Different response of amino acids on muscle protein synthesis and degradation under nutritional stresses

1.低营养条件下的亮氨酸功能：饲喂1周无蛋白饲料的小鼠骨骼肌重量下降，添加1.5%亮氨酸的小鼠体重下降受到抑制。以3-甲基组氨酸的A-V差值来评价肌肉蛋白质的降解也被添加亮氨酸的饲料所抑制。肌肉蛋白质合成速率无明显变化。在添加亮氨酸的饲料中，虽然泛素蛋白酶体的表达和钙蛋白酶活性没有降低，但LC-3II的表达却降低了。这些结果表明，亮氨酸促进了肌肉蛋白质的降解而不是合成，在低营养条件下，这种降解可能受到自噬的调节。重复摄入亮氨酸对大鼠肌肉蛋白质合成和降解的影响间隔6小时给药可降低肌原纤维蛋白的降解，通过从离体肌释放3-甲基组氨酸来评价。然而，通过磷酸化评价S6K1的合成并没有，这表明重复补充亮氨酸对肌肉萎缩是有益的。亮氨酸对废用模型大鼠肌肉萎缩的影响后肢制动增加了肌肉蛋白质的降解，进而导致肌肉萎缩。在该模型中，补充亮氨酸的饮食减轻了由于肌肉蛋白降解引起的肌肉萎缩，特别是泛素-蛋白酶体。

1. The function of leucine under low nutritional conditions Skeletal muscle weight decreased in feeding of one week protein free diet, however the weight loss was suppressed in 1.5% leucine supplementation. Muscle protein degradation evaluated by A-V difference of 3-methylhistidine concentration was also suppressed by leucine supplemented diet. The rate of muscle protein synthesis was not changed. Although ubiquitin-proteasome expression and calpain activity were not reduced in feeding of leucine supplemented diet, LC-3II expression was decreased. These results suggested that leucine enhances muscle protein degradation rather synthesis and the degradation might be regulated by autophagy in low nutritional conditions.2. Effects of repeated leucine consumption on muscle protein synthesis and degrdation Six hours interval leucine administration to rats decreased myofibrillar protein degradation evaluated by 3-methylhistidine release from the isolated muscle. However, synthesis evaluated by phosphrylation of S6K1 did not, suggesting that repeated leucine supplementation is beneficial for muscle atrophy.3. Effects of leucine on muscle atrophy in disuse model Immobilization of hind limb increased muscle protein degradation and then resulted muscle atrophy. In this model, leucine supplemented diet attenuated muscle atrophy due to muscle protein degradation especially ubiquitin-proteasome.