Nitric oxide trapping of the tyrosyl radical of prostaglandin H synthase-2 leads to tyrosine iminoxyl radical and nitrotyrosine formation

The diacetyliminoxyl radical in oxidative functionalization of alkenes

The intermolecular oxime radical addition to CC bonds was observed and studied for the first time. The diacetyliminoxyl radical was proposed as a model radical reagent for the study of oxime radical reactivity towards unsaturated substrates, which is important in the light of the active development of synthetic applications of oxime radicals. In the present work it was found that the diacetyliminoxyl radical reacts with vinylarenes and conjugated dienes to give radical addition products, whereas unconjugated alkenes can undergo radical addition or allylic hydrogen substitution by diacetyliminoxyl depending on the substrate structure. Remarkably, substituted alkenes give high yields of C-O coupling products despite the significant steric hindrance, whereas unsubstituted alkenes give lower yields of the C-O coupling products. The observed atypical C-O coupling yield dependence on the alkene structure was explained by the discovered ability of the diacetyliminoxyl radical to attack alkenes with the formation of a C-N bond instead of a C-O bond giving side products. This side process is not expected for sterically hindered alkenes due to lower steric availability of the N-atom in diacetyliminoxyl than that of the O-atom.

Radiolysis Studies of Oxidation and Nitration of Tyrosine and Some Other Biological Targets by Peroxynitrite-Derived Radicals

The widespread interest in free radicals in biology extends far beyond the effects of ionizing radiation, with recent attention largely focusing on reactions of free radicals derived from peroxynitrite (i.e., hydroxyl, nitrogen dioxide, and carbonate radicals). These radicals can easily be generated individually by reactions of radiolytically-produced radicals in aqueous solutions and their reactions can be monitored either in real time or by analysis of products. This review first describes the general principles of selective radical generation by radiolysis, the yields of individual species, the advantages and limitations of either pulsed or continuous radiolysis, and the quantitation of oxidizing power of radicals by electrode potentials. Some key reactions of peroxynitrite-derived radicals with potential biological targets are then discussed, including the characterization of reactions of tyrosine with a model alkoxyl radical, reactions of tyrosyl radicals with nitric oxide, and routes to nitrotyrosine formation. This is followed by a brief outline of studies involving the reactions of peroxynitrite-derived radicals with lipoic acid/dihydrolipoic acid, hydrogen sulphide, and the metal chelator desferrioxamine. For biological diagnostic probes such as ‘spin traps’ to be used with confidence, their reactivities with radical species have to be characterized, and the application of radiolysis methods in this context is also illustrated.

The effects of nitric oxide or oxygen on the stable products formed from the tyrosine phenoxyl radical

Tyrosine is a critical component of many proteins and can be the subject of oxidative posttranslational modifications. Furthermore, the oxidation of tyrosine residues to phenoxyl radicals, sometimes quite stable, is essential for some enzymatic functions. The lifetime and fate of tyrosine phenoxyl radicals in biological systems are largely driven by the availability and proximity of oxidants and reductants. Tyrosine phenoxyl radicals have extremely low reactivity with molecular oxygen whereas reactions with nitric oxide are diffusion controlled. This is in contrast to equivalent reactions with tryptophanyl and cysteinyl radicals where reactions with oxygen are much faster. Despite, the quite disparate apparent reactivity of tyrosine phenoxyl radicals with oxygen and nitric oxide being known, the products of the reactions are not well established. Changes in the levels from expected basal concentrations of stable products resulting from tyrosine phenoxyl radicals, for example naturally occurring 3,3'-dityrosine, 3-nitrotyrosine, and 3-hydroxytyrosine, can be indicative of oxidative and/or nitrosative stress. Using the radiolytic generation of specific oxidizing radicals to form tyrosine phenoxyl radicals in an aqueous solution at a known rate, we have compared the products in the absence and presence of nitric oxide or oxygen. Possible reactions of the phenoxyl radicals with oxygen remain unclear although we show evidence for a small decrease in the yield of dityrosine and loss of tyrosine in the presence of 20% oxygen. Low concentrations of nitric oxide in anoxic conditions react with tyrosine phenoxyl radicals, by what is most probably through the formation of an unstable intermediate, regenerating tyrosine and forming nitrite.

3-Nitrotyrosine and related derivatives in proteins: precursors, radical intermediates and impact in function

Oxidative post-translational modification of proteins by molecular oxygen (O2)- and nitric oxide (•NO)-derived reactive species is a usual process that occurs in mammalian tissues under both physiological and pathological conditions and can exert either regulatory or cytotoxic effects. Although the side chain of several amino acids is prone to experience oxidative modifications, tyrosine residues are one of the preferred targets of one-electron oxidants, given the ability of their phenolic side chain to undergo reversible one-electron oxidation to the relatively stable tyrosyl radical. Naturally occurring as reversible catalytic intermediates at the active site of a variety of enzymes, tyrosyl radicals can also lead to the formation of several stable oxidative products through radical-radical reactions, as is the case of 3-nitrotyrosine (NO2Tyr). The formation of NO2Tyr mainly occurs through the fast reaction between the tyrosyl radical and nitrogen dioxide (•NO2). One of the key endogenous nitrating agents is peroxynitrite (ONOO-), the product of the reaction of superoxide radical (O2•-) with •NO, but ONOO--independent mechanisms of nitration have been also disclosed. This chemical modification notably affects the physicochemical properties of tyrosine residues and because of this, it can have a remarkable impact on protein structure and function, both in vitro and in vivo. Although low amounts of NO2Tyr are detected under basal conditions, significantly increased levels are found at pathological states related with an overproduction of reactive species, such as cardiovascular and neurodegenerative diseases, inflammation and aging. While NO2Tyr is a well-established stable oxidative stress biomarker and a good predictor of disease progression, its role as a pathogenic mediator has been laboriously defined for just a small number of nitrated proteins and awaits further studies.

Oxime radicals: generation, properties and application in organic synthesis

N-Oxyl radicals (compounds with an N-O• fragment) represent one of the richest families of stable and persistent organic radicals with applications ranging from catalysis of selective oxidation processes and mechanistic studies to production of polymers, energy storage, magnetic materials design and spectroscopic studies of biological objects. Compared to other N-oxyl radicals, oxime radicals (or iminoxyl radicals) have been underestimated for a long time as useful intermediates for organic synthesis, despite the fact that their precursors, oximes, are extremely widespread and easily available organic compounds. Furthermore, oxime radicals are structurally exceptional. In these radicals, the N-O• fragment is connected to an organic moiety by a double bond, whereas all other classes of N-oxyl radicals contain an R2N-O• fragment with two single C-N bonds. Although oxime radicals have been known since 1964, their broad synthetic potential was not recognized until the last decade, when numerous selective reactions of oxidative cyclization, functionalization, and coupling mediated by iminoxyl radicals were discovered. This review is focused on the synthetic methods based on iminoxyl radicals developed in the last ten years and also contains some selected data on previous works regarding generation, structure, stability, and spectral properties of these N-oxyl radicals. The reactions of oxime radicals are classified into intermolecular (oxidation by oxime radicals, oxidative C-O coupling) and intramolecular. The majority of works are devoted to intramolecular reactions of oxime radicals. These reactions are classified into cyclizations involving C-H bond cleavage and cyclizations involving a double C=C bond cleavage.

