KINETICS OF BLOOD-FREE AND MILK CASEIN-AMINO ACID LABELING IN THE DAIRY GOAT AT 2 STAGES OF LACTATION

The kinetics of blood free amino acids (AA) transfer into milk casein were compared in goats (n 4) at 61 (SE 5) 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; E xpt 2, 16 h) intravenous infusions of mixtures of L-[1-C-13]leucine an d L-[1-(13)]phenylalanine with either L-[1-C-13]valine (Expt 1) or L-[ 5-C-13]methionine (Expt 2). The C-13 enrichments of blood free and cas ein-bound AA were fitted to a single exponential model to estimate iso topic plateaux and the fractional rate constant for milk casein labell ing. Milk protein output and its contribution to whole-body flux was h igher in Expt 1 (post-peak) than in Expt 2 (late lactation), but the k inetics of C-13 labelling of the casein-hound 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 po ol for milk casein biosynthesis. Plateau enrichments of casein-bound A A were generally higher than those for the corresponding blood free AA in both experiments. These results indicate that the relative contrib utions of different AA sources to the immediate precursor pool for mil k casein biosynthesis are similar at different stages of lactation des pite major changes in the partitioning of whole-body flux towards milk protein output. Non-milli protein fluxes were also similar in post-pe ak and late lactation.