Spectral and enzymatic properties of human recombinant myeloperoxidase: comparison with the mature enzyme

In Vitro Evaluation of the Antiamoebic Activity of Kaempferol against Trophozoites of Entamoeba histolytica and in the Interactions of Amoebae with Hamster Neutrophils

Entamoeba histolytica (E. histolytica) is a parasite in humans that provokes amoebiasis. The most employed drug is metronidazole (MTZ); however, some studies have reported that this drug induces genotoxic effects. Therefore, it is necessary to explore new compounds without toxicity that can eliminate E. histolytica. Flavonoids are polyphenolic compounds that have demonstrated inhibition of growth and dysregulation of amoebic proteins. Despite the knowledge acquired to date, action mechanisms are not completely understood. The present work evaluates the effect of kaempferol against E. histolytica trophozoites and in the interactions with neutrophils from hamster, which is a susceptibility model. Our study demonstrated a significant reduction in the amoebic viability of trophozoites incubated with kaempferol at 150 μM for 90 min. The gene expression analysis showed a significant downregulation of Pr (peroxiredoxin), Rr (rubrerythrin), and TrxR (thioredoxin reductase). In interactions with amoebae and neutrophils for short times, we observed a reduction in ROS (reactive oxygen species), NO (nitric oxide), and MPO (myeloperoxidase) neutrophil activities. In conclusion, we confirmed that kaempferol is an effective drug against E. histolytica through the decrease in E. histolytica antioxidant enzyme expression and a regulator of several neutrophil mechanisms, such as MPO activity and the regulation of ROS and NO.

Different behavior of myeloperoxidase in two rodent amoebic liver abscess models

The protozoan Entamoeba histolytica is the etiological agent of amoebiasis, which can spread to the liver and form amoebic liver abscesses. Histological studies conducted with resistant and susceptible models of amoebic liver abscesses (ALAs) have established that neutrophils are the first cells to contact invasive amoebae at the lesion site. Myeloperoxidase is the most abundant enzyme secreted by neutrophils. It uses hydrogen peroxide secreted by the same cells to oxidize chloride ions and produce hypochlorous acid, which is the most efficient microbicidal system of neutrophils. In a previous report, our group demonstrated that myeloperoxidase presents amoebicidal activity in vitro. The aim of the current contribution was to analyze in vivo the role of myeloperoxidase in a susceptible (hamsters) and resistant (Balb/c mice) animal models of ALAs. In liver samples of hamsters and mice inoculated intraportally with Entamoeba histolytica trophozoites, the number of neutrophils in ALAs was determined by enzymatic activity. The presence of myeloperoxidase was observed by staining, and its expression and activity were quantified in situ. A significant difference existed between the two animal models in the number of neutrophils and the expression and activity of myeloperoxidase, which may explain the distinct evolution of amoebic liver abscesses. Hamsters and mice were treated with an MPO inhibitor (4-aminobenzoic acid hydrazide). Hamsters treated with ABAH showed no significant differences in the percentage of lesions or in the percentage of amoebae damaged compared with the untreated hamsters. ABAH treated mice versus untreated mice showed larger abscesses and a decreased percentage of damaged amoebae in these lesion at all stages of evolution. Further studies are needed to elucidate the host and amoebic mechanisms involved in the adequate or inadequate activation and modulation of myeloperoxidase.

Characterization of Peroxidases Expressed in Human Antigen Presenting Cells and Analysis of the Covalent Binding of Nitroso Sulfamethoxazole to Myeloperoxidase

