Comparison of low-density lipoprotein modification by myeloperoxidase-derived hypochlorous and hypobromous acids

Prediction of Early Atherosclerotic Plaques Using a Sequence-Activated Fluorescence Probe for the Simultaneous Detection of γ-Glutamyl Transpeptidase and Hypobromous Acid

Atherosclerosis is a lipoprotein-driven disease, and there is no effective therapy to reverse atherosclerosis or existing plaques. Therefore, it is urgently necessary to create a noninvasive and reliable approach for early atherosclerosis detection to prevent initial plaque formation. Atherosclerosis is intimately associated with inflammation, which is accompanied by an excess of reactive oxygen species (ROS), leading to cells requiring more glutathione (GSH) to resist severe oxidative stress. Therefore, the GSH-hydrolyzed protein γ-glutamyl transpeptidase (GGT) and the ROS-hypobromous acid (HBrO) are potential biomarkers for predicting atherogenesis. Hence, to avoid false-positive diagnoses caused by a single biomarker, we constructed an ingenious sequence-activated double-locked TP fluorescent probe, C-HBrO-GGT, in which two sequential triggers of GGT and HBrO are meticulously designed to ensure that the probe fluoresces in response to HBrO only after GGT hydrolyzes the probe. By utilization of C-HBrO-GGT, the voltage-gated chloride channel (CLC-1)-HBrO-catalase (CAT)-GGT signaling pathway was confirmed in cellular level. Notably, the forthcoming atherosclerotic plaques were successfully predicted before the plaques could be observed via the naked eye or classical immunofluorescent staining. Collectively, this research proposed a powerful tool to indicate the precise position of mature plaques and provide early warning of atherosclerotic plaques.

Discovery and structure activity relationships of 7-benzyl triazolopyridines as stable, selective, and reversible inhibitors of myeloperoxidase

Myeloperoxidase (MPO) is a heme peroxidase found in neutrophils, monocytes and macrophages that efficiently catalyzes the oxidation of endogenous chloride into hypochlorous acid for antimicrobial activity. Chronic MPO activation can lead to indiscriminate protein modification causing tissue damage, and has been associated with chronic inflammatory diseases, atherosclerosis, and acute cardiovascular events. Triazolopyrimidine 5 is a reversible MPO inhibitor; however it suffers from poor stability in acid, and is an irreversible inhibitor of the DNA repair protein methyl guanine methyl transferase (MGMT). Structure-based drug design was employed to discover benzyl triazolopyridines with improved MPO potency, as well as acid stability, no reactivity with MGMT, and selectivity against thyroid peroxidase (TPO). Structure-activity relationships, a crystal structure of the MPO-inhibitor complex, and acute in vivo pharmacodynamic data are described herein.

The Role of Myeloperoxidase in Biomolecule Modification, Chronic Inflammation, and Disease

Significance: The release of myeloperoxidase (MPO) by activated leukocytes is critical in innate immune responses. MPO produces hypochlorous acid (HOCl) and other strong oxidants, which kill bacteria and other invading pathogens. However, MPO also drives the development of numerous chronic inflammatory pathologies, including atherosclerosis, neurodegenerative disease, lung disease, arthritis, cancer, and kidney disease, which are globally responsible for significant patient mortality and morbidity. Recent Advances: The development of imaging approaches to precisely identify the localization of MPO and the molecular targets of HOCl in vivo is an important advance, as typically the involvement of MPO in inflammatory disease has been inferred by its presence, together with the detection of biomarkers of HOCl, in biological fluids or diseased tissues. This will provide valuable information in regard to the cell types responsible for releasing MPO in vivo, together with new insight into potential therapeutic opportunities. Critical Issues: Although there is little doubt as to the value of MPO inhibition as a protective strategy to mitigate tissue damage during chronic inflammation in experimental models, the impact of long-term inhibition of MPO as a therapeutic strategy for human disease remains uncertain, in light of the potential effects on innate immunity. Future Directions: The development of more targeted MPO inhibitors or a treatment regimen designed to reduce MPO-associated host tissue damage without compromising pathogen killing by the innate immune system is therefore an important future direction. Similarly, a partial MPO inhibition strategy may be sufficient to maintain adequate bacterial activity while decreasing the propagation of inflammatory pathologies.

