important class of nonreceptor tyrosine kinases implicated in the regulation of cell communication, proliferation, migration, differentiation, and survival. Recent in vitro findings show that in erythrocytes, synaptosomes, and cerebellar primary culture cells peroxynitrite is able to inhibit phosphatases and to activate different members of the src kinase family through different mechanisms involving cysteine-dependent and -independent processes. The ability of nitrotyrosine-containing peptides with SH2 binding affinity to activate src kinases is also discussed.

On-line EPR study of free radicals induced by peroxidase/H(2)O(2) in human low-density lipoprotein

The aim of this study was to use direct electron paramagnetic resonance (EPR) spectroscopy at 37 degrees C and spin trapping techniques to study radical species formed during horseradish peroxidase/H(2)O(2)-initiated low-density lipoprotein (LDL) oxidation. Using direct EPR, we obtained evidence for the formation not only of the alpha-tocopheroxyl radical but also of a protein radical(s), assigned to a tyrosyl radical(s) of apolipoprotein B-100 (apo B-100). Spin trapping with 2-methyl-2-nitrosopropane revealed (i) the formation of a mobile adduct with beta-hydrogen coupling assigned to a lipid radical and (ii) a partially immobilised adduct detected in LDL as well as in apo B-100, assigned after proteolytic digestion to the trapping of a radical centred on a tertiary carbon atom of an aromatic residue, probably tyrosine. Our results support the hypothesis that radicals are initiators of the oxidative process, and show that their formation is an early event in peroxidase-mediated oxidation. We also tested the effects of resveratrol (RSV), a polyphenolic antioxidant present in red wine. Our data indicate that 1-10 microM RSV is able to accelerate alpha-tocopherol consumption, conjugated dienes formation and the decay kinetics of LDL-centred radicals. Since phenols are substrates for peroxidases, this result may be ascribed to a RSV-mediated catalysis of peroxidase activity.

Oxidative stress in glial cultures: detection by DAF-2 fluorescence used as a tool to measure peroxynitrite rather than nitric oxide

4,5-diaminofluorescein diacetate (DAF-2DA) is widely used as a fluorescent probe to detect endogenously produced nitric oxide (NO). Recent reports that refer to the high sensitivity of DAF-2 toward NO prompted us to test its efficiency and specificity in a mixed murine primary glial culture model, in which the NO-synthesizing enzyme inducible nitric oxide synthase (iNOS) is expressed by stimulation with lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma). Cultures were loaded with DAF-2DA and the fluorescence was measured using confocal microscopy. NO production in the cultures was determined using the ozone/chemiluminescence technique. Due to the extremely high photosensitivity of DAF-2, low laser intensities were used to avoid artifacts. No difference in DAF-2 fluorescence was observed in NO-producing cultures compared to control cultures, whereas the NO/peroxynitrite-sensitive dye 2,7-dihydrodichlorofluorescein (DCF) showed a significant fluorescence increase specifically in microglia cells. A detectable gain in fluorescence was seen when NO-containing buffer was added to the DAF-2DA-loaded cells with a minimum NO concentration at 7.7 microM. An additional gain of DAF-2 fluorescence was obtained when the cells were depleted of glutathione (GSH) with L-buthionine S,R-sulfoximine (BSO). Hence, we monitored the change in DAF-2 fluorescence intensity in the presence of NO and O(-*)(2) in a cell-free solution. The fluorescence due to NO was indeed larger when O(-*)(2) was added, implying a higher sensitivity of DAF-2 for peroxynitrite. Nevertheless, our results also indicate that measurement of DCF fluorescence is a better tool for monitoring intracellular changes in the levels of NO and/or peroxynitrite than DAF-2.

