antioxidants Review The Enzymatic and Non-Enzymatic Function of Myeloperoxidase (MPO) in Inflammatory Communication Yulia Kargapolova * , Simon Geißen, Ruiyuan Zheng, Stephan Baldus, Holger Winkels * and Matti Adam Department III of Internal Medicine, Heart Center, Faculty of Medicine and University Hospital of Cologne, 50937 North Rhine-Westphalia, Germany; [email protected] (S.G.); [email protected] (R.Z.); [email protected] (S.B.); [email protected] (M.A.) * Correspondence: [email protected] (Y.K.); [email protected] (H.W.) Abstract: Myeloperoxidase is a signature enzyme of polymorphonuclear neutrophils in mice and humans. Being a component of circulating white blood cells, myeloperoxidase plays multiple roles in various organs and tissues and facilitates their crosstalk. Here, we describe the current knowledge on the tissue- and lineage-specific expression of myeloperoxidase, its well-studied enzymatic activity and incoherently understood non-enzymatic role in various cell types and tissues. Further, we elaborate on Myeloperoxidase (MPO) in the complex context of cardiovascular disease, innate and autoimmune response, development and progression of cancer and neurodegenerative diseases. Keywords: myeloperoxidase; oxidative burst; NETs; cellular internalization; immune response; cancer; neurodegeneration Citation: Kargapolova, Y.; Geißen, S.; Zheng, R.; Baldus, S.; Winkels, H.; Adam, M. The Enzymatic and Non-Enzymatic Function of 1. Introduction. MPO Conservation Across Species, Maturation in Myeloid Progenitors, Myeloperoxidase (MPO) in and its Role in Immune Responses Inflammatory Communication. Myeloperoxidase (MPO) is a lysosomal protein and part of the organism’s host-defense Antioxidants 2021, 10, 562. https:// system. MPOs’ pivotal function is considered to be its enzymatic activity in response to doi.org/10.3390/antiox10040562 invading pathogenic agents. During infection, the bactericidal and fungicidal properties of MPO contribute to pathogen clearing inside the phagosome. Academic Editors: Carlo Cervellati, Mature MPO is a basic metalloprotein with a molecular weight of 140 kilodaltons Antonella Pantaleo, Christian Secchi (kD), composed of two heavy and two light chains of approximately 55 and 13.5 kDa, and Marco Orecchioni respectively. MPO is a member of a diverse protein family, which is comprised of myeloper- oxidase, eosinophil peroxidase, thyroid peroxidase, salivary peroxidase, lactoperoxidase, Received: 9 February 2021 ovoperoxidase, peroxidasin, and others. The members of this family share 500 amino acid Accepted: 30 March 2021 residues in length [1]. Interestingly, the first-ever orthologue of MPO can be found in Published: 5 April 2021 the Australian saltwater crocodile (Crocodylus porosus), in which 39,19% of the orthologue sequence match the human MPO sequence [2]. Mouse and human MPO share 85% amino Publisher’s Note: MDPI stays neutral acid identity (90% homology) [3]. with regard to jurisdictional claims in published maps and institutional affil- Upon protein synthesis, the MPO polypeptide matures through several steps (Figure1). iations. During the first step of maturation, the protein undergoes a series of proteolytic cleavages and posttranslational modifications, for example, N-glycosylation, and is converted from 80 kDa MPO into 90 kDa apoproMPO [4]. Two chaperones, calreticulin (CRT) and calnexin (CLN), are important for apoproMPO maturation. Subsequently, apoproMPO acquires a heme group while maturing in the endoplasmic reticulum to proMPO [5]. The inhibition of Copyright: © 2021 by the authors. heme synthesis by succinyl acetone results in a maturation arrest of MPO [6]. Finally, in the Licensee MDPI, Basel, Switzerland. last step of MPO maturation, the N-terminal protein domain is cleaved by proconvertase This article is an open access article distributed under the terms and and intramolecular proteolytic cleavage leads to the formation of heavy and light chains conditions of the Creative Commons of the enzyme. The mature protein is transported into the azurophilic granules where it Attribution (CC BY) license (https:// is stored [7,8], whereas the remaining proMPO is secreted from the cell. MPO is the only creativecommons.org/licenses/by/ member of the peroxidase-cyclooxygenase superfamily, which functions as a mature dimer, 4.0/). yet each hemi-MPO is enzymatically active. Neutrophils circulating in the blood have Antioxidants 2021, 10, 562. https://doi.org/10.3390/antiox10040562 https://www.mdpi.com/journal/antioxidants Antioxidants 2021, 10, x FOR PEER REVIEW 2 of 18 Antioxidants 2021, 10, 562 2 of 18 as a mature dimer, yet each hemi-MPO is enzymatically active. Neutrophils circulating in the blood have circadian oscillations of cell granularity. The maximal