Pilkis S J, Riou J P, Claus T H
J Biol Chem. 1976 Dec 25;251(24):7841-52.
The hormonal control of [14C]glucose synthesis from [U-14C-A1dihydroxyacetone was studied in hepatocytes from fed and starved rats. In cells from fed rats, glucagon lowered the concentration of substrate giving half-half-maximal rates of incorporation while it had little or no effect on the maximal rate. Inhibitors of gluconeogenesis from pyruvate had no effect on the ability of the hormone to stimulate the synthesis of [14C]glucose from dihydroxyacetone. The concentrations of glucagon and epinephrine giving half-maximal stimulation from dihydroxacetone were 0.3 to 0.4 mM and 0.3 to 0.5 muM, respectively. The meaximal catecholamine stimulation was much less than the maximal stimulation by glucagon and was mediated largely by the alpha receptor. Insulin had no effect on the basal rate of [14C]clucose synthesis but inhibited the effect of submaximal concentration of glucagon or of any concentration of catecholamine. Glucagon had no effect on the uptake of dihydroxyacetone but suppressed its conversion to lactate and pyruvate. This suppression accounted for most of the increase in glucose synthesis. In cells from gasted rats, where lactate production is greatly reduced and the rate of glucose synthesis is elevated, glucagon did not stimulate gluconeogenesis from dihydroxyacetone. Findings with glycerol as substrate were similar to those with dihyroxyacetone. Ethanol also stimulated glucose production from dihydroxyacetone while reducing proportionately the production of lactate. Ethanol is known to generate reducing equivalents fro clyceraldehyde-3-phosphate dehydrogenase and presumably thereby inhibits carbon flux to lactate at this site. Its effect was additive with that of glucagon. Estimates of the steady state levels of intermediary metabolites and flux rates suggested that glucagon activated conversion of fructose diphosphate to fructose 6-phosphate and suppressed conversion of phosphoenolpyruvate to pyruvate. More direct evidence for an inhibition of pyruvate kinase was the observation that brief exposure of cells to glucagon caused up to 70% inhibition of the enzyme activity in homogenates of these cells. The inhibition was not seen when the enzyme was assayed with 20 muM fructose diphosphate. The effect of glucagon to lower fructose diphosphate levels in intact cells may promote the inhibition of pyruvate kinase. The inhibition of pyruvate kinase may reduce recycling in the pathway of gluconeogenesis from major physiological substrates and probably accounts fromsome but not all the stimulatory effect of glucagon.
研究了喂食和饥饿大鼠肝细胞中[14C]葡萄糖从[U-14C]二羟基丙酮合成的激素调控。在喂食大鼠的细胞中，胰高血糖素降低了使掺入速率达到半最大速率时的底物浓度，而对最大速率几乎没有影响。丙酮酸糖异生抑制剂对该激素刺激二羟基丙酮合成[14C]葡萄糖的能力没有影响。使二羟基丙酮产生半最大刺激的胰高血糖素和肾上腺素浓度分别为0.3至0.4 mM和0.3至0.5 μM。儿茶酚胺的最大刺激远小于胰高血糖素的最大刺激，且主要由α受体介导。胰岛素对[14C]葡萄糖合成的基础速率没有影响，但抑制了亚最大浓度胰高血糖素或任何浓度儿茶酚胺的作用。胰高血糖素对二羟基丙酮的摄取没有影响，但抑制其转化为乳酸和丙酮酸。这种抑制作用占葡萄糖合成增加的大部分。在饥饿大鼠的细胞中，乳酸产生大大减少而葡萄糖合成速率升高，胰高血糖素不刺激二羟基丙酮的糖异生。以甘油为底物的结果与二羟基丙酮相似。乙醇也刺激二羟基丙酮产生葡萄糖，同时相应减少乳酸的产生。已知乙醇从甘油醛-3-磷酸脱氢酶产生还原当量，大概由此抑制该位点的碳流向乳酸。其作用与胰高血糖素的作用相加。中间代谢产物稳态水平和通量率的估计表明，胰高血糖素激活果糖二磷酸向果糖6-磷酸的转化，并抑制磷酸烯醇丙酮酸向丙酮酸的转化。丙酮酸激酶受抑制的更直接证据是观察到细胞短暂暴露于胰高血糖素会导致这些细胞匀浆中该酶活性高达70%的抑制。当用20 μM果糖二磷酸测定该酶时未观察到抑制作用。胰高血糖素降低完整细胞中果糖二磷酸水平的作用可能促进丙酮酸激酶的抑制。丙酮酸激酶的抑制可能减少主要生理底物糖异生途径中的循环，可能是胰高血糖素部分但不是全部刺激作用的原因。