Free radical-dependent inhibition of prostaglandin endoperoxide H Synthase-2 by nitro-arachidonic acid

Prostaglandin endoperoxide H synthase (PGHS) is a heme-enzyme responsible for the conversion of arachidonic acid (AA) to prostaglandin H₂ (PGH₂). PGHS have both oxygenase (COX) and peroxidase (POX) activities and is present in two isoforms (PGHS-1 and -2) expressed in different tissues and cell conditions. It has been reported that PGHS activity is inhibited by the nitrated form of AA, nitro-arachidonic acid (NO₂AA), which in turn could be synthesized by PGHS under nitro-oxidative conditions. Specifically, NO₂AA inhibits COX in PGHS-1 as well as POX in both PGHS-1 and -2, in a dose and time-dependent manner. NO₂AA inhibition involves lowering the binding stability and displacing the heme group from the active site. However, the complete mechanism remains to be understood. This review describes the interactions of PGHS with NO₂AA, focusing on mechanisms of inhibition and nitration. In addition, using a novel approach combining EPR-spin trapping and mass spectrometry, we described possible intermediates formed during PGHS-2 catalysis and inhibition. This literature revision as well as the results presented here strongly suggest a free radical-dependent inhibitory mechanism of PGHS-2 by NO₂AA. This is of relevance towards understanding the underlying mechanism of inhibition of PGHS by NO₂AA and its anti-inflammatory potential.

Detection and quantification of nitric oxide-derived oxidants in biological systems

The free radical nitric oxide (NO^•) exerts biological effects through the direct and reversible interaction with specific targets (e.g. soluble guanylate cyclase) or through the generation of secondary species, many of which can oxidize, nitrosate or nitrate biomolecules. The NO^•-derived reactive species are typically short-lived, and their preferential fates depend on kinetic and compartmentalization aspects. Their detection and quantification are technically challenging. In general, the strategies employed are based either on the detection of relatively stable end products or on the use of synthetic probes, and they are not always selective for a particular species. In this study, we describe the biologically relevant characteristics of the reactive species formed downstream from NO^•, and we discuss the approaches currently available for the analysis of NO^•, nitrogen dioxide (NO₂ ^•), dinitrogen trioxide (N₂O₃), nitroxyl (HNO), and peroxynitrite (ONOO^-/ONOOH), as well as peroxynitrite-derived hydroxyl (HO^•) and carbonate anion (CO₃ ^•-) radicals. We also discuss the biological origins of and analytical tools for detecting nitrite (NO₂ ^-), nitrate (NO₃ ^-), nitrosyl-metal complexes, S-nitrosothiols, and 3-nitrotyrosine. Moreover, we highlight state-of-the-art methods, alert readers to caveats of widely used techniques, and encourage retirement of approaches that have been supplanted by more reliable and selective tools for detecting and measuring NO^•-derived oxidants. We emphasize that the use of appropriate analytical methods needs to be strongly grounded in a chemical and biochemical understanding of the species and mechanistic pathways involved.

Human Mn-superoxide dismutase inactivation by peroxynitrite: a paradigm of metal-catalyzed tyrosine nitration in vitro and in vivo

Nitration of human MnSOD at active site Tyr34 represents a biologically-relevant oxidative post-translational modification that causes enzyme inactivation.

Toward an Understanding of the Ambiguous Electron Paramagnetic Resonance Spectra of the Iminoxy Radical from o-Fluorobenzaldehyde Oxime: Density Functional Theory and ab Initio Studies

Iminoxy radicals (R1R2C═N—O•) possess an inherent ability to exist as E and Z isomers. Although isotropic hyperfine couplings for the species with R1 = H allow one to distinguish between E and Z, unequivocal assignment of the parameters observed in the EPR spectra of the radicals without the hydrogen atom at the azomethine carbon to the right isomer is not a simple task. The iminoxyl derived from o-fluoroacetophenone oxime (R1 = CH3 and R2 = o-FC6H5) appears to be a case in point. Moreover, for its two isomers the rotation of the o-FC6H5 group brings into existence the syn and anti conformers, depending on the mutual orientation of the F atom and C═N—O• group, making a description of hyperfine couplings to structure even more challenging. To accomplish this, a vast array of theoretical methods (DFT, OO-SCS-MP2, QCISD) was used to calculate the isotropic hyperfine couplings. The comparison between experimental and theoretical values revealed that the E isomer is the dominant radical form, for which a fast interconversion between anti and syn conformers is expected. In addition, the origin of the significant AF increase with solvent polarity was analyzed.

Metal-catalyzed protein tyrosine nitration in biological systems

Protein tyrosine nitration is an oxidative postranslational modification that can affect protein structure and function. It is mediated in vivo by the production of nitric oxide-derived reactive nitrogen species (RNS), including peroxynitrite (ONOO(-)) and nitrogen dioxide ((•)NO₂). Redox-active transition metals such as iron (Fe), copper (Cu), and manganese (Mn) can actively participate in the processes of tyrosine nitration in biological systems, as they catalyze the production of both reactive oxygen species and RNS, enhance nitration yields and provide site-specificity to this process. Early after the discovery that protein tyrosine nitration can occur under biologically relevant conditions, it was shown that some low molecular weight transition-metal centers and metalloproteins could promote peroxynitrite-dependent nitration. Later studies showed that nitration could be achieved by peroxynitrite-independent routes as well, depending on the transition metal-catalyzed oxidation of nitrite (NO₂(-)) to (•)NO₂ in the presence of hydrogen peroxide. Processes like these can be achieved either by hemeperoxidase-dependent reactions or by ferrous and cuprous ions through Fenton-type chemistry. Besides the in vitro evidence, there are now several in vivo studies that support the close relationship between transition metal levels and protein tyrosine nitration. So, the contribution of transition metals to the levels of tyrosine nitrated proteins observed under basal conditions and, specially, in disease states related with high levels of these metal ions, seems to be quite clear. Altogether, current evidence unambiguously supports a central role of transition metals in determining the extent and selectivity of protein tyrosine nitration mediated both by peroxynitrite-dependent and independent mechanisms.

Interactions between (+)-catechin and quercetin during their oxidation by nitrite under the conditions simulating the stomach

When foods that contain catechins and quercetin glycosides are ingested, quercetin glycosides are hydrolyzed to quercetin during mastication by hydrolytic enzymes derived from oral bacteria and the generated quercetin aglycone is mixed with catechins in saliva. The present study deals with the interactions between (+)-catechin and quercetin during their reactions with nitrous acid under the conditions simulating the gastric lumen. Nitrous acid reacted with (+)-catechin producing 6,8-dinitrosocatechin, and quercetin partially suppressed the dinitrosocatechin formation. Nitric oxide, which was produced by not only (+)-catechin/nitrous acid but also quercetin/nitrous acid systems, was used to produce 6,8-dinitrosocatechin. Furthermore, 6,8-dinitrosocatechin was oxidized by nitrous acid to the quinone form. The quinone formation was significantly suppressed by quercetin. Quercetin-dependent suppression of the above reactions accompanied the oxidation of quercetin, which was observed with the formation of 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. Taking the above results into account, we proposed a possible mechanism of 6,8-dinitrosocatechin formation and discuss the importance of quercetin to prevent the quinone formation from 6,8-dinitrosocatechin in the gastric lumen, taking the interactions between quercetin and catechins into account.

