Gaussin V, Gailly P, Gillis J M, Hue L
Hormone and Metabolic Research Unit, International Institute of Cellular and Molecular Pathology, ICP-UCL 7529, Avenue Hippocrate, 75, B-1200 Brussels, Belgium.
Biochem J. 1997 Sep 15;326 ( Pt 3)(Pt 3):823-7. doi: 10.1042/bj3260823.
In rat hepatocytes subjected to a fructose load, ATP content decreased from 3.8 to 2.6 micromol/g of cells. Under these conditions, the intracellular free Mg2+ ion concentration,as measured with mag-fura 2, increased from 0.25 to 0.43 micromol/g of cells and 0.35 micromol of Mg2+ ions were released per g of cells in the extracellular medium. Therefore the increase in the intracellular free Mg2+ ion concentration was less than expected from the decrease in ATP, indicating that approx. 80% of the Mg2+ ions released from MgATP2- were buffered inside the cells. When this buffer capacity was challenged with an extra Mg2+ ion load by blocking the fructose-induced Mg2+ efflux, again approx. 80% of the extra Mg2+ ion load was buffered. The remaining 20% appearing as free Mg2+ions in fructose-treated hepatocytes could act as second messenger for enzymes having a Km for Mg2+ in the millimolar range. Fructose activated glycogen synthase and glycogen phosphorylase, although both the time course and the dose-dependence of activation were different. This was reflected in a stimulation of glycogen synthesis with concentrations of fructose below 5 mM. Indeed, activation of glycogen synthase reached a maximum at 30 min of incubation and was observed with small (5 mM or less) concentrations of fructose, whereas the activation of glycogen phosphorylase was almost immediate (within 5 min) and maximal with large doses of fructose. The fructose-induced activation of glycogen phosphorylase, but not that of glycogen synthase, could be related to an increase in free Mg2+ ion concentration.
在接受果糖负荷的大鼠肝细胞中，ATP含量从3.8微摩尔/克细胞降至2.6微摩尔/克细胞。在这些条件下，用mag-fura 2测量的细胞内游离Mg2+离子浓度从0.25微摩尔/克细胞增加到0.43微摩尔/克细胞，并且每克细胞在细胞外培养基中有0.35微摩尔的Mg2+离子释放。因此，细胞内游离Mg2+离子浓度的增加低于ATP减少所预期的值，表明从MgATP2-释放的Mg2+离子中约80%在细胞内被缓冲。当通过阻断果糖诱导的Mg2+外流用额外的Mg2+离子负荷挑战这种缓冲能力时，同样约80%的额外Mg2+离子负荷被缓冲。在果糖处理的肝细胞中作为游离Mg2+离子出现的其余20%可以作为对Mg2+的Km在毫摩尔范围内的酶的第二信使。果糖激活了糖原合酶和糖原磷酸化酶，尽管激活的时间进程和剂量依赖性都不同。这反映在果糖浓度低于5 mM时对糖原合成的刺激上。事实上，糖原合酶的激活在孵育30分钟时达到最大值，并且在低浓度（5 mM或更低）的果糖中观察到，而糖原磷酸化酶的激活几乎是即时的（在5分钟内）并且在大剂量果糖时最大。果糖诱导的糖原磷酸化酶的激活，而不是糖原合酶的激活，可能与游离Mg2+离子浓度的增加有关。
