Chlorination of bacterial and neutrophil proteins during phagocytosis and killing of Staphylococcus aureus

Voltage-gated proton channels maintain pH in human neutrophils during phagocytosis

Phagocytosis of microbial invaders represents a fundamental defense mechanism of the innate immune system. The subsequent killing of microbes is initiated by the respiratory burst, in which nicotinamide adenine dinucleotide phosphate (NADPH) oxidase generates vast amounts of superoxide anion, precursor to bactericidal reactive oxygen species. Cytoplasmic pH regulation is crucial because NADPH oxidase functions optimally at neutral pH, yet produces enormous quantities of protons. We monitored pH(i) in individual human neutrophils during phagocytosis of opsonized zymosan, using confocal imaging of the pH sensing dye SNARF-1, enhanced by shifted excitation and emission ratioing, or SEER. Despite long-standing dogma that Na(+)/H(+) antiport regulates pH during the phagocyte respiratory burst, we show here that voltage-gated proton channels are the first transporter to respond. During the initial phagocytotic event, pH(i) decreased sharply, and recovery required both Na(+)/H(+) antiport and proton current. Inhibiting myeloperoxidase attenuated the acidification, suggesting that diffusion of HOCl into the cytosol comprises a substantial acid load. Inhibiting proton channels with Zn(2+) resulted in profound acidification to levels that inhibit NADPH oxidase. The pH changes accompanying phagocytosis in bone marrow phagocytes from HVCN1-deficient mice mirrored those in control mouse cells treated with Zn(2+). Both the rate and extent of acidification in HVCN1-deficient cells were twice larger than in control cells. In summary, acid extrusion by proton channels is essential to the production of reactive oxygen species during phagocytosis.

DNase I inhibits a late phase of reactive oxygen species production in neutrophils

Neutrophils kill bacteria on extracellular complexes of DNA fibers and bactericidal proteins known as neutrophil extracellular traps (NETs). The NET composition and the bactericidal mechanisms they use are not fully understood. Here, we show that treatment with deoxyribonuclease (DNase I) impairs a late oxidative response elicited by Gram-positive and Gram-negative bacteria and also by phorbol ester. Isoluminol-dependent chemiluminescence elicited by opsonized Listeria monocytogenes-stimulated neutrophils was inhibited by DNase I, and the DNase inhibitory effect was also evident when phagocytosis was blocked, suggesting that DNase inhibits an extracellular mechanism of reactive oxygen species (ROS) generation. The DNase inhibitory effect was independent of actin polymerization. Phagocytosis and cell viability were not impaired by DNase I. Immunofluorescence analysis shows that myeloperoxidase is present on NETs. Furthermore, granular proteins were detected in NETs from Rab27a-deficient neutrophils which have deficient exocytosis, suggesting that exocytosis and granular protein distribution on NETs proceed by independent mechanisms. NADPH oxidase subunits were also detected on NETs, and the detection of extracellular trap-associated NADPH oxidase subunits was abolished by treatment with DNase I and dependent on cell stimulation. In vitro analyses demonstrate that MPO and NADPH oxidase activity are not directly inhibited by DNase I, suggesting that its effect on ROS production depends on NET disassembly. Altogether, our data suggest that inhibition of ROS production by microorganism-derived DNase would contribute to their ability to evade killing.

Neutrophil bleaching of GFP-expressing staphylococci: probing the intraphagosomal fate of individual bacteria

Successful host defense against bacteria such as Staphylococcus aureus (SA) depends on a prompt response by circulating polymorphonuclear leukocytes (PMN). Stimulated PMN create in their phagosomes an environment inhospitable to most ingested bacteria. Granules that fuse with the phagosome deliver an array of catalytic and noncatalytic antimicrobial peptides, while activation of the NADPH oxidase at the phagosomal membrane generates reactive oxygen species within the phagosome, including hypochlorous acid (HOCl), formed by the oxidation of chloride by the granule protein myeloperoxidase in the presence of H(2)O(2). In this study, we used SA-expressing cytosolic GFP to provide a novel probe of the fate of SA in human PMN. PMN bleaching of GFP in SA required phagocytosis, active myeloperoxidase, H(2)O(2) from the NADPH oxidase, and chloride. Not all ingested SA were bleached, and the number of cocci within PMN-retaining fluorescent GFP closely correlated with the number of viable bacteria remaining intracellularly. The percent of intracellular fluorescent and viable SA increased at higher multiplicity of infection and when SA presented to PMN had been harvested from the stationary phase of growth. These studies demonstrate that the loss of GFP fluorescence in ingested SA provides a sensitive experimental probe for monitoring biochemical events within individual phagosomes and for identifying subpopulations of SA that resist intracellular PMN cytotoxicity. Defining the molecular basis of SA survival within PMN should provide important insights into bacterial and host properties that limit PMN antistaphylococcal action and thus contribute to the pathogenesis of staphylococcal infection.

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.

Mammalian heme peroxidases: from molecular mechanisms to health implications

A marked increase in interest has occurred over the last few years in the role that mammalian heme peroxidase enzymes, primarily myeloperoxidase, eosinophil peroxidase, and lactoperoxidase, may play in both disease prevention and human pathologies. This increased interest has been sparked by developments in our understanding of polymorphisms that control the levels of these enzymes, a greater understanding of the basic chemistry and biochemistry of the oxidants formed by these species, the development of specific biomarkers that can be used in vivo to detect damage induced by these oxidants, the detection of active forms of these peroxidases at most, if not all, sites of inflammation, and a correlation between the levels of these enzymes and a number of major human pathologies. This article reviews recent developments in our understanding of the enzymology, chemistry, biochemistry and biologic roles of mammalian peroxidases and the oxidants that they generate, the potential role of these oxidants in human disease, and the use of the levels of these enzymes in disease prognosis.

Role of Nox2 in elimination of microorganisms

NADPH oxidase of the phagocytic cells (Nox2) transfers electrons from cytosolic NADPH to molecular oxygen in the extracellular or intraphagosomal space. The produced superoxide anion (O*2) provides the source for formation of all toxic oxygen derivatives, but continuous O*2 generation depends on adequate charge compensation. The vital role of Nox2 in efficient elimination of microorganisms is clearly indicated by human pathology as insufficient activity of the enzyme results in severe, recurrent bacterial infections, the typical symptoms of chronic granulomatous disease. The goals of this contribution are to provide critical review of the Nox2-dependent cellular processes that potentially contribute to bacterial killing and degradation and to indicate possible targets of pharmacological interventions.