Redox control of enzymatic functions: The electronics of life's circuitry

The field of redox biology has changed tremendously over the past 20 years. Formerly regarded as bi-products of the aerobic metabolism exclusively involved in tissue damage, reactive oxygen species (ROS) are now recognized as active participants of cell signaling events in health and in disease. In this sense, ROS and the more recently defined reactive nitrogen species (RNS) are, just like hormones and second messengers, acting as fundamental orchestrators of cell signaling pathways. The chemical modification of enzymes by ROS and RNS (that result in functional enzymatic alterations) accounts for a considerable fraction of the transient and persistent perturbations imposed by variations in oxidant levels. Upregulation of ROS and RNS in response to stress is a common cellular response that foments adaptation to a variety of physiologic alterations (hypoxia, hyperoxia, starvation, and cytokine production). Frequently, these are beneficial and increase the organisms' resistance against subsequent acute stress (preconditioning). Differently, the sustained ROS/RNS-dependent rerouting of signaling produces irreversible alterations in cellular functioning, often leading to pathogenic events. Thus, the duration and reversibility of protein oxidations define whether complex organisms remain "electronically" healthy. Among the 20 essential amino acids, four are particularly susceptible to oxidation: cysteine, methionine, tyrosine, and tryptophan. Here, we will critically review the mechanisms, implications, and repair systems involved in the redox modifications of these residues in proteins while analyzing well-characterized prototypic examples. Occasionally, we will discuss potential consequences of amino acid oxidation and speculate on the biologic necessity for such events in the context of adaptative redox signaling. © 2014 IUBMB Life, 66(3):167-181, 2014.

Detection of S-nitrosothiols

BACKGROUND

S-nitrosothiols have been recognized as biologically-relevant products of nitric oxide that are involved in many of the diverse activities of this free radical.

SCOPE OF REVIEW

This review serves to discuss current methods for the detection and analysis of protein S-nitrosothiols. The major methods of S-nitrosothiol detection include chemiluminescence-based methods and switch-based methods, each of which comes in various flavors with advantages and caveats.

MAJOR CONCLUSIONS

The detection of S-nitrosothiols is challenging and prone to many artifacts. Accurate measurements require an understanding of the underlying chemistry of the methods involved and the use of appropriate controls.

GENERAL SIGNIFICANCE

Nothing is more important to a field of research than robust methodology that is generally trusted. The field of S-nitrosation has developed such methods but, as S-nitrosothiols are easy to introduce as artifacts, it is vital that current users learn from the lessons of the past. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Autocatalytic nitration of prostaglandin endoperoxide synthase-2 by nitrite inhibits prostanoid formation in rat alveolar macrophages

AIMS

Prostaglandin endoperoxide H(2) synthase (PGHS) is a well-known target for peroxynitrite-mediated nitration. In several experimental macrophage models, however, the relatively late onset of nitration failed to coincide with the early peak of endogenous peroxynitrite formation. In the present work, we aimed to identify an alternative, peroxynitrite-independent mechanism, responsible for the observed nitration and inactivation of PGHS-2 in an inflammatory cell model.

RESULTS

In primary rat alveolar macrophages stimulated with lipopolysaccharide (LPS), PGHS-2 activity was suppressed after 12 h, although the prostaglandin endoperoxide H(2) synthase (PGHS-2) protein was still present. This coincided with a nitration of the enzyme. Coincubation with a nitric oxide synthase-2 (NOS-2) inhibitor preserved PGHS-2 nitration and at the same time restored thromboxane A(2) (TxA(2)) synthesis in the cells. Formation of reactive oxygen species (ROS) was maximal at 4 h and then returned to baseline levels. Nitrite (NO(2)(-)) production occurred later than ROS generation. This rendered generation of peroxynitrite and the nitration of PGHS-2 unlikely. We found that the nitrating agent was formed from NO(2)(-), independent from superoxide ((•)O(2)(-)). Purified PGHS-2 treated with NO(2)(-) was selectively nitrated on the active site Tyr(371), as identified by mass spectrometry (MS). Exposure to peroxynitrite resulted in the nitration not only of Tyr(371), but also of other tyrosines (Tyr).

INNOVATION AND CONCLUSION

The data presented here point to an autocatalytic nitration of PGHS-2 by NO(2)(-), catalyzed by the enzyme's endogenous peroxidase activity and indicate a potential involvement of this mechanism in the termination of prostanoid formation under inflammatory conditions.

Protein damage, repair and proteolysis

Proteins are continuously affected by various intrinsic and extrinsic factors. Damaged proteins influence several intracellular pathways and result in different disorders and diseases. Aggregation of damaged proteins depends on the balance between their generation and their reversal or elimination by protein repair systems and degradation, respectively. With regard to protein repair, only few repair mechanisms have been evidenced including the reduction of methionine sulfoxide residues by the methionine sulfoxide reductases, the conversion of isoaspartyl residues to L-aspartate by L-isoaspartate methyl transferase and deglycation by phosphorylation of protein-bound fructosamine by fructosamine-3-kinase. Protein degradation is orchestrated by two major proteolytic systems, namely the lysosome and the proteasome. Alteration of the function for both systems has been involved in all aspects of cellular metabolic networks linked to either normal or pathological processes. Given the importance of protein repair and degradation, great effort has recently been made regarding the modulation of these systems in various physiological conditions such as aging, as well as in diseases. Genetic modulation has produced promising results in the area of protein repair enzymes but there are not yet any identified potent inhibitors, and, to our knowledge, only one activating compound has been reported so far. In contrast, different drugs as well as natural compounds that interfere with proteolysis have been identified and/or developed resulting in homeostatic maintenance and/or the delay of disease progression.

