Block K P, Buse M G
Department of Medicine, Medical University of South Carolina, Charleston 29425.
Med Sci Sports Exerc. 1990 Jun;22(3):316-24.
Branched-chain alpha-keto acid dehydrogenase (BCKAD) is a multisubunit complex regulated by phosphorylation and is considered to be rate-limiting for branched-chain amino acid (BCAA) metabolism in skeletal muscle. Glucocorticoids increase net protein degradation in muscle; associated with this increased breakdown of muscle protein is an elevated rate of BCAA oxidation. The effects of glucocorticoids on skeletal muscle BCKAD were investigated in different rat models. BCKAD was activated after glucocorticoid treatment (both acutely, within 2 h, and chronically). The amount of enzyme per muscle cell increased after 5 d of cortisone acetate treatment. Insulin administration partially blocked the acute effects of glucocorticoids on muscle BCKAD. Activation was also observed during metabolic acidosis, insulinopenic diabetes mellitus, and endotoxic shock, three conditions characterized by elevated circulating glucocorticoids, increased BCAA oxidation, and increased net protein breakdown. Activation of BCKAD may account for the increased oxidation of BCAA observed during hypercortisolemia. The sequelae of this accelerated catabolism may include increased glutamine and alanine production for gluconeogenesis and provision of ATP for muscle work.
支链α-酮酸脱氢酶（BCKAD）是一种受磷酸化调节的多亚基复合物，被认为是骨骼肌中支链氨基酸（BCAA）代谢的限速酶。糖皮质激素会增加肌肉中的净蛋白质降解；与这种肌肉蛋白分解增加相关的是BCAA氧化速率的提高。在不同的大鼠模型中研究了糖皮质激素对骨骼肌BCKAD的影响。糖皮质激素治疗后（急性，2小时内，以及慢性）BCKAD被激活。醋酸可的松治疗5天后，每个肌肉细胞中的酶量增加。胰岛素给药部分阻断了糖皮质激素对肌肉BCKAD的急性作用。在代谢性酸中毒、胰岛素缺乏型糖尿病和内毒素休克期间也观察到了激活，这三种情况的特征是循环糖皮质激素升高、BCAA氧化增加和净蛋白质分解增加。BCKAD的激活可能解释了高皮质醇血症期间观察到的BCAA氧化增加。这种加速分解代谢的后遗症可能包括为糖异生增加谷氨酰胺和丙氨酸的产生，以及为肌肉工作提供ATP。
