The source of proline for casein biosynthesis.

Previous work has shown that the mammary gland takes the free essential amino acids from the blood stream to provide the essential amino acids of casein (Barry, 1952; Campbell & Work, 1952; Askonas, Campbell, Godin & Work, 1955); that it takes free glutamine and glutamic acid to provide the glutamine and glutamic acid respectively of casein (Barry, 1956; Sheldon-Peters & Barry, 1956); and that it takes free asparagine to provide asparagine, but not aspartic acid, residues of casein (Sansom & Barry, 1958). Experiments described in this paper show that the mammary gland of the goat takes free proline from the blood to provide at least 50% of the proline residues of casein. It is calculated from this and previous work that at least half of the residues of non-essential amino acids in casein are not synthesized within the mammary gland, but come directly from the blood stream.