Albumin Oxidation to Diverse Radicals by the Peroxidase Activity of Cu,Zn−Superoxide Dismutase in the Presence of Bicarbonate or Nitrite: Diffusible Radicals Produce Cysteinyl and Solvent-Exposed and -Unexposed Tyrosyl Radicals

Histone oxidation as a new mechanism of metabolic control over gene expression

The emergence of aerobic respiration created unprecedented bioenergetic advantages, while imposing the need to protect critical genetic information from reactive byproducts of oxidative metabolism (i.e., reactive oxygen species, ROS). The evolution of histone proteins fulfilled the need to shield DNA from these potentially damaging toxins, while providing the means to compact and structure massive eukaryotic genomes. To date, several metabolism-linked histone post-translational modifications (PTMs) have been shown to regulate chromatin structure and gene expression. However, whether and how PTMs enacted by metabolically produced ROS regulate adaptive chromatin remodeling remain relatively unexplored. Here, we review novel mechanistic insights into the interactions of ROS with histones and their consequences for the control of gene expression regulation, cellular plasticity, and behavior.

Mining reactive triplet carbonyls in biological systems

Aliphatic triplet carbonyls can be treated as short-lived radicals, since both species share similar reactions such as hydrogen atom abstraction, cyclization, addition, and isomerization. Importantly, enzyme-generated triplet carbonyls excite triplet molecular oxygen to the highly reactive, electrophilic singlet state by resonance energy transfer, which can react with proteins, lipids, and DNA. Carbonyl triplets, singlet oxygen, and radicals are endowed with the potential to trigger both normal and pathological responses. In this paper, we present a short review of easy, fast, and inexpensive preliminary tests for the detection of transient triplet carbonyls in chemical and biological systems. This paper covers direct and indirect methods to look for triplet carbonyls based on their spectral distribution of chemiluminescence, photoproduct analysis, quenching of light emission by conjugated dienes, and enhancement of light emission by the sensitizer 9,10-dibromoanthracence-2-sulfonate ion (DBAS).

A copper-zinc-superoxide dismutase (CuZnSOD) from redlip mullet, Liza haematocheila: Insights to its structural characteristics, immune responses, antioxidant activity, and potent antibacterial properties

Copper-zinc superoxide dismutase (CuZnSOD) is a nuclear-encoded metalloenzyme responsible for scavenging harmful reactive oxygen species (ROS). In this study, the CuZnSOD homolog from redlip mullet (Liza haematochelia) (MuCuZnSOD) was structurally and functionally characterized to evaluate its antioxidant capacity, antibacterial properties, and protective level in various pathogenic stress conditions. Structural characteristics of MuCuZnSOD were evaluated using different bioinformatics tools. Pairwise sequence comparison and evolutionary tree structure revealed that the MuCuZnSOD sequence was closely related to the CuZnSOD sequence of Oplegnathus fasciatus with a 94.2% sequence identity. Sequence alignment analysis indicated that the CuZnSOD domain was well conserved. The highest transcriptional expression of MuCuZnSOD was identified in the blood. Immune challenge with lipopolysaccharide (LPS), Lactococcus garvieae, and polyinosinic-polycytidylic acid (poly I:C) exhibited an increased MuCuZnSOD mRNA expression in the blood and liver. Transfected green fluorescent protein-fused MuCuZnSOD was localized in the cytoplasm. Recombinant MuCuZnSOD (rMuCuZnSOD) was overexpressed in a bacterial system. The rMuCuZnSOD possessed significant antioxidant properties as determined by conventional xanthine oxidase assay. The optimum pH and temperature of rMuCuZnSOD were found to be pH 9 and 25 °C, respectively. rMuCuZnSOD enzyme activity increased in a concentration-dependent manner. Treatment with potassium cyanide highly inhibited the rMuCuZnSOD activity. rMuCuZnSOD possessed a significant peroxidation activity in the presence of HCO₃- ions as demonstrated by the increased viability in cells treated with the enzyme in the presence of hydrogen peroxide. Antibacterial assays showed that rMuCuZnSOD had significant growth-inhibitory effects on both gram-positive and gram-negative bacteria. Collectively, our findings demonstrate that MuCuZnSOD is an essential antioxidant protein, which regulates the host defense mechanisms and innate immunity under oxidative stress.

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

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

Oxidative Assets Toward Biomolecules and Cytotoxicity of New Oxindolimine-Copper(II) and Zinc(II) Complexes

A new oxindolimine ligand derived from isatin (1H-indole-2,3-dione) and 2-aminomethylbenzimidazole was synthesized, leading to two novel complexes after metalation with copper(II) perchlorate or zinc(II) chloride, [Cu(isambz)2](ClO4)2 (complex 1) and [Zn(isambz)Cl2] (complex 2). This new ligand was designed as a more lipophilic compound, in a series of oxindolimine–metal complexes with antitumor properties, having DNA, mitochondria, and some proteins, such as CDK1 kinase and topoisomerase IB, as key targets. The new complexes had their reactivity to human serum albumin (HSA) and DNA, and their cytotoxicity toward tumor cells investigated. The binding to CT-DNA was monitored by circular dichroism (CD) spectroscopy and fluorescence measurements using ethidium bromide in a competitive assay. Consequent DNA cleavage was verified by gel electrophoresis with complex 1, in nmolar concentrations, with formation of linear DNA (form III) after 60 min incubation at 37 °C, in the presence of hydrogen peroxide, which acts as a reducing agent. Formation of reactive oxygen species (ROS) was observed, monitored by spin trapping EPR. Interaction with HSA lead to α-helix structure disturbance, and formation of a stable radical species (HSA–Tyr·) and carbonyl groups in the protein. Despite showing oxidative ability to damage vital biomolecules such as HSA and DNA, these new complexes showed moderate cytotoxicity against hepatocellular carcinoma (HepG2) and neuroblastoma (SHSY5Y) cells, similarly to previous compounds in this series. These results confirm DNA as an important target for these compounds, and additionally indicate that oxidative damage is not the leading mechanism responsible for their cytotoxicity. Additionally, this work emphasizes the importance of ligand characteristics and of speciation in activity of metal complexes.

