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Kinetics of blood free and milk casein-amino acid labelling in the dairy goat at two stages of lactation

Published online by Cambridge University Press:  09 March 2007

B. J. Bequette ,
F. R. C. Backwell ,
M. S. Dhanoa ,
A. Walker ,
A. G. Calder ,
D. Wray-Cahen ,
J. A. Metcalf ,
J. D. Sutton ,
D. E. Beever  and
G. E. Lobley
...Show all authors
B. J. Bequette
Affiliation:
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB
F. R. C. Backwell
Affiliation:
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB
M. S. Dhanoa
Affiliation:
AFRC Institute for Grassland and Environmental Research Plas Gogerddan, Aberystwyth SY23 3EB
A. Walker
Affiliation:
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB
A. G. Calder
Affiliation:
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB
D. Wray-Cahen
Affiliation:
Centre for Dairy Research, University of Reading, Reading RG6 2AT
J. A. Metcalf
Affiliation:
Centre for Dairy Research, University of Reading, Reading RG6 2AT
J. D. Sutton
Affiliation:
Centre for Dairy Research, University of Reading, Reading RG6 2AT AFRC Institute for Grassland and Environmental Research Plas Gogerddan, Aberystwyth SY23 3EB
D. E. Beever
Affiliation:
Centre for Dairy Research, University of Reading, Reading RG6 2AT
G. E. Lobley
Affiliation:
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB
J. C. MacRae
Affiliation:
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB
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Abstract

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The kinetics of blood free amino acids (AA) transfer into milk casein were compared in goats (n 4) at 61 (SE5)d (Expt 1; post-peak, 4.51 (SE 0.26) kg milk/d) and at 180 (SE 6) d (Expt 2; late, 2.36 (SE 0.16) kg milk/d) of lactation during non-primed, continuous (Expt 1, 12 h; Expt 2, 16 h) intravenous infusions of mixtures of L-[1-13C]leucine and L-[1-13C]phenylalanine with either L-[1-13C]valine (Expt 1) or L-[5-13Cmethionine (Expt 2). The 13C enrichments of blood free and casein-bound AA were fitted to a single exponential model to estimate isotopic plateaux and the fractional rate constant for milk casein labelling. Milk protein output and its contribution to whole-body flux was higher in Expt 1 (post-peak) than in Expt 2 (late lactation), but the kinetics of 13C labelling of the casein-bound AA were similar for all AA tracers in both experiments. At both stages of lactation the delay (6–8 h) between the attainment of isotopic plateau for the blood free AA and the corresponding attainment of plateau for the casein-bound AA indicated that the blood free pool was not the immediate precursor pool for milk casein biosynthesis. Plateau enrichments of casein-bound AA were generally higher than those for the corresponding blood free AA in both experiments. These results indicate that the relative contributions of different AA sources to the immediate precursor pool for milk casein biosynthesis are similar at different stages of lactation despite major changes in the partitioning of whole-body flux towards milk protein output. Non-milk protein fluxes were also similar in post-peak and late lactation.

Keywords

Milk proteinAmino acid kineticsCaseinLactating goats
Type
Kinetics of amino acid metabolism during lactation
Information
British Journal of Nutrition , Volume 72 , Issue 2 , August 1994 , pp. 211 - 220
Copyright
Copyright © The Nutrition Society 1994

References

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