The use of asparagine and glutamine for the biosynthesis of casein and plasma proteins.

The mammary gland is a system in which the components of the blood stream which are used by body tissues for protein synthesis can be readily investigated (Barry, 1956). 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), and takes free glutamine and glutamic acid to provide the glutamine and glutamic acid residues respectively of casein (Barry, 1956; Sheldon-Peters & Barry, 1956). Experiments described in this paper show that the mammary gland of the goat takes free aspara'gine from the blood stream to provide asparagine, but not aspartic acid, residues of casein. They also show that asparagine and glutamine are used in the biosynthesis of plasma proteins to provide asparagine and glutanine residues. These results provide the first evidence for the function of asparagine in animals, and show that the use of glutamine in protein biosynthesis is not confined to casein synthesis. In this paper 'casein asparagine' refers to the asparagine residues in casein, 'casein aspartic acid' to the aspartic acid residues in casein, and 'casein aspartic acid (acid hydrolysis)' to the aspartic acid in an acid hydrolysate, which is derived from both the casein asparagine and casein aspartic acid.