Mammalian endogenous peroxidases as cellular markers and as biosynthetic endpoints of hormone-mediated activity: Viewpoint from cytochemistry

The lactoperoxidase (LPO)-type of endogenous peroxidases are synthesized by the acinar cells of the salivary, Harderian, lacrimal and mammary glands and are present in their secretions. These LPO-type enzymes, that are inhibited by cyanide and aminotriazole, appear to operate extracellularly as bactericidal agents in milk and in other biological fluids. In the mammary gland, lactoperoxidase is a consistent marker enzyme for differentiated acinar cells engaged in lactogenesis. Myeloperoxidase (MPO)-type endogenous peroxidase are prominent markers for the GERL endomembrane system and differentiated lysosomes in certain cells of the reticuloendothelial system and phagocytes. MPO is prominent within eosinophils, peritoneal macrophages and in Kupffer cells. The MPO-type andogenous peroxidases function primarily within lysosomes as bactericidal agents. Thyroid peroxidase (TPO) is relegated to the cisternae of the granular endoplasmic reticulum and Golgi apparatus, to apical cytoplasmic vesicles and to the luminal cell membrane surface of acinar cells. The enzyme is probably activated at release and functions both in the organification reaction (T→T°) and in the biosynthesis of thyroxine. Thyroid stimulating hormone (TSH) appears to play a key role in the regulation of TPO levels and activity in the thyroid gland