Substrate cycling between glucose 6-phosphate and glycogen occurs in Schistosoma mansoni

The regulation of glycogen metabolism in Schistosoma mansoni was studied in vitro with special emphasis on the possible occurrence of substrate ('futile') cycling. The partition of label between carbon atoms 1 and 6 of the glucose units in glycogen was analysed after the incubation of intact worm pairs in the presence of [6-14C]glucose. Under all conditions tested, more than 99% of the label in glycogen was still in the 6 position, demonstrating that glycogen was synthesised not via an indirect pathway involving 3-carbon units, but directly, from glucose. Increasing the glucose concentration stimulated glycogen synthase and decreased the activity of glycogen phosphorylase. An inverse relationship was shown between the actual glycogen content and the rate of glycogenesis. Substrate cycling occurred between glucose 6-phosphate and glycogen. Glucose was incorporated into glycogen during periods of net glycogen breakdown, and vice versa: glycogen degradation occurred during periods of net glycogen synthesis. Under our experimental conditions of net glycogen degradation, the rate of glycogen synthesis as a percentage of that of glycogen breakdown was dependent on the external glucose concentration and ranged from 5 to 68% for 2 to 100 mM glucose, respectively. The synthesis of glycogen during periods of net glycogen breakdown was shown to occur in each individual worm pair.