GLYCOGEN-SYNTHESIS IN AMPHIBIAN OOCYTES - EVIDENCE FOR AN INDIRECT PATHWAY

Glycogen is the main end product of glucose metabolism in amphibian oo cytes. However, in the first few minutes after [U-C-14]glucose microin jection most of the label is found in lactate. The burst of lactate pr oduction and the shape of the time curves for the labelling of glucose 6-phosphate, fructose 6-phosphate, glucose l-phosphate and glycogen s uggest a precursor-product relationship of lactate with respect to gly cogen and its intermediates. Expansion (by microinjection) of the pool of glycolytic intermediates, such as dihydroxyacetone phosphate, glyc eraldehyde 3-phosphate, 3-phosphoglycerate or phosphoenolpyruvate, res ults in a marked decrease in [U-C-14]glucose incorporation into glycog en. After co-injection of doubly labelled glucoses, extensive detritia tion (93%) of the glycosyl units of glycogen was observed with [2-H-3, U-C-14]glucose and partial detritiation with [3-H-3,U-C-14]glucose (34 %) or [5-H-3,U-C-14]glucose (46%). After injection of [6-H-3,U-C-14]gl ucose, a small but significant and reproducible detritiation (13%) in glycogen was observed. Co-injection of [U-C-14]glucose and 3-mercaptop icolinate resulted in marked inhibition of glycogen labelling. Half-ma ximal inhibition was observed at 0.58 mM 3-mercaptopicolinate, which a grees with the IC50 value (0.47 mM) for the inhibition in vitro of pho sphoenolpyruvate carboxykinase activity. We conclude that in frog oocy tes most of the glucosyl units are incorporated into glycogen by an in direct pathway involving breakdown of glucose to lactate, which is the n converted into glycogen via gluconeogenesis. Both processes, glycoly tic degradation of glucose to lactate and subsequent reconversion of t he latter into hexose phosphates and glycogen, occur in the same cell.