Paul H S, Adibi S A
Am J Physiol. 1978 May;234(5):E494-9. doi: 10.1152/ajpendo.1978.234.5.E494.
The effect of L-carnitine (0.5-2.0 mM) on the rates of alpha-decarboxylation of 1-14C-labeled branched-chain amino acids by gastrocnemius muscle and liver homogenates of fed rats was investigated. Carnitine increased the rate of alpha-decarboxylation of leucine (125%) and valine (28%) by muscle, but it was without effect on the oxidation of these amino acids by liver. Carnitine increased the rate of alpha-decarboxylation of alpha-ketoisocaproate by both tissues. This effect was more pronounced in muscle (130% increase) than in liver (41% increase). The activity of carnitine acyltransferase, with isovaleryl-CoA as a substrate, was 18 times higher in muscle mitochondria than in liver mitochondria. Both starvation and diabetes increased the rate of alpha-decarboxylation of leucine by muscle without having a remarkable effect on the concentration of carnitine or the activity of carnitine acyltransferase. We conclude that: a) carnitine stimulates decarboxylation of branched-chain amino acids by increasing the conversion of their ketoanalogues into carnitine esters, b) a greater carnitine acyltransferase activity in muscle than in liver may be responsible for the greater carnitine effect in muscle, c) carnitine does not appear responsible for the enhancement of leucine oxidation by muscle of starved and diabetic rats.
研究了左旋肉碱（0.5 - 2.0 mM）对喂食大鼠腓肠肌和肝脏匀浆中1-14C标记的支链氨基酸α-脱羧速率的影响。肉碱增加了肌肉中亮氨酸（125%）和缬氨酸（28%）的α-脱羧速率，但对肝脏中这些氨基酸的氧化没有影响。肉碱增加了两种组织中α-酮异己酸的α-脱羧速率。这种作用在肌肉中（增加130%）比在肝脏中（增加41%）更明显。以异戊酰辅酶A为底物时，肉碱酰基转移酶在肌肉线粒体中的活性比在肝脏线粒体中高18倍。饥饿和糖尿病都增加了肌肉中亮氨酸的α-脱羧速率，而对肉碱浓度或肉碱酰基转移酶活性没有显著影响。我们得出以下结论：a）肉碱通过增加支链氨基酸酮类似物向肉碱酯的转化来刺激其脱羧作用；b）肌肉中肉碱酰基转移酶活性高于肝脏可能是肉碱在肌肉中作用更强的原因；c）肉碱似乎不是饥饿和糖尿病大鼠肌肉中亮氨酸氧化增强的原因。