Mechanism of peroxynitrite interaction with ferric hemoglobin and identification of nitrated tyrosine residues. CO(2) inhibits heme-catalyzed scavenging and isomerization

Hemoproteins are one of the major targets of peroxynitrite in vivo. It has been proposed that the bimolecular heme/peroxynitrite interaction results in both peroxynitrite inactivation (scavenging) and catalysis of tyrosine nitration. In this study, we used spectroscopic techniques to analyze the reaction of peroxynitrite with human methemoglobin (metHb). Although conventional differential spectroscopy did not reveal heme changes, our results suggest that, in the absence of bicarbonate, the heme in metHb reacts bimolecularly with peroxynitrite but is quickly back-reduced by the reaction products. This hypothesis is based on two indirect observations. First, metHb prevents the peroxynitrite-mediated nitration of a target dipeptide, Ala-Tyr, and second, it promotes the isomerization of peroxynitrite to nitrate. Both the scavenging and the isomerization activities of metHb were heme-dependent and inhibited by CO(2). Ferrous cytochrome c was an efficient scavenger of peroxynitrite, but in the ferric form did not show either scavenging or isomerization activities. We found no evidence of an increase in Ala-Tyr nitration with these hemoproteins. Peroxynitrite-treated metHb induced the formation of a long-lived radical assigned to tyrosine by spin-trapping studies. This radical, however, did not allow us to predict an interaction of peroxynitrite with heme. Hb was nitrated by peroxynitrite/CO(2) mainly in tyrosines beta 130, alpha 42, and alpha 140 and, to a lesser extent, alpha 24. The nitration of alpha chain tyrosines more exposed to the solvent (alpha 140 and alpha 24) was higher in CO-Hb and metHb, while nitration of alpha 42, the tyrosine nearest to the heme, was higher in oxyHb. We deduce that the heme/peroxynitrite interaction, which is inhibited in CO-Hb and metHb, affects alpha tyrosine nitration in two opposite ways, i.e., by protecting exposed residues and by promoting nitration of the residue nearest to the heme. Conversely, nitration of beta Tyr 130 was comparable in oxyHb, metHb, and CO-Hb, suggesting a mechanism involving only nitrating species formed during peroxynitrite decay.

Cytoprotective antioxidant function of tyrosine and tryptophan residues in transmembrane proteins

The transmembrane domains of integral membrane proteins show an astounding accumulation of tyrosine and tryptophan residues, especially in the region of the highest lipid density. We found that these residues perform vital antioxidant functions inside lipid bilayers and protect cells from oxidative destruction. First, tyrosine- and tryptophan-containing peptides representing stretches from the transmembrane domains of different integral membrane proteins, including presenilin and the cystic fibrosis transmembrane conductance regulator, prevent oxidative lysis in clonal and primary cells. Second, long-chain acylated tyrosine and tryptophan, but not phenylalanine or short-chain acylated derivatives, are potent inhibitors of lipid peroxidation and oxidative cell death. The antioxidant functions of tyrosine and tryptophan may provide a specific explanation for (a) their unique transmembrane distribution pattern and (b) the high vulnerability of low-protein neuronal membranes to oxidative stress, as seen in neurodegenerative disorders.

Scavenging of peroxynitrite by oxyhemoglobin and identification of modified globin residues

Peroxynitrite is a strong oxidant involved in cell injury. In tissues, most of peroxynitrite reacts preferentially with CO(2) or hemoproteins, and these reactions affect its fate and toxicity. CO(2) promotes tyrosine nitration but reduces the lifetime of peroxynitrite, preventing, at least in part, membrane crossing. The role of hemoproteins is not easily predictable, because the heme intercepts peroxynitrite, but its oxidation to ferryl species and tyrosyl radical(s) may catalyze tyrosine nitration. The modifications induced by peroxynitrite/CO(2) on oxyhemoglobin were determined by mass spectrometry, and we found that alphaTyr42, betaTyr130, and, to a lesser extent, alphaTyr24 were nitrated. The suggested nitration mechanism is tyrosyl radical formation by long-range electron transfer to ferrylhemoglobin followed by a reaction with (*)NO(2). Dityrosine (alpha24-alpha42) and disulfides (beta93-beta93 and alpha104-alpha104) were also detected, but these cross-linkings were largely due to modifications occurring under the denaturing conditions employed for mass spectrometry. Moreover, immunoelectrophoretic techniques showed that the 3-nitrotyrosine content of oxyhemoglobin sharply increased only in molar excess of peroxynitrite, thus suggesting that this hemoprotein is not a catalyst of nitration. The noncatalytic role may be due to the formation of the nitrating species (*)NO(2) mainly in molar excess of peroxynitrite. In agreement with this hypothesis, oxyhemoglobin strongly inhibited tyrosine nitration of a target dipeptide (Ala-Tyr) and of membrane proteins from ghosts resealed with oxyhemoglobin. Erythrocytes were poor inhibitors of Ala-Tyr nitration on account of the membrane barrier. However, at the physiologic hematocrit, Ala-Tyr nitration was reduced by 65%. This "sink" function was facilitated by the huge amount of band 3 anion exchanger on the cell membrane. We conclude that in blood oxyhemoglobin is a peroxynitrite scavenger of physiologic relevance.

