Antioxidant binding of caeruloplasmin to myeloperoxidase: Myeloperoxidase is inhibited,but oxidase,peroxidase and immunoreactive properties of caeruloplasmin remain intact

The neutrophil enzyme myeloperoxidase (MPO) purposefully makes hypochlorous acid (HOCl) as part of the cells defence against microbial infections. During cell lysis, however, MPO will be released into the extracellular environment where production of HOCl, a powerful oxidant, will lead to molecular damage. Extracellular MPO binds to the copper-containing protein caeruloplasmin (Cp) and prevents MPO making HOCl. Cp has several important antioxidant functions in extracellular fluids associated with its ability to catalyse oxidation of ferrous ions and to remove peroxides. The binding of MPO to Cp did not inhibit these important extracellular antioxidant activities of Cp, but in so doing it provided additional antioxidant protection against formation of HOCl.     

Cytotoxicity towards human endothelial cells, induced by neutrophil myeloperoxidase: protection by ceftazidime

We investigated the effects of the antibiotic ceftazidime (CAZ) on the cytolytic action of the neutrophil myeloperoxidase-hydrogen peroxide-chloride anion system (MPO/H(2)O(2)/Cl(-)). In this system, myeloperoxidase catalyses the conversion of H(2)O(2) and CI(-) to the cytotoxic agent HOCl. Stimulated neutrophils can release MPO into the extracellular environment and then may cause tissue injury through direct endothelial cells lysis. We showed that human umbilical vein endothelial cells (HUVEC) were capable of taking up active MPO. In presence of H(2)O(2) (10(-4) M), this uptake was accompanied by cell lysis. The cytolysis was estimated by the release of (51)Cr from HUVEC and expressed as an index of cytotoxicity (IC). Dose dependent protection was obtained for CAZ concentrations ranging from 10(-5) to 10(-3) M;this can be attributed to inactivation of HOCl by the drug. This protection is comparable to that obtained with methionine and histidine, both of which are known to neutralize HOCl. This protection by CAZ could also be attributed to inactivation of H(2)O(2), but when cytolysis was achieved with H(2)O(2) or O(2) (-) generating enzymatic systems, no protection by CAZ was observed. Moreover, the peroxidation activity of MPO (action on H(2)O(2)) was not affected by CAZ, while CAZ prevented the chlorination activity of MPO (chlorination of monochlorodimedon). So, we concluded that CAZ acts via HOCl inactivation. These antioxidant properties of CAZ may be clinically useful in pathological situations where excessive activation of neutrophils occurs, such as in sepsis.     

Ceruloplasmin has two nearly identical sites that bind myeloperoxidase

Ceruloplasmin (Cp) is a copper-containing ferroxidase with potent antioxidant activity. Cp is expressed by hepatocytes and activated macrophages and has been known as physiologic inhibitor of myeloperoxidase (MPO). Enzymatic activity of MPO produces anti-microbial agents and strong prooxidants such as hypochlorous acid and has a potential to damage host tissue at the sites of inflammation and infection. Thus Cp–MPO interaction and inhibition of MPO has previously been suggested as an important control mechanism of excessive MPO activity. Our aim in this study was to identify minimal Cp domain or peptide that interacts with MPO. We first confirmed Cp–MPO interaction by ELISA and surface plasmon resonance (SPR). SPR analysis of the interaction yielded 30 nM affinity between Cp and MPO. We then designed and synthesized 87 overlapping peptides spanning the entire amino acid sequence of Cp. Each of the peptides was tested whether it binds to MPO by direct binding ELISA. Two of the 87 peptides, P18 and P76 strongly interacted with MPO. Amino acid sequence analysis of identified peptides revealed high sequence and structural homology between them. Further structural analysis of Cp’s crystal structure by PyMOL software unfolded that both peptides represent surface-exposed sites of Cp and face nearly the same direction. To confirm our finding we raised anti-P18 antisera in rabbit and demonstrated that this antisera disrupts Cp–MPO binding and rescues MPO activity. Collectively, our results confirm Cp–MPO interaction and identify two nearly identical sites on Cp that specifically bind MPO. We propose that inhibition of MPO by Cp requires two nearly identical sites on Cp to bind homodimeric MPO simultaneously and at an angle of at least 120 degrees, which, in turn, exerts tension on MPO and results in conformational change.     

