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

饲喂20%酪蛋白饲料1h后，用血清3-甲基组氨酸(MeHis)浓度测定的肌原纤维蛋白降解率和离体肌中MeHis的释放率显著降低，而饲喂无蛋白饲料的大鼠则无明显变化。这种抑制作用随着饲料蛋白质含量的增加而增加，当饲料蛋白质含量不变，能量降低时，抑制作用不变。这些结果有力地表明，饲料蛋白直接调节摄食后肌原纤维蛋白的降解。膳食蛋白质中的亮氨酸主要影响抑制作用，因为这些变化在饲喂亮氨酸饲料或强迫饲喂亮氨酸悬浮液的大鼠中也可以看到。通过蛋白分解抑制剂分析了喂养后降解抑制的机制。当溶酶体和钙依赖的蛋白分解系统被抑制时，没有看到抑制作用，这表明钙蛋白酶可能是关键酶。然而，当依赖于ATP的蛋白水解系统被抑制时，摄食后的肌原纤维蛋白降解就受到了抑制。与蛋白质合成相关的p70S6激酶的磷酸化立即被蛋白质和亮氨酸摄食激活。因此，通过摄食，蛋白质合成可能比蛋白质降解更快地被激活。

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.