Subcellular and ultrastructural localization of alkaline phosphatase in lactating rat mammary glands

The subcellular fractions of lactating rat mammary glands were isolated by differential centrifugation. The mean specific activity of alkaline phosphatase in various fractions was in order greatest to least: microsomes, Golgi, mitochondria, nuclei, and cytosol. Alkaline phosphatase was examined cytochemically by transmission electron microscopy. Alkaline phosphatase activity was localized on myoepithelial membranes, basal and possibly lateral membranes of secretory epithelial cells, and endothelial cells. This finding agreed with biochemical data associating this enzyme activity with microsomes. However, intracellular activities could not be detected on Golgi, secretory vesicles, or apical plasma membranes. Saponin uncovered the activity in that portion of the endoplasmic reticulum of secretory cells adjacent to myoepithelial cells. The identify of this enzyme was further confirmed by selective inhibition studies using dithiothreitol and levamisole. Alkaline phosphatase activities were detected biochemically in lipid droplet "membranes" of secretory epithelium and fat globule membranes. Activity decreased with increasing globule size, indicating that milk alkaline phosphatase originates from lipid droplets of secretory epithelium. The predominance of alkaline phosphatase activity in myoepithelial cell plasma membranes suggests that this enzyme could be involved in cell surface reactions related to oxytocin-mediated milk ejection. In secretory epithelium, it was associated with basal and possibly lateral membranes and lipid droplets that lead to the secretion of milk fat.