Lembert N, Idahl L A
Department of Histology and Cell Biology, Umeå University, Sweden.
Diabetes. 1998 Mar;47(3):339-44. doi: 10.2337/diabetes.47.3.339.
The ability of alpha-ketoisocaproate (KIC) to induce ATP production in isolated mitochondria from pancreatic beta-cells was examined with a bioluminometric method. There was no ATP production from KIC when tested alone or in combination with malate (1 mmol/l), nor did DL-beta-hydroxybutyrate induce mitochondrial ATP production, whereas palmitoyl-carnitine and pyruvate were efficient stimulators of mitochondrial ATP production in the presence of an equimolar concentration of malate. However, KIC stimulated the mitochondrial ATP production when tested in combination with glutamate (10 mmol/l). The concentration necessary to obtain half-maximal stimulation was approximately 50 micromol/l KIC, and maximal activity, comparable to that obtained with fatty acids, was reached at 1 mmol/l KIC. Higher KIC concentrations inhibited the mitochondrial ATP production, whereas a plateau was attained at 1 mmol/l KIC in the presence of glutamine. Ca2+ stimulated the maximal mitochondrial ATP production induced by KIC. Maximal stimulation was obtained with 300 nmol/l Ca2+ in the presence of 0.3 mmol/l KIC. Ca2+ reduced the concentration of KIC necessary for half-maximal stimulation to <30 micromol/l. Leucine stimulated the mitochondrial ATP production in the presence of glutamate to the same extent as KIC. Half-maximal stimulation was observed with 2 mmol/l leucine. There were no additive effects on mitochondrial ATP production when KIC and leucine were tested in combination. The results demonstrate that KIC by itself is not a mitochondrial substrate for ATP production. KIC must transaminate with glutamate or glutamine to yield alpha-ketoglutarate and leucine. Since leucine allosterically activates glutamate dehydrogenase, which also produces alpha-ketoglutarate, the insulinogenic effect of KIC may in part be due to the intramitochondrial generation of alpha-ketoglutarate. Since KIC-induced ATP production reaches a plateau already at micromolar concentrations (i.e., far below the concentrations at which KIC induces insulin release), it is proposed here that the catabolism of KIC may induce additional signals related to insulin release.
采用生物发光法检测了α-酮异己酸（KIC）在胰腺β细胞分离线粒体中诱导ATP生成的能力。单独检测KIC或其与苹果酸（1 mmol/L）联合检测时均未产生ATP，DL-β-羟基丁酸也未诱导线粒体ATP生成，而在等摩尔浓度苹果酸存在时，棕榈酰肉碱和丙酮酸是线粒体ATP生成的有效刺激剂。然而，当与谷氨酸（10 mmol/L）联合检测时，KIC刺激了线粒体ATP生成。获得半数最大刺激所需的浓度约为50 μmol/L KIC，在1 mmol/L KIC时达到最大活性，与脂肪酸刺激的活性相当。更高浓度的KIC抑制线粒体ATP生成，而在谷氨酰胺存在时，1 mmol/L KIC时达到平台期。Ca²⁺刺激了KIC诱导的最大线粒体ATP生成。在0.3 mmol/L KIC存在时，300 nmol/L Ca²⁺可获得最大刺激。Ca²⁺将半数最大刺激所需的KIC浓度降低至<30 μmol/L。亮氨酸在谷氨酸存在时刺激线粒体ATP生成的程度与KIC相同。2 mmol/L亮氨酸可观察到半数最大刺激。联合检测KIC和亮氨酸时，对线粒体ATP生成没有相加作用。结果表明，KIC本身不是ATP生成的线粒体底物。KIC必须与谷氨酸或谷氨酰胺进行转氨作用才能产生α-酮戊二酸和亮氨酸。由于亮氨酸可别构激活谷氨酸脱氢酶，后者也产生α-酮戊二酸，KIC的促胰岛素作用可能部分归因于线粒体内α-酮戊二酸的生成。由于KIC诱导的ATP生成在微摩尔浓度时就已达到平台期（即远低于KIC诱导胰岛素释放的浓度），本文提出KIC的分解代谢可能诱导与胰岛素释放相关的其他信号。