The Interaction of Hypotaurine and Other Sulfinates with Reactive Oxygen and Nitrogen Species: A Survey of Reaction Mechanisms

Considerable strides have been made in understanding the oxidative mechanisms involved in the final steps of the cysteine pathway leading to taurine. The oxidation of sulfinates, hypotaurine and cysteine sulfinic acid, to the respective sulfonates, taurine and cysteic acid, has never been associated with any specific enzyme. Conversely, there is strong evidence that in vivo formation of taurine and cysteic acid is the result of sulfinate interaction with a variety of biologically relevant oxidants. In the last decade, many experiments have been performed to understand whether peroxynitrite, nitrogen dioxide and carbonate radical anion could be included in the biologically relevant reactive species capable of oxidizing sulfinates. Thanks to this work, it has been possible to highlight two possible reaction mechanisms (direct and indirect reaction) of sulfinates with reactive oxygen and nitrogen species.The sulfinates oxidation, mediated by peroxynitrite, is an example of both reaction mechanisms: through a two-electron-direct-reaction with peroxynitrite or through a one-electron-indirect-transfer reaction. In the indirect mechanism, the peroxynitrite homolysis releases hydroxyl and nitrogen dioxide radical and in addition the degradation of short-lived adduct formed by peroxynitrite and CO2 can generate carbonate radical anion. The reaction of hypotaurine and cysteine sulfinic acid with peroxynitrite-derived radicals is accompanied by extensive oxygen uptake with the generation of transient intermediates, which can begin a reaction by an oxygen-dependent mechanism with the sulfonates, taurine, and cysteic acid as final products. Due to pulse radiolysis studies, it has been shown that transient sulfonyl radicals (RSO2•) have been produced during the oxidation of both sulfinates by one-electron transfer reaction.The purpose is to analyze all the aspects of the reactive mechanism in the sulfinic group oxidation of hypotaurine and cysteine sulfinic acid through the results obtained from our laboratory in recent years.

Two metalloenzymes from rockfish (Sebastes schligellii): Deciphering their potential involvement in redox homeostasis against oxidative stress

Disturbance in the balance between pro-oxidants and anti-oxidants result oxidative stress in aerobic organisms. However, oxidative stress can be inhibited by enzymatic and non-enzymatic defense mechanisms. Superoxide dismutases (SODs) are well-known scavengers of superoxide radicals, and they protect cells by detoxifying hazardous reactive oxygen species. Here, we have identified and characterized two different SODs, CuZnSOD and MnSOD, from black rockfish (RfCuZnSOD and RfMnSOD, respectively). In silico analysis revealed the well-conserved molecular structures comprising all essential properties of CuZnSOD and MnSOD. Phylogenetic analysis revealed that both RfCuZnSOD and RfMnSOD cladded with their fish counterparts. The recombinant RfSOD proteins demonstrated their potential superoxide scavenging abilities through a xanthine oxidase assay. The optimum temperature and pH conditions for both rRfSODs were 25 °C and pH 8, respectively. Moreover, the potential peroxidation function of rRfCuZnSOD was observed in the presence of HCO₃^-. The highest peroxidation activity was observed at 100 μg/mL of rRfCuZnSOD using the MTT cell viability assay and flow cytometry. The analogous tissue-specific expression profile indicated ubiquitous expression of both RfCuZnSOD and RfMnSOD in selected tissues of healthy juvenile rockfish. An immune challenge experiment illustrated the altered expression profiles of both RfCuZnSOD and RfMnSOD against lipopolysaccharide, Streptococcus iniae, and polyinosinic-polycytidylic acid (poly I:C). Collectively, these results strengthen the general understanding of the structural and functional characteristics of SODs within the host defense system.

Peroxynitrite preferentially oxidizes the dithiol redox motifs of protein-disulfide isomerase

Protein-disulfide isomerase (PDI) is a ubiquitous dithiol-disulfide oxidoreductase that performs an array of cellular functions, such as cellular signaling and responses to cell-damaging events. PDI can become dysfunctional by post-translational modifications, including those promoted by biological oxidants, and its dysfunction has been associated with several diseases in which oxidative stress plays a role. Because the kinetics and products of the reaction of these oxidants with PDI remain incompletely characterized, we investigated the reaction of PDI with the biological oxidant peroxynitrite. First, by determining the rate constant of the oxidation of PDI's redox-active Cys residues (Cys53 and Cys397) by hydrogen peroxide (k = 17.3 ± 1.3 m-1 s-1 at pH 7.4 and 25 °C), we established that the measured decay of the intrinsic PDI fluorescence is appropriate for kinetic studies. The reaction of these PDI residues with peroxynitrite was considerably faster (k = (6.9 ± 0.2) × 104 m-1 s-1), and both Cys residues were kinetically indistinguishable. Limited proteolysis, kinetic simulations, and MS analyses confirmed that peroxynitrite preferentially oxidizes the redox-active Cys residues of PDI to the corresponding sulfenic acids, which reacted with the resolving thiols at the active sites to produce disulfides (i.e. Cys53-Cys56 and Cys397-Cys400). A fraction of peroxynitrite, however, decayed to radicals that hydroxylated and nitrated other active-site residues (Trp52, Trp396, and Tyr393). Excess peroxynitrite promoted further PDI oxidation, nitration, inactivation, and covalent oligomerization. We conclude that these PDI modifications may contribute to the pathogenic mechanism of several diseases associated with dysfunctional PDI.

Copper(ii) mediated phenol ring nitration by nitrogen dioxide

Addition of nitrogen dioxide in the THF solutions of Cu(ii) complexes of N₂O₂type ligands, L¹H₂and L²H₂resulted in the nitration at the 4-position of coordinated equatorial phenolate ring of the ligand frameworks. Spectroscopic evidence suggests that the reaction proceeds through a phenoxyl radical complex formation.