Immunohistochemical evidence for the myeloperoxidase/H2O2/halide system in human atherosclerotic lesions: colocalization of myeloperoxidase and hypochlorite-modified proteins.

The 'oxidation theory' of atherosclerosis proposes that oxidation of low density lipoprotein (LDL) contributes to atherogenesis. Although the precise mechanisms of in vivo oxidation are widely unknown, increasing evidence suggests that myeloperoxidase (MPO, EC 1.11.1.7), a protein secreted by activated phagocytes, generates modified/oxidized (lipo)proteins via intermediate formation of hypochlorous acid (HOCl). In vitro generation of HOCl transforms lipoproteins into high uptake forms for macrophages giving rise to cholesterol-engorged foam cells. To identify HOCl-modified-epitopes in human plaque tissues we have raised monoclonal antibodies (directed against human HOCl-modified LDL) that do not cross-react with other LDL modifications, i.e. peroxynitrite-LDL, hemin-LDL, Cu2+-oxidized LDL, 4-hydroxynonenal-LDL, malondialdehyde-LDL, glycated-LDL, and acetylated-LDL. The antibodies recognized a specific epitope present on various proteins after treatment with OCl- added as reagent or generated by the MPO/H2O2/halide system. Immunohistochemical studies revealed pronounced staining for HOCl-modified-epitopes in fibroatheroma (type V) and complicated (type VI) lesions, while no staining was observed in aortae of lesion-prone location (type I). HOCl-oxidation-specific epitopes are detected in cells in the majority of atherosclerotic plaques but not in control segments. Staining was shown to be inside and outside monocytes/macrophages, endothelial cells, as well as in the extracellular matrix. A similar staining pattern using immunohistochemistry could be obtained for MPO. The colocalization of immunoreactive MPO and HOCl-modified-epitopes in serial sections of human atheroma (type IV), fibroatheroma (type V) and complicated (type VI) lesions provides further convincing evidence for MPO/H2O2/halide system-mediated oxidation of (lipo)proteins under in vivo conditions. We propose that MPO could act as an important link between the development of atherosclerotic plaque in the artery wall and chronic inflammatory events.     

Inhibition of peroxidase activity and scavenging of reactive oxygen species by astilbin isolated from Dimorphandra mollis (Fabaceae, Caesalpinioideae)

Astilbin (5,7,3',4'-tetrahydroxy-2,3-dihydroflavonol-3-?-o-rhamnoside), a flavonoid with a large range of biological activities, was isolated from Dimorphandra mollis, a shrub common to the Brazilian Cerrado. The purpose of this study is to verify the effects of astilbin on myeloperoxidase (MPO) and horseradish peroxidase (HRP), and its antioxidant activity against hypochlorous acid (HOCl) and total antioxidant activity (TAC) by the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS?+). Astilbin inhibited MPO and HRP activities in a concentration-dependent relationship and effectively scavenged HOCl. The TAC by ABTS?+ of astilbin (IC50 ~ 20 mM) was higher than that of uric acid, which was used as a positive control. These data demonstrate that astilbin is a potent antioxidant and that it inhibits MPO and HRP activities efficiently.     

The free amino acid tyrosine enhances the chlorinating activity of human myeloperoxidase

A key function of neutrophil myeloperoxidase (MPO) is the synthesis of hypochlorous acid (HOCl), a potent oxidizing agent that plays a cytotoxic role against invading bacteria and viruses at inflammatory sites and in phagosomes. MPO displayed a chlorinating activity preferably at acidic pH but at neutral pH MPO catalyzes mainly reactions of the peroxidase cycle. In the present work effects of tyrosine on the chlorinating activity of MPO were studied. At pH 7.4 we detected an increased HOCl production in the presence of tyrosine not only by the MPO-H₂O₂-Cl^- system but also in suspensions of zymosan-activated neutrophils. An excess of H₂O₂ is known to cause an accumulation of compound II of MPO blocking the generation of HOCl at neutral pH. As evidenced by spectral changes, tyrosine-induced activation of MPO to synthesize HOCl was due to the ability of tyrosine to reduce compound II back to the native state, thus accelerating the enzyme turnover. MPO-induced oxidation of tyrosine is relevant to what can be in vivo; we detected MPO-catalyzed formation of dityrosine in the presence of plasma under experimental conditions when tyrosine concentration was about three magnitudes of order less than the Cl⁻ concentration. At acidic pH formation of compound II was impaired in the presence of chloride and dityrosine couldn't be detected in plasma. In conclusion, the ability of tyrosine to increase the chlorinating activity of MPO at neutral pH and enhanced values of H₂O₂ may be very effective for the specific enhancement of HOCl production under acute inflammation.     