Chlorinated Phospholipids and Fatty Acids: (Patho)physiological Relevance, Potential Toxicity, and Analysis of Lipid Chlorohydrins

Chlorinated phospholipids are formed by the reaction of hypochlorous acid (HOCl), generated by the enzyme myeloperoxidase under inflammatory conditions, and the unsaturated fatty acyl residues or the head group. In the first case the generated chlorohydrins are both proinflammatory and cytotoxic, thus having a significant impact on the structures of biomembranes. The latter case leads to chloramines, the properties of which are by far less well understood. Since HOCl is also widely used as a disinfecting and antibacterial agent in medicinal, industrial, and domestic applications, it may represent an additional source of danger in the case of abuse or mishandling. This review discusses the reaction behavior of in vivo generated HOCl and biomolecules like DNA, proteins, and carbohydrates but will focus on phospholipids. Not only the beneficial and pathological (toxic) effects of chlorinated lipids but also the importance of these chlorinated species is discussed. Some selected cleavage products of (chlorinated) phospholipids and plasmalogens such as lysophospholipids, (chlorinated) free fatty acids and α-chloro fatty aldehydes, which are all well known to massively contribute to inflammatory diseases associated with oxidative stress, will be also discussed. Finally, common analytical methods to study these compounds will be reviewed with focus on mass spectrometric techniques.

Tripping up Trp: Modification of protein tryptophan residues by reactive oxygen species, modes of detection, and biological consequences

Proteins comprise a majority of the dry weight of a cell, rendering them a major target for oxidative modification. Oxidation of proteins can result in significant alterations in protein molecular mass such as breakage of the polypeptide backbone and/or polymerization of monomers into dimers, multimers, and sometimes insoluble aggregates. Protein oxidation can also result in structural changes to amino acid residue side chains, conversions that have only a modest effect on protein size but can have widespread consequences for protein function. There are a wide range of rate constants for amino acid reactivity, with cysteine, methionine, tyrosine, phenylalanine, and tryptophan having the highest rate constants with commonly encountered biological oxidants. Free tryptophan and tryptophan protein residues react at a diffusion-limited rate with hydroxyl radical and also have high rate constants for reactions with singlet oxygen and ozone. Although oxidation of proteins in general and tryptophan residues specifically can have effects detrimental to the health of cells and organisms, some modifications are neutral, whereas others contribute to the function of the protein in question or may act as a signal that damaged proteins need to be replaced. This review provides a brief overview of the chemical mechanisms by which tryptophan residues become oxidized, presents both the strengths and the weaknesses of some of the techniques used to detect these oxidative interactions, and discusses selected examples of the biological consequences of tryptophan oxidation in proteins from animals, plants, and microbes.

Comparative reactivity of myeloperoxidase-derived oxidants with mammalian cells

Myeloperoxidase is an important heme enzyme released by activated leukocytes that catalyzes the reaction of hydrogen peroxide with halide and pseudo-halide ions to form various hypohalous acids. Hypohalous acids are chemical oxidants that have potent antibacterial, antiviral, and antifungal properties and, as such, play key roles in the human immune system. However, increasing evidence supports an alternative role for myeloperoxidase-derived oxidants in the development of disease. Excessive production of hypohalous acids, particularly during chronic inflammation, leads to the initiation and accumulation of cellular damage that has been implicated in many human pathologies including atherosclerosis, neurodegenerative disease, lung disease, arthritis, inflammatory cancers, and kidney disease. This has sparked a significant interest in developing a greater understanding of the mechanisms involved in myeloperoxidase-derived oxidant-induced mammalian cell damage. This article reviews recent developments in our understanding of the cellular reactivity of hypochlorous acid, hypobromous acid, and hypothiocyanous acid, the major oxidants produced by myeloperoxidase under physiological conditions.

Effect of chemical oxidation on the sorption tendency of dissolved organic matter to a model hydrophobic surface

The application of chemical oxidants may alter the sorption properties of dissolved organic matter (DOM), such as humic and fulvic acids, proteins, polysaccharides, and lipids, affecting their fate in water treatment processes, including attachment to other organic components, activated carbon, and membranes (e.g., organic fouling). Similar reactions with chlorine (HOCl) and bromine (HOBr) produced at inflammatory sites in vivo affect the fate of biomolecules (e.g., protein aggregation). In this study, quartz crystal microbalance with dissipation monitoring (QCM-D) was used to evaluate changes in the noncovalent interactions of proteins, polysaccharides, fatty acids, and humic and fulvic acids with a model hydrophobic surface as a function of increasing doses of HOCl, HOBr, and ozone (O3). All three oxidants enhanced the sorption tendency of proteins to the hydrophobic surface at low doses but reduced their sorption tendency at high doses. All three oxidants reduced the sorption tendency of polysaccharides and fatty acids to the hydrophobic surface. HOCl and HOBr increased the sorption tendency of humic and fulvic acids to the hydrophobic surface with maxima at moderate doses, while O3 decreased their sorption tendency. The behavior observed with two water samples was similar to that observed with humic and fulvic acids, pointing to the importance of these constituents. For chlorination, the highest sorption tendency to the hydrophobic surface was observed within the range of doses typically applied during water treatment. These results suggest that ozone pretreatment would minimize membrane fouling by DOM, while chlorine pretreatment would promote DOM removal by activated carbon.