Oxidation of the tryptophan 32 residue of human superoxide dismutase 1 caused by its bicarbonate-dependent peroxidase activity triggers the non-amyloid aggregation of the enzyme

The role of oxidative post-translational modifications of human superoxide dismutase 1 (hSOD1) in the amyotrophic lateral sclerosis (ALS) pathology is an attractive hypothesis to explore based on several lines of evidence. Among them, the remarkable stability of hSOD1(WT) and several of its ALS-associated mutants suggests that hSOD1 oxidation may precede its conversion to the unfolded and aggregated forms found in ALS patients. The bicarbonate-dependent peroxidase activity of hSOD1 causes oxidation of its own solvent-exposed Trp(32) residue. The resulting products are apparently different from those produced in the absence of bicarbonate and are most likely specific for simian SOD1s, which contain the Trp(32) residue. The aims of this work were to examine whether the bicarbonate-dependent peroxidase activity of hSOD1 (hSOD1(WT) and hSOD1(G93A) mutant) triggers aggregation of the enzyme and to comprehend the role of the Trp(32) residue in the process. The results showed that Trp(32) residues of both enzymes are oxidized to a similar extent to hSOD1-derived tryptophanyl radicals. These radicals decayed to hSOD1-N-formylkynurenine and hSOD1-kynurenine or to a hSOD1 covalent dimer cross-linked by a ditryptophan bond, causing hSOD1 unfolding, oligomerization, and non-amyloid aggregation. The latter process was inhibited by tempol, which recombines with the hSOD1-derived tryptophanyl radical, and did not occur in the absence of bicarbonate or with enzymes that lack the Trp(32) residue (bovine SOD1 and hSOD1(W32F) mutant). The results support a role for the oxidation products of the hSOD1-Trp(32) residue, particularly the covalent dimer, in triggering the non-amyloid aggregation of hSOD1.

Phenol ring nitration induced by the unprecedented reduction of the Cu(II) centre by nitrogen dioxide

Nitrogen dioxide (˙NO2) induces tyrosine nitration through a radical mechanism in biological systems. Two copper(II) complexes, 1 and 2, with ligands L₁ and L₂ [L₁ = 2,4-di-tert-butyl-6-(((2-(dimethylamino)ethyl)(isopropyl)amino)methyl)phenol; L₂ = 6,6'-(((2-(dimethylamino)ethyl)azanediyl)bis(methylene))bis(2,4-di-tert-butylphenol)], respectively, have been made to react with ˙NO2. In both cases, the reduction of the copper(II) center was observed in the presence of ˙NO2 which induces phenol ring nitration through nitronium ion (NO2(+)) formation.

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.

Ras GTPases are both regulators and effectors of redox agents

Redox agents have been historically considered pathological agents which can react with and damage many biological macromolecules including DNA, proteins, and lipids. However, a growing number of reports have suggested that mammalian cells can rapidly respond to ligand stimulation with a change in intracellular ROS thus indicating that the production of intracellular redox agents is tightly regulated and that they serve as intracellular signaling molecules being involved in a variety of cell signaling pathways. Numerous observations have suggested that some members of the Ras GTPase superfamily appear to regulate the production of redox agents and that oxidants can function as effector molecules for the small GTPases, thus contributing to their overall biological function. In addition, many of the Ras superfamily small GTPases have been shown to be redox sensitive, thanks to the presence of redox-sensitive sequences in their primary structure. The action of redox agents on these redox-sensitive GTPases is similar to that of guanine nucleotide exchange factors in that they perturb GTPase nucleotide-binding interactions that result in the enhancement of the guanine nucleotide exchange of small GTPases. Thus, Ras GTPases may act both as upstream regulators and downstream effectors of redox agents. Here we overview current understanding concerning the interplay between Ras GTPases and redox agents, also taking into account pathological implications of misregulation of this cross talk and highlighting the potentiality of these cellular pathways as new therapeutical targets for different pathologies.

Antioxidant promotion of tyrosine nitration in the presence of copper(II)

Copper(II) is known to catalyze the generation of reactive nitrogen species in the presence of hydrogen peroxide, nitrite or nitric oxide, leading to tyrosine nitration, a biomarker for free radical species associated diseases. Here, we find that biological antioxidants such as ascorbic acid can promote tyrosine nitration in the presence of copper(II) and nitrite under aerobic and weak acidic conditions. Tyrosine nitration is demonstrated on both the β-amyloid peptide and angiotensin I. These studies show that (i) ascorbic acid works as a pro-oxidant in the presence of copper(II) to induce oxidation and nitration on peptides, (ii) both free and coordinated copper(II) can catalyze peptide oxidation and nitration, (iii) nitration occurs under mild acidic conditions (pH = 6.0-6.5).

The carbonylation and covalent dimerization of human superoxide dismutase 1 caused by its bicarbonate-dependent peroxidase activity is inhibited by the radical scavenger tempol

The nitroxide tempol inhibited the carbonylation and covalent dimerization of human superoxide dismutase 1 caused by its bicarbonate-dependent peroxidase activity. Tempol acted by scavenging the produced carbonate radical and by recombining with hSOD1-Trp32• radicals as indicated by MS/MS evidence.

Kinetics of the oxidation of reduced Cu,Zn-superoxide dismutase by peroxymonocarbonate

Kinetic evidence is reported for the role of the peroxymonocarbonate, HOOCO(2)(-), as an oxidant for reduced Cu,Zn-superoxide dismutase-Cu(I) (SOD1) during the peroxidase activity of the enzyme. The formation of this reactive oxygen species results from the equilibrium between hydrogen peroxide and bicarbonate. Recently, peroxymonocarbonate has been proposed to be a key substrate for reduced SOD1 and has been shown to oxidize SOD1-Cu(I) to SOD1-Cu(II) much faster than H(2)O(2). We have reinvestigated the kinetics of the reaction between SOD1-Cu(I) and HOOCO(2)(-) by using conventional stopped-flow spectrophotometry and obtained a second-order rate constant of k=1600±100M(-1)s(-1) for SOD1-Cu(I) oxidation by HOOCO(2)(-). Our results demonstrate that peroxymonocarbonate oxidizes SOD1-Cu(I) to SOD1-Cu(II) and is in turn reduced to the carbonate anion radical. It is proposed that the dissociation of His61 from the active site Cu(I) in SOD-Cu(I) contributes to this chemistry by facilitating the binding of larger anions, such as peroxymonocarbonate.

