Deglycosylation influences the oxidation activity and antigenicity of myeloperoxidase

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Kidneys are one of the most common target organs of antineutrophil cytoplasmic autoantibodies (ANCA)‐associated systemic vasculitis; the condition may sometimes present as ‘kidney‐limited’ vasculitis. Proteinase‐3 (PR3) and myeloperoxidase (MPO) are the two major target antigens of ANCA. MPO‐ANCA was reported to be more predominant in Chinese patients.1 Both MPO‐ANCA complex and MPO itself have been shown to be pathogenic.2 Furthermore, linear epitopes on MPO were reported in ANCA‐negative vasculitis,6 and occult antibodies against these linear epitopes were blocked in serum by a ceruloplasmin fragment, which further proves the pathogenicity of MPO itself and occurrence of special anti‐MPO autoantibodies in ANCA‐negative vasculitis.
Myeloperoxidase is also one of the major proteins of the antimicrobial system in mammalian neutrophils.2 It catalyzes the production of hypochlorous acid, a potent reactive oxygen species, from hydrogen peroxide (H2O2) and chloride ions (Cl−).7 However, the reactive oxygen species produced by the catalytic activity of MPO has been shown to contribute to tissue damage in certain inflammatory diseases such as, rheumatoid arthritis, atherosclerosis and multiple sclerosis.2 Under physiological conditions, the peroxidase activity of MPO is inhibited by ceruloplasmin, which is a multi‐functional copper containing protein that oxidizes the highly toxic ferrous ions of MPO to ferric state.7 Increased levels of ceruloplasmin and its high affinity for binding to MPO has been demonstrated in patients with active MPO‐ANCA vasculitis.6 However, the inhibitory effect of ceruloplasmin on MPO can be disturbed in the presence of MPO‐ANCA, which may result in the persistent generation of reactive oxygen by MPO and lead to endothelial insult.4
Myeloperoxidase is a glycosylated hemo‐enzyme, which is most abundantly present in neutrophils. The mature MPO protein consists of two light and two heavy polypeptide chains with each heavy chain binding to prosthetic heme group.10 Mass spectrometry has demonstrated the presence of five N‐linked glycosylation sites in each MPO heavy chain, which are occupied by either complex or high mannose glycan structures.11 The overall structures of MPO glycan and the biological effects of glycosylation patterns on MPO function are not well characterized.
In our previous study involving patients with ANCA‐associated vasculitis, the MPO‐ANCA positive sera recognized linear peptides in the N‐terminal part of MPO heavy chain, especially H1279–409.12 We further found that the major epitopes in this peptide were located at H4 and H11.13 Each of these two regions contain one N‐linked glycosylation site on MPO. We hypothesized that the glycosylation of MPO may influence its antigenicity as well as its functions as a peroxidase.
Oxidation and antimicrobial activity are two major functions of MPO under physiological conditions, and ceruloplasmin prevents overactivation of MPO. In ANCA‐associated vasculitis, the inhibitory effect of ceruloplasmin on MPO is disturbed because of MPO‐ANCA. Therefore, we conducted a study to investigate the influence of glycans on the chlorination activity and microbicidal effect of MPO, on the binding capacity of ceruloplasmin to MPO, and the effect of MPO‐ANCA on this binding. The objective was to explore the influence of glycosylation patterns on MPO functions and the potential role in the pathogenesis of ANCA associated vasculitis.
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