# Myeloperoxidase

Myeloperoxidase (MPO) is a peroxidase enzyme that in humans is encoded by the MPO gene on chromosome 17. 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. It is a lysosomal protein stored in azurophilic granules of the neutrophil and released into the extracellular space during degranulation. Neutrophil myeloperoxidase has a heme pigment, which causes its green color in secretions rich in neutrophils, such as mucus and sputum. The green color contributed to its outdated name verdoperoxidase.
Myeloperoxidase is found in many different organisms including mammals, birds, fish, reptiles, and amphibians. Myeloperoxidase deficiency is a well-documented disease among humans resulting in impaired immune function.

## Function

MPO is a member of the XPO subfamily of peroxidases and produces hypochlorous acid (HOCl) from hydrogen peroxide (H$_2$O$_2$) 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.
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. In addition, MPO mediates protein nitrosylation and the formation of 3-chlorotyrosine and dityrosine crosslinks.
Myeloperoxidase is the first and so far only 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.

### Role in innate immunity

Neutrophils use myeloperoxidase to produce the substances needed for their respiratory burst. Hypochlorous acid and tyrosyl radical are cytotoxic, so they are used by the neutrophil to kill bacteria and certain types of fungi.

## 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. 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.
Variation in glycosylation and the identity of the heavy chain lead to variations in molecular weight within the 135-200 kDa range. In mice, three isoforms exist, differing only by the heavy chain.
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.

### 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. This cycle continues for as long as the immune system requires.

## Clinical significance
### Myeloperoxidase deficiency

Myeloperoxidase deficiency is a hereditary deficiency of the enzyme, which causes a mild immune deficiency against certain pathogens. 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.

### 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.

### 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.

### Medical tests

An initial 2003 study suggested that MPO could serve as a sensitive predictor for myocardial infarction in patients presenting with chest pain. 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.
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. 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.

### 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.