Drug hypersensitivity remains a major concern, as it causes high morbidity and mortality. Understanding the mechanistic basis of drug hypersensitivity is complicated by the multiple risk factors implicated. This study utilized sulfamethoxazole (SMX) as a model drug to (1) relate SMX metabolism in antigen presenting cells (APCs) to the activation of T-cells and (2) characterize covalent adducts of SMX and myeloperoxidase, which might represent antigenic determinants for T-cells. The SMX metabolite nitroso-SMX (SMX-NO) was found to bind irreversibly to APCs. Time- and concentration-dependent drug-protein adducts were also detected when APCs were cultured with SMX. Metabolic activation of SMX was significantly reduced by the oxygenase/peroxidase inhibitor methimazole. Similarly, SMX-NO-specific T-cells were activated by APCs pulsed with SMX, and the response was inhibited by pretreatment with methimazole or glutaraldehyde, which blocks antigen processing. Western blotting, real-time polymerase chain reaction (RT-PCR), and mass spectrometry analyses suggested the presence of low concentrations of myeloperoxidase in APCs. RT-PCR revealed mRNA expression for flavin-containing monooxygenases (FMO1-5), thyroid peroxidase, and lactoperoxidase, but the corresponding proteins were not detected. Mass spectrometric characterization of SMX-NO-modified myeloperoxidase revealed the formation of N-hydroxysulfinamide adducts on Cys309 and Cys398. These data show that SMX's metabolism in APCs generates antigenic determinants for T-cells. Peptides derived from SMX-NO-modified myeloperoxidase may represent one form of functional antigen.

Molecular heterogeneity and alternative splicing of human lactoperoxidase

Human lactoperoxidase (LPO) exists as two distinct molecules independent of glycosylation. The N-terminus of one form is blocked and has not been identified while the other is proteolytically processed at the N-terminus similar to myeloperoxidase. Our analysis identified alternatively spliced human LPO mRNAs that may explain the observed molecular heterogeneity of LPO. Two mRNAs omit propeptide encoding exons while retaining the 5' exon encoding the secretion signal, consistent with the heterogeneity and suggesting a possible functional role for the propeptide. Two LPO forms were expressed using baculovirus and both showed similar enzyme activity. LC/MS/MS analysis of trypsin digested, partially purified, salivary LPO confirmed the larger unprocessed LPO is present in saliva. To compare variant expression patterns, antisera were raised against purified recombinant (rhLPO) as well as against an antigenic peptide sequence within the exons encoding the propeptide region. Immunohistochemistry demonstrated proLPO was differently localized within gland cells compared to other forms of LPO. The data suggested splice variants may contribute to LPO molecular heterogeneity and its regulation by intracellular compartmental localization.

Myeloperoxidase: a target for new drug development?

Myeloperoxidase (MPO), a member of the haem peroxidase-cyclooxygenase superfamily, is abundantly expressed in neutrophils and to a lesser extent in monocytes and certain type of macrophages. MPO participates in innate immune defence mechanism through formation of microbicidal reactive oxidants and diffusible radical species. A unique activity of MPO is its ability to use chloride as a cosubstrate with hydrogen peroxide to generate chlorinating oxidants such as hypochlorous acid, a potent antimicrobial agent. However, evidence has emerged that MPO-derived oxidants contribute to tissue damage and the initiation and propagation of acute and chronic vascular inflammatory disease. The fact that circulating levels of MPO have been shown to predict risks for major adverse cardiac events and that levels of MPO-derived chlorinated compounds are specific biomarkers for disease progression, has attracted considerable interest in the development of therapeutically useful MPO inhibitors. Today, detailed information on the structure of ferric MPO and its complexes with low- and high-spin ligands is available. This, together with a thorough understanding of reaction mechanisms including redox properties of intermediates, enables a rationale attempt in developing specific MPO inhibitors that still maintain MPO activity during host defence and bacterial killing but interfere with pathophysiologically persistent activation of MPO. The various approaches to inhibit enzyme activity of MPO and to ameliorate adverse effects of MPO-derived oxidants will be discussed. Emphasis will be put on mechanism-based inhibitors and high-throughput screening of compounds as well as the discussion of physiologically useful HOCl scavengers.