The role of chloride anion and CFTR in killing of Pseudomonas aeruginosa by normal and CF neutrophils

Chloride anion is essential for myeloperoxidase (MPO) to produce hypochlorous acid (HOCl) in polymorphonuclear neutrophils (PMNs). To define whether chloride availability to PMNs affects their HOCl production and microbicidal capacity, we examined how extracellular chloride concentration affects killing of Pseudomonas aeruginosa (PsA) by normal neutrophils. PMN-mediated bacterial killing was strongly dependent on extracellular chloride concentration. Neutrophils in a chloride-deficient medium killed PsA poorly. However, as the chloride level was raised, the killing efficiency increased in a dose-dependent manner. By using specific inhibitors to selectively block NADPH oxidase, MPO, and cystic fibrosis transmembrane conductance regulator (CFTR) functions, neutrophil-mediated killing of PsA could be attributed to three distinct mechanisms: CFTR-dependent and oxidant-dependent; chloride-dependent but not CFTR- and oxidant-dependent; and independent of any of the tested factors. Therefore, chloride anion is involved in oxidant- and nonoxidant-mediated bacterial killing. We previously reported that neutrophils from CF patients are defective in chlorination of ingested bacteria, suggesting that the chloride channel defect might impair the MPO-hydrogen peroxide-chloride microbicidal function. Here, we compared the competence of killing PsA by neutrophils from normal donors and CF patients. The data demonstrate that the killing rate by CF neutrophils was significantly lower than that by normal neutrophils. CF neutrophils in a chloride-deficient environment had only one-third of the bactericidal capacity of normal neutrophils in a physiological chloride environment. These results suggest that CFTR-dependent chloride anion transport contributes significantly to killing PsA by normal neutrophils and when defective as in CF, may compromise the ability to clear PsA.

Neutrophil microbicides induce a pathogen survival response in community-associated methicillin-resistant Staphylococcus aureus

In recent years, there has been a dramatic increase in the incidence of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections. MW2 (pulsed-field type USA400), the prototype CA-MRSA strain, is highly virulent and has enhanced ability to evade killing by neutrophils. Although progress has been made, the molecular basis for enhanced virulence of CA-MRSA remains incompletely defined. To that end, we studied resistance of MW2 to key microbicides of human neutrophils. Hydrogen peroxide (H2O2), hypochlorous acid, and azurophilic granule proteins had significant bacteriostatic but limited staphylocidal activity toward MW2 under the conditions tested. An MW2-specific microarray revealed common changes in S. aureus gene expression following exposure to each microbicide, such as up-regulation of transcripts involved in gene regulation (e.g., saeRS and kdpDE) and stress response. Azurophilic granule proteins elicited the greatest number of changes in MW2 transcripts, including up-regulation of mRNAs encoding multiple toxins and hemolysins (e.g., hlgA, hlgB, hlgC, hla, lukS-PV, lukF-PV, sec4, and set17-26). Notably, H2O2 triggered up-regulation of transcripts related to heme/iron uptake (e.g., isdA, isdB, and isdCDEFsrtBisdG), and an isogenic isdAB-negative strain of MW2 had increased susceptibility to H2O2 (p<0.001) and human neutrophils (p<0.05) compared with the wild-type parental strain. These findings reveal a S. aureus survival response wherein Iron-regulated surface determinant (Isd) proteins are important for resistance to innate host defense. Collectively, the data provide an enhanced view of the mechanisms used by S. aureus to circumvent destruction by the innate immune system.

How human neutrophils kill and degrade microbes: an integrated view

Neutrophils constitute the dominant cell in the circulation that mediates the earliest innate immune human responses to infection. The morbidity and mortality from infection rise dramatically in patients with quantitative or qualitative neutrophil defects, providing clinical confirmation of the important role of normal neutrophils for human health. Neutrophil-dependent anti-microbial activity against ingested microbes represents the collaboration of multiple agents, including those prefabricated during granulocyte development in the bone marrow and those generated de novo following neutrophil activation. Furthermore, neutrophils cooperate with extracellular agents as well as other immune cells to optimally kill and degrade invading microbes. This brief review focuses attention on two examples of the integrated nature of neutrophil-mediated anti-microbial action within the phagosome. The importance and complexity of myeloperoxidase-mediated events illustrate a collaboration of anti-microbial responses that are endogenous to the neutrophil, whereas the synergy between the phagocyte NADPH (nicotinamide adenine dinucleotide phosphate) oxidase and plasma-derived group IIA phospholipase A(2) exemplifies the collective effects of the neutrophil with an exogenous factor to achieve degradation of ingested staphylococci.

The function of the NADPH oxidase of phagocytes and its relationship to other NOXs in plants, invertebrates, and mammals

The NADPH (nicotinamide adenine dinucleotide phosphate) oxidase (NOX) of ‘professional’ phagocytic cells transfers electrons across the wall of the phagocytic vacuole, forming superoxide in the lumen. It is generally accepted that this system promotes microbial killing through the generation of reactive oxygen species and through the activity of myeloperoxidase. An alternative scenario exists in which the passage of electrons across the membrane alters the pH and generates a charge that drives ions into, and out of, the vacuole. It is proposed that the primary function of the oxidase is to produce these pH changes and ion fluxes, and the issues surrounding these processes are considered. The neutrophil oxidase is the prototype of a whole family of NOXs that exist throughout biology, from plants to man, which might function, at least in part, in a similar fashion. Some examples of how these other NOXs might influence ion fluxes are examined.

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.

Evaluation of amino acids as mediators for the antibacterial activity of iodine-lithium-alpha-dextrin in vitro and in vivo

OBJECTIVES

The systemic therapeutic application of iodophores has not yet been accepted due to limited availability of safe and effective ionized iodine preparations. Here we evaluated the antibacterial activity of iodine-lithium-alpha-dextrin (ILalphaD) both in vitro and in vivo.

METHODS

The MIC values of ILalphaD against 189 bacterial isolates in various growth media and in vivo toxicity and protective efficacy of ILalphaD in preventing mortality of rats infected with Staphylococcus aureus were determined. The intracellular killing of S. aureus by neutrophils in the presence of ILalphaD and myeloperoxidase (MPO)-catalysed oxidation of iodide was also determined.