The biological buffer bicarbonate/CO2 potentiates H2O2-mediated inactivation of protein tyrosine phosphatases

Hydrogen peroxide is a cell signaling agent that inactivates protein tyrosine phosphatases (PTPs) via oxidation of their catalytic cysteine residue. PTPs are inactivated rapidly during H(2)O(2)-mediated cellular signal transduction processes, but, paradoxically, hydrogen peroxide is a rather sluggish PTP inactivator in vitro. Here we present evidence that the biological buffer bicarbonate/CO(2) potentiates the ability of H(2)O(2) to inactivate PTPs. The results of biochemical experiments and high-resolution crystallographic analysis are consistent with a mechanism involving oxidation of the catalytic cysteine residue by peroxymonocarbonate generated via the reaction of H(2)O(2) with HCO(3)(-)/CO(2).

Regulation of Ras proteins by reactive nitrogen species

Ras GTPases have been a subject of intense investigation since the early 1980s, when single point mutations in Ras were shown to cause deregulated cell growth control. Subsequently, Ras was identified as the most prevalent oncogene found in human cancer. Ras proteins regulate a host of pathways involved in cell growth, differentiation, and apoptosis by cycling between inactive GDP-bound and active GTP-bound states. Regulation of Ras activity is controlled by cellular factors that alter guanine nucleotide cycling. Oncogenic mutations prevent protein regulatory factors from down-regulating Ras activity, thereby maintaining Ras in a chronically activated state. The central dogma in the field is that protein modulatory factors are the primary regulators of Ras activity. Since the mid-1990s, however, evidence has accumulated that small molecule reactive nitrogen species (RNS) can also influence Ras guanine nucleotide cycling. Herein, we review the basic chemistry behind RNS formation and discuss the mechanism through which various RNS enhance nucleotide exchange in Ras proteins. In addition, we present studies that demonstrate the physiological relevance of RNS-mediated Ras activation within the context of immune system function, brain function, and cancer development. We also highlight future directions and experimental methods that may enhance our ability to detect RNS-mediated activation in cell cultures and in vivo. The development of such methods may ultimately pave new directions for detecting and elucidating how Ras proteins are regulated by redox species, as well as for targeting redox-activated Ras in cancer and other disease states.

(Bi)sulfite oxidation by copper, zinc-superoxide dismutase: Sulfite-derived, radical-initiated protein radical formation

BACKGROUND

Sulfur dioxide, formed during the combustion of fossil fuels, is a major air pollutant near large cities. Its two ionized forms in aqueous solution, sulfite and (bi)sulfite, are widely used as preservatives and antioxidants to prevent food and beverage spoilage. (Bi)sulfite can be oxidized by peroxidases to form the very reactive sulfur trioxide anion radical (*SO(3)-). This free radical further reacts with oxygen to form the peroxymonosulfate anion radical (-O(3)SOO*) and sulfate anion radical (SO(4)*-).

OBJECTIVE

To explore the critical role of these radical intermediates in further oxidizing biomolecules, we examined the ability of copper,zinc-superoxide dismutase (Cu,Zn-SOD) to initiate this radical chain reaction, using human serum albumin (HSA) as a model target.

METHODS

We used electron paramagnetic resonance, optical spectroscopy, oxygen uptake, and immuno-spin trapping to study the protein oxidations driven by sulfite-derived radicals.

RESULTS

We found that when Cu,Zn-SOD reacted with (bi)sulfite, *SO(3)- was produced, with the concomitant reduction of SOD-Cu(II) to SOD-Cu(I). Further, we demonstrated that sulfite oxidation mediated by Cu,Zn-SOD induced the formation of radical-derived 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin-trapped HSA radicals.

CONCLUSIONS

The present study suggests that protein oxidative damage resulting from (bi)sulfite oxidation promoted by Cu,Zn-SOD could be involved in oxidative damage and tissue injury in (bi)sulfite-exacerbated allergic reactions.

Bicarbonate and active site zinc modulate the self-peroxidation of bovine copper-zinc superoxide dismutase

Peroxidation reactions of copper-zinc superoxide dismutase (CuZn-SOD1) or its zinc-depleted form (CuE-SOD1) that likely also involve a component of bicarbonate buffer have been implicated in the pathophysiology of the neurodegenerative diseases amyotrophic lateral sclerosis (ALS), Alzheimer's Disease and Parkinson's Disease. Neither removal of the zinc ion nor adding bicarbonate had large effects on the self-peroxidation reaction of bovine SOD1, but the combination of zinc-deficiency and added bicarbonate caused major changes to the spin trapped SOD1-centred free radical. Removal of the active site zinc ion greatly decreased the formation of an unassigned SOD1-centred free radical in the reaction with the inorganic peroxide peroxynitrite. The results suggest that under cellular conditions ( approximately 5 mM bicarbonate) zinc-deficient SOD1 peroxidation could play a pathogenic role in neurodegenerative diseases.

Direct magnetic resonance evidence for peroxymonocarbonate involvement in the cu,zn-superoxide dismutase peroxidase catalytic cycle

Cu,Zn-superoxide dismutase (SOD1) is a copper- and zinc-dependent enzyme. The main function of SOD1 is believed to be the scavenging and detoxification of superoxide radicals. Nevertheless, the last 30 years have seen a rapid accumulation of evidence indicating that SOD1 may also act as a peroxidase, an alternative function that was implicated in the onset and progression of familial amyotrophic lateral sclerosis. Although SOD1 peroxidase activity and its dependence on carbon dioxide have been well described, the molecular basis of the SOD1 peroxidase cycle remains obscure, because none of the proposed catalytic intermediates have so far been identified. In view of recent observations, we hypothesized that the SOD1 peroxidase cycle relies on two steps: 1) reduction of SOD-Cu(II) by hydrogen peroxide followed by 2) oxidation of SOD-Cu(I) by peroxymonocarbonate, the product of the spontaneous reaction of bicarbonate with hydrogen peroxide, to produce SOD-Cu(II) and carbonate radical anion. This hypothesis has been investigated through electron paramagnetic resonance and nuclear magnetic resonance to provide direct evidence for a peroxycarbonate-driven, SOD1-catalyzed carbonate radical production. The results gathered herein indicate that peroxymonocarbonate (HOOCO(2)(-)) is a key intermediate in the SOD1 peroxidase cycle and identify this species as the precursor of carbonate radical anions.