Cyclooxygenase reaction mechanism of PGHS--evidence for a reversible transition between a pentadienyl radical and a new tyrosyl radical by nitric oxide trapping

Incubation of prostaglandin H synthase-1 (PGHS-1) under anaerobic conditions with peroxide and arachidonic acid leads to two major radical species: a pentadienyl radical and a radical with a narrow EPR spectrum. The proportions of the two radicals are sensitive to temperature, favoring the narrow radical species at 22 °C. The EPR characteristics of this latter radical are somewhat similar to the previously reported narrow-singlet tyrosine radical NS1a and are insensitive to deuterium labeling of AA. To probe the origin and structure of this radical, we combined EPR analysis with nitric oxide (NO) trapping of tyrosine and substrate derived radicals for both PGHS-1 and -2. Formation of 3-nitrotyrosine in the proteins was analyzed by immunoblotting, whereas NO adducts to AA and AA metabolites were analyzed by mass spectrometry and by chromatography of (14)C-labeled products. The results indicate that both nitrated tyrosine residues and NO-AA adducts formed upon NO trapping. The NO-AA adduct was predominantly an oxime at C11 of AA with three conjugated double bonds, as indicated by absorption at 275 nm and by mass spectral analysis. This adduct amounted to 10% and 20% of the heme concentration of PGHS-1 and -2, respectively. For PGHS-1, the yield of NO-AA adduct matched the yield of the narrow radical signal obtained in parallel EPR experiments. High frequency EPR characterization of this narrow radical, reported in an accompanying paper, supports assignment to a new tyrosyl radical, NS1c, rather than an AA-based radical. To reconcile the results from EPR and NO-trapping studies, we propose that the NS1c is in equilibrium with an AA pentadienyl radical, and that the latter reacts preferentially with NO.

Functional consequences of actin nitration: in vitro and in disease states

To link the phenomena of inflammatory-induced increases in protein nitrotyrosine (NO(2)Tyr) derivatives to protein dysfunction and consequent pathological conditions, the evaluation of discrete NO(2)Tyr modifications on specific proteins must be undertaken. Mass spectrometric (MS) proteomics-based strategies allow for the identification of all individual proteins that are nitrated by separating tissue homogenates using 2D gel electrophoresis, detecting the nitrated proteins using an anti-NO(2)Tyr antibody, and then identifying the peptides generated during an in-gel proteolytic digest using matrix-assisted laser desorption ionization/time-of-flight (MALDI-TOF) MS. Actin, one of the most abundant proteins in eukaryotic cells, constitutes 5% or more of cell protein and serves with other cytoskeletal proteins as a critical target for nitration-induced functional impairment. Herein, examples of actin nitration detected under physiological conditions in various models of human disease or in clinically derived tissues are given and the impact that this post-translational protein modification can have on cell and organ function is discussed.

Cyclooxygenase competitive inhibitors alter tyrosyl radical dynamics in prostaglandin H synthase-2

Reaction of prostaglandin H synthase (PGHS) isoforms 1 or 2 with peroxide forms a radical at Tyr385 that is required for cyclooxygenase catalysis and another radical at Tyr504, whose function is unknown. Both tyrosyl radicals are transient and rapidly dissipated by reductants, suggesting that cyclooxygenase catalysis might be vulnerable to suppression by intracellular antioxidants. Our initial hypothesis was that the two radicals are in equilibrium and that their proportions and stability are altered upon binding of fatty acid substrate. As a test, we examined the effects of three competitive inhibitors (nimesulide, flurbiprofen, and diclofenac) on the proportions and stability of the two radicals in PGHS-2 pretreated with peroxide. Adding nimesulide after ethyl peroxide led to some narrowing of the tyrosyl radical signal detected by EPR spectroscopy, consistent with a small increase in the proportion of the Tyr504 radical. Neither flurbiprofen nor diclofenac changed the EPR line width when added after peroxide. In contrast, the effects of cyclooxygenase inhibitors on the stability of the preformed tyrosyl radicals were dramatic. The half-life of total tyrosyl radical was 4.1 min in the control, >10 h with added nimesulide, 48 min with flurbiprofen, and 0.8 min with diclofenac. Stabilization of the tyrosyl radicals was evident even at substoichiometric levels of nimesulide. Thus, the inhibitors had potent, structure-dependent, effects on the stability of both tyrosyl radicals. This dramatic modulation of tyrosyl radical stability by cyclooxygenase site ligands suggests a mechanism for regulating the reactivity of PGHS tyrosyl radicals with cellular antioxidants.

Cardiovascular consequences when nitric oxide and lipid signaling converge

The identification of nitric oxide ((*)NO) as an endogenously produced free radical mediator of endothelial-dependent relaxation and host defense has fundamentally changed concepts of cell signal transduction. Ligand-receptor oriented paradigms of cell signaling were originally centered on the concept of a high affinity and specific interaction between a ligand and its receptor, resulting in the activation of secondary signaling events such as gene expression or modulation of catalytic protein function. While (*)NO ligation of the heme iron of soluble guanylate cyclase is consistent with this perspective, the readily diffusible and broadly reactive (*)NO is increasingly appreciated to react with a vast array of target molecules that mediate paracrine vasodilator actions, inhibition of thrombosis and neointimal proliferation, and both pro- and antiinflammatory signaling reactions that are not affected by inhibitors of soluble guanylate cyclase. There is an expanding array of functionally significant "off target" collateral reactions mediated by (*)NO that are guanylate cyclase-independent and rather are dictated by anatomic distribution and the formation of secondary (*)NO-derived species. These reactions are a critical element of redox-regulated signaling and are addressed herein in the context of the oxidation of unsaturated fatty acids to vascular and inflammatory signaling mediators. Because of their abundance and the intrinsic reactivity of unsaturated lipid intermediates and eicosanoid metabolism enzymes with (*)NO and other oxides of nitrogen, lipid signaling mechanisms are a significant target for regulation by (*)NO in the vascular compartment. This convergence of (*)NO and lipid signaling pathways thus adds another level of regulation to physiological responses such as vasodilation, thrombosis, and inflammation. Herein, interactions between (*)NO and lipid signaling events are placed in the context of cardiovascular regulation.

Convergence of nitric oxide and lipid signaling: anti-inflammatory nitro-fatty acids

The signaling mediators nitric oxide ( NO) and oxidized lipids, once viewed to transduce metabolic and inflammatory information via discrete and independent pathways, are now appreciated as interdependent regulators of immune response and metabolic homeostasis. The interactions between these two classes of mediators result in reciprocal control of mediator synthesis that is strongly influenced by the local chemical environment. The relationship between the two pathways extends beyond coregulation of NO and eicosanoid formation to converge via the nitration of unsaturated fatty acids to yield nitro derivatives (NO(2)-FA). These pluripotent signaling molecules are generated in vivo as an adaptive response to oxidative inflammatory conditions and manifest predominantly anti-inflammatory signaling reactions. These actions of NO(2)-FA are diverse, with these species serving as a potential chemical reserve of NO, reacting with cellular nucleophiles to posttranslationally modify protein structure, function, and localization. In this regard these species act as potent endogenous ligands for peroxisome proliferator-activated receptor gamma. Functional consequences of these signaling mechanisms have been shown in multiple model systems, including the inhibition of platelet and neutrophil functions, induction of heme oxygenase-1, inhibition of LPS-induced cytokine release in monocytes, increased insulin sensitivity and glucose uptake in adipocytes, and relaxation of preconstricted rat aortic segments. These observations have propelled further in vitro and in vivo studies of mechanisms of NO(2)-FA signaling and metabolism, highlighting the therapeutic potential of this class of molecules as anti-inflammatory drug candidates.