Proatherogenic modification of LDL by surface-bound myeloperoxidase

One of the factors promoting oxidative/halogenating modification of low-density lipoproteins (LDL) is myeloperoxidase (MPO). We have shown previously that MPO binds to the LDL surfaces. The LDL-MPO complex is uncoupled in the presence of peptide EQIQDDCTGDED that corresponds to a fragment of apoB-100 (445-456). In this paper we studied how this peptide, as well as inhibitors and modulators of halogenating activity of MPO such as ceruloplasmin (CP), 4-aminobenzoic acid hydrazide (ABAH) and thiocyanate (SCN(-)) affect the accumulation of cholesterol and its esters in monocytes/macrophages after incubation with LDL subjected to different kinds of MPO-dependent oxidative/halogenating modification. In the presence of H2O2 and halides MPO causes stronger proatherogenic modification of LDL than exogenous reactive halogen species (HOCl and HOBr). Both monocytes, which differentiate into macrophages, and neutrophils secrete MPO in response to the presence of damaged LDL. The peptide EQIQDDCTGDED preventing interaction between MPO and LDL reduces the uptake of modified LDL and MPO by monocytes/macrophages and thus precludes the accumulation of intracellular cholesterol. Our results indicate that binding to MPO is important for LDL to become modified and acquire proatherogenic properties. The peptide EQIQDDCTGDED, CP, ABAH, and SCN(-) can play the role of anti-atherogenic factors reducing the deleterious effect of catalytically active MPO on LDL and accumulation of cholesterol in macrophages.

Low-density lipoprotein modified by myeloperoxidase in inflammatory pathways and clinical studies

Oxidation of low-density lipoprotein (LDL) has a key role in atherogenesis. Among the different models of oxidation that have been studied, the one using myeloperoxidase (MPO) is thought to be more physiopathologically relevant. Apolipoprotein B-100 is the unique protein of LDL and is the major target of MPO. Furthermore, MPO rapidly adsorbs at the surface of LDL, promoting oxidation of amino acid residues and formation of oxidized lipoproteins that are commonly named Mox-LDL. The latter is not recognized by the LDL receptor and is accumulated by macrophages. In the context of atherogenesis, Mox-LDL accumulates in macrophages leading to foam cell formation. Furthermore, Mox-LDL seems to have specific effects and triggers inflammation. Indeed, those oxidized lipoproteins activate endothelial cells and monocytes/macrophages and induce proinflammatory molecules such as TNFα and IL-8. Mox-LDL may also inhibit fibrinolysis mediated via endothelial cells and consecutively increase the risk of thrombus formation. Finally, Mox-LDL has been involved in the physiopathology of several diseases linked to atherosclerosis such as kidney failure and consequent hemodialysis therapy, erectile dysfunction, and sleep restriction. All these issues show that the investigations of MPO-dependent LDL oxidation are of importance to better understand the inflammatory context of atherosclerosis.

Design, synthesis, and structure-activity relationship studies of novel 3-alkylindole derivatives as selective and highly potent myeloperoxidase inhibitors

Due to its production of potent antimicrobial oxidants including hypochlorous acid, human myeloperoxidase (MPO) plays a critical role in innate immunity and inflammatory diseases. Thus MPO is an attractive target in drug design. (Aminoalkyl)fluoroindole derivatives were detected to be very potent MPO inhibitors; however, they also promote inhibition of the serotonin reuptake transporter (SERT) at the same concentration range. Via structure-based drug design, a new series of MPO inhibitors derived from 3-alkylindole were synthesized and their effects were assessed on MPO-mediated taurine chlorination and low-density lipoprotein oxidation as well as on inhibition of SERT. The fluoroindole compound with three carbons in the side chain and one amide group exhibited a selectivity index of 35 (Ki/IC50) with high inhibition of MPO activity (IC50 = 18 nM), whereas its effect on SERT was in the micromolar range. Structure-function relationships, mechanism of action, and safety of the molecule are discussed.

Halogenated phospholipids regulate secretory phospholipase A2 group IIA activity

Secretory phospholipase A2 group IIA (sPLA2-IIA) is an active participant of inflammation. The enzyme destroys bacterial cell wall and induces production of biologically active lipid mediators. It is involved in various pathological processes and high serum content and activity of sPLA2-IIA are associated with adverse cardiovascular events. Study of sPLA2-IIA regulation is of great physiological and clinical importance and is necessary for better understanding of mechanisms underlying inflammation. Another major participant of inflammatory response is the enzyme myeloperoxidase (MPO) which is secreted by neutrophils in the focus of inflammation and catalyzes formation of HOCl and HOBr. Both halogenated (chloro- and bromohydrins) and oxidized lipids are formed due to interaction between HOCl and HOBr with unsaturated bonds of phospholipid acyl chains. Previously we showed that oxidized phospholipids stimulate sPLA2-IIA activity. In this study we examined the effects of chloro- and bromohydrins of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) on sPLA2-IIA activity. In contrast to POPC, chloro- and bromohydrins of POPC (POPC-Cl and POPC-Br, respectively) were not hydrolyzed by sPLA2-IIA. In addition, phospholipids which are sPLA2-IIA substrates, were not cleaved by the enzyme in the presence of POPC-Cl and POPC-Br. Halogenohydrins of POPC prevented the activity of both purified and serum sPLA2-IIA. Blocking effects of POPC-Cl and POPC-Br were abolished by increased concentrations of phospholipid-substrate. These results suggest that halogenated phospholipids formed in MPO-dependent reactions can be considered as a new class of biologically active compounds potentially capable of regulating sPLA2-IIA activity in the areas of inflammation and producing the effects opposite to those of oxidized phospholipids. Control over sPLA2-IIA can be useful in the therapy of diseases involving systemic inflammation.

