Bequette B J, Backwell F R, Dhanoa M S, Walker A, Calder A G, Wray-Cahen D, Metcalf J A, Sutton J D, Beever D E, Lobley G E
Rowett Research Institute, Bucksburn, Aberdeen.
Br J Nutr. 1994 Aug;72(2):211-20. doi: 10.1079/bjn19940025.
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; 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-[5-13C]methionine (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.
在山羊（n = 4）泌乳的第61（标准误5）天（实验1；泌乳高峰期后，日产奶量4.51（标准误0.26）千克）和第180（标准误6）天（实验2；泌乳后期，日产奶量2.36（标准误0.16）千克），通过非预充、连续（实验1为12小时；实验2为16小时）静脉输注L-[1-¹³C]亮氨酸和L-[1-¹³C]苯丙氨酸与L-[1-¹³C]缬氨酸（实验1）或L-[5-¹³C]蛋氨酸（实验2）的混合物，比较了游离血氨基酸（AA）向乳中酪蛋白转移的动力学。将游离血和酪蛋白结合的AA的¹³C富集度拟合到单指数模型中，以估计同位素平台期和乳中酪蛋白标记的分数速率常数。实验1（泌乳高峰期后）的乳蛋白产量及其对全身通量的贡献高于实验2（泌乳后期），但在两个实验中，所有AA示踪剂标记酪蛋白结合AA的¹³C动力学相似。在泌乳的两个阶段，游离血AA达到同位素平台期与酪蛋白结合AA相应达到平台期之间的延迟（6 - 8小时）表明，游离血池不是乳中酪蛋白生物合成的直接前体池。在两个实验中，酪蛋白结合AA的平台期富集度通常高于相应的游离血AA。这些结果表明，尽管全身通量向乳蛋白产量的分配发生了重大变化，但在泌乳的不同阶段，不同AA来源对乳中酪蛋白生物合成直接前体池的相对贡献相似。泌乳高峰期后和泌乳后期的非乳蛋白通量也相似。