The role of nitrite ion in phagocyte function--perspectives and puzzles

Macrophages and neutrophils are essential elements of host cellular defense systems that function, at least in part, by generating respiration-driven oxidative toxins in response to external stimuli. In both cells, encapsulation by phagocytosis provides a mechanism to direct the toxins against the microbes. The toxic chemicals formed by these two phagocytic cells differ markedly, as do the enzymatic catalysts that generate them. Nitrite ion is microbicidal under certain conditions, is generated by activated macrophages, and is present at elevated concentration levels at infection sites. In this review, we consider potential roles that nitrite might play in cellular disinfection by these phagocytes within the context of available experimental information. Although the suggested roles are plausible, based upon the chemical and biochemical reactivity of NO2(-), studies to date provide little support for their implementation within phagosomes.

Protein tyrosine nitration--an update

Tyrosine nitration is a covalent post-translational protein modification derived from the reaction of proteins with nitrating agents. Tyrosine nitration has been used as a marker of oxidant burden in human diseases. However, it remains unclear whether protein nitration is responsible for alterations in protein function that imparts an increased risk for disease development or unfavorable outcomes. Emerging data implicate tyrosine nitration as a mediator of immune responses suggesting a novel biological function for this protein modification.

The chemistry of cell signaling by reactive oxygen and nitrogen species and 4-hydroxynonenal

During the past several years, major advances have been made in understanding how reactive oxygen species (ROS) and nitrogen species (RNS) participate in signal transduction. Identification of the specific targets and the chemical reactions involved still remains to be resolved with many of the signaling pathways in which the involvement of reactive species has been determined. Our understanding is that ROS and RNS have second messenger roles. While cysteine residues in the thiolate (ionized) form found in several classes of signaling proteins can be specific targets for reaction with H(2)O(2) and RNS, better understanding of the chemistry, particularly kinetics, suggests that for many signaling events in which ROS and RNS participate, enzymatic catalysis is more likely to be involved than non-enzymatic reaction. Due to increased interest in how oxidation products, particularly lipid peroxidation products, also are involved with signaling, a review of signaling by 4-hydroxy-2-nonenal (HNE) is included. This article focuses on the chemistry of signaling by ROS, RNS, and HNE and will describe reactions with selected target proteins as representatives of the mechanisms rather attempt to comprehensively review the many signaling pathways in which the reactive species are involved.

Endogenous peroxynitrite modulates PGHS-1-dependent thromboxane A2 formation and aggregation in human platelets

Aggregation of activated platelets is considerably mediated by the autocrine action of thromboxane A2 (TxA2) which is formed in a prostaglandin endoperoxide H2 synthase-1 (PGHS-1 or COX-1)-dependent manner. The activity of PGHS-1 can be stimulated by peroxides, an effect termed "peroxide tone", that renders PGHS-1 the key regulatory enzyme in the formation of TxA2. Activated platelets release nitric oxide (*NO) and superoxide (O*2) but their interactions with the prostanoid pathway have been controversially discussed in platelet physiology and pathophysiology. The current study demonstrates that endogenously formed peroxynitrite at nanomolar concentrations, originating from the interaction of *NO and *O2, potently activated PGHS-1, which parallels TxA2 formation and aggregation in human platelets. Inhibition of the endogenous formation of either *NO or O*2 resulted in a concentration-dependent decline of PGHS-1 activity, TxA2 release, and aggregation. The concept of peroxynitrite as modulator of TxA2 formation and aggregation explains the interaction of *NO and O*2 with the PGHS pathway and suggests a mechanism by which antioxidants can regulate PGHS-1-dependent platelet aggregation. This may provide a molecular explanation for the clinically observed hyperreactivity of platelets in high-risk patients and serve as a basis for novel therapeutic interventions.

Neurodegeneration and peroxidases

Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative diseases that affect different parts of the central nervous system. However, a review of the literature indicates that certain biochemical reactions involved in neurodegeneration in these three diseases are quite similar and could be partly identical. This article critically examines the similarities and, based on data from our own and other laboratories, proposes a novel explanation for neurodegeneration in these three diseases. We identified about 20 commonalities that exist in the neurodegenerative process of each disease. We hypothesize that there are two enzyme-catalyzed pathways that operate in affected neurons: an oxidative pathway leading to destruction of various neuronal proteins and lipids, and an apoptotic pathway which the body normally uses to remove unwanted and dysfunctional cells. Data from many laboratories indicate that oxidative reactions are primarily responsible for neurodegeneration, whereas apoptosis may well be a secondary response to the presence of neurons that have already been severely damaged by oxidative reactions. Attempts to inhibit apoptosis for the purpose of attenuating progression of these diseases may therefore be only of marginal benefit. Specific oxidative reactions within affected neurons led us to propose that one or more heme peroxidases may be the catalyst(s) involved in oxidation of proteins and lipids. Support for this proposal is provided by the recent finding that amyloi-beta peptide may act as a peroxidase in AD. Possible participation of the peroxidase activity of cytochrome c, herein designated as cytochrome c(px) to distinguish it from yeast cytochrome c peroxidase, is discussed. Of special interest is our recent finding that many compounds that cause attenuation of neurodegeneration are inhibitors of the peroxidase activity of cytochrome c. Several inhibitors were subsequently identified as suicide substrates. Such inhibitors could be ideally suited for targeted clinical approaches aimed at arresting progression of neurodegeneration. Finally, it is possible that immobilized yet still active peroxidase(s) may be present in protein aggregates in AD, PD, and ALS. This activity could be the catalyst for the slow, self-perpetuating and irreversible degeneration of affected neurons that occurs over long periods of time in these neurodegenerative diseases.

Pathways for intracellular generation of oxidants and tyrosine nitration by a macrophage cell line

Two transformed murine macrophage cell lines (RAW 264.7 ATCC TIB-71 and CRL-2278) were examined for oxidant production at various times following activation by using a set of fluorescence and ESR-active probes. Stimulation with a soluble agonist or activation with bacterial lipopolysaccharide plus gamma-interferon caused only very small initial increases in O2 consumption above basal rates; however, at 2-4 h post-activation, respiration increased to 2-3-fold and remained at these elevated levels over the subsequent lifetime of the cell (20-30 h). Oxidation reactions were confined primarily within the cell, as was demonstrated by using phagocytosable dichlorodihydrofluorescein-conjugated latex beads and cyclic hydroxylamines with differing membrane permeabilities. From the intrinsic reactivities of these probes and the time course of their oxidations, one infers the induction of apparent peroxidase activity beginning at approximately 2 h post-activation coinciding with the increase in overall respiratory rate; this acquired capability was accompanied by accumulation of a stable horseradish peroxidase-reactive oxidant, presumably H2O2, in the extracellular medium. Nitrite ion rapidly accumulated in the extracellular medium over a period of 5-8 h post-activation in both cell lines, indicating the presence of active nitric oxide synthase (iNOS) during that period. Prostaglandin endoperoxide H synthase (COX-2) activity was detected at 15-20 h post-activation by the use of a sensitive peroxide assay in conjunction with a COX-2 specific inhibitor (DuP-697). Superoxide formation was detected by reaction with hydroethidine within the first hour following activation, but not thereafter. Consistent with the absence of significant respiratory stimulation, the amount of O2*- formed was very small; comparative reactions of cyclic hydroxylamine probes indicated that virtually none of the O2*- was discharged into the external medium. Myeloperoxidase (MPO) activity was probed at various times post-activation by using fluorescein-conjugated polyacrylamide beads, which efficiently trap MPO-generated HOCl in neutrophils to give stable chlorofluorescein products. However, chlorination of the dye was not detected under any conditions in RAW cells, virtually precluding MPO involvement in their intracellular reactions. This same probe was used to determine changes in intraphagosomal pH, which increased slowly from approximately 6.5 to approximately 8.2 over a 20 h post-phagocytosis period. The cumulative data suggest that activation is followed by sequential induction of an endogenous peroxidase, iNOS, and COX-2, with NADPH oxidase-derived O2*- playing a minimal role in the direct generation of intracellular oxidants. To account for reported observations of intracellular tyrosine nitration late in the life cycles of macrophages, we propose a novel mechanism wherein iNOS-generated NO2- is used by COX-2 to produce NO2* as a terminal microbicidal oxidant and nitrating agent.

