Aoki T T, Finley R J, Cahill G F
Biochem Soc Symp. 1978(43):17-29.
Traditionally, regulation of amino acid metabolism in both postabsorptive and prolonged-fasted man has been generally regarded as being hormonal in nature. In particular, insulin, and to a lesser extent glucagon, have been nominated for key roles in this process. More recently, however, reconsideration of previous studies involving insulin, glucagon, and protein meals as well as previously unreported studies (cortisol and tri-iodothyronine) from this laboratory, have suggested another means of regulating amino acid metabolism in fasting man. This new hypothesis is centered on the redox state of muscle of fasting man, which is remarkably reduced in both cytosolic and mitochondrial compartments. It was found that insulin, and to a lesser extent glucagon, when infused into fasting subjects (1) rendered muscle significantly more reduced, and (2) resulted in a diminution in urinary nitrogen excretion. In contrast, when either tri-iodothyronine or cortisol were administered to fasting individuals (1) muscle was found to become more oxidized when compared with the control period, and (2) increased urinary nitrogen excretion was observed in both cases. It was noteworthy that the ingestion of a protein meal by a nitrogen-depleted individual was followed by a dramatic change in muscle redox state (the muscle became more reduced), together with marked uptakes of a variety of amino acids. It is therefore proposed that the protein conservation evidenced by fasting man may be dependent on the reduced state of muslce tissue.
传统上，人们普遍认为，在吸收后和长期禁食的人体中，氨基酸代谢的调节本质上是由激素控制的。特别是胰岛素，以及在较小程度上的胰高血糖素，被认为在这一过程中起关键作用。然而，最近，对先前涉及胰岛素、胰高血糖素和蛋白质餐的研究以及本实验室先前未报道的研究（皮质醇和三碘甲状腺原氨酸）进行重新审视后，提出了另一种调节禁食人体氨基酸代谢的方式。这一新假说的核心是禁食人体肌肉的氧化还原状态，其在细胞溶质和线粒体区室中均显著降低。研究发现，当将胰岛素以及在较小程度上的胰高血糖素注入禁食受试者体内时，（1）肌肉的还原程度显著增加，（2）尿氮排泄减少。相反，当向禁食个体施用三碘甲状腺原氨酸或皮质醇时，（1）与对照期相比，肌肉被发现氧化程度更高，（2）在这两种情况下均观察到尿氮排泄增加。值得注意的是，氮缺乏个体摄入蛋白质餐后，肌肉氧化还原状态会发生显著变化（肌肉变得更加还原），同时会大量摄取各种氨基酸。因此，有人提出，禁食人体所表现出的蛋白质保存可能取决于肌肉组织的还原状态。