Absorption and disposition of bromate in F344 rats

Bromate (BrO(3)(-)) is a ubiquitous by-product of using ozone to disinfect water containing bromide (Br(-)). The reactivity of BrO(3)(-) with biological reductants suggests that its systemic absorption and distribution to target tissues may display non-linear behavior as doses increase. The intent of this study is to determine the extent to which BrO(3)(-) is systemically bioavailable via oral exposure and broadly identify its pathways of degradation. In vitro experiments of BrO(3)(-) degradation in rat blood indicate a rapid initial degradation immediately upon addition that is >98% complete at concentrations up to 66μM in blood. As initial concentrations are increased, progressively lower fractions are lost prior to the first measurement. Secondary to this initial loss, a slower and predictable first order degradation rate was observed (10%/min). Losses during both phases were accompanied by increases in Br(-) concentrations indicating that the loss of BrO(3)(-) was due to its reduction. In vivo experiments were conducted using doses of BrO(3)(-) ranging from 0.077 to 15.3mg/kg, administered intravenously (IV) or orally (gavage) to female F344 rats. The variable nature and uncertain source of background concentrations of BrO(3)(-) limited derivation of terminal half-lives, but the initial half-life was approximately 10min for all dose groups. The area under the curve (AUC) and peak concentrations (C(t=5')) were linearly related to IV dose up to 0.77mg/kg; however, disproportionate increases in the AUC and C(t=5') and a large decrease in the volume of distribution was observed when IV doses of 1.9 and 3.8mg/kg were administered. The average terminal half-life of BrO(3)(-) from oral administration was 37min, but this was influenced by background levels of BrO(3)(-) at lower doses. With oral doses, the AUC and C(max) increased linearly with dose up to 15.3mgBrO(3)(-)/kg. BrO(3)(-) appeared to be 19-25% bioavailable without an obvious dose-dependency between 0.077 and 1.9mg/kg. The urinary elimination of BrO(3)(-) and Br(-) was measured from female F344 rats for four days following administration of single doses of 8.1mgKBrO(3)/kg and for 15 days after a single dose of 5.0mgKBr/kg. BrO(3)(-) elimination was detected over the first 12h, but Br(-) elimination from BrO(3)(-) over the first 48h was 18% lower than expected based on that eliminated from an equimolar dose of Br(-) (15.5±1.6 vs. 18.8±1.2μmol/kg, respectively). The cumulative excretion of Br(-) from KBr vs. KBrO(3) was equivalent 72h after administration. The recovery of unchanged administered BrO(3)(-) in the urine ranged between 6.0 and 11.3% (creatinine corrected) on the 27th day of treatment with concentrations of KBrO(3) of 15, 60, and 400mg/L of drinking water. The recovery of total urinary bromine as Br(-)+BrO(3)(-) ranged between 61 and 88%. An increase in the fraction of the daily BrO(3)(-) dose recovered in the urine was observed at the high dose to both sexes. The deficit in total bromine recovery raises the possibility that some brominated biochemicals may be produced in vivo and more slowly metabolized and eliminated. This was supported by measurements of dose-dependent increases of total organic bromine (TOBr) that was eliminated in the urine. The role these organic by-products play in BrO(3)(-)-induced cancer remains to be established.

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.

Kinetics of hypobromous acid-mediated oxidation of lipid components and antioxidants