RESULTS

The MIC values of ILalphaD against 189 Gram-positive cocci and Gram-negative bacilli ranged between 124-512 mg/L in growth media and 6.2-12.5 mg/L in buffer solution, and were highly variable in the presence of amino acids. We observed protection of S. aureus-infected rats from death with significant reduction of bacterial growth in organs upon intravenous administration of ILalphaD at doses that are 4-12 times lower than maximal in vivo tolerability dose. Intracellular killing of S. aureus by neutrophils increased in the presence of ILalphaD probably due to MPO-catalysed oxidation of iodide into hypoiodous acid. The pattern of ILalphaD reaction with amino acids at different pH or halide ion content determined both the generation of long-lived secondary oxidants and antibacterial activity.

CONCLUSIONS

Systemic application of ILalphaD proved to be successful in the rat infection model by promoting host defence. Probable mechanisms are increased intracellular killing of bacteria by production of hypoiodous acid and iodamines as well as anti-inflammatory activity.

Modeling the reactions of superoxide and myeloperoxidase in the neutrophil phagosome: implications for microbial killing

Neutrophils kill bacteria by ingesting them into phagosomes where superoxide and cytoplasmic granule constituents, including myeloperoxidase, are released. Myeloperoxidase converts chloride and hydrogen peroxide to hypochlorous acid (HOCl), which is strongly microbicidal. However, the role of oxidants in killing and the species responsible are poorly understood and the subject of current debate. To assess what oxidative mechanisms are likely to operate in the narrow confines of the phagosome, we have used a kinetic model to examine the fate of superoxide and its interactions with myeloperoxidase. Known rate constants for reactions of myeloperoxidase have been used and substrate concentrations estimated from neutrophil morphology. In the model, superoxide is generated at several mm/s. Most react with myeloperoxidase, which is present at millimolar concentrations, and rapidly convert the enzyme to compound III. Compound III turnover by superoxide is essential to maintain enzyme activity. Superoxide stabilizes at approximately 25 microM and hydrogen peroxide in the low micromolar range. HOCl production is efficient if there is adequate chloride supply, but further knowledge on chloride concentrations and transport mechanisms is needed to assess whether this is the case. Low myeloperoxidase concentrations also limit HOCl production by allowing more hydrogen peroxide to escape from the phagosome. In the absence of myeloperoxidase, superoxide increases to >100 microM but hydrogen peroxide to only approximately 30 microM. Most of the HOCl reacts with released granule proteins before reaching the bacterium, and chloramine products may be effectors of its antimicrobial activity. Hydroxyl radicals should form only after all susceptible protein targets are consumed.

CFTR Expression in human neutrophils and the phagolysosomal chlorination defect in cystic fibrosis

Production of hypochlorous acid (HOCl) in neutrophils, a critical oxidant involved in bacterial killing, requires chloride anions. Because the primary defect of cystic fibrosis (CF) is the loss of chloride transport function of the CF transmembrane conductance regulator (CFTR), we hypothesized that CF neutrophils may be deficient in chlorination of bacterial components due to a limited chloride supply to the phagolysosomal compartment. Multiple approaches, including RT-PCR, immunofluorescence staining, and immunoblotting, were used to demonstrate that CFTR is expressed in resting neutrophils at the mRNA and protein levels. Probing fractions of resting neutrophils isolated by Percoll gradient fractionation and free flow electrophoresis for CFTR revealed its presence exclusively in secretory vesicles. The CFTR chloride channel was also detected in phagolysosomes, a special organelle formed after phagocytosis. Interestingly, HL-60 cells, a human promyelocytic leukemia cell line, upregulated CFTR expresssion when induced to differentiate into neutrophils with DMSO, strongly suggesting its potential role in mature neutrophil function. Analyses by gas chromatography and mass spectrometry (GC-MS) revealed that neutrophils from CF patients had a defect in their ability to chlorinate bacterial proteins from Pseudomonas aeruginosa metabolically prelabeled with [(13)C]-l-tyrosine, unveiling defective intraphagolysosomal HOCl production. In contrast, both normal and CF neutrophils exhibited normal extracellular production of HOCl when stimulated with phorbol ester, indicating that CF neutrophils had the normal ability to produce this oxidant in the extracellular medium. This report provides evidence which suggests that CFTR channel expression in neutrophils and its dysfunction affect neutrophil chlorination of phagocytosed bacteria.

Phagocyte-derived reactive species: salvation or suicide?

Activated phagocytes produce "reactive oxygen, halogen and nitrogen species" that help to kill some types of microorganism. How these species destroy microorganisms remains, however, an enigma: both direct oxidative damage and indirect damage (whereby reactive species promote the actions of other antibacterial agents) are involved, and no single mechanism is likely to account for the killing of all microorganisms. Phagocyte-derived reactive species are known to injure human tissues and to contribute to inflammation. Recently, however, we have learned that they can also be anti-inflammatory by modulating the immune response. These data have implications for the proposed use of antioxidants to treat inflammation.

Staphyloxanthin plays a role in the fitness of Staphylococcus aureus and its ability to cope with oxidative stress

Staphyloxanthin is a membrane-bound carotenoid of Staphylococcus aureus. Here we studied the interaction of staphyloxanthin with reactive oxygen substances (ROS) and showed by comparative analysis of the wild type (WT) and an isogenic crtM mutant that the WT is more resistant to hydrogen peroxide, superoxide radical, hydroxyl radical, hypochloride, and neutrophil killing.

Effects of biological oxidants on the catalytic activity and structure of group VIA phospholipase A2

Group VIA phospholipase A(2) (iPLA(2)beta) is expressed in phagocytes, vascular cells, pancreatic islet beta-cells, neurons, and other cells and plays roles in transcriptional regulation, cell proliferation, apoptosis, secretion, and other events. A bromoenol lactone (BEL) suicide substrate used to study iPLA(2)beta functions inactivates iPLA(2)beta by alkylating Cys thiols. Because thiol redox reactions are important in signaling and some cells that express iPLA(2)beta produce biological oxidants, iPLA(2)beta might be subject to redox regulation. We report that biological concentrations of H(2)O(2), NO, and HOCl inactivate iPLA(2)beta, and this can be partially reversed by dithiothreitol (DTT). Oxidant-treated iPLA(2)beta modifications were studied by LC-MS/MS analyses of tryptic digests and included DTT-reversible events, e.g., formation of disulfide bonds and sulfenic acids, and others not so reversed, e.g., formation of sulfonic acids, Trp oxides, and Met sulfoxides. W(460) oxidation could cause irreversible inactivation because it is near the lipase consensus sequence ((463)GTSTG(467)), and site-directed mutagenesis of W(460) yields active mutant enzymes that exhibit no DTT-irreversible oxidative inactivation. Cys651-sulfenic acid formation could be one DTT-reversible inactivation event because Cys651 modification correlates closely with activity loss and its mutagenesis reduces sensitivity to inhibition. Intermolecular disulfide bond formation might also cause reversible inactivation because oxidant-treated iPLA(2)beta contains DTT-reducible oligomers, and oligomerization occurs with time- and temperature-dependent iPLA(2)beta inactivation that is attenuated by DTT or ATP. Subjecting insulinoma cells to oxidative stress induces iPLA(2)beta oligomerization, loss of activity, and subcellular redistribution and reduces the rate of release of arachidonate from phospholipids. These findings raise the possibility that redox reactions affect iPLA(2)beta functions.

