Utilization of phosphorus for casein biosynthesis in the mammary gland. I. Incorporation in vivo of P32 into phosphoprotein of milk and of mammary gland.

It is now well established that the synthesis of the major protein constituents of milk takes place exclusively in the mammary gland, utilizing as building materials the free amino acids of the blood (1). The research of Aten and Hevesy (2), confirmed subsequently by other workers (3, 4), leaves little room for doubt that the precursor of the casein phosphorus is the inorganic phosphorus of the blood. The rapid rate at which phosphorus incorporation into casein is effected has been noticed by several workers. The specific activity of casein phosphorus reaches a maximum 3 hours after intravenous injection of labeled phosphorus to the lactating animal (5) and is generally much higher than the activities in hexose monophosphate and phospholipid (2). These experiments do not, however, exclude the possibility that the casein owes its high activity to phosphoprotein enzymes possessing high rates of phosphorus turnover, which may be present as contaminants in the preparation. In view of recent findings on the phosphoprotein nature of various enzymes involved in phosphate transfer (6-9) a detailed characterization of casein with reference to the nature of its phosphorus appears desirable. A study of this aspect and a characterization of the phosphoprotein of mammary gland with a view to establishing its relationship with casein have been undertaken in the present investigation. It was also considered of interest to compare the specific activity of casein phosphorus with that in liver phosphoprotein since the latter has been shown to exhibit phosphorus turnover at a remarkably high rate (10-12).