MacDonald M J
Childrens Diabetes Center, University of Wisconsin Medical School, Madison 53706.
Arch Biochem Biophys. 1993 Jan;300(1):201-5. doi: 10.1006/abbi.1993.1028.
Previous work demonstrated that methyl esters of succinate are potent insulin secretagogues in pancreatic islets, while unesterified succinate is not. This can be explained by studies reported here, which show that 14C-labeled dimethyl succinate is metabolized to 14CO2 by pancreatic islets, but that 14C-labeled succinic acid is not metabolized. Islets maintained at 1 mM glucose in tissue culture medium for 1 day lose the ability to release insulin in response to glucose and glucose metabolism is decreased 50-80%. The metabolism of dimethyl [1,4-14C]succinate and dimethyl [2,3-14C]succinate is decreased 50-60% in these incapacitated islets relative to islets maintained at 20 mM glucose. From the ratio of 14CO2 formed from dimethyl [1,4-14C]succinate, relative to that from dimethyl [2,3-14C]succinate, "acetate" ratios of 4.9-6.2 were calculated and from the ratio of 14CO2 formed from [2-14C]glucose, relative to that from [6-14C]glucose, "pyruvate ratios" of 1.6-1.7 were calculated. According to the 14CO2 ratios method, these ratios indicate that 53-66% of pyruvate derived from glucose enters the citric acid cycle via carboxylation and 34-47% enters via decarboxylation. Malic enzyme, which carboxylates pyruvate in the cytosol, was normal in islets maintained at 1 mM glucose. Previous work indicated that inhibition of glucose metabolism in islets maintained at low glucose is due to decreased net synthesis of the mitochondrial enzymes pyruvate dehydrogenase and pyruvate carboxylase [J. Biol. Chem. (1991) 266, 22392-22397], which decarboxylate and carboxylate pyruvate, respectively. Acetate (1 mM) but not pyruvate, when added to islets maintained at low glucose, increased dimethyl succinate metabolism to almost that of islets maintained at high glucose. This is consistent with a low amount of pyruvate dehydrogenase being unable to supply acetyl-CoA for condensation with oxalacetate (derived from succinate) and that the rate of the citric acid cycle could be enhanced by adding acetate which can bypass the reaction catalyzed by pyruvate dehydrogenase.
先前的研究表明，琥珀酸甲酯是胰岛中有效的胰岛素促分泌剂，而未酯化的琥珀酸则不是。本文报道的研究对此作出了解释，这些研究表明，胰腺胰岛可将14C标记的琥珀酸二甲酯代谢为14CO2，但14C标记的琥珀酸却不会被代谢。组织培养基中葡萄糖浓度维持在1 mM的胰岛培养1天后，失去了对葡萄糖作出反应释放胰岛素的能力，葡萄糖代谢降低了50 - 80%。相对于葡萄糖浓度维持在20 mM的胰岛，这些功能丧失的胰岛中[1,4 - 14C]琥珀酸二甲酯和[2,3 - 14C]琥珀酸二甲酯的代谢降低了50 - 60%。根据[1,4 - 14C]琥珀酸二甲酯生成的14CO2与[2,3 - 14C]琥珀酸二甲酯生成的14CO2的比例，计算出“乙酸盐”比例为4.9 - 6.2；根据[2 - 14C]葡萄糖生成的14CO2与[6 - 14C]葡萄糖生成的14CO2的比例，计算出“丙酮酸比例”为1.6 - 1.7。根据14CO2比例法，这些比例表明，源自葡萄糖的丙酮酸有53 - 66%通过羧化作用进入柠檬酸循环，34 - 47%通过脱羧作用进入。在胞质溶胶中将丙酮酸羧化的苹果酸酶，在葡萄糖浓度维持在1 mM的胰岛中是正常的。先前的研究表明，低葡萄糖浓度下胰岛中葡萄糖代谢的抑制是由于线粒体酶丙酮酸脱氢酶和丙酮酸羧化酶的净合成减少所致[《生物化学杂志》(1991年)266, 22392 - 22397]，这两种酶分别使丙酮酸脱羧和羧化。当向低葡萄糖浓度下培养的胰岛中添加乙酸盐(1 mM)而非丙酮酸时，琥珀酸二甲酯的代谢增加至几乎与高葡萄糖浓度下培养的胰岛相同。这与低水平的丙酮酸脱氢酶无法为与草酰乙酸(源自琥珀酸)缩合提供乙酰辅酶A一致，并且添加可绕过丙酮酸脱氢酶催化反应的乙酸盐可提高柠檬酸循环的速率。