Disruption of the aspartate to heme ester linkage in human myeloperoxidase: impact on ligand binding, redox chemistry, and interconversion of redox intermediates

In human heme peroxidases the prosthetic group is covalently attached to the protein via two ester linkages between conserved glutamate and aspartate residues and modified methyl groups on pyrrole rings A and C. Here, monomeric recombinant myeloperoxidase (MPO) and the variants D94V and D94N were produced in Chinese hamster ovary cell lines. Disruption of the Asp(94) to heme ester bond decreased the one-electron reduction potential E'(0) [Fe(III)/Fe(II)] from 1 to -55 mV at pH 7.0 and 25 degrees C, whereas the kinetics of binding of low spin ligands and of compound I formation was unaffected. By contrast, in both variants rates of compound I reduction by chloride and bromide (but not iodide and thiocyanate) were substantially decreased compared with the wild-type protein. Bimolecular rates of compound II (but not compound I) reduction by ascorbate and tyrosine were slightly diminished in D94V and D94N. The presented biochemical and biophysical data suggest that the Asp(94) to heme linkage is no precondition for the autocatalytic formation of the other two covalent links found in MPO. The findings are discussed with respect to the known active site structure of MPO and its complexes with ligands.

Expression of Human Myeloperoxidase by Macrophages Promotes Atherosclerosis in Mice

Background— Myeloperoxidase (MPO) colocalizes with macrophages in the human artery wall, and its characteristic oxidation products have been detected in atherosclerotic lesions. Thus, oxidants produced by the enzyme might promote atherosclerosis. However, macrophages in mouse atherosclerotic tissue do not express MPO. Therefore, mice are an inappropriate model for testing the role of MPO in vascular disease. To overcome this problem, we generated and studied transgenic (Tg) mice that contained the human MPO gene.

Methods and Results— We produced human MPO -Tg mice with use of a Visna virus promoter. To confine MPO expression to macrophages, we lethally irradiated LDL receptor–deficient mice and repopulated their bone marrow with cells from wild-type mice or MPO -Tg mice. Despite having similarly high levels of cholesterol after maintenance on a high-fat, high-cholesterol diet, the MPO -Tg animals developed a 2-fold greater atherosclerotic area in the aorta than did mice transplanted with wild-type bone marrow ( P =0.00003).

Conclusions— Our observations indicate that expression of human MPO in macrophages promotes atherosclerosis in hypercholesterolemic mice, raising the possibility that the enzyme might be a potential therapeutic target for preventing cardiovascular disease in humans.

Organic and inorganic substrates as probes for comparing native bovine lactoperoxidase and recombinant human myeloperoxidase

The interaction of native bovine lactoperoxidase (nbLPO) with four substrates has been studied and compared with that of recombinant human myeloperoxidase (rhMPO). Kinetic, spectroscopic and binding parameters extrapolated for each enzyme-substrate adduct have been interpreted in the light of the structural data available for myeloperoxidase (X-ray structure) and lactoperoxidase (3D-model), respectively. The differences in the reactivity and affinity of nbLPO and rhMPO towards SCN(-), catechol, dopamine and 3,4-dihydroxyphenylpropionic acid are here discussed and related to a different structure of the organic substrate access channel as well as to a different accessibility of the heme pocket in the two enzymes.

A transient kinetic study on the reactivity of recombinant unprocessed monomeric myeloperoxidase

