View source for Myeloperoxidase

{{infobox enzyme
| Name = Myeloperoxidase
| EC_number = 1.11.2.2
| CAS_number = 
| IUBMB_EC_number = 1/11/2/2
| GO_code = 
| image = 
| width = 
| caption = 
}}
{{Infobox_gene}}

'''Myeloperoxidase''' ('''MPO''') is a [[peroxidase]] [[enzyme]] that in humans is encoded by the ''MPO'' [[gene]] on [[chromosome 17]].<ref name="entrez">{{cite web | title = Entrez Gene: Myeloperoxidase | url = https://www.ncbi.nlm.nih.gov/sites/entrez?db=gene&cmd=retrieve&list_uids=4353 }}</ref> MPO is most abundantly expressed in [[neutrophil granulocyte]]s (a subtype of [[white blood cell]]s), and produces hypohalous acids to carry out their [[antimicrobial]] activity.<ref name="entrez"/><ref name="pmid15689384">{{cite journal | vauthors = Klebanoff SJ | title = Myeloperoxidase: friend and foe | journal = Journal of Leukocyte Biology | volume = 77 | issue = 5 | pages = 598–625 | date = May 2005 | pmid = 15689384 | doi = 10.1189/jlb.1204697 }}</ref> It is a [[lysosome|lysosomal]] protein stored in [[azurophilic granules]] of the neutrophil and released into the [[extracellular space]] during degranulation.<ref name="pmid6192815">{{cite journal | vauthors = Kinkade JM, Pember SO, Barnes KC, Shapira R, Spitznagel JK, Martin LE | title = Differential distribution of distinct forms of myeloperoxidase in different azurophilic granule subpopulations from human neutrophils | journal = Biochemical and Biophysical Research Communications | volume = 114 | issue = 1 | pages = 296–303 | date = Jul 1983 | pmid = 6192815 | doi=10.1016/0006-291x(83)91627-3}}</ref> Neutrophil myeloperoxidase has a [[heme]] pigment, which causes its green color in secretions rich in [[neutrophils]], such as [[pus]] and some forms of [[mucus]]. The green color contributed to its outdated name verdoperoxidase.

== Structure ==

The 150-[[kiloDalton|kDa]] MPO protein is a cationic [[homodimer]] consisting of two 15-kDa light chains and two variable-weight glycosylated heavy chains bound to a prosthetic [[heme]] group.<ref name="ELISA">{{cite web|url=http://bioaimscientific.com/sites/default/files/manual/MouseMPOELISA.pdf|title=Mouse MPO EasyTestTM ELISA Kit}}</ref><ref name="pmid9553712">{{cite journal | vauthors = Mathy-Hartert M, Bourgeois E, Grülke S, Deby-Dupont G, Caudron I, Deby C, Lamy M, Serteyn D | title = Purification of myeloperoxidase from equine polymorphonuclear leucocytes | journal = Canadian Journal of Veterinary Research = Revue Canadienne De Recherche Vétérinaire | volume = 62 | issue = 2 | pages = 127–32 | date = Apr 1998 | pmid = 9553712 | pmc = 1189459 | doi =  }}</ref><ref name="pmid21297906">{{cite journal | vauthors = Davies MJ | title = Myeloperoxidase-derived oxidation: mechanisms of biological damage and its prevention | journal = Journal of Clinical Biochemistry and Nutrition | volume = 48 | issue = 1 | pages = 8–19 | date = Jan 2011 | pmid = 21297906 | doi = 10.3164/jcbn.11-006FR | pmc=3022070}}</ref> The light chains are [[glycosylate]]d and contain the modified iron [[protoporphyrin IX]] [[active site]]. Together, the light and heavy chains form two identical 73-kDa monomers connected by a [[cystine]] bridge at Cys153. The protein forms a deep crevice which holds the heme group at the bottom, as well as a [[hydrophobic]] pocket at the entrance to the distal heme cavity which carries out its catalytic activity.<ref name="pmid21297906"/>

Three [[isoform]]s have been identified, differing only in the size of the heavy chains.<ref name="ELISA"/>

MPO contains a [[calcium]] binding site with seven [[ligand]]s, forming a pentagonal pyramid conformation.{{citation needed|date=August 2015}} One of the ligands is the [[carbonyl]] group of Asp 96. Calcium-binding is important for structure of the active site because of Asp 96's close proximity to the catalytic His95 [[Substituent|side chain]].<ref name="pmid11237766">{{cite journal | vauthors = Shin K, Hayasawa H, Lönnerdal B | title = Mutations affecting the calcium-binding site of myeloperoxidase and lactoperoxidase | journal = Biochemical and Biophysical Research Communications | volume = 281 | issue = 4 | pages = 1024–9 | date = Mar 2001 | pmid = 11237766 | doi = 10.1006/bbrc.2001.4448 }}</ref>
 