Nitrotyrosine and Chlorotyrosine: Clinical Significance and Biological Functions in the Vascular System

The heme-containing enzyme myeloperoxidase (MPO) is both present and active in inflammatory conditions. This enzyme is potentiated by its formation of multiple inflammatory mediators. The two most common mediators are the modified tyrosines: nitrotyrosine and 3-chlorotyrosine. Along with other modified tyrosines, these molecules have been found to be elevated in atherosclerosis, lung disease, sepsis, vasculitis, and other inflammatory diseases. By treating some of these diseases, the levels of modified tyrosines have been shown to decrease. There have been a wide range of animal models designed to study the in vivo effects of these tyrosine molecules. In addition, there are also several reports in the literature of the in vitro actions of modified tyrosine molecules demonstrated by various cell-culture models. The purpose of this review is to evaluate the clinical significance and biological functions of these modified tyrosine molecules in atherosclerosis as well as a variety of other inflammatory conditions. It is timely information because of their association with diseases as well as lack of overview of their molecular actions. This review will focus on the formation, clinical significance, and animal and cell-culture models of these important molecules.

Charge compensation during the phagocyte respiratory burst

The phagocyte NADPH oxidase produces superoxide anion (O(2)(.-)) by the electrogenic process of moving electrons across the cell membrane. This charge translocation must be compensated to prevent self-inhibition by extreme membrane depolarization. Examination of the mechanisms of charge compensation reveals that these mechanisms perform several other vital functions beyond simply supporting oxidase activity. Voltage-gated proton channels compensate most of the charge translocated by the phagocyte NADPH oxidase in human neutrophils and eosinophils. Quantitative modeling of NADPH oxidase in the plasma membrane supports this conclusion and shows that if any other conductance is present, it must be miniscule. In addition to charge compensation, proton flux from the cytoplasm into the phagosome (a) helps prevent large pH excursions both in the cytoplasm and in the phagosome, (b) minimizes osmotic disturbances, and (c) provides essential substrate protons for the conversion of O(2)(*-) to H(2)O(2) and then to HOCl. A small contribution by K+ or Cl- fluxes may offset the acidity of granule contents to keep the phagosome pH near neutral, facilitating release of bactericidal enzymes. In summary, the mechanisms used by phagocytes for charge compensation during the respiratory burst would still be essential to phagocyte function, even if NADPH oxidase were not electrogenic.

Synergy between extracellular group IIA phospholipase A2 and phagocyte NADPH oxidase in digestion of phospholipids of Staphylococcus aureus ingested by human neutrophils

Acute inflammatory responses to invading bacteria such as Staphylococcus aureus include mobilization of polymorphonuclear leukocytes (PMN) and extracellular group IIA phospholipase A2 (gIIA-PLA2). Although accumulating coincidentally, the in vitro anti-staphylococcal activities of PMN and gIIA-PLA2 have thus far been studied separately. We now show that degradation of S. aureus phospholipids during and after phagocytosis by human PMN requires the presence of extracellular gIIA-PLA2. The concentration of extracellular gIIA-PLA2 required to produce bacterial digestion was reduced 10-fold by PMN. The effects of added gIIA-PLA2 were greater when present before phagocytosis but even apparent when added after S. aureus were ingested by PMN. Related group V and X PLA2, which are present within PMN granules, do not contribute to bacterial phospholipid degradation during and after phagocytosis even when added at concentrations 30-fold higher than that needed for action of the gIIA-PLA2. The action of added gIIA-PLA2 required catalytically active gIIA-PLA2 and, in PMN, a functional NADPH oxidase but not myeloperoxidase. These findings reveal a novel collaboration between cellular oxygen-dependent and extracellular oxygen-independent host defense systems that may be important in the ultimate resolution of S. aureus infections.

How neutrophils kill microbes

Neutrophils provide the first line of defense of the innate immune system by phagocytosing, killing, and digesting bacteria and fungi. Killing was previously believed to be accomplished by oxygen free radicals and other reactive oxygen species generated by the NADPH oxidase, and by oxidized halides produced by myeloperoxidase. We now know this is incorrect. The oxidase pumps electrons into the phagocytic vacuole, thereby inducing a charge across the membrane that must be compensated. The movement of compensating ions produces conditions in the vacuole conducive to microbial killing and digestion by enzymes released into the vacuole from the cytoplasmic granules.

Hypochlorous acid-mediated mitochondrial dysfunction and apoptosis in human hepatoma HepG2 and human fetal liver cells: role of mitochondrial permeability transition

Liver cirrhosis is often preceded by overt signs of hepatitis, including parenchymal cell inflammation and infiltration of polymorphonuclear (PMN) leukocytes. Activated PMNs release both reactive oxygen species and reactive halogen species, including hypochlorous acid (HOCl), which are known to be significantly cytotoxic due to their oxidizing potential. Because the role of mitochondria in the hepatotoxicity attributed to HOCl has not been elucidated, we investigated the effects of HOCl on mitochondrial function in the human hepatoma HepG2 cell line, human fetal liver cells, and isolated rat liver mitochondria. We show here that HOCl induced mitochondrial dysfunction, and apoptosis was dependent on the induction of the mitochondrial permeability transition (MPT), because HOCl induced mitochondrial swelling and collapse of the mitochondrial membrane potential with the concomitant release of cytochrome c. These biochemical events were inhibited by the classical MPT inhibitor cyclosporin A (CSA). Cell death induced by HOCl exhibited several classical hallmarks of apoptosis, including annexin V labeling, caspase activation, chromatin condensation, and cell body shrinkage. The induction of apoptosis by HOCl was further supported by the finding that CSA and caspase inhibitors prevented cell death. For the first time, these results show that HOCl activates the MPT, which leads to the induction of apoptosis and provides a novel insight into the mechanisms of HOCl-mediated cell death at sites of chronic inflammation.

