Buse M G, Reid S S
J Clin Invest. 1975 Nov;56(5):1250-61. doi: 10.1172/JCI108201.
Incorporation of radiolabeled precursors into muscle proteins was studied in isolated rat hemidiaphragms. A mixture of three branched-chain amino acids (0.3 mM each) added to media containing glucose stimulated the incorporation of [14C]lysine into proteins. When tested separately, valine was ineffective, isoleucine was inhibitory, but 0.5 mM leucine increased the specific activity of muscle proteins during incubation with [14C]lysine or [14C]acetate in hemidiaphragms from fed or fasted rats incubated with or without insulin. Preincubation with 0.5 mM leucine increased the specific activity of muscle proteins during a subsequent 30- or 60-min incubation with [14C]lysine or [14C]pyruvate without leucine. Preincubation with other amino acids (glutamate, histidine, methionine, phenylalanine, or tryptophan) did not exert this effect. When hemidiaphragms were incubated with a mixture of amino acids at concentrations found in rat serum and a [14C]lysine tracer, the specific activity of muscle proteins increased when leucine in the medium was raised from 0.1 to 0.5 mM. Experiments with actinomycin D and cycloheximide suggested that neither RNA synthesis nor protein synthesis are required for the initiation of the leucine effect. Leucine was not effective when added after 1 h preincubation without leucine. The concentration of lysine in the tissue water of diaphragms decreased during incubation with 0.5 mM leucine in the presence or absence of cycloheximide, suggesting that leucine inhibited protein degradation. During incubation with [3h]tyrosine (0.35 mM) the addition of 0.5 mM leucine increased the specific activity of muscle proteins, while the specific activity of intracellular tyrosine remained constant and its concentration decreased, suggesting that leucine also promoted protein synthesis. The concentration of leucine in muscle cells or a compartment thereof may play a role in regulating the turnover of muscle proteins and influence the transition to negative nitrogen balance during fasting, uncontrolled diabetes, and the posttraumatic state. Leucine may play a pivotal role in the protein-sparing effect of amino aicds.
在分离的大鼠半膈肌中研究了放射性标记前体掺入肌肉蛋白的情况。向含有葡萄糖的培养基中添加三种支链氨基酸的混合物（每种0.3 mM）可刺激[14C]赖氨酸掺入蛋白质。单独测试时，缬氨酸无效，异亮氨酸有抑制作用，但0.5 mM亮氨酸在喂食或禁食大鼠的半膈肌与[14C]赖氨酸或[14C]乙酸盐一起孵育时，无论有无胰岛素，均可提高肌肉蛋白的比活性。用0.5 mM亮氨酸预孵育后，在随后30或60分钟与不含亮氨酸的[14C]赖氨酸或[14C]丙酮酸一起孵育时，肌肉蛋白的比活性增加。用其他氨基酸（谷氨酸、组氨酸、蛋氨酸、苯丙氨酸或色氨酸）预孵育未产生此效果。当半膈肌与大鼠血清中发现的浓度的氨基酸混合物和[14C]赖氨酸示踪剂一起孵育时，培养基中亮氨酸浓度从0.1 mM提高到0.5 mM时，肌肉蛋白的比活性增加。用放线菌素D和环己酰亚胺进行的实验表明，亮氨酸效应的启动既不需要RNA合成也不需要蛋白质合成。在无亮氨酸预孵育1小时后添加亮氨酸无效。在有或无环己酰亚胺存在的情况下，用0.5 mM亮氨酸孵育期间，膈肌组织水中赖氨酸的浓度降低，表明亮氨酸抑制蛋白质降解。在用[3H]酪氨酸（0.35 mM）孵育期间，添加0.5 mM亮氨酸可提高肌肉蛋白的比活性，而细胞内酪氨酸的比活性保持不变，其浓度降低，表明亮氨酸也促进蛋白质合成。肌肉细胞或其一个区室中亮氨酸的浓度可能在调节肌肉蛋白周转中起作用，并影响禁食、未控制的糖尿病和创伤后状态期间向负氮平衡的转变。亮氨酸可能在氨基酸的蛋白质节省效应中起关键作用。