Spectral and kinetic features of the redox intermediates of human recombinant unprocessed monomeric myeloperoxidase (recMPO), purified from an engineered Chinese hamster ovary cell line, were studied by the multi-mixing stopped-flow technique. Both the ferric protein and compounds I and II showed essentially the same kinetic behavior as the mature dimeric protein (MPO) isolated from polymorphonuclear leukocytes. Firstly, hydrogen peroxide mediated both oxidation of ferric recMPO to compound I (1.9 x 10(7) M(-1) s(-1), pH 7 and 15 degrees C) and reduction of compound I to compound II (3.0 x 10(4) M(-1) s(-1), pH 7 and 15 degrees C). With chloride, bromide, iodide and thiocyanate compound I was reduced back to the ferric enzyme (3.6 x 10(4) M(-1) s(-1), 1.4 x 10(6) M(-1) s(-1), 1.4 x 10(7) M(-1) s(-1) and 1.4 x 10(7) M(-1) s(-1), respectively), whereas the endogenous one-electron donor ascorbate mediated transformation of compound I to compound II (2.3 x 10(5) M(-1) s(-1)) and of compound II back to the resting enzyme (5.0 x 10(3) M(-1) s(-1)). Comparing the data of this study with those known from the mature enzyme strongly suggests that the processing of the precursor enzyme (recMPO) into the mature form occurs without structural changes at the active site and that the subunits in the mature dimeric enzyme work independently.

Inhibition of the myeloperoxidase chlorinating activity by non-steroidal anti-inflammatory drugs investigated with a human recombinant enzyme

Non-steroidal anti-inflammatory drugs (NSAIDs) were investigated for their ability to affect the chlorinating activity of human myeloperoxidase and to scavenge HOCl, the main myeloperoxidase system product. Fourteen drugs representative of various NSAIDs families were tested with the chlorination of taurine used as a detection system. All were unable to inhibit taurine chlorination in a system without myeloperoxidase. In contrast, most of them induced a dose-dependent inhibition of the taurine chlorination mediated by a myeloperoxidase/H2O2/Cl- system. This took place at variable drug concentrations and IC50 were calculated. The inhibitory effect was therefore due to a direct interaction with the enzyme rather than to HOCl scavenging. A spectroscopic method used to measure the myeloperoxidase compound II lifetime in presence of the different drugs showed that all the drugs, which inhibited chlorination activity were able to induce accumulation of compound II. The extent of chlorinating activity inhibition (IC50) was inversely related to the duration of the block of enzyme in compound II form. This further demonstrates that myeloperoxidase is an interesting target for anti-inflammatory therapy. The recombinant myeloperoxidase used for the first time in this kind of study was as convenient for pharmacological purposes as the purified one.

The endothelium and cytokine secretion: the role of peroxidases as immunoregulators

The endothelium is frequently exposed to many proinflammatory mediators. The present study was done to determine the effects of human recombinant myeloperoxidase (MPO) and porcine eosinophil peroxidase (EPO) on certain endothelial cell (HUVEC) functions. The following areas were evaluated: (1) production of reactive oxygen intermediates (ROI), (2) cytokine secretion, and (3) regulation of mRNA cytokine transcripts. Both MPO and EPO induced the production of ROI, but an enzymatically inactive form of MPO (iMPO) was the most effective. Enzymatically inactive MPO, but not MPO, induced the secretion of interleukins 6 and 8 and granulocyte-monocyte colony-stimulating factor. A ribonuclease protection assay indicated that both iMPO and MPO upregulated mRNA cytokine transcripts; however, the former was markedly more effective. The simultaneous addition of EPO and iMPO resulted in a decrease in cytokine-specific mRNA. These data indicate a major role for peroxidases in the regulation of inflammation.

The sulfonium ion linkage in myeloperoxidase. Direct spectroscopic detection by isotopic labeling and effect of mutation

The heme group of myeloperoxidase is covalently linked via two ester bonds to the protein and a unique sulfonium ion linkage involving Met(243). Mutation of Met(243) into Thr, Gln, and Val, which are the corresponding residues of eosinophil peroxidase, lactoperoxidase, and thyroid peroxidase, respectively, and into Cys was performed. The Soret band in the optical absorbance spectrum in the oxidized mutants is now found at approximately 411 nm. Both the pyridine hemochrome spectra and resonance Raman spectra of the mutants are affected by the mutation. In the Met(243) mutants the affinity for chloride has decreased 100-fold. All mutants have lost their chlorination activity, except for the M243T mutant, which still has 15% activity left. By Fourier transform infared difference spectroscopy it was possible to specifically detect the (13)CD(3)-labeled methionyl sulfonium ion linkage. We conclude that the sulfonium ion linkage serves two roles. First, it serves as an electron-withdrawing substituent via its positive charge, and, second, together with its neighboring residue Glu(242), it appears to be responsible for the lower symmetry of the heme group and distortion from the planar conformation normally seen in heme-containing proteins.