Neutrophils in the war against Staphylococcus aureus: predator-prey models to the rescue

To address the question of whether a minimum concentration of blood neutrophils is necessary to decrease Staphylococcus aureus concentration in mastitic milk, literature was searched for studies in which neutrophils were incubated with Staph. aureus. Different mathematical models that describe the changes in Staph. aureus population as a function of neutrophilic concentrations were applied to the collected data. The best fitted model established (1) that the rate of bacterial killing depended on the ratio of neutrophils to bacteria with neutrophilic attack rate accelerating at first before decelerating as the ratio increases, and (2) that neutrophil concentration should be within a limited range to trigger a decline in the bacterial population. Outcomes of this model are supported by what is known about neutrophilic functions and laboratory findings in bovine and human neutrophils. These results may be of assistance in setting selection goals for a better resilience to Staph. aureus mastitis in dairy cattle. Indeed, an optimal neutrophilic concentration appears to exist for successful clearance of Staph. aureus infection, which is neither the lowest nor the highest one.

Myeloperoxidase plays critical roles in killing Klebsiella pneumoniae and inactivating neutrophil elastase: effects on host defense

Activated neutrophils use myeloperoxidase (MPO) to generate an array of potent toxic oxidants. In the current studies we used genetically altered mice deficient in MPO to investigate the role of the enzyme in host defense against the Gram-negative bacterium Klebsiella pneumoniae, an important human pathogen. For comparison, we used mice deficient in the antimicrobial molecule, neutrophil elastase (NE). When challenged i.p., mice deficient in either MPO or NE were markedly more susceptible to bacterial infection and death. In vitro studies suggested that MPO impairs the morphology of bacteria in a distinctive way. Of importance, our in vitro studies found that MPO mediated oxidative inactivation of NE, an enzyme that has been widely implicated in the pathogenesis of various tissue-destructive diseases. This pathway of oxidative inactivation may be physiologically relevant, because activated neutrophils isolated from MPO-deficient mice exhibited increased elastase activity. Our observations provide strong evidence that MPO, like NE, is a key player in the killing of K. pneumoniae bacteria. They also suggest that MPO may modulate NE to protect the host from the tissue-degrading activity of this proteinase.

PKC-induced sensitization of Ca2+-dependent exocytosis is mediated by reducing the Ca2+ cooperativity in pituitary gonadotropes

The highly cooperative nature of Ca2+-dependent exocytosis is very important for the precise regulation of transmitter release. It is not known whether the number of binding sites on the Ca2+ sensor can be modulated or not. We have previously reported that protein kinase C (PKC) activation sensitizes the Ca2+ sensor for exocytosis in pituitary gonadotropes. To further unravel the underlying mechanism of how the Ca2+ sensor is modulated by protein phosphorylation, we have performed kinetic modeling of the exocytotic burst and investigated how the kinetic parameters of Ca2+-triggered fusion are affected by PKC activation. We propose that PKC sensitizes exocytosis by reducing the number of calcium binding sites on the Ca2+ sensor (from three to two) without significantly altering the Ca2+-binding kinetics. The reduction in the number of Ca2+-binding steps lowers the threshold for release and up-regulates release of fusion-competent vesicles distant from Ca2+ channels.

Myeloperoxidase: friend and foe

Neutrophilic polymorphonuclear leukocytes (neutrophils) are highly specialized for their primary function, the phagocytosis and destruction of microorganisms. When coated with opsonins (generally complement and/or antibody), microorganisms bind to specific receptors on the surface of the phagocyte and invagination of the cell membrane occurs with the incorporation of the microorganism into an intracellular phagosome. There follows a burst of oxygen consumption, and much, if not all, of the extra oxygen consumed is converted to highly reactive oxygen species. In addition, the cytoplasmic granules discharge their contents into the phagosome, and death of the ingested microorganism soon follows. Among the antimicrobial systems formed in the phagosome is one consisting of myeloperoxidase (MPO), released into the phagosome during the degranulation process, hydrogen peroxide (H2O2), formed by the respiratory burst and a halide, particularly chloride. The initial product of the MPO-H2O2-chloride system is hypochlorous acid, and subsequent formation of chlorine, chloramines, hydroxyl radicals, singlet oxygen, and ozone has been proposed. These same toxic agents can be released to the outside of the cell, where they may attack normal tissue and thus contribute to the pathogenesis of disease. This review will consider the potential sources of H2O2 for the MPO-H2O2-halide system; the toxic products of the MPO system; the evidence for MPO involvement in the microbicidal activity of neutrophils; the involvement of MPO-independent antimicrobial systems; and the role of the MPO system in tissue injury. It is concluded that the MPO system plays an important role in the microbicidal activity of phagocytes.

Antimicrobial reactive oxygen and nitrogen species: concepts and controversies

Phagocyte-derived reactive oxygen and nitrogen species are of crucial importance for host resistance to microbial pathogens. Decades of research have provided a detailed understanding of the regulation, generation and actions of these molecular mediators, as well as their roles in resisting infection. However, differences of opinion remain with regard to their host specificity, cell biology, sources and interactions with one another or with myeloperoxidase and granule proteases. More than a century after Metchnikoff first described phagocytosis, and more than four decades after the discovery of the burst of oxygen consumption that is associated with microbial killing, the seemingly elementary question of how phagocytes inhibit, kill and degrade microorganisms remains controversial. This review updates the reader on these concepts and the topical questions in the field.

The myeloperoxidase product hypochlorous acid oxidizes HDL in the human artery wall and impairs ABCA1-dependent cholesterol transport

Although oxidatively damaged lipoproteins are implicated in vascular injury, there is little information regarding the role of high-density lipoprotein (HDL) oxidation in atherogenesis. One potential pathway involves hypochlorous acid (HOCl) produced by myeloperoxidase (MPO), a heme protein secreted by phagocytes. We previously showed that 3-chlorotyrosine is a specific product of HOCl. Therefore, to explore the role of oxidized HDL in the pathogenesis of vascular disease, we used MS to quantify 3-chlorotyrosine in HDL isolated from plasma and atherosclerotic tissue. HDL from human aortic atherosclerotic intima had an 8-fold higher level of 3-chlorotyrosine than plasma HDL. Tandem MS analysis identified MPO as a component of lesion HDL, suggesting that the two interact in the artery wall. Moreover, immunohistochemical studies found that specific epitopes derived from HOCl colocalized with apolipoprotein A-I, the major protein of HDL. These observations strongly support the hypothesis that MPO promotes HDL oxidation in the human artery wall. Levels of 3-chlorotyrosine were elevated in HDL isolated from the blood of humans with established coronary artery disease, suggesting that circulating levels of oxidized HDL represent a unique marker for clinically significant atherosclerosis. HDL or lipid-free apolipoprotein A-I exposed to HOCl was less able to remove cholesterol from cultured cells by a pathway requiring the cell membrane transporter ATP-binding cassette transporter A1. The detection of 3-chlorotyrosine in HDL isolated from vascular lesions raises the possibility that MPO, by virtue of its ability to form HOCl, may promote atherogenesis by counteracting the established antiatherogenic effects of HDL and the ATP-binding cassette transporter A1 pathway.

