EosinophilPeroxidase

Eosinophil peroxidase (EPO) is a heme-containing peroxidase enzyme found in the granules of eosinophils, a type of white blood cell involved in defense against parasites and in allergic inflammation. When eosinophils are activated, EPO can be released into the extracellular milieu or into phagolysosomes, where it uses hydrogen peroxide produced by the cell to catalyze the oxidation of halide and pseudohalide substrates.

Compared with myeloperoxidase (MPO) of neutrophils, EPO has relatively low activity with chloride and is more efficient at oxidizing substrates such as thiocyanate and bromide. This leads to the generation of reactive species including hypobromous acid (HOBr) and hypothiocyanous acid (HOSCN), which contribute to antimicrobial activity. The substrate preference of EPO is particularly relevant at mucosal surfaces where thiocyanate is present in secretions such as saliva and airway lining fluid.

EPO participates in host defense by assisting eosinophils in killing parasites and certain microbes. Its oxidants can also modify host biomolecules and tissues, and excessive or mislocalized EPO activity has been implicated in tissue damage associated with eosinophilic and allergic inflammatory conditions.

Genetically, EPO is encoded by the EPO gene. The enzyme is stored in eosinophil granules and released upon activation. Like other heme peroxidases, its activity can be inhibited by classic inhibitors such as cyanide or azide, and its function is influenced by the local chemical environment, including the availability of substrates like thiocyanate.