Characterization of the Asp94 and Glu242 mutants in myeloperoxidase, the residues linking the heme group via ester bonds

The heme group of all mammalian peroxidases is covalently linked to the protein matrix via two esterbonds, as we have recently shown by Fourier transform infrared (FTIR) difference spectroscopy [Kooter, I. M., Pierik, A.J., Merkx, M., Averill, B.A., Moguilevsky, N., Bollen, A. & Wever, R. (1997) J. Am. Chem. Soc. 119, 11542-11543]. We have examined the effects of mutation of Asp94 and Glu242, responsible for those ester bonds in myeloperoxidase, on the spectroscopic properties and catalytic activity of this enzyme. Mutation of Asp94 in myeloperoxidase results in two species. The first species has spectroscopic characteristics similar to that of wild-type myeloperoxidase. The second species has spectroscopic characteristics similar to that of Met243-->Gln mutant, and it is therefore concluded that, besides loss of the ester bond involving Asp94, this species also has lost the sulfonium ion linkage that is also characteristic of myeloperoxidase. The Asp94-->Asn mutant still has about 30% residual peroxidase activity while for the Asp94-->Val mutant only a few percentage activity is left. When Glu242 is mutated the sulfonium ion linkage is not affected, but this residue together with its neighbouring residue Met243, according to resonance Raman spectra, is responsible for the low symmetry of the heme group. Mutation of either of these residues results in loss of the bowed distortion from the planar conformation, and in a heme group with higher symmetry. For the Glu242-->Gln mutant 8% residual peroxidase activity is found.

The role of the propeptide for processing and sorting of human myeloperoxidase

Myeloperoxidase (MPO), stored in azurophil granules of neutrophils, is critical for an optimal oxygen-dependent microbicidal activity of these cells. Pro-MPO goes through a stepwise proteolytic trimming with elimination of an amino-terminal propeptide to yield one heavy and one light polypeptide chain. The propeptide of MPO may have a role in retention and folding of the nascent protein into its tertiary structure or in targeting of pro-MPO for processing and storage in granules. A propeptide-deleted pro-MPO mutant (MPODeltapro) was constructed to determine if deletion of the propeptide interferes with processing and targeting after transfection to the myeloid 32D cell line. Transfection of full-length cDNA for human MPO results in normal processing and targeting of MPO to cytoplasmic dense organelles. Although the efficiency of incorporation was lower for MPODeltapro, both pro-MPO and MPODeltapro showed heme incorporation indicating that the propeptide is not critical for this process. Deletion of the propeptide results in synthesis of a protein that lacks processing into mature two-chain forms but rather is degraded intracellularly or secreted. The finding of continued degradation of MPODeltapro in the presence of lysosomotrophic agents or brefeldin A rules out that the observed degradation takes place after transfer to granules. Intracellular pro-MPO has high mannose oligosaccharide side chains, whereas stored mature MPO was found to have both high mannose and complex oligosaccharide side chains as judged by only partial sensitivity to endoglycosidase H. The propeptide may normally interfere with the generation of certain complex oligosaccharide chain(s) supported by the finding of high mannose side chains in secreted pro-MPO and lack of them in MPODeltapro that contained complex oligosaccharide side chains only. In conclusion, elimination of the propeptide of pro-MPO blocks the maturation process and abolishes accumulation of the final product in granules suggesting a critical role of the propeptide for late processing of pro-MPO and targeting for storage in granules.

