Distinct chromatographic forms of human hemi-myeloperoxidase obtained by reductive cleavage of the dimeric enzyme. Evidence for subunit heterogeneity

Presence of melatonin-catabolizing non-specific enzymes myeloperoxidase and indoleamine 2,3-dioxygenase in the ram reproductive tract

The melatonin catabolism is very complex and not completely understood. Melatonin can be metabolized by free radical interaction, but also pseudo-enzymatically or by enzymatic pathways. We have previously detected the existence of melatonin-synthesizing enzymes and melatonin receptors MT1 and MT2 in the ram reproductive tract; thus, in order to start to elucidate melatonin catabolism in these organs, we have investigated the presence of the melatonin-catabolizing enzymes indoleamine 2,3-dioxygenase (IDO, both IDO1 and IDO2 isoforms) and myeloperoxidase (MPO) in testis, epididymis and accessory glands. Gene expression analyses by real-time PCR showed the presence of MPO, IDO1 and IDO2 in all the organs of the ram reproductive tract and revealed that MPO is the main melatonin-catabolizing enzyme, which is mainly expressed in the testis and the bulbourethral glands (p < .05). These results were further corroborated by immunohistochemical staining, and by Western blot. Likewise, MPO was also evidenced in epididymal and ejaculated spermatozoa by indirect immunofluorescence and Western blot. In conclusion, melatonin-catabolizing enzymes MPO, IDO1 and IDO2 are expressed in the ram reproductive tract, and MPO is the most expressed one, mainly in the testis and the bulbourethral glands. The presented results warrant further studies on the function of these enzymes and their melatonin-metabolizing activity.

[Enzymatic and bactericidal activity of monomeric and dimeric forms of myeloperoxidase]

This study was carried out to compare the enzymatic and bactericidal activity of mature, dimeric myeloperoxidase (MPO) and its monomeric form. Dimeric MPO was isolated from HL-60 cells. Hemi-MPO obtained from dimeric MPO by reductive cleavage of a disulfide bond between protomeric subunits was used as the monomeric form. Both peroxidase and halogenating (chlorinating) activities of MPO were assayed, each of them by two methods. Bactericidal activity of the MPO/Н2О2/Cl- system was tested using the Escherichia coli laboratory strain DH5a. No difference in the enzymatic and bactericidal activity between dimeric MPO and hemi-MPO was found. Both forms of the enzyme also did not differ in the resistance to HOCl, the main product of MPO. HOCl caused a dose-dependent decrease in peroxidase and chlorinating activity, and the pattern of this decrease was identical for dimeric MPO and hemi-MPO. At equal heme concentration, a somewhat higher bactericidal effect was observed for the hemi-MPO/Н2О2/Cl- system compared with the dimeric MPO/Н2О2/Cl- system. However, this is most likely not related to some specific property of hemi-MPO and can be accounted for by the higher probability of contacting between bacterial surface and hemi-MPO molecules due to their two-fold greater number relative to that of dimeric MPO molecules at the same heme concentration. By using Western-blotting with antibodies to MPO, we showed, for the first time, that the dimeric molecule of MPO could be cleaved into two monomeric subunits by HOCl, most probably due to oxidation of the disulfide bond between these subunits. This finding suggests that appearance in blood of MPO corresponding in mass to its monomer may result from the damage of dimeric MPO by reactive halogen species, especially upon their overproduction underlying oxidative/halogenative stress in inflammatory diseases.

Identification of a myeloperoxidase-like ortholog from rock bream (Oplegnathus fasciatus), deciphering its transcriptional responses to induced pathogen stress

Myeloperoxidases (MPOs) are heme-linked oxidative stress-generating enzymes found abundantly in azurophilic granules of polymorphonuclear neutrophils. Mature MPOs act as potent antimicrobial agents by producing hypohalous acids using hydrogen peroxide and halide ions as substrates. These acids can readily oxidize reactive groups of biomolecules on invading microbes. In this study, we identified and characterized a homolog of MPO from rock bream (Oplegnathus fasciatus), designated as RbMPO. We analyzed the RbMPO gene for its basal expression level in physiologically important tissues and for transcriptional changes under different pathogenic stress conditions. The complete coding sequence of RbMPO consisted of 2652 nucleotides encoding an 884 amino acid sequence with a predicted molecular mass of 99.7 kDa. Our in silico analysis confirmed the typical MPO domain arrangement in RbMPO, including the propeptide, large chain and heavy chain, along with the heme peroxidase signature. Intriguingly, a C1q domain was also identified in the C-terminal region of the derived amino acid sequence. Most of the known functionally important residues of MPOs are found to be well conserved in RbMPO, showing a close evolutionary relationship with other teleostan MPOs, particularly with that of mandarin fish. RbMPO exhibited a ubiquitous basal expression in physiologically relevant tissues, with particularly high expression levels in blood cells. Basal transcript levels of RbMPO in gill and spleen tissues were found to change upon different pathogen or pathogen-derived mitogen stimulation, with detectable inductive responses. Together, these data suggest the potential involvement of RbMPO in the innate immune response in rock bream.