== Function ==
MPO is a member of the XPO subfamily of peroxidases and produces [[hypochlorous acid]] (HOCl) from [[hydrogen peroxide]] (H<sub>2</sub>O<sub>2</sub>) and [[chloride]] [[anion]] (Cl<sup>−</sup>) (or the equivalent from a non-chlorine halide) during the neutrophil's [[respiratory burst]]. It requires heme as a [[Cofactor (biochemistry)|cofactor]]. Furthermore, it oxidizes [[tyrosine]] to tyrosyl radical using [[hydrogen peroxide]] as an [[oxidizing agent]].<ref name="ELISA"/><ref name="pmid8390491">{{cite journal | vauthors = Heinecke JW, Li W, Francis GA, Goldstein JA | title = Tyrosyl radical generated by myeloperoxidase catalyzes the oxidative cross-linking of proteins | journal = The Journal of Clinical Investigation | volume = 91 | issue = 6 | pages = 2866–72 | date = Jun 1993 | pmid = 8390491 | pmc = 443356 | doi = 10.1172/JCI116531 }}</ref> [[Hypochlorous acid]] and tyrosyl radical are [[cytotoxic]], so they are used by the neutrophil to kill [[bacteria]] and other [[pathogen]]s.<ref name="pmid9787133">{{cite journal | vauthors = Hampton MB, Kettle AJ, Winterbourn CC | title = Inside the neutrophil phagosome: oxidants, myeloperoxidase, and bacterial killing | journal = Blood | volume = 92 | issue = 9 | pages = 3007–17 | date = Nov 1998 | pmid = 9787133 }}</ref> However, this hypochlorous acid may also cause [[oxidative damage]] in host tissue. Moreover, MPO oxidation of [[apolipoprotein A|apoA]]-I reduces HDL-mediated inhibition of [[apoptosis]] and inflammation.<ref>{{vcite2 journal | vauthors = Shao B, Oda MN, Oram JF, Heinecke JW | title = Myeloperoxidase: an oxidative pathway for generating dysfunctional high-density lipoprotein | journal = Chemical Research in Toxicology | volume = 23 | issue = 3 | pages = 447–54 | date = Mar 2010 | pmid = 20043647 | doi = 10.1021/tx9003775 | pmc=2838938}}</ref> In addition, MPO mediates protein [[nitrosylation]] and the formation of 3-chlorotyrosine and dityrosine [[crosslink]]s.<ref name="ELISA"/>

== Clinical significance ==

[[Myeloperoxidase deficiency]] is a hereditary deficiency of the enzyme, which predisposes to [[immune deficiency]].<ref name="pmid11111115">{{cite journal | vauthors = Kutter D, Devaquet P, Vanderstocken G, Paulus JM, Marchal V, Gothot A | title = Consequences of total and subtotal myeloperoxidase deficiency: risk or benefit ? | journal = Acta Haematologica | volume = 104 | issue = 1 | pages = 10–5 | year = 2000 | pmid = 11111115 | doi = 10.1159/000041062 }}</ref>

[[Antibody|Antibodies]] against MPO have been implicated in various types of [[vasculitis]], most prominently three clinically and pathologically recognized forms: granulomatosis with polyangiitis (GPA, formerly Wegener's granulomatosis), microscopic polyangiitis (MPA); and [[eosinophilic granulomatosis with polyangiitis]] (EGPA, formerly Churg-Strauss syndrome). Antibodies are also known as [[anti-neutrophil cytoplasmic antibody|anti-neutrophil cytoplasmic antibodies]] (ANCAs), though ANCAs have also been detected in [[staining]] of the perinuclear region.<ref name="pmid25629443">{{cite journal | vauthors = Flint SM, McKinney EF, Smith KG | title = Emerging concepts in the pathogenesis of antineutrophil cytoplasmic antibody-associated vasculitis | journal = Current Opinion in Rheumatology | volume = 27 | issue = 2 | pages = 197–203 | date = Mar 2015 | pmid = 25629443 | doi = 10.1097/BOR.0000000000000145 }}</ref>

Recent studies have reported an association between elevated myeloperoxidase levels and the severity of [[coronary artery disease]].<ref name="pmid11694155">{{cite journal | vauthors = Zhang R, Brennan ML, Fu X, Aviles RJ, Pearce GL, Penn MS, Topol EJ, Sprecher DL, Hazen SL | title = Association between myeloperoxidase levels and risk of coronary artery disease | journal = JAMA | volume = 286 | issue = 17 | pages = 2136–42 | date = Nov 2001 | pmid = 11694155 | doi = 10.1001/jama.286.17.2136 }}</ref>  And Heslop et al. reported that elevated MPO levels more than doubled the risk for cardiovascular mortality over a 13-year period.<ref name="pmid20223364">{{cite journal | vauthors = Heslop CL, Frohlich JJ, Hill JS | title = Myeloperoxidase and C-reactive protein have combined utility for long-term prediction of cardiovascular mortality after coronary angiography | journal = Journal of the American College of Cardiology | volume = 55 | issue = 11 | pages = 1102–9 | date = Mar 2010 | pmid = 20223364 | doi = 10.1016/j.jacc.2009.11.050 }}</ref> It has also been suggested that myeloperoxidase plays a significant role in the development of the [[atherosclerotic]] [[lesion]] and rendering [[Atheroma|plaques]] unstable.<ref name="pmid15790935">{{cite journal | vauthors = Nicholls SJ, Hazen SL | title = Myeloperoxidase and cardiovascular disease | journal = Arteriosclerosis, Thrombosis, and Vascular Biology | volume = 25 | issue = 6 | pages = 1102–11 | date = Jun 2005 | pmid = 15790935 | doi = 10.1161/01.ATV.0000163262.83456.6d }}</ref><ref name="pmid16476484">{{cite journal | vauthors = Lau D, Baldus S | title = Myeloperoxidase and its contributory role in inflammatory vascular disease | journal = Pharmacology & Therapeutics | volume = 111 | issue = 1 | pages = 16–26 | date = Jul 2006 | pmid = 16476484 | doi = 10.1016/j.pharmthera.2005.06.023 }}</ref>

