Niculescu L, Van Schaftingen E
Laboratoire de Chimie Physiologique, ICP and Université Catholique de Louvain, Brussels, Belgium.
Diabetologia. 1998 Aug;41(8):947-54. doi: 10.1007/s001250051012.
When tested in the presence of an inhibitor of sorbitol dehydrogenase, both mannitol and sorbitol caused a progressive inhibition of the detritiation of [2-3H]glucose in isolated rat hepatocytes. The purpose of the present work was to investigate the possibility that this effect was mediated by the regulatory protein of glucokinase. When added to hepatocytes, mannitol decreased the apparent affinity of glucokinase for glucose and increased the concentration of fructose required to stimulate detritiation, without affecting the concentration of fructose 1-phosphate. Its effect could be attributed to the formation of mannitol 1-phosphate, a potent agonist of the regulatory protein, which, similarly to fructose 6-phosphate, reinforces its inhibitory action. Formation of mannitol 1-phosphate in hepatocytes was dependent on the presence of mannitol and was stimulated by compounds that increase the concentration of glucose 6-phosphate. Liver extracts catalysed the conversion of mannitol to mannitol 1-phosphate about 7 times more rapidly in the presence of glucose 6-phosphate than of ATP. The glucose 6-phosphate-dependent formation was entirely accounted for by a microsomal enzyme, glucose-6-phosphatase and was not due to a loss of latency of this enzyme. In hepatocytes in primary culture, mannitol decreased the detritiation rate and counteracted the effect of fructose to stimulate glucokinase translocation. Taken together, these results strongly support a central role played by the regulatory protein in the control of glucokinase activity and translocation in the liver, as well as a feedback control exerted by fructose 6-phosphate on this enzyme.
在山梨醇脱氢酶抑制剂存在的情况下进行测试时，甘露醇和山梨醇均会导致分离的大鼠肝细胞中[2-³H]葡萄糖的去氚化作用逐渐受到抑制。本研究的目的是探讨这种效应是否由葡萄糖激酶的调节蛋白介导。当添加到肝细胞中时，甘露醇降低了葡萄糖激酶对葡萄糖的表观亲和力，并增加了刺激去氚化所需的果糖浓度，而不影响1-磷酸果糖的浓度。其作用可归因于1-磷酸甘露醇的形成，1-磷酸甘露醇是调节蛋白的强效激动剂，与6-磷酸果糖类似，可增强其抑制作用。肝细胞中1-磷酸甘露醇的形成依赖于甘露醇的存在，并受到增加6-磷酸葡萄糖浓度的化合物的刺激。在6-磷酸葡萄糖存在的情况下，肝脏提取物催化甘露醇转化为1-磷酸甘露醇的速度比在ATP存在的情况下快约7倍。6-磷酸葡萄糖依赖性形成完全由微粒体酶葡萄糖-6-磷酸酶引起，而不是由于该酶潜伏性的丧失。在原代培养的肝细胞中，甘露醇降低了去氚化速率，并抵消了果糖刺激葡萄糖激酶易位的作用。综上所述，这些结果有力地支持了调节蛋白在肝脏中控制葡萄糖激酶活性和易位方面所起的核心作用，以及6-磷酸果糖对该酶施加的反馈控制。