Role of the carbonate radical anion in tyrosine nitration and hydroxylation by peroxynitrite

Peroxynitrite has been receiving increasing attention as the pathogenic mediator of nitric oxide cytotoxicity. In most cases, the contribution of peroxynitrite to diseases has been inferred from detection of 3-nitrotyrosine in injured tissues. However, presently it is known that other nitric oxide-derived species can also promote protein nitration. Mechanistic details of protein nitration remain under discussion even in the case of peroxynitrite, although recent literature data strongly suggest a free radical mechanism. Here, we confirm the free radical mechanism of tyrosine modification by peroxynitrite in the presence and in the absence of the bicarbonate-carbon dioxide pair by analyzing the stable tyrosine products and the formation of the tyrosyl radical at pH 5.4 and 7.4. Stable products, 3-nitrotyrosine, 3-hydroxytyrosine, and 3, 3-dityrosine, were identified by high performance liquid chromatography and UV spectroscopy. The tyrosyl radical was detected by continuous-flow and spin-trapping electron paramagnetic resonance (EPR). 3-Hydroxytyrosine was detected at pH 5.4 and its yield decreased in the presence of the bicarbonate-carbon dioxide pair. In contrast, the yields of the tyrosyl radical increased in the presence of the bicarbonate-carbon dioxide pair and correlated with the yields of 3-nitrotyrosine under all tested experimental conditions. Taken together, the results demonstrate that the promoting effects of carbon dioxide on peroxynitrite-mediated tyrosine nitration is due to the selective reactivity of the carbonate radical anion as compared with that of the hydroxyl radical. Colocalization of 3-hydroxytyrosine and 3-nitrotyrosine residues in proteins may be useful to discriminate between peroxynitrite and other nitrating species.

Decrease in 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) EPR signal in ozone-treated erythrocyte membranes

In ozone-treated erythrocyte membrane suspension a slow decrease occurs in the EPR signal of 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO). Because of the absence of such a phenomenon in control membranes and ozonized buffer, this effect must be caused by reaction of nitroxide radicals with products of ozone reactions with membrane components. To find out which components are responsible for the decrease in EPR signal we studied this effect in simple model systems. The same phenomenon was observed both in lipid and protein systems treated by ozone. For unsaturated fatty acids, the correlation between the rate of decrease in EPR signal and the number of double bonds in the lipid molecule was very strong. This suggests that the observed decrease in the nitroxide radical TEMPO EPR signal in ozone-treated erythrocyte membranes is a complex process, but probably the most important reaction is recombination of nitroxide radicals with organic free radicals produced both in the process of lipid peroxidation and ozonolysis of double bonds.

Peroxynitrite induces long-lived tyrosyl radical(s) in oxyhemoglobin of red blood cells through a reaction involving CO2 and a ferryl species

