Myeloperoxidase

Template:Cs1 config Template:Short description Template:Infobox gene Template:Infobox enzyme Myeloperoxidase (MPO) is a peroxidase enzyme that in humans is encoded by the MPO gene on chromosome 17.<ref name="entrez">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> MPO is most abundantly expressed in neutrophils (a subtype of white blood cells), and produces hypohalous acids to carry out their antimicrobial activity, including hypochlorous acid, the sodium salt of which is the chemical in bleach.<ref name="entrez"/><ref name="pmid15689384">Template:Cite journal</ref> It is a lysosomal protein stored in azurophilic granules of the neutrophil and released into the extracellular space during degranulation.<ref name="pmid6192815">Template:Cite journal</ref> Neutrophil myeloperoxidase has a heme pigment, which causes its green color in secretions rich in neutrophils, such as mucus and sputum.<ref>Template:Cite book</ref> The green color contributed to its outdated name verdoperoxidase.

Myeloperoxidase is found in many different organisms including mammals, birds, fish, reptiles, and amphibians.Template:Cn Myeloperoxidase deficiency is a well-documented disease among humans resulting in impaired immune function.<ref name="pmid11111115"/>

Function

MPO is a member of the XPO subfamily of peroxidases and produces hypochlorous acid (HOCl) from hydrogen peroxide (H₂O₂) and chloride anion (Cl⁻) (or hypobromous acid if Br- is present) during the neutrophil's respiratory burst. It requires heme as a cofactor. Furthermore, it oxidizes tyrosine to tyrosyl radical using hydrogen peroxide as an oxidizing agent.<ref name="ELISA" /><ref name="pmid8390491">Template:Cite journal</ref>

However, this hypochlorous acid may also cause oxidative damage in host tissue. Moreover, MPO oxidation of apoA-I reduces HDL-mediated inhibition of apoptosis and inflammation.<ref>Template:Cite journal</ref> In addition, MPO mediates protein nitrosylation and the formation of 3-chlorotyrosine and dityrosine crosslinks.<ref name="ELISA" />

Myeloperoxidase is the first and so far onlyTemplate:Update inline human enzyme known to break down carbon nanotubes, 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">Template:Cite journal

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Role in innate immunity

Neutrophils use myeloperoxidase to produce the substances needed for their respiratory burst.<ref name=":0">Template:Cite journal</ref> Hypochlorous acid and tyrosyl radical are cytotoxic, so they are used by the neutrophil to kill bacteria and certain types of fungi.<ref name=":0" /><ref name="pmid9787133">Template:Cite journal</ref><ref>Template:Cite journal</ref>

Structure

The 150-kDa MPO protein is a cationic heterotetramer consisting of two 15-kDa light chains and two variable-weight glycosylated heavy chains bound to a prosthetic heme group complex with calcium ions, arranged as a homodimer of heterodimers. Both are proteolytically generated from the precursor peptide encoded by the MPO gene.<ref name="Davey1996">Template:Cite journal</ref><ref name="ELISA">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="pmid9553712">Template:Cite journal</ref><ref name="pmid21297906">Template:Cite journal</ref> The light chains are glycosylated 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"/>

Variation in glycosylation and the identity of the heavy chain lead to variations in molecular weight within the 135-200 kDa range.<ref>Template:Cite journal</ref><ref name="Davey1996" /> In mice, three isoforms exist, differing only by the heavy chain.<ref name="ELISA"/>

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 side chain.<ref name="pmid11237766">Template:Cite journal</ref>

Reaction mechanism

The central heme group acts as the active site. The reaction starts when hydrogen peroxide donates an oxygen to the heme group, converting it to an activated form called "Compound I". This compound then oxidizes the chloride ions to form the hypochlorous acid and Compound II, which can be reduced back down to its original heme state.Template:How This cycle continues for as long as the immune system requires.Template:Cn

Clinical significance

Myeloperoxidase deficiency

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Myeloperoxidase deficiency is a hereditary deficiency of the enzyme, which causes a mild immune deficiency against certain pathogens.<ref name="pmid11111115">Template:Cite journal</ref><ref name=":0" /> People with myeloperoxidase deficiency are most at risk of infection by Candida species, which are pathogenic fungi. The most common species found in humans is Candida albicans. There may be an increased risk of certain other infections, such as with Klebsiella pneumoniae, but recurrent candidiasis is the only common clinical consequence, if the patient is noticeably affected at all.<ref name=":0" />

Vasculitis

Antibodies against MPO have been implicated in various types of vasculitis, most prominently three clinically and pathologically recognized forms: granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA); and eosinophilic granulomatosis with polyangiitis (EGPA). Antibodies are also known as anti-neutrophil cytoplasmic antibodies (ANCAs), though ANCAs have also been detected in staining of the perinuclear region.<ref name="pmid25629443">Template:Cite journal</ref>

Atherosclerosis and heart disease

Myeloperoxidase is known to contribute to atherosclerosis and diseases related to it, including coronary artery disease. MPO oxidizes LDL cholesterol, and as a result, the LDL receptor in liver cells becomes unable to bind to LDL and remove it from the blood stream. However, in its oxidized state, LDL can still contribute to foam cell formation and other atherosclerotic processes. Thus, elevated levels of MPO are a risk factor for atherosclerosis.<ref>Template:Cite journal</ref>

Medical tests

Template:Update 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">Template:Cite journal</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">Template:Cite journal</ref>

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 lymphomas, which can otherwise have a similar appearance.<ref name = "Leong_2003">Template:Cite book</ref> In the case of screening patients for vasculitis, 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">Template:Cite journal</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">Template:Cite journal</ref>

See also

• Chloroma

External links

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