Evidence for the utilisation of peptides for milk protein synthesis
in the lactating dairy goat in vivo.
Backwell, F. R. Colette, Brian J. Bequette, Dana Wilson, John A.
Metcalf, Mike F. Franklin, David E. Beever, Gerald E. Lobley, and
John C. Macrae.
Rowett Research Institute, Aberdeen, AB2 9SB. UK, University of
Reading, Reading RG6 2AT. UK
APStracts 3:0137R, 1996.
Precursors for milk protein synthesis have been examined in lactating
dairy goats using arterio-venous difference and isotope kinetic
techniques. Certain amino acids, such as phenylalanine and histidine,
are taken up by the mammary gland in quantities which are
insufficient to account for their output in milk protein. Some amino
acids have been shown to be present in significant quantities (10 -
30 % of total non-protein bound amino acids) as peptides (<1500
Da) in the arterial supply to the mammary gland, although
methodological considerations make it difficult to accurately assess
the extent of their uptake across the tissue bed. Indirect evidence
for the utilisation of peptides for milk protein synthesis in?vivo
has been obtained, however, by examination of the kinetics of milk
casein labelling during long-term (24 h) systemic infusion of [1
-13C]-phenylalanine and [1-13C]leucine. Comparison of plateaux
enrichments for blood, plasma and casein indicate that while, for
leucine, the plasma free pool seems to provide all the leucine for
milk protein synthesis, sources other than the labelled plasma free
amino acids contribute phenylalanine (10 - 20 %) for casein
biosynthesis. These findings raise questions relating to the type and
source of amino acid precursors used by tissues for protein
synthesis.
Received 8 December 1995; accepted in final form 2 April 1996.
APS Manuscript Number R775-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 16 April 96