Peroxynitrite-mediated oxidative chemistry is currently the subject of intense investigation owing to the toxic side effects associated with nitric oxide overproduction. Using direct electron spin resonance spectroscopy (ESR) at 37 degrees C, we observed that in human erythrocytes peroxynitrite induced a long-lived singlet signal at g = 2.004 arising from hemoglobin. This signal was detectable in oxygenated red blood cells and in purified oxyhemoglobin but significantly decreased after deoxygenation. The formation of the g = 2.004 radical required the presence of CO2 and pH values higher than the pKa of peroxynitrous acid (pKa = 6.8), indicating the involvement of a secondary oxidant formed in the interaction of ONOO- with CO2. The g = 2.004 radical yield leveled off at a 1:1 ratio between peroxynitrite and oxyhemoglobin, while CO-hemoglobin formed less radical and methemoglobin did not form the radical at all. These results suggest that the actual oxidant is or is derived from the ONOOCO2- adduct interacting with oxygenated FeII-heme. Spin trapping with 2-methyl-2-nitrosopropane (MNP) of the g = 2.004 radical and subsequent proteolytic digestion of the MNP/hemoglobin adduct revealed the trapping of a tyrosyl-centered radical(s). A similar long-lived unresolved g = 2.004 singlet signal is a common feature of methemoglobin/H2O2 and metmyoglobin/H2O2 systems. We show by spin trapping that these g = 2.004 signals generated by H2O2 also indicated trapping of radicals centered on tyrosine residues. Analysis of visible spectra of hemoglobin treated with peroxynitrite revealed that, in the presence of CO2, oxyhemoglobin was oxidized to a ferryl species, which rapidly decayed to lower iron oxidation states. The g = 2.004 radical may be an intermediate formed during ferrylhemoglobin decay. Our results describe a new pathway of peroxynitrite-dependent hemoglobin oxidation of dominating importance in CO2-containing biological systems and identify the g = 2.004 radical(s) formed in the process as tyrosyl radical(s).

Formation of long-lived radicals on proteins by radical transfer from heme enzymes--a common process?

Incubation of Fe(III)myoglobin (Fe(III)Mb) with H2O2 in the presence of bovine serum albumin (BSA) has been shown previously to give albumin-derived radicals as a result of radical transfer from myoglobin to BSA. In this study the occurrence of similar processes with peroxidases has been investigated using horseradish peroxidase (HRP)/H2O2, in the presence and absence of added tyrosine. Incubation of HRP with H2O2 and bovine or human serum albumins, in the presence and absence of tyrosine, gave long-lived albumin-derived radicals as detected by EPR spectroscopy. Evidence has been obtained for these albumin radicals being located on buried tyrosine residues on the basis of blocking experiments. The effect of protein conformation on radical transfer has been investigated using partial proteolytic digestion prior to protein oxidation. With HRP/H2O2/BSA and Fe(III)Mb/H2O2/BSA increased radical concentrations were observed after limited digestion, although this effect was less marked with the HRP/H2O2/BSA system than with Fe(III)Mb/H2O2/BSA, consistent with different modes of radical transfer. More extensive digestion of BSA decreased the radical concentration to levels below those detected with native albumin, indicating that the tertiary structure of the target protein plays an important role in determining the rate of radical transfer and/or the stability of the resultant species. These results are consistent with a mechanism for the HRP/H2O2/no free tyrosine system involving radical transfer to the albumin via the heme edge of the peroxidase. In contrast, albumin radical formation by the HRP/H2O2/free tyrosine system was only marginally affected by proteolysis, consistent with free tyrosine phenoxyl radicals being the mediators of radical transfer, without significant protein-protein interaction. These protein-to-protein radical transfer reactions may have important consequences for understanding protein oxidation in biological systems.

Myoglobin-induced oxidative damage: evidence for radical transfer from oxidized myoglobin to other proteins and antioxidants

Reaction of equine Fe(III) myoglobin with H2O2 gives rise to an Fe(IV)-oxo species at the heme center and protein (globin)-derived radicals. Studies have shown that there are two (or more) sites for the protein-derived radical: at tyrosine (Tyr-103) or tryptophan (Trp-14). The latter radical reacts rapidly with oxygen to give a Trp-derived peroxyl radical. The formation of both the tyrosine phenoxyl radical and the tryptophan-derived peroxyl species have been confirmed in the present study; the latter appears to be the major initial radical, with the phenoxyl radical appearing at longer reaction times, possibly via secondary reactions. We have investigated, by EPR spectroscopy, the reactivity of the Trp-14 peroxyl radical with amino acids, peptides, proteins, and antioxidants, with the aim of determining whether this species can damage other targets, i.e., whether intermolecular protein-to-protein radical transfer and hence chain-oxidation occurs, and the factors that control these reactions. Three amino acids show significant reactivity: Tyr, Trp, and Cys, with Cys the least efficient. Evidence has also been obtained for (inefficient) hydrogen abstraction at peptide alpha-carbon sites; this may result in backbone cleavage in the presence of oxygen. The myoglobin Trp-14 peroxyl radical has been shown to react rapidly with a wide range of proteins to give long-lived secondary radicals on the target protein. These reactions appear to mainly involve Tyr residues on the target protein, although evidence for reaction at Trp has also been obtained. Antioxidants (GSH, ascorbate, Trolox C, vitamin E, and urate) react with the myoglobin-derived peroxyl radical; in some cases antioxidant-derived radicals are detected. These reactions are only efficient at high antioxidant concentrations, suggesting that protein-to-protein damage transfer and protein chain-oxidation may occur readily in biological systems.