=== Medical uses ===

An initial 2003 study suggested that MPO could serve as a sensitive predictor for [[myocardial infarction]] in patients presenting with [[chest pain]].<ref name="pmid14573731">{{cite journal | vauthors = Brennan ML, Penn MS, Van Lente F, Nambi V, Shishehbor MH, Aviles RJ, Goormastic M, Pepoy ML, McErlean ES, Topol EJ, Nissen SE, Hazen SL | title = Prognostic value of myeloperoxidase in patients with chest pain | journal = The New England Journal of Medicine | volume = 349 | issue = 17 | pages = 1595–604 | date = Oct 2003 | pmid = 14573731 | doi = 10.1056/NEJMoa035003 }}</ref> Since then, there have been over 100 published studies documenting the utility of MPO testing. The 2010 Heslop et al. study reported that measuring both MPO and CRP (C-reactive protein; a general and cardiac-related marker of inflammation) provided added benefit for risk prediction than just measuring CRP alone.<ref name="pmid20223364"/>

[[immunohistochemistry|Immunohistochemical]] staining for myeloperoxidase used to be administered in the diagnosis of [[acute myeloid leukemia]] to demonstrate that the leukemic cells were derived from the [[myeloid]] lineage. Myeloperoxidase staining is still important in the diagnosis of [[myeloid sarcoma]], contrasting with the negative staining of [[lymphoma]]s, which can otherwise have a similar appearance.<ref name = "Leong_2003">{{cite book | author = Leong A S-Y, Cooper K, Leong, FJ W-M | title = Manual of Diagnostic Antibodies for Immunohistology | publisher = Greenwich Medical Media | location = London | year = 2003 | pages = 325–326 | isbn = 1-84110-100-1 }}</ref> In the case of screening patients for vasculitis, [[flow cytometry|flow cytometric assays]] have demonstrated comparable sensitivity to [[immunofluorescence]] tests, with the additional benefit of simultaneous detection of multiple autoantibodies relevant to vasculitis. Nonetheless, this method still requires further testing.<ref name="pmid24890776">{{cite journal | vauthors = Csernok E, Moosig F | title = Current and emerging techniques for ANCA detection in vasculitis | journal = Nature Reviews. Rheumatology | volume = 10 | issue = 8 | pages = 494–501 | date = Aug 2014 | pmid = 24890776 | doi = 10.1038/nrrheum.2014.78 }}</ref>

Myeloperoxidase is the first and so far only human enzyme known to break down [[carbon nanotube]]s, allaying a concern among clinicians that using nanotubes for targeted delivery of medicines would lead to an unhealthy buildup of nanotubes in tissues.<ref name="pmid20364135">{{cite journal | vauthors = Kagan VE, Konduru NV, Feng W, Allen BL, Conroy J, Volkov Y, Vlasova II, Belikova NA, Yanamala N, Kapralov A, Tyurina YY, Shi J, Kisin ER, Murray AR, Franks J, Stolz D, Gou P, Klein-Seetharaman J, Fadeel B, Star A, Shvedova AA | title = Carbon nanotubes degraded by neutrophil myeloperoxidase induce less pulmonary inflammation | journal = Nature Nanotechnology | volume = 5 | issue = 5 | pages = 354–9 | date = May 2010 | pmid = 20364135 | doi = 10.1038/nnano.2010.44 | laysummary = http://www.popsci.com/science/article/2010-04/enzyme-blood-cells-breaks-down-carbon-nanotubes-paving-way-nano-delivered-drugs | laysource = popsci.com }}</ref>

=== Inhibitors of MPO ===

[[Azide]] has been used traditionally as an MPO inhibitor, but 4-aminobenzoic acid hydrazide (4-ABH) is a more specific inhibitor of MPO.<ref name="pmid9020887">{{cite journal | vauthors = Kettle AJ, Gedye CA, Winterbourn CC | title = Mechanism of inactivation of myeloperoxidase by 4-aminobenzoic acid hydrazide | journal = The Biochemical Journal | volume = 321 | issue =  2| pages = 503–8 | date = Jan 1997 | pmid = 9020887 | pmc = 1218097 | doi =  10.1042/bj3210503| series = 321 }}</ref>

== See also ==
* [[Chloroma]]

== References ==
{{Reflist|33em}}

{{PDB_Gallery|geneid=4353}}
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{{Peroxidases}}
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[[Category:EC 1.11.1]]
[[Category:Hemoproteins]]