The use of glutamine and glutamic acid by the mammary gland for casein synthesis

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Abstract
Experiments with glutamine labeled with C14 and N15, and glutamic acid labeled with C14 show that the lactating mammary gland of the goat takes free glutamine and glutamic acid from the blood plasma to incorporate them into casein as two independent amino acid residues. There is little synthesis of casein glutamine or glutamic acid from glucose within the mammary gland. Evidence is presented that transpeptidation reactions, involving gamma-glutamyl peptides formed from glutamine are not important in casein synthesis. The results also suggest that the glutamine of blood plasma is synthesized from plasma glutamic acid; the average life of a plasma glutamine molecule is about 50 minutes.