Myeloperoxidase (MPO) is a peroxidase enzyme (EC 1.11.1.7) most abundantly present in neutrophil granulocytes (a subtype of white blood cells). It is a lysosomal protein stored in azurophilic granules of the neutrophil. MPO has a heme pigment, which causes its green color in secretions rich in neutrophils, such as pus and some forms of mucus.

Function

MPO produces hypochlorous acid (HOCl) from hydrogen peroxide (H₂O₂) and chloride anion (Cl^-) during the neutrophil's respiratory burst. It requires heme as a cofactor. Hypochlorous acid is cytotoxic, so it is used by the neutrophil to kill bacteria and other pathogens.

Genetics

The gene is located on chromosome 17 (17q23.1).

Role in disease

Myeloperoxidase deficiency is a rare hereditary deficiency of the enzyme, which leads to immune deficiency.

Antibodies against MPO have been implicated in various types of vasculitis, most prominently crescentic glomerulonephritis and Churg-Strauss syndrome. They are detected as perinuclear ANCAs (pANCAs), as opposed to the cytoplasmic ANCAs (cANCAs) against proteinase 3 (PR3), which are strongly associated with Wegener's granulomatosis.

Diagnostic use

Routine testing of MPO is not performed. A 2003 study (Brennan et al) suggested that it could serve as a sensitive predictor for myocardial infarction in patients presenting with chest pain.

References

• Brennan M-L, Penn MS, Van Lente F, Nambi V, Shishehbor MH, Aviles RJ, Goormastic M, Pepoy ML, McErlean ES, Topol EJ, Nissen SE, Hazen SL. Prognostic value of myeloperoxidase in patients with chest pain. N Engl J Med 2003;349:1595-604. PMID 14573731.

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