[5][6] It is a lysosomal protein stored in azurophilic granules of the neutrophil and released into the extracellular space during degranulation.

Myeloperoxidase is found in many different organisms including mammals, birds, fish, reptiles, and amphibians.

[citation needed] Myeloperoxidase deficiency is a well-documented disease among humans resulting in impaired immune function.

[14] Hypochlorous acid and tyrosyl radical are cytotoxic, so they are used by the neutrophil to kill bacteria and certain types of fungi.

Calcium-binding is important for structure of the active site because of Asp 96's close proximity to the catalytic His95 side chain.

The reaction starts when hydrogen peroxide donates an oxygen to the heme group, converting it to an activated form called "Compound I".

[9][14] People with myeloperoxidase deficiency are most at risk of infection by Candida species, which are pathogenic fungi.

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.

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