Interaction of antimycobacterial drugs with the anti-Mycobacterium avium complex effects of antimicrobial effectors, reactive oxygen intermediates, reactive nitrogen intermediates, and free fatty acids produced by macrophages

The profiles of the interaction of antimycobacterial drugs with macrophage (MPhi) antimicrobial mechanisms have yet to be elucidated in detail. We examined the effects of various antimycobacterial drugs on the anti-Mycobacterium avium complex (MAC) antimicrobial activity of reactive oxygen intermediates (ROIs), especially of an H(2)O(2)-halogen (H(2)O(2)-Fe(2+)-NaI)-mediated bactericidal system, reactive nitrogen intermediates (RNIs), and free fatty acids (FFAs), which are known as central antimicrobial effectors of host MPhis against mycobacterial pathogens. We have found that certain drugs, such as rifampin (RIF), rifabutin (RFB), isoniazid (INH), clofazimine (CLO), and some fluoroquinolones, strongly or moderately reduced the anti-MAC activity of the H(2)O(2)-Fe(2+)-NaI system, primarily by inhibiting the generation of hypohalite ions and in part by interfering with the halogenation reaction of bacterial cell components due to the H(2)O(2)-Fe(2+)-NaI system. This phenomenon is specific to the H(2)O(2)-Fe(2+)-NaI system, since these drugs did not reduce the anti-MAC activity of RNIs and FFAs. From the perspective of the chemotherapy of MAC infections, the present findings indicate an important possibility that certain antimycobacterial drugs, such as rifamycins (RIF and RFB), INH, CLO, and also some types of fluoroquinolones, may interfere with the ROI-mediated antimicrobial mechanisms of host MPhis against intracellular MAC organisms.

The NADPH oxidase of professional phagocytes--prototype of the NOX electron transport chain systems

The NADPH oxidase is an electron transport chain in "professional" phagocytic cells that transfers electrons from NADPH in the cytoplasm, across the wall of the phagocytic vacuole, to form superoxide. The electron transporting flavocytochrome b is activated by the integrated function of four cytoplasmic proteins. The antimicrobial function of this system involves pumping K+ into the vacuole through BKCa channels, the effect of which is to elevate the vacuolar pH and activate neutral proteases. A number of homologous systems have been discovered in plants and lower animals as well as in man. Their function remains to be established.

Morphological and physiological changes of Staphylococcus aureus exposed to hypochlorous acid

AIMS

To characterize hypochlorous acid (HOCl) stress resistance of Staphylococcus aureus and to assess physiological state and changes in cell morphology.

METHODS AND RESULTS

Clinical wild-type strain of S. aureus was used in the stress with HOCl at concentrations ranging from 0 to 4 mg l(-1). Concentrations below 1.5 mg l(-1) caused no significant drop in viability. During 2 h of HOCl stress at 2 mg l(-1), there was appearance of minicells capable of passing through the 0.45 microm pores of filtration membranes. Intracellular proteins increased gradually to reach a level of 51% of dry weight and an enhanced synthesis of at least two proteins of 23 and 220 kDa was concluded.

CONCLUSIONS, SIGNIFICANCE AND IMPACT OF THE STUDY

Staphylococcus aureus can undergo morphological and physiological changes during 2 h of exposure to 2 mg l(-1) of HOCl, which represents an adaptative response towards the hypochlorous acid stress. This evolution limits the use of 0.45 microm pores size membrane filters for research on S. aureus in waters and the clinical environment.

HOCl-mediated cell death and metabolic dysfunction in the yeast Saccharomyces cerevisiae

The nature of oxidative damage to Saccharomyces cerevisiae caused by levels of HOCl that inhibit cell replication was explored with the intent of identifying the loci of lethal lesions. Functions of cytosolic enzymes and organelles that are highly sensitive to inactivation by HOCl, including aldolase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and the mitochondrion, were only marginally affected by exposure of the yeast to levels of HOCl that completely inhibited colony formation. Loss of function in membrane-localized proteins, including the hexose transporters and PMA1 H(+)-ATPase, which is the primary proton pump located within the S. cerevisiae plasma membrane, was also marginal and K(+) leak rates to the extracellular medium increased only slowly with exposure to increasing amounts of HOCl, indicating that the plasma membrane retained its intrinsic impermeability to ions and metabolites. Adenylate phosphorylation levels in fermenting yeast declined in parallel with viability; however, yeast grown on respiratory substrates maintained near-normal phosphorylation levels at HOCl doses several-fold greater than that required for killing. This overall pattern of cellular response to HOCl differs markedly from that previously reported for bacteria, which appear to be killed by inhibition of plasma membrane proteins involved in energy transduction. The absence of significant loss of function in critical oxidant-sensitive cellular components and retention of ATP-synthesizing capabilities in respiring yeast cells exposed to lethal levels of HOCl suggests that toxicity in this case may arise by programmed cell death.

Lysine Residues Direct the Chlorination of Tyrosines in YXXK Motifs of Apolipoprotein A-I When Hypochlorous Acid Oxidizes High Density Lipoprotein

Oxidized lipoproteins may play an important role in the pathogenesis of atherosclerosis. Elevated levels of 3-chlorotyrosine, a specific end product of the reaction between hypochlorous acid (HOCl) and tyrosine residues of proteins, have been detected in atherosclerotic tissue. Thus, HOCl generated by the phagocyte enzyme myeloperoxidase represents one pathway for protein oxidation in humans. One important target of the myeloperoxidase pathway may be high density lipoprotein (HDL), which mobilizes cholesterol from artery wall cells. To determine whether activated phagocytes preferentially chlorinate specific sites in HDL, we used tandem mass spectrometry (MS/MS) to analyze apolipoprotein A-I that had been oxidized by HOCl. The major site of chlorination was a single tyrosine residue located in one of the protein's YXXK motifs (where X represents a nonreactive amino acid). To investigate the mechanism of chlorination, we exposed synthetic peptides to HOCl. The peptides encompassed the amino acid sequences YKXXY, YXXKY, or YXXXY. MS/MS analysis demonstrated that chlorination of tyrosine in the peptides that contained lysine was regioselective and occurred in high yield if the substrate was KXXY or YXXK. NMR and MS analyses revealed that the N(epsilon) amino group of lysine was initially chlorinated, which suggests that chloramine formation is the first step in tyrosine chlorination. Molecular modeling of the YXXK motif in apolipoprotein A-I demonstrated that these tyrosine and lysine residues are adjacent on the same face of an amphipathic alpha-helix. Our observations suggest that HOCl selectively targets tyrosine residues that are suitably juxtaposed to primary amino groups in proteins. This mechanism might enable phagocytes to efficiently damage proteins when they destroy microbial proteins during infection or damage host tissue during inflammation.