Sulfenic acid--a key intermediate in albumin thiol oxidation

The single thiol of human serum albumin (HSA-SH) is the predominant plasma thiol. Both circulating albumin and pharmaceutical preparations are heterogeneous regarding the thiol redox status, as revealed by anion-exchange-hydrophobic interaction chromatography. Sulfenic acid (HSA-SOH) is an intermediate in HSA-SH oxidation processes that was detected through different techniques including mass spectrometry. Recently, quantitative data led to the determination of rate constants. The preferred fate of HSA-SOH is the formation of mixed disulfides. Alternatively, HSA-SOH can be further oxidized to sulfinic and sulfonic acids. Oxidized forms increase under disease conditions, underscoring the importance of HSA-SH as a plasma scavenger of intravascular oxidants. We here provide a critical review of the oxidation of HSA-SH in the context of the intravascular compartment, with emphasis in the methodological approaches of mass spectrometry and chromatography for the analysis of albumin thiol redox states.

Buffer modulation of menadione-induced oxidative stress in Saccharomyces cerevisiae

The objective of this study was to compare, in vivo, the effects of bicarbonate and phosphate buffers on survival and menadione-induced oxidative stress in yeast cells. The latter were treated with different concentrations of menadione in the presence of these two buffers. At 25 mM concentration of buffers, menadione only slightly reduced yeast surviving; at 50 mM concentration, cell killing by menadione was much more pronounced in bicarbonate than in phosphate buffer. Although the content of protein carbonyl groups did not show development of oxidative stress under menadione-induced stress, inactivation of aconitase and decrease in glutathione level mirrored its induction. However, cellular glutathione and aconitase activity decrease did not correlate with yeast survival. In vitro, aconitase was more quickly inactivated in 50 mM carbonate, than in 50 mM phosphate buffer. The possible involvement of the carbonate radical in these processes is discussed.

Cu,Zn-superoxide dismutase-driven free radical modifications: copper- and carbonate radical anion-initiated protein radical chemistry

The understanding of the mechanism, oxidant(s) involved and how and what protein radicals are produced during the reaction of wild-type SOD1 (Cu,Zn-superoxide dismutase) with H2O2 and their fate is incomplete, but a better understanding of the role of this reaction is needed. We have used immuno-spin trapping and MS analysis to study the protein oxidations driven by human (h) and bovine (b) SOD1 when reacting with H2O2 using HSA (human serum albumin) and mBH (mouse brain homogenate) as target models. In order to gain mechanistic information about this reaction, we considered both copper- and CO3(*-) (carbonate radical anion)-initiated protein oxidation. We chose experimental conditions that clearly separated SOD1-driven oxidation via CO(*-) from that initiated by copper released from the SOD1 active site. In the absence of (bi)carbonate, site-specific radical-mediated fragmentation is produced by SOD1 active-site copper. In the presence of (bi)carbonate and DTPA (diethylenetriaminepenta-acetic acid) (to suppress copper chemistry), CO(*-) produced distinct radical sites in both SOD1 and HSA, which caused protein aggregation without causing protein fragmentation. The CO(*-) produced by the reaction of hSOD1 with H2O2 also produced distinctive DMPO (5,5-dimethylpyrroline-N-oxide) nitrone adduct-positive protein bands in the mBH. Finally, we propose a biochemical mechanism to explain CO(*-) production from CO2, enhanced protein radical formation and protection by (bi)carbonate against H2O2-induced fragmentation of the SOD1 active site. Our present study is important for establishing experimental conditions for studying the molecular mechanism and targets of oxidation during the reverse reaction of SOD1 with H2O2; these results are the first step in analysing the critical targets of SOD1-driven oxidation during pathological processes such as neuroinflammation.

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.

Immunolocalization of hypochlorite-induced, catalase-bound free radical formation in mouse hepatocytes

The establishment of oxidants as mediators of signal transduction has renewed the interest of investigators in oxidant production and metabolism. In particular, H(2)O(2) has been demonstrated to play pivotal roles in mediating cell differentiation, proliferation, and death. Intracellular concentrations of H(2)O(2) are modulated by its rate of production and its rate of decomposition by catalase and peroxidases. In inflammation and infection, some of the H(2)O(2) is converted to hypochlorous acid, a key mediator of the host immune response against pathogens. In vivo HOCl production is mediated by myeloperoxidase, which uses excess H(2)O(2) to oxidize Cl(-). Mashino and Fridovich (Biochim. Biophys. Acta 956:63-69; 1988) observed that a high excess of HOCl over catalase inactivated the enzyme by mechanisms that remain unclear. The potential relevance of this as an alternative mechanism for catalase activity control and its potential impact on H(2)O(2)-mediated signaling and HOCl production compelled us to explore in depth the HOCl-mediated catalase inactivation pathways. Here, we demonstrate that HOCl induces formation of catalase protein radicals and carbonyls, which are temporally correlated with catalase aggregation. Hypochlorite-induced catalase aggregation and free radical formation that paralleled the enzyme loss of function in vitro were also detected in mouse hepatocytes treated with the oxidant. Interestingly, the novel immuno-spin-trapping technique was applied to image radical production in the cells. Indeed, in HOCl-treated hepatocytes, catalase and protein-DMPO nitrone adducts were colocalized in the cells' peroxisomes. In contrast, when hepatocytes from catalase-knockout mice were treated with hypochlorous acid, there was extensive production of free radicals in the plasma membrane. Because free radicals are short-lived species with fundamental roles in biology, the possibility of their detection and localization to cell compartments is expected to open new and stimulating research venues in the interface of chemistry, biology, and medicine.

