Localization in the electron microscope by acid phosphatase of casein particles in lactating mammary glands of rats.

Abstract

SUMMARYCaseins are phosphoproteins and contain phosphate in monoester form; acid phosphatase is a specific hydrolysing enzyme for such phosphate monoesters; and electron‐dense particles of lead phosphate can be precipitated from a solution containing lead by the hydrolysed phosphate radical. We have made use of these facts to verify that the previously described ‘protein droplets’ located within the vacuoles of mammary epithelial cells contain phosphoprotein. Since these protein particles respond to an enzyme having Phosphomonoesterase activity at the correct pH it seems likely that they contain phosphate monoester in their structure and that they are therefore most likely to be casein particles.The method appears to have applications in the study of lactating mammary glands and the secretory activity of mammary tumours.