Glycogen synthetases and phosphorylases in rat hepatomas

Activity of glycogen metabolizing enzymes in glucose deprived HT 29 adenocarcinoma cell-line

When deprived of glucose, the cultured HT 29 adenocarcinoma cells are able to mobilize their glycogen within 4 hours. Glycogen phosphorylase is strongly activated during the first hour of glucose starvation. Then, while the a/a + b ratio for phosphorylase is declining, glycogen synthase is partially converted into the a form; this conversion does occur although glycogen phosphorylase is far from being totally inactivated. After 4 hours, activity of both a and total forms of glycogen synthase decrease. Cell UDP-glucose and glucose-6-P levels are declining during the 24 hours period of glucose starvation. Cell ATP content decreases by only 50 percent over the same period of time.

Modifications of the activities of key enzymes and intracellular levels of cyclic nucleotides, in correlation with the glyogen deposition in a cultured hepatoma cell line

Glycogen accumulation in growing cultures of ZHC cells (originally derived from the Zajdela ascitic hepatoma) is accompanied by an increase in glycogen synthetase (E.C. 2.4.1.11) and phosphorylase (E.C. 2.4.1.1) activities. Essentially the synthetase b and the phosphorylase a are involved in this process. The glycogen accumulation in ZHC cells us preceeded by a noticeable peak of cAMP, whereas cGMP rises early after replating and then decreases simultaneously with the growth rate. The present results suggest that these cultured hepatoma cells undergo throughout every passage an induction process involved in glycogen synthesis storage. Since the original ascites cells growing in vivo (which lack glycogen) and the cultured ZHC cells exhibit similar glycogen synthetase and phosphorylase activities, the resurgence of the glycogenic function (Staedel and Beck, 1978) in the in vitro cultureed cells does not seem related to a change in these two enzymes. By contrast, the high cyclic nucleotide levels in the cultured cells, as compared to those in the ascites cells, offer a possible explanation.