Factor of suppression of tissue protein degradation after feeding and its mechanism

饲喂20%酪蛋白饲料1h后，大鼠肌原纤维蛋白降解速率（用血清3-甲基组氨酸（MeHis）浓度测定）和MeHis从离体肌肉中的释放速率均显著降低，而饲喂无蛋白饲料时无明显变化。抑制作用随饲粮蛋白质含量的增加而增强，当饲粮蛋白质含量不变而能量降低时，抑制作用无明显变化。这些结果有力地表明，膳食蛋白质直接调节肌原纤维蛋白质降解后，喂养。蛋白质中的Leu主要影响蛋白质降解的抑制作用，因为这些变化在喂食Leu饲料或强制喂食Leu混悬液的大鼠中也可见。当溶酶体和钙依赖的蛋白水解系统被抑制时，抑制作用不明显，表明钙蛋白酶可能是关键酶。当ATP依赖的蛋白水解系统被抑制时，与蛋白质合成相关的p70 S6激酶磷酸化被蛋白质和Leu直接激活，而肌原纤维蛋白降解被抑制。因此，蛋白质合成可能比蛋白质降解更快地通过进食被激活。

After feeding of 20% casein diet for 1h, the rate of myofibrillar protein degradation measured by the serum concentration of 3-methyl-histidine (MeHis) and the release rate of MeHis from the isolated muscle significantly decreased, although they did not change when rats fed protein free diet. The suppression was increased with content of dietary protein content and it was not changed when feeding protein content was constant but energy was decreased. These results strongly suggest that dietary protein directly regulate myofibrillar protein degradation after feeding. Leu in dietary protein mainly affect the suppression, since these change was also seen in rats fed on Leu diet or force fed on Leu suspension.Mechanism of suppression of degradation after feeding was analyzed by proteolysis inhibitors. When lysosome and calcium-dependent proteolytic systems were inhibited, the suppression was not seen, indicating that calpain may key enzyme. However, when ATP-dependent proteolytic system was inhibited, the suppression of myofibrillar protein degradation after feeding was demonstrated.Phosphorylation of p70S6 kinase which correlates with protein synthesis was immediately activated by protein and Leu feeding. Therefore, protein synthesis may be activated more rapidly than protein degradation by feeding.