Modulation of protein tyrosine nitration and inflammatory mediators by isoprenylhydroquinone glucoside

The nitration of tyrosine caused by peroxynitrite and other reactive nitrogen species is clearly detrimental for some physiological processes; however, its signalling role is still open to controversy. Among the natural phenolics known for their ability to oppose free tyrosine nitration, isoprenylhydroquinone glucoside is investigated due to its unusual structure, which contains a simple hydroxybenzene alkylated by a hemiterpenoid moiety. This hydroquinone was shown to be an effective inhibitor of peroxynitrite-induced protein tyrosine nitration in 3T3 fibroblasts. When tested on bovine seroalbumin nitration, however, the potency was reduced by half and the effect was almost abolished in the presence of bicarbonate. In contrast, addition of this anion had no effect on the nitrite/hydrogen peroxide/hemin system. Isoprenylhydroquinone glucoside was also active in the microM range on intra- and extracellular protein-bound tyrosine nitration by phorbol 12-myristate 13-acetate-stimulated neutrophils. The effects on nitric oxide synthase expression, interleukin-1beta and tumor necrosis factor-alpha production by lipopolysaccharide-stimulated macrophages were quite moderate. Thus, isoprenylhydroquinone glucoside is an inhibitor of protein nitration in situ, but lacks effect on the generation of either nitric oxide or inflammatory cytokines.

Reprint of “Oxidative alterations of cyclooxygenase during atherogenesis” [Prostag. Oth. Lipid. M. 80 (2006) 1–14]

Nitric oxide (*NO) and eicosanoids are critical mediators of physiological and pathophysiological processes. They include inflammation and atherosclerosis. *NO production and eicosanoid synthesis become disrupted during atherosclerosis and thus, it is important to understand the mechanisms that may contribute to this outcome. We, and others, have shown that nitrogen oxide (NOx) species modulate cyclooxygenase (COX; also known as prostaglandin H2 synthase) activity and alter eicosanoid production. We have determined that peroxynitrite (ONOO-) has multiple effects on COX activity. ONOO- can provide the peroxide tone necessary for COX activation, such that simultaneous exposure of COX to its arachidonic acid substrate and ONOO- results in increased eicosanoid production. Alternatively, in the absence of arachidonic acid, ONOO- can modify COX through nitration of an essential tyrosine residue (Tyr385) such that it is incapable of catalysis. In this regard, we have shown that COX nitration occurs in human atherosclerotic tissue and in aortic lesions from ApoE-/- mice kept on a high fat diet. Additionally, we have demonstrated that Tyr nitration in ApoE-/- mice is dependent on the inducible form of NO synthase (iNOS). Under conditions where ONOO- persists and arachidonic acid is not immediately available, the cell may try to correct the situation by responding to ONOO- and releasing arachidonic acid via a signaling pathway to favor COX activation. Other post-translational modifications of COX by NOx species include S-nitrosation of cysteine (Cys) residues (which may have an activating effect) and Cys oxidation. The central focus of this review will include a discussion of how NOx species alter COX activity at the molecular level and how these modifications may contribute to altered eicosanoid output during atherosclerosis and lesion development.

Investigation of tyrosine nitration and nitrosylation of angiotensin II and bovine serum albumin with electrospray ionization mass spectrometry

Protein tyrosine nitration is one of the important regulatory mechanisms in various cellular phenomena such as cell adhesion, endo/exo-cytosis of cellular materials, and signal transduction. In the present study, electrospray ionization tandem mass spectrometry (ESI-MS/MS) with a linear ion-trap mass spectrometer was applied for identification of nitrated proteins and localization of the modified tyrosine residues. When angiotensin II(DRVYIHPF) was nitrated in vitro with tetranitromethane (TNM), the mass spectrum showed a shift of +45 Da which corresponded to tyrosine nitration. An additional +29 Da mass shift was also detected by ESI-MS. This differed from nitrated peptide analysis with matrix-associated laser desorption/ionization mass spectrometry (MALDI-MS), which showed oxygen neutral loss from the nitrated tyrosine residues upon laser irradiation. Hence the +29 Da mass shift of the nitrated peptide observed by ESI-MS suggested the introduction of an NO group for nitrosylation of tyrosine residues. To confirm this in vitro nitrosylation on the protein level, bovine serum albumin was in vitro nitrated with TNM and analyzed by ESI-MS/MS. As expected, +29 as well as +45 Da mass shifts were detected, and the +29 Da mass shift was found to correspond to the modification on tyrosine residues by NO. Although the chemical mechanism by which this occurs in ESI-MS is not clear, the +29 Da mass shift could be a new potential marker of nitrosylated peptides.

Protein tyrosine nitration in hydrophilic and hydrophobic environments

In this review we address current concepts on the biological occurrence, levels and consequences of protein tyrosine nitration in biological systems. We focused on mechanistic aspects, emphasizing on the free radical mechanisms of protein 3-nitrotyrosine formation and critically analyzed the restrictions for obtaining large tyrosine nitration yields in vivo, mainly due to the presence of strong reducing systems (e.g. glutathione) that can potently inhibit at different levels the nitration process. Evidence is provided to show that the existence of metal-catalyzed processes, the assistance of nitric oxide-dependent nitration steps and the facilitation by hydrophobic environments, provide individually and/or in combination, feasible scenarios for nitration in complex biological milieux. Recent studies using hydrophobic tyrosine analogs and tyrosine-containing peptides have revealed that factors controlling nitration in hydrophobic environments such as biomembranes and lipoproteins can differ to those in aqueous compartments. In particular, exclusion of key soluble reductants from the lipid phase will more easily allow nitration and lipid-derived radicals are suggested as important mediators of the one-electron oxidation of tyrosine to tyrosyl radical in proteins associated to hydrophobic environments. Development and testing of hydrophilic and hydrophobic probes that can compete with endogenous constituents for the nitrating intermediates provide tools to unravel nitration mechanisms in vitro and in vivo; additionally, they could also serve to play cellular and tissue protective functions against the toxic effects of protein tyrosine nitration.