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.

Some patients with anti-myeloperoxidase autoantibodies have a C-ANCA pattern

Rapidly progressive glomerulonephritis with or without other signs of systemic vasculitis is often accompanied by antibodies to myeloperoxidase. Such antibodies normally produce a perinuclear pattern on ethanol-fixed neutrophils (perinuclear anti-neutrophil cytoplasm antibodies (P-ANCA)) at indirect immunofluorescence. We report here sera from three patients that are anti-myeloperoxidase-positive in ELISA that instead produce a cytoplasmic pattern (classical anti-neutrophil cytoplasmic antibodies (C-ANCA)), a pattern normally seen in conjunction with antibodies to proteinase 3. These sera did not react with proteinase 3. For two of the sera the specificity of the anti-myeloperoxidase reaction was confirmed with inhibition-ELISA experiments and with immunoblotting. A mouse anti-myeloperoxidase MoAb that produces a cytoplasmic pattern is also described. Competition ELISA experiments show that this antibody and anti-myeloperoxidase sera with cytoplasmic pattern recognize epitopes that are separate from epitopes recognized by another perinuclear pattern producing anti-myeloperoxidase MoAb. 'Cytoplasmic pattern' epitopes as well as 'perinuclear pattern' epitopes can be found on all three major myeloperoxidase isoforms, after separation by ion exchange chromatography. Affinity chromatography, using the cytoplasmic pattern producing anti-myeloperoxidase monoclonal antibody, shows that the epitope recognized by this MoAb is present on all myeloperoxidase molecules. This epitope is not confined to any special subpopulation. These findings indicate that all myeloperoxidase do not relocate after ethanol fixation, and that C-ANCA and P-ANCA epitopes exist simultaneously on the same myeloperoxidase molecule. We propose that the two immunofluorescence patterns arise due to different availabilities of the epitopes in the microenvironment where myeloperoxidase is present.

Expression of recombinant myeloperoxidase using a baculovirus expression system

Myeloperoxidase (MPO) is a glycosylated heme-containing enzyme present in the azurophilic granules of normal human polymorphonuclear neutrophils. This enzyme plays a major role in the microbicidal activity of the host defense system by catalyzing the formation of the potent oxidant, hypochlorous acid. Although the amino acid sequence of MPO has been deduced from the cDNA, the structural basis for the observed heterogeneity of this enzyme is not known. Furthermore, the nature of the prosthetic group and its mode of linkage to the apoprotein has not been determined. To address questions regarding the structural features of MPO, which arise during the complex posttranslational processing of this enzyme, we utilized a baculovirus system to express MPO in Sf9 insect cells. Two glycosylated, single-chain precursor species of MPO were observed: an 84 kDa species that was secreted and a 74 kDa species that was cell-associated. This is the first report of an expression system in which a cell-associated MPO precursor undergoes posttranslational proteolytic processing.

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.

The early and late processing of lysosomal enzymes: proteolysis and compartmentation

Lysosomal enzymes are subjected to a number of modifications including carbohydrate restructuring and proteolytic maturation. Some of these reactions support lysosomal targeting, others are necessary for activation or keeping the enzyme inactive before being segregated, while still others may be adventitious. The non-segregated fraction of the enzyme is secreted and can be isolated from the medium. It is considered that the secreted lysosomal enzymes fulfill certain physiological and pathophysiological roles. By comparing the secreted and the intracellular enzymes it is possible to distinguish between the reactions that occur before and after the segregation. In this review the reactions that may influence the segregation are referred to as the early processing and those characteristic for the enzymes isolated from lysosomal compartments as the late processing. The early processing is characterized mainly by modifications of carbohydrate side chains. In the late processing, proteolytic fragmentation represents the most conspicuous changes. The review focuses on the compartmentation of the reactions and the proteolytic fragmentation of lysosomal enzyme precursors. While a plethora of proteolytic reactions are involved, our knowledge of the proteinases responsible for the particular maturation reactions remains very limited. The review points also to work with cells from patients affected with lysosomal storage disorders, which contributed to our understanding of the lysosomal apparatus.