Biosynthesis of proteins. 3. Precursors in the synthesis of casein and β-lactoglobulin

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Abstract
Part 2: Askonas, Campbell and Work (1954a). Methods have been developed for the isolation of peptides from partial acid hydrolysates of casein and 0-lactoglobulin. Radioactive casein and [beta]-lactoglobulin were obtained from milk collected 4 and 3 hours after injection of radioactive amino acids. Partial acid hydrolysis of these proteins and isolation of radioactive peptides permitted the determination of specific radioactivity of the same amino acid at different points in the protein chain. Radioactivity was uniformly distributed within the protein molecule. Results indicate that casein and [beta]-lactoglobulin are synthesized from the free amino acids of the blood and that peptides for milk protein synthesis are not supplied by partial hydrolysis of plasma protein. Intermediate peptides cannot be stored in the mammary gland in significant quantity. It is suggested that peptides as intermediates in protein synthesis have only transitory existence. An apparatus is described which simplifies the recovery of amino acids and peptides from ion exchange column effluents.