Kinetics of peroxynitrite reaction with amino acids and human serum albumin

An initial rate approach was used to study the reaction of peroxynitrite with human serum albumin (HSA) through stopped-flow spectrophotometry. At pH 7.4 and 37 degreesC, the second order rate constant for peroxynitrite reaction with HSA was 9.7 +/- 1.1 x 10(3) M-1 s-1. The rate constants for sulfhydryl-blocked HSA and for the single sulfhydryl were 5.9 +/- 0.3 and 3.8 +/- 0.8 x 10(3) M-1 s-1, respectively. The corresponding values for bovine serum albumin were also determined. The reactivity of sulfhydryl-blocked HSA increased at acidic pH, whereas plots of the rate constant with the sulfhydryl versus pH were bell-shaped. The kinetics of peroxynitrite reaction with all free L-amino acids were determined under pseudo-first order conditions. The most reactive amino acids were cysteine, methionine, and tryptophan. Histidine, leucine, and phenylalanine (and by extension tyrosine) did not affect peroxynitrite decay rate, whereas for the remaining amino acids plots of kobs versus concentration were hyperbolic. The sum of the contributions of the constituent amino acids of the protein to HSA reactivity was comparable to the experimentally determined rate constant, where cysteine and methionine (seven residues in 585) accounted for an estimated 65% of the reactivity. Nitration of aromatic amino acids occurred in HSA following peroxynitrite reaction, with nitration of sulfhydryl-blocked HSA 2-fold higher than native HSA. Carbon dioxide accelerated peroxynitrite decomposition, enhanced aromatic amino acid nitration, and partially inhibited sulfhydryl oxidation of HSA. Nitration in the presence of carbon dioxide increased when the sulfhydryl was blocked. Thus, cysteine 34 was a preferential target of peroxynitrite both in the presence and in the absence of carbon dioxide.

Peroxynitrite induces covalent dimerization of epidermal growth factor receptors in A431 epidermoid carcinoma cells

Irreversible tyrosine modifications by inflammatory oxidants such as peroxynitrite (ONOO-) can affect signal transduction pathways involving tyrosine phosphorylation. The epidermal growth factor receptor (EGFR), a member of the c-ErbB receptor tyrosine kinase family, is involved in regulation of epithelial cell growth and differentiation, and possible modulation of EGFR-dependent signaling by ONOO- was studied. Exposure of epidermoid carcinoma A431 cells to 0.1-1.0 mM ONOO- resulted in tyrosine nitration on EGFR and other proteins but did not significantly affect EGFR tyrosine autophosphorylation. A high molecular mass tyrosine-phosphorylated protein (approximately 340 kDa) was detected in A431 cell lysates after exposure to ONOO-, most likely representing a covalently dimerized form of EGFR, based on immunoprecipitation and/or immunoblotting with alpha-EGFR antibodies and co-migration with ligand-induced EGFR dimers cross-linked with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. Covalent EGFR dimerization by ONOO- probably involved intermolecular dityrosine cross-linking and was enhanced after receptor activation with epidermal growth factor. Furthermore, irreversibly cross-linked EGFR was more extensively tyrosine-phosphorylated compared with the monomeric form, indicating that ONOO- preferentially cross-links activated EGFR. Exposure of A431 cells to ONOO- markedly reduced the kinetics of tyrosine phosphorylation of a downstream EGFR substrate, phospholipase C-gamma1, which may be related to covalent alterations in EGFR. Alteration of EGFR signaling by covalent EGFR dimerization by inflammatory oxidants such as ONOO- may affect conditions of increased EGFR activation such as epithelial repair or tumorigenesis.