Comparison of HOCl traps with myeloperoxidase inhibitors in prevention of low density lipoprotein oxidation

In this study, the production of the highly toxic oxidant hypochlorous acid (HOCl) by the phagocytic enzyme myeloperoxidase (MPO) was quantitated and the concomitant alterations of low density lipoprotein (LDL) were analyzed in view of the potential role of LDL in atherosclerosis. Using the monochlorodimedone assay, it was found that HOCl is produced in micromolar concentrations. The kinetics of the decrease of tryptophan fluorescence appeared to be a sensitive method to monitor LDL alterations under near in vivo conditions. Therefore, this method was used to subsequently compare the effectiveness of MPO inhibitors that block production of HOCl with compounds that act as HOCl traps. The efficiency of MPO inhibitors to prevent LDL damage increased in the series benzohydroxamic acid < salicylhydroxamic acid < 3-amino-1,2,4-triazole < sodium azide < potassium cyanide < p-hydroxy-benzoic acid hydrazide, while for the HOCl traps the protective efficiency increased in the series glycine < taurine < methionine. We conclude that HOCl traps may have high potential therapeutic impact in vivo due to their low toxicity, although high concentrations of them would have to reach sites of inflammation. In contrast, only low concentrations of a specific MPO inhibitor would be required to irreversibly inhibit the enzyme.     

The reactivity of myeloperoxidase compound I formed with hypochlorous acid

The reaction of human myeloperoxidase (MPO) with hypochlorous acid (HOCl) was investigated by conventional stopped-flow spectroscopy at pH 5, 7, and 9. In the reaction of MPO with HOCl, compound I is formed. Its formation is strongly dependent on pH. HOCl (rather than OCl-) reacts with the unprotonated enzyme in its ferric state. Apparent second-order rate constants were determined to be 8.1 x 10(7) M(-1)s(-1) (pH 5), 2.0 x 10(8) M(-1)s(-1) (pH 7) and 2.0 x 10(6) M(-1)s(-1) (pH 9) at 15 degrees C. Furthermore, the kinetics and spectra of the reactions of halides and thiocyanate and of physiologically relevant one-electron donors (ascorbate, nitrite, tyrosine and hydrogen peroxide) with this compound I were investigated using the sequential-mixing technique. The results show conclusively that the redox intermediates formed upon addition of either hydrogen peroxide or hypochlorous acid to native MPO exhibit the same spectral features and reactivities and thus are identical. In stopped-flow investigations, the MPO/HOCl system has some advantage since: (i) in contrast to H2O2, HOCl cannot function as a one-electron donor of compound I; and (ii) MPO can easily be prevented from cycling by addition of methionine as HOCl scavenger. As a consequence, the observed absorbance changes are bigger and errors in data analysis are smaller.     

Neutrophil dysfunction induced by hyperglycemia: modulation of myeloperoxidase activity

Our data suggest that impaired activity of myeloperoxidase (MPO) may play an important role in the dysfunction of neutrophils from hyperglycemic rats. Neutrophil biochemical pathways include the NADPH oxidase system and the MPO enzyme. They both play important role in the killing function of neutrophils. The effect of hyperglycemia on the activity of these enzymes and the consequences with regard to Candida albicans phagocytosis and the microbicidal property of rat peritoneal neutrophils is evaluated here. The NADPH oxidase system activity was measured using chemiluminescence and cytochrome C reduction assays. MPO activity was measured by monitoring HOCl production, and MPO protein expression was analysed using Western blot and immunofluorescence. C. albicans phagocytosis and death were evaluated by optical microscopy using the May-Grunwald-Giemsa staining method. ROS generation kinetic was slightly delayed in the diabetic group. MPO expression levels were higher in diabetic neutrophils; however, MPO activity was decreased in these same neutrophils compared with the controls. C. albicans phagocytosis and killing were lower in the diabetic neutrophils. Based on our experimental model, the phagocytic and killing functions of neutrophil phagocytosis are impaired in diabetic rats because of the decreased production of HOCl, highlighting the importance of MPO in the microbicidal function of neutrophils. Copyright ? 2012 John Wiley & Sons, Ltd.     

