EVIDENCE FOR THE UTILIZATION OF PEPTIDES FOR MILK PROTEIN-SYNTHESIS IN THE LACTATING DAIRY GOAT IN-VIVO

Precursors for milk protein synthesis have been examined in lactating dairy goats using arteriovenous difference and isotope kinetic techniq ues. Certain amino acids, such as phenylalanine and histidine, are tak en up by the mammary gland in quantities that are insufficient to acco unt for their output in milk protein. Some amino acids have been shown to be present in significant quantities (10-30% of total nonprotein-b ound amino acids) as peptides (<1,500 Da) in the arterial supply to th e mammary gland, although methodological considerations make it diffic ult to accurately assess the extent of their uptake across the tissue bed. Indirect evidence for the utilization of peptides for milk protei n synthesis in vivo has been obtained, however, by examination of the kinetics of milk casein labeling during long-term (24 h) systemic infu sion of [1-C-13]phenylalanine and [1-C-13]leucine. Comparison of plate au enrichments for blood, plasma, and casein indicate that, although, for leucine, the plasma free pool seems to provide all the leucine for milk protein synthesis, sources other than the labeled plasma free am ino acids contribute phenylalanine (10-20%) for casein biosynthesis. T hese findings raise questions relating to the type and source of amino acid precursors used by tissues for protein synthesis.