Human hemi‐myeloperoxidase

Abstract

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 (HOC1) 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 protomers, '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 I1 and 111, 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 HzOz for complete conversion. Hemi-MPO was less stable at a high temperature (SO'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 (ICso = 32 pM HOC1) compared to dimeric MPO (ICs0 = 50 pM 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 enzyme myeloperoxidase (donor: H 2 0 2 oxidoreductase; MPO) is a major azurophil granule component of normal human ncutrophils and makes up 2 -5% of the total dry mass of these cells [l -41. MPO is involved in the defence mechanism against invading micro-organisms. In the presence of H202, which is generated from 0; produced by NADPH oxidase [5, 61, MPO so-called 'hemi-MPO' [18]. From kinetic studies of the chlorinating and peroxidase (in the absence of CI -) activities in the pH range of 4.4-6.2 of hemi-MPO (generated from normal dimeric MPO [20]), and of monomeric MPO (isolated from HL-60 cells [21]) no significant differences were found between dimeric enzyme, and hemi-or monomeric enzyme. At neutral pH, however, the peroxidase activities of monomeric MPO (79 kDa) [23], both isolated from human leukemia cells, were slightly lower as compared to those of the dimeric or 'large' MPO.Based on the assumption that dimcrization might confer higher catalytic activity or increased stability to the enzyme, we investigated and compared some properties of hemi-MPO with those of dimeric MPO. If the dimer was more stable or more active, this could explain its exclusive occurrence in normal human neutrophils. Besides chlorinating activity at a neutral physiological pH, we also examined compound I1 and compound 111 formation, since both compounds are inactive forms of MPO resulting from side-reactions in the catalytic process [24, 251. Furthermore, we investigated the heat stability and the resistance of both enzyme preparations towards their own product HOCl and the secondary oxid...

The effect of myeloperoxidase isoforms on biophysical properties of red blood cells

“…Recently, we have shown that hemi-MPO induced cytosolic Ca 2+ -rise, as well as lysozyme and elastase degranulation in human neutrophils, but these efects were much weaker than observed in the case of dimeric MPO [20]. It should be noted that hemi-MPO has the same as dimeric MPO peroxidase and chlorinating activity and retains its bactericidal ability [16,17,21].…”

Enzymatic and bactericidal activity of monomeric and dimeric forms of myeloperoxidase

“…В связи с тем, что синтез МПО не заканчивается мономерной формой, возник вопрос о преимуществах преобладающей в организме димерной формы. Показано, что димерная молекула МПО более термостабильна и конформационно устойчива, чем мономерная [7][8][9]. Исследования каталитических свойств двух форм МПО выявили только небольшие различия в функционировании мономерной и димерной МПО, и большинство исследователей сходятся во мнении об отсутствии принципиальной разницы между этими формами в отношении и пероксидазной и галогенирующей активностей [8][9][10][11].…”

“…При проведении этих и других экспериментов концентрацию МПО и геми-МПО уравнивали по гему. Результаты не выявили различий в каталитической активности между димерной МПО и геми-МПО (таблица), что согласуется с данными других исследователей [8][9][10].…”

The Production and Use of Reactive Oxidants by Phagocytes

“…Myeloperoxidase is composed of a pair of identical heterodimers, each consisting of two polypeptides of ~60 and ~15 kDa (Andrews and Krinsky, 1981;Zuurbier et al, 1992;Zeng and Fenna, 1992). The heterodimer is formed by posttranslational processing of a single 88-kDa polypeptide, and its two subunits are held together by noncovalent forces.…”