Aftring R P, Manos P N, Buse M G
Metabolism. 1985 Aug;34(8):702-11. doi: 10.1016/0026-0495(85)90018-6.
In vitro catabolism of branched-chain amino acids, leucine and valine, was investigated using diaphragm muscles from normal, streptozotocin-diabetic and overnight fasted rats. Oxidation and transamination of [1-14C] branched-chain amino acids were both stimulated to a similar extent by diabetes or fasting, when diaphragms were incubated with glucose. Transamination of leucine and valine was increased when diaphragms were incubated with pyruvate; stimulation of transamination was greatest in diaphragms from diabetic rats. Leucine and valine oxidation by control diaphragms was inhibited by pyruvate while it was unchanged or slightly stimulated in diaphragms from fasted or diabetic rats. Thus diaphragms from diabetic rats oxidized two to threefold more branched-chain amino acids than controls when they were incubated with pyruvate. The specific radioactivity of extracellular alpha-ketoisocaproate (KIC; the product of leucine transamination) produced by diaphragms incubated with [14C]leucine was similar for all groups (fed, fasted, or diabetic) in the presence or absence of pyruvate. Oxidation of [1-14C]KIC by diaphragms from fasted or diabetic rats, incubated with glucose, was the same or less than KIC oxidation by control diaphragms. Incubation with pyruvate inhibited KIC oxidation by control diaphragms to a significantly greater degree than that by diaphragms from diabetic or fasted rats. These data suggest the following Flux through branched-chain amino acid transaminase is limited by the availability of amino group acceptors in diaphragms from normal and overnight fasted rats, and to a greater extent in diaphragms from diabetic rats. Flux through the transaminase may be a major determinant of accelerated branched-chain amino acid oxidation by diaphragms in fasting and diabetes. In diaphragms of fasted and diabetic rats, flux through the branched-chain alpha-ketoacid dehydrogenase complex is resistant to inhibition by pyruvate, which is normally observed in controls.
利用正常大鼠、链脲佐菌素诱导的糖尿病大鼠以及禁食过夜大鼠的膈肌，研究了支链氨基酸（亮氨酸和缬氨酸）的体外分解代谢。当膈肌与葡萄糖一起孵育时，糖尿病或禁食会使[1-¹⁴C]支链氨基酸的氧化和转氨作用受到相似程度的刺激。当膈肌与丙酮酸一起孵育时，亮氨酸和缬氨酸的转氨作用增强；糖尿病大鼠膈肌中转氨作用的刺激最为显著。丙酮酸抑制对照膈肌中亮氨酸和缬氨酸的氧化，而在禁食或糖尿病大鼠的膈肌中，这种氧化作用不变或略有增强。因此，当与丙酮酸一起孵育时，糖尿病大鼠的膈肌氧化支链氨基酸的量比对照膈肌多两到三倍。在有或没有丙酮酸存在的情况下，所有组（喂食、禁食或糖尿病）的膈肌在与[¹⁴C]亮氨酸孵育时产生的细胞外α-酮异己酸（KIC；亮氨酸转氨作用的产物）的比放射性相似。禁食或糖尿病大鼠的膈肌在与葡萄糖孵育时对[1-¹⁴C]KIC的氧化与对照膈肌的KIC氧化相同或更低。与丙酮酸一起孵育时，对照膈肌中KIC氧化受到的抑制程度明显大于糖尿病或禁食大鼠的膈肌。这些数据表明：在正常和禁食过夜大鼠的膈肌中，支链氨基酸转氨酶的通量受氨基受体可用性的限制，在糖尿病大鼠的膈肌中这种限制程度更大。转氨酶的通量可能是禁食和糖尿病状态下膈肌中支链氨基酸氧化加速的主要决定因素。在禁食和糖尿病大鼠的膈肌中，支链α-酮酸脱氢酶复合体的通量对丙酮酸的抑制具有抗性，而在对照中通常可观察到这种抑制作用。