The reactivity of myeloperoxidase compound I formed with hypochlorous acid

Abstract

The reaction of human myeloperoxidase (MPO) with hypochlorous acid (HOCl) was investigated by conventional stopped-flow spectroscopy at pH 5, 7, and 9. In the reaction of MPO with HOCl, compound I is formed. Its formation is strongly dependent on pH. HOCl (rather than OCl^-) reacts with the unprotonated enzyme in its ferric state. Apparent second-order rate constants were determined to be 8.1×10⁷ M^-1s^-1 (pH 5), 2.0×10⁸ M^-1s^-1 (pH 7) and 2.0×10⁶ M^-1s^-1 (pH 9) at 15°C. Furthermore, the kinetics and spectra of the reactions of halides and thiocyanate and of physiologically relevant one-electron donors (ascorbate, nitrite, tyrosine and hydrogen peroxide) with this compound I were investigated using the sequential-mixing technique. The results show conclusively that the redox intermediates formed upon addition of either hydrogen peroxide or hypochlorous acid to native MPO exhibit the same spectral features and reactivities and thus are identical. In stopped-flow investigations, the MPO/HOCl system has some advantage since: (i) in contrast to H₂O₂, HOCl cannot function as a one-electron donor of compound I; and (ii) MPO can easily be prevented from cycling by addition of methionine as HOCl scavenger. As a consequence, the observed absorbance changes are bigger and errors in data analysis are smaller.     

The reactions of hypochlorous acid, the reactive oxygen species produced by myeloperoxidase, with lipids

Myeloperoxidase (MPO), an abundant enzyme in phagocytes, has been implicated in the pathogenesis of various inflammatory diseases including atherosclerosis. The major oxidant produced by MPO, hypochlorous acid (HOCl), is able to modify a great variety of biomolecules by chlorination and/or oxidation. In this paper the reactions of lipids (preferentially unsaturated fatty acids and cholesterol) with either reagent HOCl or HOCl generated by the MPO-hydrogen peroxide-chloride system are reviewed. One of the major issues has been whether the reaction of HOCl with lipids of low density lipoprotein (LDL) yields predominantly chlorohydrins or lipid hydroperoxides. Electrospray mass spectrometry provided direct evidence that chlorohydrins rather than peroxides are the major products of HOCl- or MPO-treated LDL phosphatidylcholines. Nevertheless lipid peroxidation is a possible alternative reaction of HOCl with polyunsaturated fatty acids if an additional radical source such as pre-formed lipid hydroperoxides is available. In phospholipids carrying a primary amino group such as phosphatidylethanolamine chloramines are the preferred products compared to chlorohydrins. Cholesterol can be converted by HOCl to great variety of oxysterols besides three isomers of chlorohydrins. For the situation in vivo it appears that the type of reaction occurring between HOCl and lipids would very much depend on the circumstances, e.g. the pH and the presence of radical initiators. The biological effects of lipid chlorohydrins are not yet well understood. It has been shown that chlorohydrins of both unsaturated fatty acids as well as of cholesterol may cause lysis of target cells, possibly by disruption of membrane structures.     