Hypohalous acids are generated from the oxidation of halide ions by myeloperoxidase and eosinophil peroxidase in the presence of H2O2. These oxidants are potent antibacterial agents, but excessive production can result in host tissue damage, with this implicated in a number of human pathologies. Rate constants for HOCl with lipid components and antioxidants have been established. Here, the corresponding reactions of HOBr have been examined to determine whether this species shows similar reactivity. The second-order rate constants for the reaction of HOBr with 3-pentenoic acid and sorbate, models of unsaturated lipids, are 1.1x10(4) and 1.3x10(3) M(-1) s(-1), respectively, while those for reaction of HOBr with phosphoryl-serine and phosphoryl-ethanolamine are ca. 10(6) M(-1) s(-1). The second-order rate constants (M(-1) s(-1)) for reactions of HOBr with Trolox (6.4x10(4)), hydroquinone (2.4x10(5)), and ubiquinol-0 (2.5x10(6)) were determined, as models of the lipid-soluble antioxidants, alpha-tocopherol, and ubiquinol-10; all of these rate constants are ca. 50-2000-fold greater than for HOCl. In contrast, the second-order rate constants for the reaction of HOBr with the water-soluble antioxidants, ascorbate and urate, are ca. 10(6) M(-1) s(-1) and closer in magnitude to those for HOCl. Kinetic models have been developed to predict the sites of HOBr attack on low-density lipoproteins. The data obtained indicate that HOBr reacts to a much greater extent with fatty acid side chains and lipid-soluble antioxidants than HOCl; this has important implications for HOBr-mediated damage to cells and lipoproteins.

Thiocyanate-dependent induction of endothelial cell adhesion molecule expression by phagocyte peroxidases: a novel HOSCN-specific oxidant mechanism to amplify inflammation

Both eosinophil peroxidase (EPO) and neutrophil myeloperoxidase (MPO) preferentially oxidize SCN(-) to generate HOSCN, a weak, sulfhydryl-reactive oxidant, as a major physiologic product. We here show that HOSCN is a uniquely potent phagocyte oxidant inducer of E-selectin, ICAM-1, and VCAM-1 expression in HUVEC as detected by Western blot and flow cytometry. EMSA and inhibitor studies show that HOSCN up-regulation of these adhesion molecules is transcriptionally mediated through a mechanism that is dependent upon activation of the NF-kappaB p65/p50 transcription factor and constitutively suppressed by PI3K-Akt pathway activity. HUVEC monolayers exposed to HOSCN bind 8-fold more neutrophils and 3- to 4-fold more Aml14.3D10 cells (a differentiated cell line model of mature eosinophils) than control monolayers. Blocking Ab studies confirm the involvement of E-selectin and ICAM-1 but not VCAM-1 in neutrophil adhesion and of all three in Aml14.3D10 adhesion. Intraperitoneal injection of HOSCN evoked an 8-fold increase in neutrophil peritoneal extravasation. In addition to NF-kappaB, HOSCN also activates the potentially proinflammatory transcription factors Stat4, CDP, GRE, CBF, Ets-1/PEA3, and TFIID, a pattern easily distinguishable from that induced by LPS. These results suggest that phagocyte peroxidases function to amplify inflammation through a novel, HOSCN-specific oxidant mechanism.

pH-dependent regulation of myeloperoxidase activity

The balance between peroxidase and chlorinating activities of myeloperoxidase (MPO) is very important for the enhancement of antimicrobial action and prevention of damage caused by hypochlorite. In the present paper, the peroxidase and chlorinating activities have been studied at various pH values. The possibility of using neutrophil protein solution for the evaluation of MPO activity has been demonstrated. It is shown that at neutral pH MPO had higher affinity to peroxidase substrate guaiacol: at pH 7.4, chloride ions did not compete with guaiacol up to the concentration of 150 mM. At acidic pH, chlorinating activity of MPO dominates: only hypochlorite production can be detected at equal chloride and guaiacol concentrations of 15 mM. However, horseradish peroxidase does not exhibit any difference in activity in the presence of chloride ions even at acidic pH values. It was demonstrated by MALDI-TOF mass-spectrometry that the amount of hypochlorite produced is sufficient to modify phospholipids (with formation of Cl- and Br-hydrins and lyso-derivatives) only at acidic pH (5.0). Thus, in the presence of phenolic peroxidase substrate, MPO chlorinating activity can be displayed at acidic pH only. It can lead to elimination of hypochlorite production in normal tissues at neutral pH (7.4) and its enhancement in phagosomes where the pH range is 4.7-6.0.

Leukocytic myeloperoxidase-mediated formation of bromohydrins and lysophospholipids from unsaturated phosphatidylcholines

Using MALDI-TOF mass spectrometry, we have shown that leukocytic myeloperoxidase (MPO) in the presence of its substrates (H2O2 and Br?) does not induce any changes in saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. Incubation of liposomes prepared from mono-unsaturated phosphatidylcholine (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) with the (MPO + H2O2 + Br-) system resulted in formation of bromohydrins as the main products. 1-Palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (lysophosphatidylcholine) was the main product of the reaction of polyunsaturated phosphatidylcholine (1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine) with the (MPO + H2O2 + Br-) system. The formation of lysophospholipids as well as of bromohydrins was not observed when the enzyme or one of its substrates (H2O2 or Br-) was absent from the incubation medium, or if an inhibitor of MPO (sodium azide) or hypobromite scavengers (taurine or methionine) were added. Thus, it can be postulated that the formation of bromohydrins as well as lysophospholipids by the (MPO + H2O2 + Br-) system results from reactions of hypobromite formed during MPO catalysis with double bonds of acyl chains of phosphatidylcholine. Such destructive processes may take place in vivo in membrane- or lipoprotein-associated unsaturated lipids in centers of inflammation.

