Myeloperoxidase (MPO)

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Myeloperoxidase (MPO) is a human enzyme in the azurophilic granules of

neutrophils and in the lysosomes of monocytes. Its major role is to aid in

microbial killing. Although MPO received little clinical attention until 1966,

the enzyme was first isolated in 1941, and deficiency of MPO was first described

in 1954. Some patients with MPO deficiency have impaired microbial killing, but

most are asymptomatic.

MPO, a heme-containing protein, is found in the azurophilic granules of

neutrophils and in the lysosomes of monocytes in humans; however, monocytes

contain only about a third of the MPO present in neutrophils. When neutrophils

become activated during phagocytosis, they undergo a process referred to as a

respiratory burst. This respiratory burst causes production of superoxide,

hydrogen peroxide, and other reactive oxygen derivatives, which are all toxic to

microbes. During respiratory bursts, granule contents are released into the

phagolysosomes and outside the cell, allowing released contents to come into

contact with any microbes present. Experiments conducted in the 1960s showed

that MPO catalyzes the conversion of hydrogen peroxide and chloride ions (Cl)

into hypochlorous acid.1 Hypochlorous acid is 50 times more potent in microbial

killing than hydrogen peroxide.

http://emedicine.medscape.com/article/887599-overview