Nitrite-mediated protection against hypochlorous acid-induced chondrocyte toxicity: a novel cytoprotective role of nitric oxide in the inflamed joint?

OBJECTIVE

To examine the potential consequences of overproduction of nitric oxide (NO) and nitrite (NO(2) (-)) in the inflamed rheumatoid joint.

METHODS

Human articular chondrocytes in culture were exposed to HOCl (hypochlorous acid, a physiologic oxidant formed in increased amounts at sites of chronic inflammation), and assays of cell viability, intracellular ATP and glutathione (GSH), and lactate dehydrogenase (LDH) were performed. HOCl-induced lipid peroxidation and activation of the MAP kinases ERK-1/2, JNK-1/2, and p38 were also measured. The modulatory effects of NO-derived nitrite (NO(2) (-)) and nitrate (NO(3) (-)) on HOCl-mediated chondrocyte toxicity were investigated.

RESULTS

Exposure of human articular chondrocytes to HOCl resulted in a concentration- and time-dependent loss of viability, decrease in ATP and GSH levels, LDH leakage, and cell death. HOCl induced significant lipid peroxidation as well as activation of the MAP kinases ERK-1/2 and p38 but not JNK-1/2. However, the presence of NO(2) (-) but not NO(3) (-) substantially decreased HOCl-dependent cellular toxicity even when NO(2) (-) was added at low (microM) concentrations. In sharp contrast, NO(2) (-) (1 mM) did not inhibit superoxide-, hydroxyl radical-, H(2)O(2)-, or peroxynitrite-mediated cytotoxicity. Furthermore, culture media from cells treated with interleukin-1beta (to generate NO and NO(2) (-)) offered significantly more protection against HOCl-mediated cytotoxicity than culture media from untreated cells.

CONCLUSION

These data suggest that NO(2) (-) accumulation at chronically inflamed sites where both HOCl and NO are overproduced may be cytoprotective against damage induced by HOCl. Accumulation of NO(2) (-) could represent a novel cytoprotective role of NO in inflamed joints. A mechanism for this is suggested.

Myeloperoxidase-derived hypochlorous acid antagonizes the oxidative stress-mediated activation of iron regulatory protein 1

Hypochlorous acid (HOCl) is a highly reactive product generated by the myeloperoxidase reaction during the oxidative burst of activated neutrophils, which is implicated in many bactericidal and cytotoxic responses. Recent evidence suggests that HOCl may also play a role in the modulation of redox sensitive signaling pathways. The short half-life of HOCl and the requirement for a continuous presence of H2O2 as a substrate for its myeloperoxidase-catalyzed generation make the study of HOCl-mediated responses very difficult. We describe here an enzymatic model consisting of glucose/glucose oxidase, catalase, and myeloperoxidase (GOX/CAT/MPO) that allows the controlled generation of both HOCl and H2O2 and thus, mimics the oxidative burst of activated neutrophils. By employing this model we show that HOCl prevents the H2O2-mediated activation of iron regulatory protein 1 (IRP1), a central post-transcriptional regulator of mammalian iron metabolism. Activated IRP1 binds to (R)iron-responsive elements" (IREs) within the mRNAs encoding proteins of iron metabolism and thereby controls their translation or stability. The inhibitory effect of HOCl is not a result of a direct modification of IRP1 by this oxidant. Kinetics experiments provide evidence that HOCl intervenes with the signaling cascade, which results in the activation of IRP1. We further demonstrate that HOCl antagonizes the H2O2-mediated increase in the levels of transferrin receptor, which is a downstream target of IRP1. Our findings suggest that HOCl can modulate signaling pathways in a concerted action with H2O2. The GOX/CAT/MPO system provides a valuable tool for studying the regulatory function of HOCl.

Reassessment of the microbicidal activity of reactive oxygen species and hypochlorous acid with reference to the phagocytic vacuole of the neutrophil granulocyte

During phagocytosis, neutrophils undergo a burst of respiration in which oxygen is reduced to superoxide (O(-)(2)), which dismutates to form H(2)O(2). Myeloperoxidase (MPO) is discharged from the cytoplasmic granules into the phagosome following particle ingestion. It is thought to utilize H(2)O(2) to oxidize halides, which then react with and kill ingested microbes. Recent studies have provided new information as to the concentration of O(-)(2) and proteins, and the pH, within the vacuole. This study was conducted to examine the antimicrobial effect of O(-)(2), H(2)O(2) and hypochlorous acid under these conditions and it was found that the previously described bactericidal effect of these agents was reversed in the presence of granule proteins or MPO. To establish which cellular proteins were iodinated by MPO, cellular proteins and bacterial proteins, iodinated in neutrophils phagocytosing bacteria in the presence of (125)I, were separated by 2D gel electrophoresis. Iodinated spots were detected by autoradiography and the oxidized proteins were identified by MS. The targets of these iodination reactions were largely those of the host cell rather than those of the engulfed microbe.

MCLA-dependent chemiluminescence suggests that singlet oxygen plays a pivotal role in myeloperoxidase-catalysed bactericidal action in neutrophil phagosomes

Bacteria ingested by a neutrophil are located in phagosomes in which H(2)O(2) is produced through the NADPH oxidase-dependent respiratory burst. Myeloperoxidase (MPO) plays important role in the bactericidal action of phagosomes. MPO catalyses the reaction of H(2)O(2) and Cl(-) to produce HClO. The chemical mechanism behind the bactericidal action of the MPO-H(2)O(2)-Cl(-) system is unclear. Bactericidal action may result from (a) the direct reactions of HOCl with biological components (through amine chlorination) or (b) (1)O(2), formed non-enzymatically from HOCl and H(2)O(2), that mainly works to kill microorganisms through bacterial respiratory chain injury. To answer this question, we developed a Cypridina luciferin analogue (MCLA)-dependent chemiluminescence method to determine the rate of formation of (1)O(2) from a (1)O(2) source at pH 4.5-9.0. Using the MCLA-dependent chemiluminescence method, we found that the rate of formation of (1)O(2) from the MPO-H(2)O(2)-Cl(-) system peaked at pH 7.0. Segal et al. (28) reported that almost all Staphylococcus aureus is killed 2 min after phagocytosis by neutrophils where the phagosomal pH is 7.4-7.75. However, amine chlorination by HOCl did not proceed at pH > 7.0. Moreover, the bactericidal activities of the MPO-H(2)O(2)-Cl(-) system with Escherichia coli at pH 4.5 and 8.0 were paralleled by the rate of formation of (1)O(2). Combining these observations and the results reported by Segal et al., we concluded that (1)O(2) is a major chemical species in the killing of bacteria in neutrophil phagosomes.