abundance of MPO- positive granules is observed in mice at midnight. At this time, neutrophils leave the bone circadian oscillations of cell granularity. The maximal abundance of MPO-positive granules marrow and decrease with the time in circulation. Plasma elastase activity, on the other is observed in mice at midnight. At this time, neutrophils leave the bone marrow and hand, increases with time of circulation, furthermore suggesting the granule contents re- decrease with the time in circulation. Plasma elastase activity, on the other hand, increases withleased time over of circulation, time [9]. furthermore suggesting the granule contents released over time [9]. Figure 1. Structure and maturation of Myeloperoxidase. Myeloperoxidase (MPO) is translated as an 84 kDa apo-pre-pro- Figure 1. Structure and maturation of Myeloperoxidase. Myeloperoxidase (MPO) is translated as an 84 kDa apo-pre-pro- MPO protein and undergoes several maturation steps. The first step of maturation takes place in the endoplasmic reticulum, MPO protein and undergoes several maturation steps. The first step of maturation takes place in the endoplasmic reticu- where proteins acquire post-translational modifications and lose the signal peptide. The second step occurs in the Golgi lum, where proteins acquire post-translational modifications and lose the signal peptide. The second step occurs in the apparatusGolgi apparatus and early and endosomes, early endosomes, where the wher hemee the group heme is acquiredgroup is andacquired the pro-domain and the pro-domain is lost. The is third lost. stepThe third takes step place takes in lysosomes,place in lysosomes, where the where intermolecular the intermolecular S–S bounds S–S are bounds formed ar ande formed heavy and and heavy light chains and light of MPO chains are of separated MPO are withseparated the formationwith the offormation mature MPO.of mature Adapted MPO. from Adapted Laura from et al. Laura [10]. et al. [10]. − − − − MPOMPO uses uses hydrogen hydrogen peroxide peroxide and and halide halide (Cl (Cl,− Br, Br,I−, I−)) or pseudohalide (SCN -) )ions ions to tocatalyze catalyze the the production production of hypochloroushypochlorous (HOCl),(HOCl), hypobromous hypobromous (HOBr), (HOBr), hypoiodous hypoiodous (HOI)(HOI) or or hypothiocyanous hypothiocyanous (HOSCN) (HOSCN) acids acids [11 [11–15].–15]. The The enzyme enzyme is is capable capable of of catalyzing catalyzing twotwo types types of of redox redox reactions, reactions, specifically specifically the the halogenation halogenation cycle cycle and and the the peroxidase peroxidase cycle. cycle. TheThe first first step step of of thethe halogenationhalogenation and peroxida peroxidasese cycles cycles are are similar. similar. During During this this step, step, the thehydrogen hydrogen peroxide peroxide reacts reacts with with native native MPO, MPO, subsequently subsequently forming forming compound compound I, a ferryl I, a ferrylform, form, which which possesses possesses a higher a higher reduction reduction potential potential compared compared to the toferric the form ferric of form ground of ground state MPO. During the second step of the halogenation cycle, the two electrons are state MPO. During the second step of the halogenation cycle, the two electrons are trans- transferred onto the halides, catalyzing acid formation. Subsequently, in the halogenation ferred onto the halides, catalyzing acid formation. Subsequently, in the halogenation cy- cycle, compound I is reduced to the native enzyme. As Cl− is the most abundant halide in cle, compound I is reduced to the native enzyme. As Cl- is the most abundant halide in the blood, the halogenation reaction with Cl− happens most frequently and hypochlorous the blood, the halogenation reaction with Cl- happens most frequently and hypochlorous acid (HOCl) was identified as the main product of MPO reacting with hydrogen peroxide. acid (HOCl) was identified as the main product of MPO reacting with hydrogen peroxide. In the peroxidation cycle, the reduction of compound I is catalyzed by two successive In the peroxidation cycle, the reduction of compound I is catalyzed by two successive one- one-electron transfer events and via a formation of compound II. In this case possible electron transfer events and via a formation of compound II. In this case possible electron electron donors are aromatic acids [16], indole derivatives [17], nitrite [18], sulfhydryls [19], donors are aromatic acids [16], indole derivatives [17], nitrite [18], sulfhydryls [19], hydrogen peroxide [20] and other species. hydrogen peroxide [20] and other species. The accumulated oxidative MPO products are highly reactive and released into the surroundingThe accumulated tissue (reviewed oxidative