Modification of low-density lipoprotein by myeloperoxidase-derived oxidants and reagent hypochlorous acid

Substantial evidence supports the notion that oxidative processes contribute to the pathogenesis of atherosclerosis and coronary heart disease. The nature of the oxidants that give rise to the elevated levels of oxidised lipids and proteins, and decreased levels of antioxidants, detected in human atherosclerotic lesions are, however, unclear, with multiple species having been invoked. Over the last few years, considerable data have been obtained in support of the hypothesis that oxidants generated by the heme enzyme myeloperoxidase play a key role in oxidation reactions in the artery wall. In this article, the evidence for a role of myeloperoxidase, and oxidants generated therefrom, in the modification of low-density lipoprotein, the major source of lipids in atherosclerotic lesions, is reviewed. Particular emphasis is placed on the reactions of the reactive species generated by this enzyme, the mechanisms and sites of damage, the role of modification of the different components of low-density lipoprotein, and the biological consequences of such oxidation on cell types present in the artery wall and in the circulation, respectively.

Captopril inhibits the oxidative modification of apolipoprotein B-100 caused by myeloperoxydase in a comparative in vitro assay of angiotensin converting enzyme inhibitors

The oxidative modification of low-density lipoproteins (LDL) is a key event in the formation of atheromatous lesions. Indeed, oxidized derivatives accumulate in the vascular wall and promote a local inflammatory process which triggers the progression of the atheromatous plaque. Myeloperoxidase (MPO) has been mentioned as a major contributor to this oxidative process. It takes part in the oxidation both of lipids by chlorination and peroxidation and of apolipoprotein B-100. Based on recent observations with several anti-inflammatory and thiol-containing drugs, the present study was designed to test the hypothesis that anti-hypertensive agents from the angiotensin converting enzyme (ACE) inhibitors group inhibit the oxidative modifications of Apo B-100 caused by MPO. Captopril, ramipril, enalapril, lisinopril and fosinopril were assessed by measuring: their inhibiting effect on the MPO/H(2)O(2)/Cl(-) system, the accumulation of compound II, which reflects the inhibition of the synthesis of HOCl and the LDL oxidation by MPO in presence of several concentrations of ACE inhibitors. Only captopril, a thiol-containing ACE inhibitor, was able to significantly decrease the oxidative modification of LDL in a dose dependent manner and this by scavenging HOCl. This efficient anti-hypertensive drug therefore appears to also protect against the atherosclerotic process by this newly documented mechanism.

Thiol-containing molecules interact with the myeloperoxidase/H2O2/chloride system to inhibit LDL oxidation

Oxidized low-density lipoproteins (LDL) accumulate in the vascular wall and promote a local inflammatory process contributing to the progression of atheromatous plaque. The key role of myeloperoxidase (MPO) in this process has been documented and the enzyme has been involved in the oxidative modification of apolipoprotein B-100 in the intima and at the surface of endothelial cells. As the inhibition of this last phenomenon could be of relevance in pharmacological interventions, thiol-containing molecules such as glutathione, captopril, and N-acetylcysteine (NAC) and its lysinate salt (NAL) were tested in this system and their properties were compared with those of flufenamic acid (control). This last compound already demonstrated an inhibition of the production of HOCl by MPO and a more intense inhibition of MPO activity than glutathione, NAC, NAL, and captopril. However, NAC and NAL inhibited the oxidative modification of LDL more intensively than captopril and glutathione whereas flufenamic acid had no comparable inhibiting effect. This could be related to the presence of LDL close to the catalytic site of the enzyme. NAC and NAL therefore appeared as the most efficient inhibitors probably as a consequence of their relatively small size. The relevance of such effects has to be documented by in vivo studies.

The role of reactive N-bromo species and radical intermediates in hypobromous acid-induced protein oxidation

Activated eosinophils, and hypobromous acid (HOBr) generated by these cells, have been implicated in the tissue injury in asthma, allergic reactions, and some infections. Proteins are major targets for this oxidant, but limited information is available on the mechanisms of damage and intermediates formed. Reaction of HOBr with proteins is shown to result in the formation of bromamines and bromamides, from side-chain and backbone amines and amides, and 3-bromo- and 3,5-dibromo-Tyr, from Tyr residues; these materials account for ca. 70% of the oxidant consumed. Protein carbonyls, dityrosine, and 3,4-dihydroxyphenylalanine are also formed, though these are minor products (<5% of HOBr added). With BSA, extensive (selective and nonspecific) protein fragmentation and limited aggregation are also observed. The bromamines/bromamides are unstable and induce further oxidation and free radical formation as detected by EPR spin trapping. Evidence was obtained for the generation of nitrogen-centered radicals on side-chain and backbone amide groups of amino acids, peptides, and proteins. These radicals readily undergo rearrangement reactions to give carbon-centered radicals. With proteins, alpha-carbon (backbone) radicals are detected, which may play a role in protein fragmentation. A novel damage transfer pathway from Gln side-chain amide groups to backbone sites was also observed.

