myeloperoxidase

Myeloperoxidase (MPO) is a heme-containing peroxidase enzyme primarily found in neutrophils and, to a lesser extent, in monocytes. It is stored in azurophilic granules and is encoded by the MPO gene. MPO is a key component of the innate immune system and contributes to the microbicidal activity of phagocytes.

In the respiratory burst that accompanies phagocytosis, activated NADPH oxidase generates hydrogen peroxide. MPO uses this hydrogen peroxide to oxidize halide ions, especially chloride, into hypochlorous acid (HOCl), a potent antimicrobial agent. MPO can also catalyze the formation of other reactive species, including hypobromous acid and various chlorinated and oxidized substrates, which help kill engulfed pathogens but can also damage host tissue during chronic inflammation.

MPO plays a central role in antimicrobial defense within phagosomes, but its oxidants can contribute to collateral tissue injury in inflammatory conditions. Elevated MPO activity has been associated with atherosclerosis, neurodegenerative diseases, and other inflammatory states, and MPO-derived products are investigated as biomarkers of disease activity. MPO activity can be detected by biochemical assays and histochemical staining, making it a useful laboratory marker of neutrophil activity.

Clinical significance includes MPO deficiency, the most common inherited defect of neutrophil function. Most individuals with deficiency are asymptomatic, though there is a modestly increased risk of mucosal candidiasis. Therapeutic approaches targeting MPO are being explored in research settings to reduce inflammatory tissue damage while preserving host defense.