Lobley G E, Harris P M, Skene P A, Brown D, Milne E, Calder A G, Anderson S E, Garlick P J, Nevison I, Connell A
Rowett Research Institute, Bucksburn, Aberdeen.
Br J Nutr. 1992 Sep;68(2):373-88. doi: 10.1079/bjn19920096.
In ten lambs (average live weight 33 kg), five offered 300 g/d (approximately 0.6 x maintenance; L) and five 900 g/d (1.8 x maintenance; H), tissue protein synthesis was measured by three procedures simultaneously. The techniques involved continuous infusion of [U-14C]phenylalanine and [1-13C]leucine over 7-8 h followed by a terminal large dose of [15N]phenylalanine during the last 30 or 60 min. Rates of protein synthesis were then calculated based on the free amino acid or oxo-acid isotopic activity in either arterial, iliac venous blood or tissue homogenate for the continuous-infusion studies, or on plasma or tissue homogenate for the large-dose procedure. For muscle (> 99%), and to a lesser extent skin (85-93%), effective flood conditions were achieved with the [15N]phenylalanine but were either not established or maintained for liver and tissues of the gastrointestinal tract (< 50%). The large dose of phenylalanine also caused changes in the concentration and isotopic activity of blood leucine and 4-methyl-2-oxo-pentanoate. Based on the assumption that the large-dose procedure yields the closest value for the true rate of protein synthesis (L 1.97%/d, H 2.85%/d) then, for muscle, only values based on the homogenate as precursor gave comparable results for both leucine (L 1.83%/d, H 3.01%/d) and phenylalanine (L 1.67%/d, H 2.71%/d) continuous infusion. The values based on the arterial or venous amino or oxo-acid were significantly less, more so at the lower intake. In contrast, for skin, a tissue dominated by export protein synthesis, values from the large-dose procedure (L 6.37%/d, H 10.98%/d) were similar to those derived with arterial or venous metabolites as precursor (L 5.23 and 6.93%/d, H 9.98 and 11.71%/d for leucine), but much less than those based on homogenate data. Based on the large-dose technique, protein synthesis increased with intake in muscle (P < 0.001), skin (P = 0.009) and liver (26.7 v. 30.5%/d; P = 0.029). The contributions of muscle and skin to total protein synthesis were approximately equal. The incremental efficiency of conversion for muscle of synthesized protein into deposition appeared to be similar to values reported for rodents.
在10只羔羊（平均活重33千克）中，5只给予300克/天（约为维持量的0.6倍；低量组），5只给予900克/天（维持量的1.8倍；高量组），同时采用三种方法测定组织蛋白质合成。这些技术包括在7 - 8小时内持续输注[U - 14C]苯丙氨酸和[1 - 13C]亮氨酸，然后在最后30或60分钟给予一次大剂量的[15N]苯丙氨酸。对于持续输注研究，根据动脉血、髂静脉血或组织匀浆中的游离氨基酸或氧代酸同位素活性来计算蛋白质合成速率；对于大剂量法，根据血浆或组织匀浆来计算。对于肌肉（>99%），在较小程度上对于皮肤（85 - 93%），[15N]苯丙氨酸实现了有效的充盈条件，但对于肝脏和胃肠道组织（<50%），要么未建立起该条件，要么未维持该条件。大剂量的苯丙氨酸还导致血液亮氨酸和4 - 甲基 - 2 - 氧代戊酸的浓度和同位素活性发生变化。基于大剂量法得出的蛋白质合成真实速率最接近实际值这一假设（低量组为1.97%/天，高量组为2.85%/天），那么对于肌肉，只有以匀浆作为前体的数据得出的亮氨酸（低量组为1.83%/天，高量组为3.01%/天）和苯丙氨酸（低量组为1.67%/天，高量组为2.71%/天）持续输注结果具有可比性。基于动脉血或静脉血中氨基酸或氧代酸得出的值明显更低，在低摄入量时更是如此。相比之下，对于以输出蛋白质合成为主的皮肤组织，大剂量法得出的值（低量组为6.37%/天，高量组为10.98%/天）与以动脉血或静脉血代谢物作为前体得出的值相似（亮氨酸的低量组分别为5.23和6.93%/天，高量组分别为9.98和11.71%/天），但远低于基于匀浆数据得出的值。基于大剂量技术，肌肉（P < 0.001）、皮肤（P = 0.009）和肝脏（26.7对30.5%/天；P = 0.029）中的蛋白质合成随摄入量增加。肌肉和皮肤对总蛋白质合成的贡献大致相等。肌肉中合成蛋白质转化为沉积的增量效率似乎与啮齿动物报道的值相似。