The principal eosinophil peroxidase product, HOSCN, is a uniquely potent phagocyte oxidant inducer of endothelial cell tissue factor activity: a potential mechanism for thrombosis in eosinophilic inflammatory states

In vivo, bromide (Br(-)), nitrite (NO(2)(-)), and thiocyanate (SCN(-)) compete for oxidation by eosinophil peroxidase (EPO) and H(2)O(2), yielding, respectively, HOBr, NO(2)., and HOSCN. We have recently shown that SCN(-) is the strongly preferred substrate for EPO in vivo and that HOSCN, in contrast with other EPO-generated oxidants and HOCl, is a relatively weak, cell-permeant, sulfhydryl (SH)-reactive oxidant. We here show that HOSCN is a uniquely potent (up to 100-fold) phagocyte oxidant inducer of tissue factor (TF) activity in human umbilical vein endothelial cells (HUVECs). This induction is attributable to transcriptional up-regulation of TF gene expression dependent upon both activation of the p65/c-Rel TF-kappaB transcription factor and activity of the ERK1/2 kinase pathway upstream of Egr-1 and was markedly further enhanced in the presence of wortmannin, an inhibitor of the PI3 kinase/Akt pathway. HOSCN also markedly activates the proinflammatory p65/p50 NF-kappaB pathway. Based on these findings we hypothesize that HOSCN generated by adherent and infiltrating eosinophils may provoke the development of a prothrombotic and proinflammatory endothelial/endocardial phenotype that promotes the pronounced thrombotic diathesis characteristic of the hypereosinophilic syndrome.

Effect of therapeutic plasma concentrations of non-steroidal anti-inflammatory drugs on the production of reactive oxygen species by activated rat neutrophils

The release of reactive oxygen specie (ROS) by activated neutrophil is involved in both the antimicrobial and deleterious effects in chronic inflammation. The objective of the present investigation was to determine the effect of therapeutic plasma concentrations of non-steroidal anti-inflammatory drugs (NSAIDs) on the production of ROS by stimulated rat neutrophils. Diclofenac (3.6 microM), indomethacin (12 microM), naproxen (160 microM), piroxicam (13 microM), and tenoxicam (30 microM) were incubated at 37 masculineC in PBS (10 mM), pH 7.4, for 30 min with rat neutrophils (1 x 10(6) cells/ml) stimulated by phorbol-12-myristate-13-acetate (100 nM). The ROS production was measured by luminol and lucigenin-dependent chemiluminescence. Except for naproxen, NSAIDs reduced ROS production: 58 +/- 2% diclofenac, 90 +/- 2% indomethacin, 33 +/- 3% piroxicam, and 45 +/- 6% tenoxicam (N = 6). For the lucigenin assay, naproxen, piroxicam and tenoxicam were ineffective. For indomethacin the inhibition was 52 +/- 5% and diclofenac showed amplification in the light emission of 181 +/- 60% (N = 6). Using the myeloperoxidase (MPO)/H2O2/luminol system, the effects of NSAIDs on MPO activity were also screened. We found that NSAIDs inhibited both the peroxidation and chlorinating activity of MPO as follows: diclofenac (36 +/- 10, 45 +/- 3%), indomethacin (97 +/- 2, 100 +/- 1%), naproxen (56 +/- 8, 76 +/- 3%), piroxicam (77 +/- 5, 99 +/- 1%), and tenoxicam (90 +/- 2, 100 +/- 1%), respectively (N = 3). These results show that therapeutic levels of NSAIDs are able to suppress the oxygen-dependent antimicrobial or oxidative functions of neutrophils by inhibiting the generation of hypochlorous acid.

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.

Myeloperoxidase-induced lipid peroxidation of LDL in the presence of nitrite. Protection by cocoa flavanols

Lipid peroxidation (LPO) of low-density lipoprotein (LDL) is believed to be a pivotal process rendering this plasma lipoprotein atherogenic. Several endogenous factors have been proposed to mediate LPO of LDL, among them myeloperoxidase (MPO), which is active in atherosclerotic lesions, and the plasma level of which has been proposed to be a prognostic parameter for cardiac events. Nitrite, a major oxidation product of nitric oxide, is substrate of MPO and a cofactor of MPO-mediated LPO under physiological conditions. Dietary flavonoids including (-)-epicatechin, a major flavan-3-ol in cocoa products, grapes and wine, are substrates of MPO as well as potent inhibitors of LPO in LDL at micromolar concentrations. Moreover, they strongly suppress protein tyrosine nitration of LDL by MPO/nitrite or peroxynitrite. By blunting undesirable MPO-mediated actions of nitrite, presumably via scavenging of the strong prooxidant and nitrating *NO2 radical, dietary flavonoids modulate NO metabolism in a favorable direction and thus counteract endothelial dysfunction. This article gives a survey on recent progress in this field with special reference to own recently published work.

