Meijer G A, Van der Meulen J, Bakker J G, Van der Koelen C J, Van Vuuren A M
Dienst Landbouwkundig Onderzoek, Department of Ruminant Nutrition, Lelystad, The Netherlands.
J Dairy Sci. 1995 May;78(5):1131-41. doi: 10.3168/jds.S0022-0302(95)76730-3.
Free AA in plasma and muscle were monitored in 36 dairy cows from 2 wk before until 15 wk after parturition. Cows were kept indoors and fed individually. The AA concentrations in plasma from 6 to 15 wk of lactation were compared with precalving concentrations. Plasma concentrations of Met, Phe, Glu, and Gln decreased by 16, 24, 25, and 25%, respectively. The ranking of the essential AA according to their decrease in plasma corresponded to the order in which they generally appear to be limiting for milk protein synthesis. The decrease of Glu and Gln in plasma exceeded that of the essential AA and contrasted strongly with all other nonessential AA. The change in the AA profile in muscle from pregnancy to lactation resembled that described for the catabolic state in mammals and suggests that muscle protein was degraded for supply of AA to the udder, despite excess protein and energy supply 15 wk into lactation. In muscle, the decrease in the pool of free Gln exceeds 25% and is higher than for any other AA. These observations support our hypothesis that Gln is potentially limiting for milk protein synthesis in the high yielding dairy cow.
对36头奶牛在分娩前2周直至分娩后15周期间的血浆和肌肉中的游离氨基酸进行了监测。奶牛饲养在室内，单独饲喂。将泌乳6至15周时血浆中的氨基酸浓度与产犊前的浓度进行了比较。蛋氨酸、苯丙氨酸、谷氨酸和谷氨酰胺的血浆浓度分别下降了16%、24%、25%和25%。必需氨基酸在血浆中下降的排序与它们通常对乳蛋白合成的限制顺序一致。血浆中谷氨酸和谷氨酰胺的下降超过了必需氨基酸，且与所有其他非必需氨基酸形成强烈对比。从怀孕到泌乳期间肌肉中氨基酸谱的变化类似于哺乳动物分解代谢状态下的情况，这表明尽管泌乳15周时蛋白质和能量供应充足，但肌肉蛋白仍被降解以向乳腺提供氨基酸。在肌肉中，游离谷氨酰胺池的下降超过25%，且高于任何其他氨基酸。这些观察结果支持了我们的假设，即谷氨酰胺可能是高产奶牛乳蛋白合成的限制因素。