EPR spin-trapping of protein radicals to investigate biological oxidative mechanisms

Presently, free radicals and oxidants are considered to mediate from signaling circuits involved in physiology and pathology to cell and tissue injury. The elucidation of these many inter-related processes requires a better understanding of cellular oxidative mechanisms many of which are mediated by protein radicals. Here, we will discuss the potentialities of EPR spin-trapping of protein radicals to unravel oxidative mechanisms. An overview of the methodology and its application to identify protein residues that are the target of specific oxidants, characterize emerging oxidants, and discriminate radical from non radical mechanisms will be presented. The examples are based on work developed in our laboratories but will be discussed in a broad scenario to emphasize that simple experiments can provide relevant insights into the biological reactivity of known and emerging biological oxidants and into signaling mechanisms.

Oxidation-state-dependent reactions of cytochrome c with the trioxidocarbonate(•1−) radical: a pulse radiolysis study

The reaction of the trioxidocarbonate(*1-) radical (CO (3) (*-) , "carbonate radical anion") with cytochrome c was studied by pulse radiolysis at alkaline pH and room temperature. With iron(III) cytochrome c, CO (3) (*-) reacts with the protein moiety with rate constants of (5.1 +/- 0.6) x 10(7) M(-1) s(-1) (pH 8.4, I approximately 0.27 M) and (1.0 +/- 0.2) x 10(8) M(-1) s(-1) (pH 10, I = 0.5 M). The absorption spectrum of the haem moiety was not changed, thus, amino acid radicals produced on the protein do not reduce the haem. The pH-dependent difference in rate constants may be attributed to differences in ionization states of amino acids and to the change in the conformation of the protein. With iron(II) cytochrome c, CO (3) (*-) oxidizes the haem quantitatively, presumably via electrostatic guidance of the radical to the solvent-accessible haem edge, with a different pH dependence: at pH 8.4, the rate constant is (1.1 +/- 0.1) x 10(9) M(-1) s(-1) and, at pH 10, (7.6 +/- 0.6) x 10(8) M(-1) s(-1). We propose that CO (3) (*-) oxidizes the iron center directly, and that the lower rate observed at pH 10 is due to the different charge distribution of iron(II) cytochrome c.

Ferrous ion-induced strand breaks in the DNA plasmid pBR322 are not mediated by hydrogen peroxide

Ferrous ion-induced generation of single and multiple strand breaks in the DNA plasmid pBR322 induces the formation of two new plasmid forms with altered electrophoretic mobility. The yield of these plasmid forms, the circular relaxed and the linear forms, depended on the applied Fe(2+) concentration. This property was independent of the presence of hydrogen peroxide in the incubation mixture indicating the lack of Fenton chemistry to explain the DNA degradation. The removal of dioxygen or the presence of superoxide dismutase diminished partially the yield of ferrous ion-induced DNA plasmid degradation, while catalase was without any effect. Autoxidation of divalent iron as followed by the formation of a coloured iron-phenanthroline complex was enhanced in a concentration-dependent manner by phosphate and bicarbonate and very efficiently using a mixture of 0.15 M NaCl, 1.2 mM phosphate, 23.8 mM bicarbonate, pH 7.4, that concentrations correspond closely to the intracellular values of buffer components. Thus, the formation of a yet unknown reactive species from Fe(2+), and dioxygen, that is complexed to buffer components especially phosphate and its contribution in DNA plasmid degradation is more likely than the often cited formation of hydroxyl radicals in result of the Fenton reaction from Fe(2+) and hydrogen peroxide.

Analysis of DHE-derived oxidation products by HPLC in the assessment of superoxide production and NADPH oxidase activity in vascular systems

Dihydroethidium (DHE) is a widely used sensitive superoxide (O2(*-)) probe. However, DHE oxidation yields at least two fluorescent products, 2-hydroxyethidium (EOH), known to be more specific for O2(*-), and the less-specific product ethidium. We validated HPLC methods to allow quantification of DHE products in usual vascular experimental situations. Studies in vitro showed that xanthine/xanthine oxidase, and to a lesser degree peroxynitrite/carbon dioxide system led to EOH and ethidium formation. Peroxidase/H2O2 but not H2O2 alone yielded ethidium as the main product. In vascular smooth muscle cells incubated with ANG II (100 nM, 4 h), we showed a 60% increase in EOH/DHE ratio, prevented by PEG-SOD or SOD1 overexpression. We further validated a novel DHE-based NADPH oxidase assay in vascular smooth muscle cell membrane fractions, showing that EOH was uniquely increased after ANG II. This assay was also adapted to a fluorescence microplate reader, providing results in line with HPLC results. In injured artery slices, shown to exhibit increased DHE-derived fluorescence at microscopy, there was approximately 1.5- to 2-fold increase in EOH/DHE and ethidium/DHE ratios after injury, and PEG-SOD inhibited only EOH formation. We found that the amount of ethidium product and EOH/ethidium ratios are influenced by factors such as cell density and ambient light. In addition, we indirectly disclosed potential roles of heme groups and peroxidase activity in ethidium generation. Thus HPLC analysis of DHE-derived oxidation products can improve assessment of O2(*-) production or NADPH oxidase activity in many vascular experimental studies.

The role of CO2 in metal-catalyzed peroxidations

Transition metals, such as Cu(+2), Mn(+2), and Co(+2), have been seen to catalyze the bicarbonate enhanced oxidation of a variety of substrates by H(2)O(2). In several of these cases it has been demonstrated that CO(2), rather than bicarbonate, is the enhancing species. Mechanisms that are in accord with the data involve a hypervalent state that may be written (MO)(+n), or (MOH)(+n+1), or (M)(+n+2). This metal centered oxidant then oxidizes CO(2) to the carbonate radical; that is then the proximal oxidant of the various substrates. Whether a similar process has in vivo reality remains to be demonstrated.