Morphine,a potential inhibitor of myeloperoxidase activity

Morphine is an opioid alkaloid commonly used in clinical practice for its analgesic properties. The phenolic hydroxyl group of that phenanthrene derivative is pivotal for binding to opioid receptors but it may also be responsible for the antioxidant behavior of morphine reported in several in vitro experiments. In this study, we assessed the effect of morphine on myeloperoxidase (MPO), a hemic enzyme from azurophilic granules of polymorphonuclear neutrophils involved in the production of cytotoxic and microbicidal reactive oxidants during inflammatory response. Specific immunological extraction followed by enzyme detection (SIEFED) and molecular modeling (docking) were performed to study the potential anti-catalytic action of morphine on MPO in comparison with the inhibitory effects of reference antioxidant molecules quercetin, gallic acid and ascorbic acid. The reducing action of morphine on the MPO peroxidase cycle has been investigated using electron paramagnetic resonance (EPR) and UV-visible absorption spectroscopy. Morphine acted as a reducing substrate in the peroxidase cycle of MPO and therefore protected the enzyme against the suicide action of its natural substrate, hydrogen peroxide. The SIEFED experiments associated with the docking study, further demonstrated a lack of strong and sustained anti-catalytic activity of morphine. In summary, from the results of this study, it appears that morphine acts as a weak and reversible inhibitor of MPO that may nonetheless contribute to immunomodulatory and antioxidant effects of this opioid analgesic in vivo.     

Human hemi-myeloperoxidase. Initial chlorinating activity at neutral pH, compound II and III formation, and stability towards hypochlorous acid and high temperature.

Human neutrophilic myeloperoxidase (MPO) is involved in the defence mechanism of the body against micro-organisms. The enzyme catalyses the generation of the strong oxidant hypochlorous acid (HOCl) from hydrogen peroxide and chloride ions. In normal neutrophils MPO is present in the dimeric form (140 kDa). The disulphide-linked protomers each consist of a heavy subunit and a light one. Reductive alkylation converts the dimeric enzyme into two promoters, 'hemi-myeloperoxidase'. We studied the initial activities of human dimeric MPO and hemi-MPO at the physiological pH of 7.2 and found no significant differences in chlorinating activity. These results indicate that, at least at neutral pH, the protomers of MPO function independently. The absorption spectra of MPO compounds II and III, both inactive forms concerning HOCl generation, and the rate constants of their formation were the same for dimeric MPO and hemi-MPO, but hemi-MPO required a slightly larger excess of H2O2 for complete conversion. Hemi-MPO was less stable at a high temperature (80 degrees C) as compared to the dimeric enzyme. Furthermore, the resistance of the chlorinating activity of hemi-MPO against its oxidative product hypochlorous acid was somewhat lower (IC50 = 32 microM HOCl) compared to dimeric MPO (IC50 = 50 microM HOCl). The higher stability of dimeric MPO in the presence of its oxidative product compared to that of monomeric MPO might be the reason for the occurrence of MPO as a dimer.     

Interaction of methyl-xanthines with myeloperoxidase. An anti-inflammatory mechanism.

1. Inhibition of myeloperoxidase (MPO)-catalyzed reactions by methyl-substituted xanthines has been investigated. 2. Except for theobromine and caffeine, all xanthines tested were potent inhibitors of the MPO-H2O2-Cl- system. 3. In contrast to methyl substitution in the 1 or 8 position of xanthine, substitution in the 3 or 7 position had a marked effect on the inhibition of MPO catalysis. 4. Two different inhibitory mechanisms were induced; scavenging of hypochlorous acid (HOCl) generated by the MPO system and accumulation of Compound II (ferryl MPO) which is inactive as a catalyst of Cl- oxidation.     

Hydrogen sulphide: a novel inhibitor of hypochlorous acid-mediated oxidative damage in the brain?

Hydrogen sulphide (H(2)S) is a cytotoxic gas that has recently been proposed as a novel neuromodulator. Endogenous levels of H(2)S in the brain range between 50 and 160 microM, and considerably lower H(2)S levels are reported in the brains of Alzheimer's disease (AD) patients. Levels of myeloperoxidase (MPO), an enzyme that catalyses the formation of the oxidant hypochlorous acid (HOCl), are elevated in the prefrontal cortex, hippocampal microglia, and neurons of AD patients where MPO co-localised with beta-amyloid plaques. Recently 3-chlorotyrosine, a bio-marker for MPO activity (and HOCl production), was shown to be elevated threefold in hippocampal proteins from AD patients. Since H(2)S and HOCl are important mediators in brain function and disease, we investigated the effects of H(2)S on HOCl-mediated damage to bio-molecules and to cultured human SH-SY5Y cells. H(2)S significantly inhibited HOCl-mediated inactivation of alpha(1)-antiproteinase and protein oxidation to a comparable extent to reduced glutathione. H(2)S also inhibited HOCl-induced cytotoxicity, intracellular protein oxidation, and lipid peroxidation in SH-SY5Y cells. These data suggest that H(2)S has the potential to act as an inhibitor of HOCl-mediated processes in vivo and that the potential antioxidant action of H(2)S deserves further study, especially since extracellular GSH levels in the brain are very low.     

