Wagenmakers A J
Department of Human Biology, Maastricht University, The Netherlands.
Exerc Sport Sci Rev. 1998;26:287-314.
Six amino acids are metabolized in resting muscle. They are leucine, isoleucine, valine, asparagine, aspartate, and glutamate. These amino acids provide the amino groups and probably the ammonia required for synthesis of glutamine and alanine, which are released in excessive amounts in the postabsorptive state and during ingestion of a protein-containing meal. Only leucine and part of the isolecine molecule can be oxidized in muscle as they are converted to acetyl-CoA. The other carbon skeletons are used solely for de novo synthesis of TCA-cycle intermediates and glutamine. The carbon atoms of the released alanine originate primarily from glycolysis of blood glucose and from muscle glycogen (about half each in resting conditions). After consumption of a protein-containing meal, BCAA and glutamate are taken up by muscle and their carbon skeletons are used for de novo synthesis of glutamine. About half of the glutamine released from muscle originates from glutamate taken up from the blood, both after overnight starvation, after prolonged starvation, and after consumption of a mixed meal. Glutamine produced by muscle is an important fuel and regulator of DNA and RNA synthesis in mucosal cells and immune system cells, and fulfils several other important functions in human metabolism. The alanine aminotransferase reaction functions to establish and maintain high concentrations of TCA-cycle intermediates in muscle during the first 10 min of exercise. The increase in concentration of TCA-cycle intermediates probably is needed to increase the flux of the TCA-cycle and meet the increased energy demand of exercise. A gradual increase in leucine oxidation subsequently leads to a carbon drain on the TCA-cycle in glycogen-depleted muscles, and may thus reduce the maximal flux in the TCA-cycle and lead to fatigue. Deamination of amino acids and glutamine synthesis present alternative anaplerotic mechanisms in glycogen-depleted muscles, but only allow exercise at 40-50% of Wmax. One-leg exercise leads to the net breakdown of muscle protein. The liberated amino acids are used for synthesis of TCA-cycle intermediates and glutamine. Today, the importance of this process in endurance exercise in the field (running or cycling) in athletes who ingest carbohydrates is not clear. It is proposed that the maximal flux in the TCA-cycle is reduced in glycogen-depleted muscles due to insufficient TCA-cycle anaplerosis, and that this presents a limitation for the maximal rate of fatty acid oxidation. Interactions between the amino acid pool and the TCA-cycle are suggested to play a central role in the energy metabolism of the exercising muscle.
六种氨基酸在静息肌肉中进行代谢。它们是亮氨酸、异亮氨酸、缬氨酸、天冬酰胺、天冬氨酸和谷氨酸。这些氨基酸提供氨基，可能还提供合成谷氨酰胺和丙氨酸所需的氨，谷氨酰胺和丙氨酸在吸收后状态以及摄入含蛋白质餐时会大量释放。只有亮氨酸和部分异亮氨酸分子在肌肉中被氧化，因为它们会转化为乙酰辅酶A。其他碳骨架仅用于从头合成三羧酸循环中间产物和谷氨酰胺。释放的丙氨酸的碳原子主要来自血糖的糖酵解和肌肉糖原（在静息状态下各占约一半）。摄入含蛋白质餐之后，支链氨基酸和谷氨酸被肌肉摄取，它们的碳骨架用于从头合成谷氨酰胺。无论是在隔夜禁食后、长期饥饿后还是摄入混合餐之后，从肌肉释放的谷氨酰胺中约一半都源自从血液中摄取的谷氨酸。肌肉产生的谷氨酰胺是黏膜细胞和免疫系统细胞中DNA和RNA合成的重要燃料及调节剂，并且在人体新陈代谢中还发挥其他几种重要功能。在运动的前10分钟，丙氨酸转氨酶反应的作用是在肌肉中建立并维持高浓度的三羧酸循环中间产物。三羧酸循环中间产物浓度的增加可能是为了增加三羧酸循环的通量，以满足运动增加的能量需求。随后亮氨酸氧化的逐渐增加会导致糖原耗竭的肌肉中三羧酸循环出现碳流失，因此可能会降低三羧酸循环的最大通量并导致疲劳。氨基酸脱氨基和谷氨酰胺合成是糖原耗竭的肌肉中另外的回补机制，但仅能支持以40 - 50%的最大摄氧量进行运动。单腿运动导致肌肉蛋白质净分解。释放的氨基酸用于合成三羧酸循环中间产物和谷氨酰胺。目前，在摄入碳水化合物的运动员进行野外耐力运动（跑步或骑自行车）时，这一过程的重要性尚不清楚。有人提出，糖原耗竭的肌肉中三羧酸循环的最大通量因三羧酸循环回补不足而降低，这对脂肪酸氧化的最大速率构成限制。氨基酸池与三羧酸循环之间的相互作用被认为在运动肌肉的能量代谢中起核心作用。