Bilirubin is an effective antioxidant of peroxynitrite-mediated protein oxidation in human blood plasma

Bilirubin is a bile pigment that may have an important role as an antioxidant. Its antioxidant potential is attributed mainly to the scavenging of peroxyl radicals. We investigated the reaction of bilirubin with peroxynitrite in phosphate buffer and in blood plasma. In phosphate buffer bilirubin was rapidly oxidized by micromolar concentrations of peroxynitrite, and its oxidation yield was higher at alkaline pH with an apparent pKa = 6.9. In contrast, the major oxidation product of bilirubin in plasma was biliverdin, and the pH profile of its oxidation yield showed a slightly increased oxidation at acidic pH without a clear inflection point. The addition of NaHCO3 to bilirubin decreased the peroxynitrite-dependent oxidation, suggesting that the reactive intermediates formed in the reaction between CO2 and peroxynitrite are less efficient oxidants of bilirubin. The antioxidant role of bilirubin was investigated in some peroxynitrite-mediated plasma protein modifications that are enhanced by CO2 (tryptophan oxidation and protein tyrosine nitration) or slightly decreased by CO2 (protein carbonyl groups). Bilirubin in the micromolar concentration range afforded a significant protection against all these oxidative modifications and, notably, protected plasma proteins even when the pigment was added 5 s after peroxynitrite (i.e., when peroxynitrite is completely decomposed). The loss of tryptophan fluorescence triggered by peroxynitrite was a relatively slow process fulfilled only after a few minutes. After this time, bilirubin was unable to reduce the tryptophan loss, and it was unable to reduce previously formed nitrated albumin or previously formed carbonyls. We deduce that bilirubin in plasma cannot react to a significant extent with peroxynitrite, and we suggest that bilirubin, through a hydrogen donation mechanism, participates as a scavenger of secondary oxidants formed in the oxidative process.

One-electron oxidation pathway of thiols by peroxynitrite in biological fluids: bicarbonate and ascorbate promote the formation of albumin disulphide dimers in human blood plasma

Recent studies have shown that peroxynitrite oxidizes thiol groups through competing one- and two-electron pathways. The two-electron pathway is mediated by the peroxynitrite anion and prevails quantitatively over the one-electron pathway, which is mediated by peroxynitrous acid or a reactive species derived from it. In CO2-containing fluids the oxidation of thiols might follow a different mechanism owing to the rapid formation of a different oxidant, the nitrosoperoxycarbonate anion (ONOOCO2(-)). Here we present evidence that in blood plasma peroxynitrite induces the formation of a disulphide cross-linked protein identified by immunological (anti-albumin antibodies) and biochemical criteria (peptide mapping) as a dimer of serum albumin. The albumin dimer did not form in plasma devoid of CO2 and its formation was enhanced by ascorbate. However, analysis of thiol groups showed that reconstituting dialysed plasma with NaHCO3 protected protein thiols against the oxidation mediated by peroxynitrite and that the simultaneouspresence of ascorbate provided further protection. Ascorbate alone did not protect thiol groups from peroxynitrite-mediated oxidation. ESR spin-trapping studies with N-t-butyl-alpha-phenylnitrone (PBN) revealed that peroxynitrite induced the formation of protein thiyl radicals and their intensity was markedly decreased by plasma dialysis and restored by reconstitution with NaHCO3. PBN completely inhibited the formation of albumin dimer. Moreover, the addition of iron-diethyldithiocarbamate to plasma demonstrated that peroxynitrite induced the formation of protein S-nitrosothiols and/or S-nitrothiols. Our results are consistent with the hypothesis that NaHCO3 favours the one-electron oxidation of thiols by peroxynitrite with formation of thiyl radicals, ;NO2, and RSNOx. Thiyl radicals, in turn, are involved in chain reactions by which thiols are oxidized to disulphides.