Interaction of myeloperoxidase with vascular NAD(P)H oxidase-derived reactive oxygen species in vasculature: implications for vascular diseases

Vascular NAD(P)H oxidase-derived reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) have emerged as important molecules in the pathogenesis of atherosclerosis, hypertension, and diabetic vascular complications. Additionally, myeloperoxidase (MPO), a transcytosable heme protein that is derived from leukocytes, is also believed to play important roles in the above-mentioned inflammatory vascular diseases. Previous studies have shown that MPO-induced vascular injury responses are H2O2 dependent. It is well known that MPO can use leukocyte-derived H2O2; however, it is unknown whether the vascular-bound MPO can use vascular nonleukocyte oxidase-derived H2O2 to induce vascular injury. In the present study, ANG II was used to stimulate vascular NAD(P)H oxidases and increase their H2O2 production in the vascular wall, and vascular dysfunction was used as the vascular injury parameter. We demonstrated that vascular-bound MPO has sustained activity in the vasculature. MPO could use the vascular NAD(P)H oxidase-derived H2O2 to produce hypochlorus acid (HOCl) and its chlorinating species. More importantly, MPO derived HOCl and chlorinating species amplified the H2O2-induced vascular injury by additional impairment of endothelium-dependent relaxation. HOCl-modified low-density lipoprotein protein (LDL), a specific biomarker for the MPO-HOCl-chlorinating species pathway, was expressed in LDL and MPO-bound vessels with vascular NAD(P)H oxidase-derived H2O2. MPO-vascular NAD(P)H oxidase-HOCl-chlorinating species may represent a common pathogenic pathway in vascular diseases and a new mechanism involved in exacerbation of vascular diseases under inflammatory conditions.     

Inactivation of myocardial dihydrolipoamide dehydrogenase by myeloperoxidase systems: effect of halides, nitrite and thiol compounds

Dihydrolipoamide dehydrogenase (LADH) lipoamide reductase activity decreased whereas enzyme diaphorase activity increased after LADH treatment with myeloperoxidase (MPO) dependent systems (MPO/H₂O₂/halide, MPO/NADH/halide and MPO/H₂O₂/nitrite systems. LADH inactivation was a function of the composition of the inactivating system and the incubation time. Chloride, iodide, bromide, and the thiocyanate anions were effective complements of the MPO/H₂O₂ system. NaOCl inactivated LADH, thus supporting hypochlorous acid (HOCl) as putative agent of the MPO/H₂O₂/NaCl system. NaOCl and the MPO/H₂O₂/NaCl system oxidized LADH thiols and NaOCl also oxidized LADH methionine and tyrosine residues. LADH inactivation by the MPO/ NADH/halide systems was prevented by catalase and enhanced by superoxide dismutase, in close agreement with H₂O₂ production by the LADH/NADH system. Similar effects were obtained with lactoperoxidase and horseradish peroxidase suplemented systems. L-cysteine, N-acetylcysteine, penicillamine, N-(2-mercaptopropionylglycine), Captopril and taurine protected LADH against MPO systems and NaOCl. The effect of the MPO/H₂O₂/NaNO₂ system was prevented by MPO inhibitors (sodium azide, isoniazid, salicylhydroxamic acid) and also by L-cysteine, L-methionine, L-tryptophan, L-tyrosine, L-histidine and reduced glutathione. The summarized observations support the hypothesis that peroxidase-generated “reactive species” oxidize essential thiol groups at LADH catalytic site.     