Pulse Radiolysis Studies of the Reactions of CO3•- and NO2• with Nitrosyl(II)myoglobin and Nitrosyl(II)hemoglobin

The reactions of carbonate radical anion [CO3*-, systematic name: trioxidocarbonate*1-] with nitrosyl(II)hemoglobin (HbFe(II)NO) and nitrosyl(II)myoglobin (MbFe(II)NO) were studied by pulse radiolysis in N2O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature. The reactions proceed in two steps: outer-sphere oxidation of the nitrosyliron(II) proteins to their corresponding nitrosyliron(III) forms and subsequent dissociation of NO*. The second-order rate constants measured for the first reaction steps were (4.3 +/- 0.2) x 10(8) and (1.5 +/- 0.3) x 10(8) M(-1) s(-1), for MbFe(II)NO and HbFe(II)NO, respectively. The reactions between nitrogen dioxide and MbFe(II)NO or HbFe(II)NO were studied by pulse radiolysis in N2O-saturated 0.1 M phosphate buffer pH 7.4 containing 5 mM nitrite. Also for the reactions of this oxidant with the nitrosyliron(II) forms of Mb and Hb a two-step reaction was observed: oxidation of the iron was followed by dissociation of NO*. The second-order rate constants measured for the first reaction steps were (2.9 +/- 0.3) x 10(7) and (1.8 +/- 0.3) x 10(7) M(-1) s(-1), for MbFe(II)NO and HbFe(II)NO, respectively. Both radicals appear to be able to oxidize the iron(II) centers of the proteins directly. Only for the reactions with HbFe(II)NO it cannot be excluded that, in a parallel reaction, CO3*- and NO2* first react with amino acid(s) of the globin, which then oxidize the nitrosyliron(II) center.

Competition between superoxide and hydrogen peroxide signaling in heterolytic enzymatic processes

Signaling functions of superoxide and hydrogen peroxide in enzymatic phosphorylation/dephosphorylation reactions are now well documented, but their mechanisms are still not always clear. Now we propose the novel signaling mechanisms, by which superoxide and hydrogen peroxide mediate the activation and inhibition of phosphorylation/dephosphorylation catalyzed by protein kinases and protein phosphatases. We suggest that as a powerful nucleophile, superoxide is able to mediate phosphorylation of numerous proteins by protein kinases through the deprotonation of protein serine or threonine residues that sharply accelerates the rates of nucleophilic reaction between kinases and phosphorylating proteins. Furthermore the role of superoxide is enhanced due to its "chain" formation in the O(2)(-)--> PI 3-kinase --> protein kinases --> NADPH oxidase --> O(2)(-) cycle. Furthermore we suggest that hydrogen peroxide signaling in the dephosphorylation reactions by protein phosphatases and in the activation of protein kinases is actually mediated by superoxide formed during the conversion of H(2)O(2) into superoxide by the oxidized superoxide dismutase. This proposal is supported by the high rates of superoxide reactions with an anion of the catalytic cysteine residue of protein tyrosine phosphatases and the inability of hydrogen peroxide to react directly with protein serine and threonine residues in the reactions of protein kinases. Understanding of specific role of superoxide in the reactions catalyzed by protein kinases and protein phosphatases can be of importance for the selection of inhibitors of these enzymes playing a big role in numerous physiological and pathological processes.

Protein oxidation by the cytochrome P450 mixed-function oxidation system

This mini-review summarizes results of studies on the oxidation of proteins and low-density lipoprotein (LDL) by various mixed-function oxidation (MFO) systems. Oxidation of LDL by the O2/FeCl3/H2O2/ascorbate MFO system is dependent on all four components and is much greater when reactions are carried out in the presence of a physiological bicarbonate/CO2 buffer system as compared to phosphate buffer. However, FeCl3 in this system could be replaced by hemin or the heme-containing protein, hemoglobin, or cytochrome c. Oxidation of LDL by the O2/cytochrome P450 cytochrome c reductase/NADPH/FeCl3 MFO system is only slightly higher (25%) in the bicarbonate/CO2 buffer as compared to phosphate buffer, but is dependent on all components except FeCl3. Omission of FeCl3 led to a 60% loss of activity. These results suggest that peroxymonobicarbonate and/or free radical derivatives of bicarbonate ion and/or CO2 might contribute to LDL oxidation by these MFO systems.

The role of CO2 in cobalt-catalyzed peroxidations

Augmentation, by CO(2)/HCO(3)(-), of Co(II)-catalyzed peroxidations was explored to clarify whether the rate enhancement was due to CO(2) or to HCO(3)(-). The rate of oxidation of NADH by Co(II) plus H(2)O(2), in Tris or phosphate, was markedly enhanced by CO(2)/HCO(3)(-). Phosphate was seen to inhibit the Co(II)-catalyzed peroxidation, probably due to its sequestration of the Co(II). When CO(2) was used, there was an initial burst of NADH oxidation followed by a slower linear rate. The presence of carbonic anhydrase eliminated this initial burst; establishing that CO(2) rather than HCO(3)(-) was the species responsible for the observed rate enhancements. Both kinetic and spectral data indicated that Co(II) was converted by H(2)O(2) into a less active form from which Co(II) could be regenerated. This less active form absorbed in both the UV and visible regions, and is assumed to be a peroxy bridged binuclear complex. The rate of formation of this absorbing form was increased by HCO(3)(-)/CO(2). A minimal mechanism consistent with these observations is proposed.

Copper-catalyzed Protein Oxidation and Its Modulation by Carbon Dioxide

It is well known that hydrogen peroxide (H2O2)-induced copper-catalyzed fragmentation of proteins follows a site-specific oxidative mechanism mediated by hydroxyl radical-like species (i.e. Cu(I)O, Cu(II)/*OH or Cu(III)) that ends in increased carbonyl formation and protein fragmentation. We have found that the nitrone spin trap DMPO (5,5-dimethyl-1-pyrroline N-oxide) prevented such processes by trapping human serum albumin (HSA)-centered radicals, in situ and in real time, before they reacted with oxygen. When (bi)carbonate (CO2, H2CO3, HCO3- and CO3(-2)) was added to the reaction mixture, it blocked fragmentation mediated by hydroxyl radical-like species but enhanced DMPO-trappable radical sites in HSA. In the past, this effect would have been explained by oxidation of (bi)carbonate to a carbonate radical anion (CO3*) by a bound hydroxyl radical-like species. We now propose that the CO3* radical is formed by the reduction of HOOCO2- (a complex of H2O2 with CO2) by the protein-Cu(I) complex. CO3* diffuses and produces more DMPO-trappable radical sites but does not fragment HSA. We were also able, for the first time, to detect discrete but highly specific H2O2-induced copper-catalyzed CO3*-mediated induction of DMPO-trappable protein radicals in functioning RAW 264.7 macrophages. We conclude that carbon dioxide modulates H2O2-induced copper-catalyzed oxidative damage to proteins by preventing site-specific fragmentation and enhancing DMPO-trappable protein radicals in functioning cells. The pathophysiological significance of our findings is discussed.

