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

Aldehyde-resistant, diaminobenzidine-stained endogenous peroxidases form ideal markers for the biochemical endpoints of hormone stimulation and differentiation of certain mammalian cells and tissues. 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 peroxidases 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 endogenous 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 luminar cell membrane surface of acinar cells. The enzyme is probably activated at release and functions both in the organification reaction (T leads to To) 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. Certain tissues displaying growth-dependency on estrogen (i.e., uterus, cervix, vagina and the DMBA-induced rat mammary tumor) synthesize and secrete endogenous peroxidase into their lumina. These enzymes serve as important marker proteins of estrogen action, in that they occur distal to the binding of estrogen to its receptor protein. Estrogen antagonists, particularly CI-628 (Parke-Davis) and Nafoxidine (Upjohn) that appear to function through the estrogen receptor mechanism, also induce synthesis of the reproductive tract endogenous peroxidase but inhibit growth of these tissues. Progesterone antagonizes the synthesis of the reproductive tract peroxidases and inhibits growth of the tissues as well, in part, through the reduction of the cytosol estrogen receptor protein. Endogenous peroxidase activity appears to represent a reliable marker for rodent breast cancer tissues displaying dependency for estrogen and is of potential interest as a diagnostic marker protein in human breast cancer. Rat uterine peroxidase (UP) has been investigated by microelectrophoretic techniques. The molecular weight of UP has been determined in the range of 100,000 by using polyacrylamide gradient gels in the absence and presence of nonionic and anionic detergents. The isoelectric point of UP is located between pH 4.5 and 5.9. Employing the two-dimensional combination of isoelectric focusing and gel gradient electrophoresis, UP was separated into two subunits, one having a molecular weight of 70,000, the other less than 20,000.