Oxidative alterations of cyclooxygenase during atherogenesis

Nitric oxide (*NO) and eicosanoids are critical mediators of physiological and pathophysiological processes. They include inflammation and atherosclerosis. *NO production and eicosanoid synthesis become disrupted during atherosclerosis and thus, it is important to understand the mechanisms that may contribute to this outcome. We, and others, have shown that nitrogen oxide (NO(x)) species modulate cyclooxygenase (COX; also known as prostaglandin H(2) synthase) activity and alter eicosanoid production. We have determined that peroxynitrite (ONOO(-)) has multiple effects on COX activity. ONOO(-) can provide the peroxide tone necessary for COX activation, such that simultaneous exposure of COX to its arachidonic acid substrate and ONOO(-) results in increased eicosanoid production. Alternatively, in the absence of arachidonic acid, ONOO(-) can modify COX through nitration of an essential tyrosine residue (Tyr385) such that it is incapable of catalysis. In this regard, we have shown that COX nitration occurs in human atherosclerotic tissue and in aortic lesions from ApoE(-/-) mice kept on a high fat diet. Additionally, we have demonstrated that Tyr nitration in ApoE(-/-) mice is dependent on the inducible form of NO synthase (iNOS). Under conditions where ONOO(-) persists and arachidonic acid is not immediately available, the cell may try to correct the situation by responding to ONOO(-) and releasing arachidonic acid via a signaling pathway to favor COX activation. Other post-translational modifications of COX by NO(x) species include S-nitrosation of cysteine (Cys) residues (which may have an activating effect) and Cys oxidation. The central focus of this review will include a discussion of how NO(x) species alter COX activity at the molecular level and how these modifications may contribute to altered eicosanoid output during atherosclerosis and lesion development.

Oxidation and cytotoxicity of 6-OHDA are mediated by reactive intermediates of COX-2 overexpressed in PC12 cells

Parkinson's disease is characterized by a progressive loss of dopaminergic neurons, likely associated with dysregulation of oxidation of catechols, such as dopamine (DA) and 6-hydroxydopamine (6-OHDA), and resulting in oxidative stress. The involvement of cyclooxygenase-2 (COX-2) in pathogenesis of Parkinson's disease has been suggested. However, specific COX-2 triggered mechanisms participating in catalysis of DA oxidation and enhanced catechol-induced cytotoxicity remain poorly characterized. Here, we demonstrate that in a model biochemical system, recombinant heme-reconstituted COX-2 induced oxidation of 6-OHDA in the course of its peroxidase (H(2)O(2)-dependent) and cyclooxygenase (arachidonic acid (AA)-dependent) catalytic half-cycles. Similarly, COX-2 was able to stimulate 6-OHDA oxidation during its peroxidase- and cyclooxygenase half-cycles and caused oxidative stress in homogenates of PC12 cells stably overexpressing the enzyme (but not in mock-transfected cells). In addition, the increased levels of COX-2 were associated with enhanced cytotoxicity of 6-OHDA in stably transfected PC12 cells. Finally, co-oxidation of 6-OHDA by COX-2 triggered production of superoxide radicals critical for both propagation of 6-OHDA oxidation and induction of oxidative stress in COX-2 overexpressing cells. Thus, we conclude that both peroxidase and cyclooxygenase half-cycles of COX-2-catalyzed reactions are essential for COX-2-dependent activation of 6-OHDA oxidation, oxygen radical production, oxidative stress, and cytotoxicity.

COX-2 contributes to the maintenance of flow-induced dilation in arterioles of eNOS-knockout mice

Our previous studies demonstrated that, in gracilis muscle arterioles of male mice deficient in the gene for endothelial nitric oxide synthase (eNOS), flow-induced dilation (FID) is mediated by endothelial PGs. Thus the present study aimed to identify the specific isoform of cyclooxygenase (COX) responsible for the compensatory mediation of FID in arterioles of eNOS-knockout (KO) mice. Experiments were conducted on gracilis muscle arterioles of male eNOS-KO and wild-type (WT) mice. Basal tone and magnitude of FID of arterioles were comparable in the two strains of mice. A role for COX isoforms in the mediation of the responses was assessed by use of valeryl salicylate (3 mM) and NS-398 (10 microM), inhibitors of COX-1 and COX-2, respectively. In eNOS-KO arterioles, valeryl salicylate or NS-398 alone inhibited FID (at maximal flow rate) by approximately 51% and approximately 58%, respectively. Administration of both inhibitors eliminated the dilation. In WT arterioles, inhibition of COX-2 did not significantly affect FID, whereas inhibition of COX-1 decreased the dilation by approximately 57%. The residual portion of the response was abolished by additional administration of Nomega-nitro-L-arginine methyl ester. Western blot analysis indicated a comparable content of COX-1 protein in arterioles of WT and eNOS-KO mice. COX-2 protein, which was not detectable in arterioles of WT mice, was strongly expressed in arterioles of eNOS-KO mice, together with an upregulation of COX-2 gene expression. Immunohistochemical staining confirmed the presence of COX-2 in the endothelium of eNOS-KO arterioles. In conclusion, COX-2-derived PGs are the mediators responsible for maintenance of FID in arterioles of eNOS-deficient mice.

Inducible nitric oxide synthase mediates prostaglandin h2 synthase nitration and suppresses eicosanoid production

Nitric oxide (NO) modulates the biological levels of arachidonate-derived cell signaling molecules by either enhancing or suppressing the activity of prostaglandin H(2) isoforms (PGHS-1 and PGHS-2). Whether NO activates or suppresses PGHS activity is determined by alternative protein modifications mediated by NO and NO-derived species. Here, we show that inducible NO synthase (iNOS) and PGHS-1 co-localize in atherosclerotic lesions of ApoE(-/-) mouse aortae. Immunoblotting and immunohistochemistry revealed Tyr nitration in PGHS-1 in aortic lesions but markedly less in adjacent nonlesion tissue. PGHS-2 was also found in lesions, but 3-nitrotyrosine incorporation was not detected. 3-Nitrotyrosine formation in proteins is considered a hallmark reaction of peroxynitrite, which can form via NO-superoxide reactions in an inflammatory setting. That iNOS-derived NO is essential for 3-nitrotyrosine modification of PGHS-1 was confirmed by the absence of 3-nitrotyrosine in lesions from ApoE(-/-)iNOS(-/-) mice. Mass spectrometric studies specifically identified the active site residue Tyr385 as a 3-nitrotyrosine modification site in purified PGHS-1 exposed to peroxynitrite. PGHS-mediated eicosanoid (PGE(2)) synthesis was more than fivefold accelerated in cultured iNOS(-/-) versus iNOS-expressing mouse aortic smooth muscle cells, suggesting that iNOS-derived NO markedly suppresses PGHS activity in vascular cells. These results further suggest a regulatory role of iNOS in eicosanoid biosynthesis in human atherosclerotic lesions.

