Glucose 6-phosphate plays a central role in the regulation of glycogen synthesis in a glycogen-storing liver cell line

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Abstract
Two substrains of the epithelial liver cell line C1 I, one storing large amounts of glycogen, the other one being very poor in glycogen were used as a model for studying glycogen synthesis. The glycogen content of glycogen‐rich cells doubled during the proliferative phase and remained high in plateau phase although glycogen synthase I activity was not significantly altered during growth cycle and was too low to account for the increase in glycogen. However, the activity of the glucose 6‐phosphate (Glc6‐P)‐dependent synthase rose continuously during growth cycle, and intracellular Glc6‐P‐concentration increased about 10‐fold in log phase cells to 0·72 μmol g−1 wet weight. A0·5 of synthase for Glc6‐P was 0·79 mM. It was also found that in contrast to the enzyme from normal liver, glycogen phosphorylase a from C1 I cells was inhibited by Glc6‐P, the apparent Ki being 0·45 mM. It was concluded that glycogen accumulation in C1I cells was due to stimulation of synthase and inhibition of phosphorylase by Glc6‐P. Findings from the glycogen‐poor cell line which revealed similar specific activities of synthase and phosphorylase but only low Glc6‐P (0·056 μmol g−1 wet weight) supported this conclusion. Addition of glucose to starved cells resulted in a transient activation of synthase in both cell lines. Net glycogen synthesis, was, however, only observed in the cells with a high Glc6‐P‐content. Thus, modulation of synthase and phosphorylase by Glc6‐P and not activation/inactivation of the enzymes seems to play a predominant role in glycogen accumulation in this cell line.

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