Effect of the R569W missense mutation on the biosynthesis of myeloperoxidase

Human neutrophil microbicidal activity is largely mediated by reactive species generated by the oxygen-dependent myeloperoxidase (MPO) system. Peroxidase-negative neutrophils from many patients with hereditary MPO deficiency possess a 90-kDa MPO-related protein. We recently identified a missense mutation, R569W, in the MPO gene of many subjects with MPO deficiency. In these studies we examined the consequences of R569W on MPO biosynthesis and processing, using stably transfected K562 cells expressing normal MPO or the R569W mutation. K562 cells expressing normal MPO mimicked faithfully many features of MPO biosynthesis in myeloid cells. 1) apopro-MPO was synthesized; 2) a functional heme group was inserted into apopro-MPO, and enzymatically active pro-MPO was thereby generated; 3) pro-MPO underwent proteolytic processing to mature MPO; and 4) hemin augmented the processing of pro-MPO. pREP-R569W cells synthesized apopro-MPO, but heme was not inserted. Neither enzymatically active pro-MPO nor mature MPO was synthesized by transfectants expressing mutated cDNA, confirming our hypothesis that the R569W mutation results in a form of apopro-MPO which does not undergo post-translational processing to enzymatically active MPO species. In addition, these data support previous suggestions that heme insertion into apopro-MPO is necessary for its subsequent proteolytic processing into mature MPO subunits.

Bactericidal activity against Pseudomonas aeruginosa is acquired by cultured human monocyte-derived macrophages after uptake of myeloperoxidase

Myeloperoxidase (MPO) is an enzyme located within polymorphonuclear neutrophils capable of producing cytotoxic oxidant species that are particularly active against bacteria with polysaccharide capsules. Pseudomonas aeruginosa (10(6) bacteria per 1ml) are killed within 1 h in vitro by a MPO/H2O2/C1- system (48mU=132ng of MPO). The question arose as to whether human macrophages would acquire cytotoxic activity when loaded with this enzyme. Monocytes were therefore isolated from human blood and cultured for up to ten days to induce maturation to macrophages. These cells lost endogenous MPO within five days while H2O2 production in response to stimulation by phorbol myristate acetate (10(-6)M) decreased to 23% within ten days. On the other hand, their capacity to take up exogenous MPO increased fourfold from day three to day ten. Human macrophages cultured from eight days (when both H2O2 production and MPO uptake were sufficient) were therefore used to study the effects of MPO uptake on cytocidal activity against Pseudomonas aeruginosa. After a 1 h MPO loading period, macrophages (5X10(5) cells per ml) were incubated in the presence of bacteria (0.5 to 2X10(6) bacteria per ml) for 2 h at 37 degrees C. At a bacteria/macrophage ratio of 1, only 34.8+/-7.0% of bacteria survived (compared to killing by non-loaded macrophages), while 74.4+/-9.3% survived at a ratio of 4. From these results, we conclude that loading macrophages with exogenous MPO could enhance their microbicidal activity, suggesting a potentially useful therapeutic application.

Candidicidal activity of the human recombinant myeloperoxidase system

The effect of the human recombinant myeloperoxidase (recMPO) system on the viability of Candida albicans blastospores was determined. Cells were incubated in the presence of the enzyme, H2O2, and various substrates. In optimal conditions, 100% mortality was reached with only 41 mU of recMPO per ml, confirming its powerful candidicidal activity.

Site-directed mutants of human myeloperoxidase. A topological approach to the heme-binding site

Two site-directed mutants of human promyeloperoxidase, MPO(His416----Ala) and MPO(His502----Ala), have been expressed in Chinese hamster ovary cells and purified. Overall purification yields and apparent molecular masses of the mutant proteins were similar to those of the wild-type enzyme. Both mutant species were analyzed spectroscopically to check the presence of the hemic iron in the proteins and were assayed for peroxidase activity. The data show that substitution of His502 leads to the loss, or to an inappropriate configuration, of the heme together with the loss of activity, suggesting that this residue could be the proximal His involved in the binding to the iron centers. On the other hand, substitution of His416 by alanine had no effect on either of the studied parameters.