Investigation of the role of branched chain amino add on stimulation of protein synthesis and application for preservation and improvement of muscle mass

支链氨基酸亮氨酸已被证明可以激活蛋白质的翻译调节。然而，亮氨酸的作用是自我限制的，因为亮氨酸通过氧化途径促进自身的处置。亮氨酸氧化途径中不可逆的限速步骤是由兰格链α-酮酸脱氢酶复合体催化的。利用RNA干扰技术研究了BCKDH复合体的活性与亮氨酸介导的蛋白质翻译激活之间的关系。结果表明，下调BCKDHE2亚单位的表达，可能取消了BCKDH复合体的活性，增加了亮氨酸对p70S6K磷酸化的影响，p70S6K是参与启动mRNA翻译的关键调控蛋白。反之，通过转导激酶-siRNA激活BCKDH复合体可减弱亮氨酸的作用。这些结果表明，亮氨酸对翻译调节因子的激活部分受BCKDH复合体活性的调节。有报道称，BCKDH在肝脏中的活性存在性别差异。因此，我们研究了亮氨酸对雄性和雌性大鼠蛋白质翻译起始的激活作用。在光照末期，无论雌雄大鼠，亮氨酸诱导的骨骼肌翻译起始激活均增加(雄性大鼠肝脏BDKCH活性高于雌性大鼠)。相反，在饥饿18h后，雄性大鼠骨骼肌中亮氨酸诱导的翻译起始激活高于雌性大鼠(雄性和雌性大鼠的肝脏BCKDH都被失活)。这些结果表明，亮氨酸诱导的骨骼肌蛋白质翻译起始激活的性别差异与肝脏BCKDH的活性无关。还需要进一步的研究来阐明亮氨酸诱导的蛋白质翻译起始激活的性别差异。

Branched-chain amino acid Ieucine has been shown to activate the protein translational regulators. However, the leucine's effects are self-limiting because leucine promotes its own disposal by an oxidative pathway. The irreversible and rate-limiting step in the leucine oxidation pathway is catalyzed by the ranched-chain α-keto acid dehydrogenase (BCKDH) complex. The relationship between the activity of BCKDH complex and leucine-mediated activation of the protein translation was investigated using the technique of RNA interference. The results demonstrated that knockdown of the BCKDH E2 subunit expression, presumably abolishing the BCKDH complex activity, increased the effect of leucine on phosphorylation of p70S6K, a key regulatory protein involved in the initiation of mRNA translation. On the contrary, activation of BCKDH complex by trancfection of kinase-siRNA decreased the leucine's effect. These results suggest that the activation of the translational regulators by leucine was partly regulated by the activity of BCKDH complex. It has been reported that BCKDH activity in liver has gender difference. So we investigated the leucine-induced activation of the protein translation initiation in male and female rats. Leucien-induced activation of the translation initiation in skeletal muscle was increased both in male and female rats at the end of light period (hepatic BDKCH activity is higher in male rats than female rats). In contrast, leucine-induced activation of the translation initiation in skeletal muscle was higher in male rats than female rats after the 18 h starvation (hepatic BCKDH is inactivated both in male and female rats). These results suggest that gender difference in leucine-induced activation of the protein translation initiation in skeletal muscle is not related to the activity of hepatic BCKDH. Further studies are needed to elucidate the gender difference in leucine-induced activation of the protein translation initiation.