Autocatalytic tyrosine nitration of prostaglandin endoperoxide synthase-2 in LPS-stimulated RAW 264.7 macrophages

In the literature, biological tyrosine nitrations have been reported to depend not only on peroxynitrite but also on nitrite/hydrogen peroxide linked to catalysis by myeloperoxidase. In endotoxin-stimulated RAW 264.7 macrophages, we have detected a major nitrotyrosine positive protein band around 72 kDa and identified it as prostaglandin endoperoxide synthase-2 (PGHS-2). Isolated PGHS-2 in absence of its substrate arachidonate was not only tyrosine-nitrated with peroxynitrite, but also with nitrite/hydrogen peroxide in complete absence of myeloperoxidase. Our data favor an autocatalytic activation of nitrite by PGHS-2 with a subsequent nitration of the essential tyrosine residue in the cyclooxygenase domain. Under inflammatory conditions, nitrite formed via NO-synthase-2 may therefore act as an endogenous regulator for PGHS-2 in stimulated macrophages. Nitration of PGHS-2 by the autocatalytic activation of nitrite further depends on the intracellular concentration of arachidonate since arachidonate reacted competitively with nitrite and could prevent PGHS-2 from nitration when excessively present.

Immunochemical detection of nitric oxide and nitrogen dioxide trapping of the tyrosyl radical and the resulting nitrotyrosine in sperm whale myoglobin

We demonstrate herein that nitric oxide (*NO) and nitrogen dioxide (*NO2) both react with the tyrosyl radical formed in sperm whale myoglobin (swMb) by reaction with hydrogen peroxide. The tyrosyl radical was detected by Western blotting using a novel anti-5,5-dimethyl-1-pyrroline N-oxide (DMPO) polyclonal antiserum that specifically recognizes protein radical-derived DMPO nitrone adducts. In the presence of DMPO, hydrogen peroxide reacts with swMb to form the DMPO tyrosyl radical as is known from both electron spin resonance and immuno-spin trapping investigations. Both *NO and NO2- significantly suppressed DMPO-Mb formation under the physiological oxygen tension of 30 mm Hg. If this inhibition of DMPO trapping of the tyrosyl radical is due, at least in part, to the reaction of the tyrosyl radical with *NO and *NO2, then nitrotyrosine should be formed. In line with this expectation, swMb treated with low concentrations of *NO or NO2- formed nitrotyrosine when hydrogen peroxide was added under 30 mm Hg oxygen tension as detected by Western blotting. The amount of nitrotyrosine generated with *NO was higher than with NO2-, implying that there are two different peroxynitrite-independent nitrotyrosine formation mechanisms and that *NO is not just a source of *NO2.

Protective action of a hexane crude extract of Pterodon emarginatus fruits against oxidative and nitrosative stress induced by acute exercise in rats

Background

The aim of the present work was to evaluate the effect of a hexane crude extract (HCE) of Pterodon emarginatus on the oxidative and nitrosative stress induced in skeletal muscle, liver and brain of acutely exercised rats.

Methods

Adult male rats were subjected to acute exercise by standardized contractions of the tibialis anterior (TA) muscle (100 Hz, 15 min) and treated orally with the HCE (once or three times with a fixed dose of 498 mg/kg), before and after acute exercise. Serum creatine kinase activity was determined by a kinetic method and macrophage infiltration by histological analyses of TA muscle. Lipid peroxidation was measured as malondialdehyde (MDA) levels. Nitric oxide production was evaluated by measuring nitrite formation, using Griess reagent, and nitrotyrosine was assessed by western blotting.

Results

Serum creatine kinase activities in the controls (111 U/L) increased 1 h after acute exercise (443 U/L). Acute exercise also increased the infiltration of macrophages into TA muscle; lipid peroxidation levels in TA muscle (967%), liver (55.5%) and brain (108.9%), as well as the nitrite levels by 90.5%, 30.7% and 60%, respectively. The pattern of nitrotyrosine formation was also affected by acute exercise. Treatment with HCE decreased macrophage infiltration, lipid peroxidation, nitrite production and nitrotyrosine levels to control values.

Conclusion

Acute exercise induced by functional electrical stimulation in rats resulted in increase in lipid peroxidation, nitrite and nitrotyrosine levels in brain, liver and skeletal muscle. The exercise protocol, that involved eccentric muscle contraction, also caused some muscle trauma, associated with over-exertion, leading to inflammation. The extract of P. emarginatus abolished most of these oxidative processes, thus confirming the high antioxidant activity of this oil which infusions are used in folk medicine against inflammatory processes.

Dietary copper supplements modulate aortic superoxide dismutase, nitric oxide and atherosclerosis

The objective was to test the hypothesis that dietary copper inhibits atherosclerosis by inducing superoxide dismutase (SOD) and potentiating nitric oxide (NO). New Zealand White rabbits were fed either a cholesterol diet (n = 8) or a cholesterol diet containing 0.02% copper acetate (n = 8) for 13 weeks. We found that the intimal area was significantly smaller in the animals supplemented with copper (P < 0.005), although integrated plasma cholesterol levels were not significantly different. This was associated with a significant increase in aortic copper content (P < 0.05), SOD activity (P < 0.05) and Cu/Zn SOD mRNA (P < 0.05) and a significant decrease in nitrotyrosine content (P < 0.05). Furthermore, there was a positive correlation between aortic copper content and SOD activity (P < 0.005, R(2) = 0.83) and a negative correlation between aortic superoxide dimutase activity and nitrotyrosine content (P < 0.005, R(2) = 0.93). In organ bath experiments, the relaxation of precontracted carotid artery rings to calcium ionophore was greater in animals supplemented with copper. No difference in response to sodium nitroprusside was observed. These data suggest that in the cholesterol-fed rabbit, copper supplements inhibit the progression of atherosclerosis by increasing SOD expression, thereby reducing the interaction of NO with superoxide, and hence potentiating NO-mediated pathways that may protect against atherosclerosis.

Involvement of phospholipid, biomembrane integrity, and NO peroxidase activity in the NO catabolism by cytochrome c oxidase

The physiological regulation of mitochondrial respiration by NO has been reported to result from the reversible binding of NO to the two-electron reduced binuclear center (Fe(2+)(a3)-Cu(1+)(B)) of cytochrome c oxidase (CcO). Although the role of CcO and its derived catalytic intermediates in the catabolism of NO has been documented, little has been established for the enzyme in its fully oxidized state (Fe(3+)(a3)-Cu(2+)(B)). We report: (1) CcO, in its fully oxidized state, represents the major component of the mitochondrial electron transport chain for NO consumption as controlled by the binding of NO to its binuclear center. Phospholipid enhances NO consumption by fully oxidized CcO, whereas the consumption of NO is slowed down by membrane structure and membrane potential when CcO is embedded in the phospholipid bilayer. (2) In the presence of H(2)O(2), CcO was shown to serve as a mitochondria-derived NO peroxidase. A CcO-derived protein radical intermediate was induced and involved in the modulation of NO catabolism.