Inhibition of hypochlorous acid-induced oxidative reactions by nitrite: is nitrite an antioxidant?

Acute and chronic inflammation result in increased nitrogen monoxide (z.rad;NO) formation and the accumulation of nitrite (NO(2)(-)). Neutrophils stimulated by various inflammatory mediators release myeloperoxidase to produce the cytotoxic agent hypochlorous acid (HOCl). At physiologically attainable concentrations, we found that NO(2)(-) significantly inhibits HOCl-mediated DNA strand breakage and ascorbate depletion. HOCl-mediated inactivation of pure alpha(1)-antiproteinase or of the elastase inhibitory capacity of human plasma was inhibited by the addition of NO(2)(-). NO(2)(-) was more effective than ascorbate, GSH, and urate at inhibiting HOCl-mediated toxicity to human HepG2 cells in culture. These data suggest that NO(2)(-) may act in an antioxidant manner by removing HOCl at sites of inflammation where both HOCl and z.rad;NO are overproduced.

3-Chlorotyrosine as a marker of protein damage by myeloperoxidase in tracheal aspirates from preterm infants: association with adverse respiratory outcome

Oxidative injury is implicated in the development of chronic lung disease in preterm infants with respiratory distress. However, direct evidence of a causal role is limited and the source of reactive oxidants has not been identified. We have previously shown that protein carbonyl levels in tracheal aspirates correlate positively with myeloperoxidase, suggesting that neutrophil oxidants could be the source of this protein injury. We have extended these observations by measuring 3-chlorotyrosine, a specific biomarker of the neutrophil oxidant, hypochlorous acid, in tracheal aspirate proteins (144 samples) from 69 infants with birth weight <1500 g. 3-Chlorotyrosine levels were higher in these infants than in larger infants without respiratory distress (median 83 compared with 13 micromol/mol tyrosine). They correlated strongly with myeloperoxidase activity (correlation coefficient 0.75, p < 0.0001) and to a lesser extent with protein carbonyls. 3-Chlorotyrosine levels (at 1 wk after birth) correlated negatively with birth weight or gestational age. They were significantly higher in infants who developed chronic lung disease (oxygen requirement at 36 wk postmenstrual age) than in those who did not (median 88 and 49 micromol/mol tyrosine, respectively) and correlated with days of supplemental oxygen. 3-Chlorotyrosine was also significantly higher in infants who had lung infection or were Ureaplasma urealyticum positive. Our results are the first evidence that chlorinated proteins are produced in the lungs of premature infants and that they are higher in infection. The higher 3-chlorotyrosine levels in infants who develop chronic lung disease suggest that neutrophil oxidants contribute to the pathology of this disease.

Inhibition of human surfactant protein A function by oxidation intermediates of nitrite

NitraNitration of protein tyrosine residues by peroxynitrite (ONOO - ) has been implicated in a variety of inflammatory diseases such as acute respiratory distress syndrome (ARDS). Pulmonary surfactant protein A (SP-A) has multiple functions including host defense. We report here that a mixture of hypochlorous acid (HOCl) and nitrite (NO 2 - ) induces nitration, oxidation, and chlorination of tyrosine residues in human SP-A and inhibits SP-A's ability to aggregate lipids and bind mannose. Nitration and oxidation of SP-A was not altered by the presence of lipids, suggesting that proteins are preferred targets in lipid-rich mixtures such as pulmonary surfactant. Moreover, both horseradish peroxidase and myeloperoxidase (MPO) can utilize NO 2 - and hydrogen peroxide (H 2 O 2 ) as substrates to catalyze tyrosine nitration in SP-A and inhibit its lipid aggregation function. SP-A nitration and oxidation by MPO is markedly enhanced in the presence of physiological concentrations of Cl - and the lipid aggregation function of SP-A is completely abolished. Collectively, our results suggest that MPO released by activated neutrophils during inflammation utilizes physiological or pathological levels of NO 2 - to nitrate proteins, and may provide an additional mechanism in addition to ONOO - formation, for tissue injury in ARDS and other inflammatory diseases associated with upregulated *NO and oxidant production.

Human neutrophils use the myeloperoxidase-hydrogen peroxide-chloride system to chlorinate but not nitrate bacterial proteins during phagocytosis

The generation of extracellular oxidants by neutrophils has been widely investigated, but knowledge about the chemical reactions that occur in the phagolysosome, the cellular compartment that kills pathogens, is more limited. One important pathway may involve the production of potent halogenating agents such as hypochlorous acid (HOCl) by the myeloperoxidase-hydrogen peroxide-halide system. However, explorations of the oxidation chemistry of phagolysosomes have been hampered by the organelle's inaccessibility. To overcome this limitation, we recovered Escherichia coli that had been internalized by human neutrophils. We then analyzed the bacterial proteins for 3-chlorotyrosine, a stable marker of damage by HOCl. Mass spectrometric analysis revealed that levels of 3-chlorotyrosine in E. coli proteins increased markedly after the bacteria were internalized by human neutrophils. This increase failed to occur in E. coli exposed to neutrophils deficient in NADPH oxidase or myeloperoxidase, implicating H(2)O(2) and myeloperoxidase in the halogenation reaction. The extent of protein chlorination by normal neutrophils paralleled bacterial killing. Our observations support the view that the phagolysosome of human neutrophils uses the myeloperoxidase-hydrogen peroxide-chloride system to chlorinate bacterial proteins. In striking contrast, human neutrophils failed to nitrate bacterial proteins unless the medium was supplemented with 1 mm nitrite, and the level of nitration was low. Protein chlorination associated with bacterial killing was unaffected by the presence of nitrite in the medium. Nitration required NADPH oxidase but appeared to be independent of myeloperoxidase, suggesting that neutrophils can nitrate proteins through a pathway that requires nitrite but is independent of myeloperoxidase.