Peroxidase is an enzyme from oxydoreductase group. It is found in myeloid cells and is then called myeloperoxidase. Myeloid cells can destroy microorganisms with the help of this enzyme. Peroxidase is present in primary granules of neutrophils, and in the granules of eosinophils and monocytes.
The identification of this enzyme in the cytoplasm of leukocytes is useful in distinguishing acute myelogenous leukemia from acute lymphocytic leukemia, and to detect or confirm diminished peroxidase activity in individuals with myeloperoxidase deficiency.
In the presence of peroxidase, H2O2 is split liberating O2 that oxidizes benzidine or 4-chlor-1-naphthol. Positive reaction goes yellow-brown stained granules in the cytoplasm of a cell. The positive reaction is different in the cells of granulocytopoiesis from promyelocyte to segmented granulocyte. Positive reaction to peroxidase is also possible in sense of monocytes. In myeloblasts, megakaryocytes, lymphocytes, thrombocytes, and erythrocytes the reaction is negative.
Positive myeloperoxidase reaction differentiates myelogenous from lymphatic acute leukemia. However, a negative result does not exclude myeloblastic leukemia.
Smears of the peripheral blood and bone marrow, in which the activity of peroxidase is to be examined, must not be older than 3 days. Anticoagulants, especially EDTA, inhibit the peroxidase reaction.