Hypochlorous Acid Inhibition by Acetoacetate: Implications on Neutrophil Functions

Type-1 diabetic patients experience hyperketonemia caused by an increase in fatty acid metabolism. Thus, the aim of this study was to measure the effect of ketone bodies as suppressors of oxidizing species produced by stimulated neutrophils. Both acetoacetate and 3-hydroxybutyrate have suppressive effect on the respiratory burst measured by luminol-enhanced chemiluminescence. Through measurements of hypochlorous acid production, using neutrophils or the myeloperoxidase/H2O2/Cl- system, it was found that acetoacetate but not 3-hydroxybutyrate is able to inhibit the generation of this antimicrobial oxidant. The superoxide anion scavenging properties were confirmed by ferricytochrome C reduction and lucigenin-enhanced chemiluminescence assays. However, ketone bodies did not alter the rate of oxygen uptake by stimulated neutrophils, measured with an oxygen electrode. A strong inhibition of the expression of the cytokine IL-8 by cultured neutrophils was also observed; this is discussed with reference to the antioxidant-like property of acetoacetate.

Hypochlorous acid generated by myeloperoxidase modifies adjacent tryptophan and glycine residues in the catalytic domain of matrix metalloproteinase-7 (matrilysin): an oxidative mechanism for restraining proteolytic activity during inflammation

Dysregulation of matrix metalloproteinase (MMP) activity is implicated in tissue destruction under inflammatory conditions. An important mechanism controlling enzymatic activity might involve reactive oxygen species generated by phagocytes. Myeloperoxidase, a heme protein secreted by neutrophils, monocytes, and macrophages, uses hydrogen peroxide to generate hypochlorous acid (HOCl). We demonstrate that HOCl inhibits the activity of human matrilysin (MMP-7) in vitro, suggesting that it might limit proteolytic activity during inflammation. When MMP-7 was exposed to HOCl generated by myeloperoxidase, the proteinase lost activity. High performance liquid chromatographic analysis of the tryptic digest of the HOCl-treated proteinase demonstrated the absence of two peptides that were present in the untreated enzyme. Tandem mass spectrometric analysis revealed that both of the lost peptides contained methionine and tryptophan-glycine residues. The methionine residue of one of the peptides had been oxidized to methionine sulfoxide. In contrast, the major product from the other peptide was 4 atomic mass units smaller than its precursor (WG-4). This novel oxidation product was derived though modification of adjacent tryptophan and glycine residues in the catalytic domain of the enzyme. Loss of proteolytic activity was associated with conversion of the precursor peptide to WG-4 but not with methionine oxidation. In contrast, hydrogen peroxide failed to oxidize MMP-7 or to inactivate the enzyme. Thus, HOCl inactivates MMP-7, perhaps by site-specific conversion of tryptophan-glycine to WG-4. This inactivation mechanism is distinct from the well studied mechanisms involving tissue inhibitors of metalloproteinases. Our findings suggest that local pericellular production of HOCl by phagocytes is a physiological mechanism for governing MMP activity during inflammation.

Formation of active oxygen species and lipid peroxidation induced by hypochlorite

Hypochlorite or its acid, hypochlorous acid, may exert both beneficial and toxic effects in vivo. In order to understand the role and action of hypochlorite, the formation of active oxygen species and its kinetics were studied in the reactions of hypochlorite with peroxides and amino acids. It was found that tert-butyl hydroperoxide and methyl linoleate hydroperoxide reacted with hypochlorite to give peroxyl and/or alkoxyl radicals with little formation of singlet oxygen in contrast to hydrogen peroxide, which gave singlet oxygen exclusively. Amino acids and ascorbate reacted with hypochlorite much faster than peroxides. Free radical-mediated lipid peroxidation of micelles and membranes in aqueous suspensions was induced by hypochlorite, the chain initiation being the decomposition of hydroperoxides by hypochlorite. It was suppressed efficiently by ebselen which reduced hydroperoxides and by alpha-tocopherol, which broke chain propagation, but less effectively by hydrophilic antioxidants present in the aqueous phase. Cysteine suppressed the oxidation, but it was poorer antioxidant than alpha-tocopherol. Ascorbate also exerted moderate antioxidant capacity, but it acted as a synergist with alpha-tocopherol. Taken together, it was suggested that the primary target of hypochlorite must be sulfhydryl and amino groups in proteins and that the lipid peroxidation may proceed as the secondary reaction, which is induced by radicals generated from sulfenyl chlorides and chloramines.