Influence of ceruloplasmin and lactoferrin on the chlorination activity of leukocyte myeloperoxidase assayed by chemiluminescence

We demonstrate that addition of H₂O₂ to a mixture of myeloperoxidase (MPO), chloride and luminol immediately evokes a short intense flash of chemiluminescence (CL). This flash is diminished in the absence of MPO or chloride, and in the complete system it is suppressed by an MPO inhibitor azide, hypochlorite scavengers taurine or methionine, or an MPO peroxidase-cycle substrate guaiacol. Hence, this CL is mostly due to the MPO halogenation function; a measure of this activity is provided by the integral CL. With three independent methods (CL, taurine chlorination, and peroxidase assay) it is shown that MPO activity is suppressed by ceruloplasmin (Cp). Lactoferrin has no effect either on MPO or on the MPO-Cp complex. It is also shown that peroxidase inhibition by Cp is the stronger the larger is the MPO substrate, which suggests steric hindrances to substrate binding in the MPO-Cp complex. Importantly, the conventional chlorination and peroxidase assays detect MPO inhibition by Cp only at a large excess of the latter, whereas the CL assay reveals it at stoichiometric ratios characteristic of the naturally occurring protein complexes.     

Novel model of inflammatory neointima formation reveals a potential role of myeloperoxidase in neointimal hyperplasia

Atherosclerosis, which is characterized by neointima formation, is an inflammatory disease. However, there is no inflammatory product-elicited neointimal model to support the causal role of inflammation in atherogenesis. We reported previously that leukocyte-derived MPO induces vascular injury responses such as endothelial dysfunction. We now test the role of MPO in inflammatory neointima formation. We infused temporarily isolated rat common carotid arteries with MPO (200 nM) and incubated for 1 h. We found that although MPO itself did not induce any neointima formation 2 wk after treatment, in the presence of its substrate, hydrogen peroxide, MPO was able to elicit neointimal hyperplasia. We further confirmed that MPO-induced neointimal hyperplasia is mediated by its product, hypochlorous acid (HOCl). HOCl elicited apoptosis both in intima and media followed by vascular proliferative response and resulted in neointima formation with a heterogeneous cell population. Both histological and functional features of HOCl-treated vessels are similar to those in atherosclerotic lesions. To our knowledge, this is the first direct in vivo demonstration of neointimal formation induced by a product of the inflammatory cascade. The results suggest that MPO may be a mediator for pathological neointima growth. This novel neointimal model could be useful for studying inflammation and atherosclerosis.     

Selective modification of apoB-100 in the oxidation of low density lipoproteins by myeloperoxidase in vitro

Oxidative modification of LDL may be important in the initiation and/or progression of atherosclerosis, but the precise mechanisms through which low density lipoprotein (LDL) is oxidized are unknown. Recently, evidence for the existence of HOCl-oxidized LDL in human atherosclerotic lesions has been reported, and myeloperoxidase (MPO), which is thought to act through production of HOCl, has been identified in human atherosclerotic lesions. In the present report we describe the formation of 2,4-dinitrophenylhydrazine (DNPH)-reactive modifications in the apolipoprotein (apo) by exposure of LDL to myeloperoxidase in vitro. In contrast with the complex mixture of peptides from oxidation of LDL with reagent HOCl, oxidation with MPO in vitro produced a major tryptic peptide showing absorbance at 365 nm. This peptide was isolated and characterized as VELEVPQL(*C)SFILK..., corresponding to amino acid residues 53-66...on apoB-100. Mass spectrometric analyses of two tryptic peptides from oxidation of LDL by HOCl indicated formation of the corresponding methionine sulfoxide (M=O), cysteinyl azo (*C), RS -N= N-DNP, derivatives of EEL(*C)T(M=O)FIR and LNDLNS VLV(M=O)PTFHVPFTDLQVPS(*C)K, which suggest oxidation to the corresponding sulfinic acids (RSO2H) by HOCl.The present results demonstrate that DNPH-reactive modifications other than aldehydes and ketones can be formed in the oxidation of proteins and illustrate how characterization of specific products of protein oxidation can be useful in assessing the relative contributions of different and unexpected mechanisms to the oxidation of LDL and other target substrates. The data also suggest a direct interaction of the LDL particle with the active site on myeloperoxidase and indicate that effects of the protein microenvironment can greatly influence product formation and stability.