Mechanism of redox-mediated guanine nucleotide exchange on redox-active Rho GTPases

Rho GTPases regulate multiple cellular processes including actin cytoskeletal rearrangements, transcriptional regulation, and oxidant production. The studies described herein demonstrate that small molecule redox agents, in addition to protein regulatory factors, can regulate the activity of redox-active Rho GTPases. A novel (GXXXXGK(S/T)C) motif, conserved in a number of Rho GTPases, appears critical for redox-mediated guanine nucleotide dissociation in vitro. A detailed molecular mechanism for redox regulation of GXXXXGK(S/T)C motif-containing Rho GTPases is proposed.

Superoxide anion radical modulates the activity of Ras and Ras-related GTPases by a radical-based mechanism similar to that of nitric oxide

Ras GTPases cycle between inactive GDP-bound and active GTP-bound states to modulate a diverse array of processes involved in cellular growth control. The activity of Ras is up-regulated by cellular agents, including both protein (guanine nucleotide exchange factors) and redox-active agents (nitric oxide (NO) and superoxide anion radical (O2*). We have recently elucidated the mechanism by which NO promotes guanine nucleotide dissociation of redox-active NKCD motif-containing Ras and Ras-related GTPases. In this study, we show that guanine nucleotide dissociation is enhanced upon exposure of the redox-active GTPases, Ras and Rap1A, to O2* and provide evidence for the efficient guanine nucleotide reassociation in the presence of the radical quenching agent ascorbate to complete guanine nucleotide exchange. In vivo, guanine nucleotide reassociation is necessary to populate Ras in its biologically active GTP-bound form after the dissociation of GDP. We further show that treatment of the redox-active GTPases with O2* releases GDP in form of an unstable the oxygenated GDP adduct, putatively assigned as 5-oxo-GDP. 5-Oxo-GDP was not produced from either the C118S or the F28L Ras variants upon the treatment of O2*, supporting the involvement of residues Cys118 and Phe28 in O2*-mediated Ras guanine nucleotide dissociation. These results indicate that the mechanism of O2*-mediated Ras guanine nucleotide dissociation is similar to that of NO/O2-mediated Ras guanine nucleotide dissociation.

Production of the Carbonate Radical Anion during Xanthine Oxidase Turnover in the Presence of Bicarbonate

Xanthine oxidase is generally recognized as a key enzyme in purine catabolism, but its structural complexity, low substrate specificity, and specialized tissue distribution suggest other functions that remain to be fully identified. The potential of xanthine oxidase to generate superoxide radical anion, hydrogen peroxide, and peroxynitrite has been extensively explored in pathophysiological contexts. Here we demonstrate that xanthine oxidase turnover at physiological pH produces a strong one-electron oxidant, the carbonate radical anion. The radical was shown to be produced from acetaldehyde oxidation by xanthine oxidase in the presence of catalase and bicarbonate on the basis of several lines of evidence such as oxidation of both dihydrorhodamine 123 and 5,5-dimethyl-1-pyrroline-N-oxide and chemiluminescence and isotope labeling/mass spectrometry studies. In the case of xanthine oxidase acting upon xanthine and hypoxanthine as substrates, carbonate radical anion production was also evidenced by the oxidation of 5,5-dimethyl-1-pyrroline-N-oxide and of dihydrorhodamine 123 in the presence of uricase. The results indicated that Fenton chemistry occurring in the bulk solution is not necessary for carbonate radical anion production. Under the conditions employed, the radical was likely to be produced at the enzyme active site by reduction of a peroxymonocarbonate intermediate whose formation and reduction is facilitated by the many xanthine oxidase redox centers. In addition to indicating that the carbonate radical anion may be an important mediator of the pathophysiological effects of xanthine oxidase, the results emphasize the potential of the bicarbonate-carbon dioxide pair as a source of biological oxidants.

Inactivation of human Cu,Zn superoxide dismutase by peroxynitrite and formation of histidinyl radical

Human recombinant copper-zinc superoxide dismutase (CuZnSOD) was inactivated by peroxynitrite, the product of the reaction between nitric oxide and superoxide. The concentration of peroxynitrite that decreased the activity by 50% (IC(50)) was approximately 100 microM at 5 microM CuZnSOD and the inactivation was higher at alkaline pH. Stopped-flow determinations showed that the second-order rate constant for the direct reaction of peroxynitrite with CuZnSOD was (9.4 +/- 1.0) x 10(3) M(-1) s(-1) per monomer at pH 7.5 and 37 degrees C. Addition of peroxynitrite (1 mM) to CuZnSOD (0.5 mM) in the presence of the spin trap 2-methyl-2-nitrosopropane led to the electron paramagnetic resonance detection of an anisotropic signal typical of a protein radical adduct. Treatment with Pronase revealed a nearly isotropic signal consistent with the formation of histidinyl radical. The effects of nitrite, hydrogen peroxide, bicarbonate, and mannitol on the inactivation were assessed. Considering the mechanism accepted for the reaction of CuZnSOD with hydrogen peroxide and the fact that CuZnSOD promotes the nitration of phenolics by peroxynitrite, we herein propose that peroxynitrite reacts with CuZnSOD leading to nitrogen dioxide plus a copper-bound hydroxyl radical species that reacts with histidine residues, forming histidinyl radical.