Regulatory role of fructose-2,6-bisP on glucose metabolism in frog oocytes: in vivo inhibition of glycogen synthesis.

Glycogen synthesis following glucose microinjection in frog oocytes proceeds preferentially by an indirect pathway involving gluconeogenesis from triose compounds. Because of the known regulatory role of fructose-2,6-bisP on glucose utilization in most vertebrate tissues we coinjected [U-14C]glucose and fructose-2,6-bisP into oocytes and observed a marked inhibition of label incorporation into glycogen, with an I50 value of 2 microM, which is similar to the value measured for the in vitro inhibition of oocyte fructose-1,6-bisphosphatase. Other hexoses-bisP were tested: 2,5-anhydromannitol-1,6-bisP was as effective as inhibitor as fructose-2,6-bisP; glucose-1,6-bisP showed some effect although 50% inhibition was obtained at a concentration 10 times higher than with fructose-2,6-bisP; fructose-1,6-bisP had no effect at all. The inhibition pattern for the in vivo glycogen synthesis by these analogs closely matched the one obtained with partially purified oocyte fructose-1,6-bisphosphatase. The intracellular concentration of fructose-2,6-bisP in unperturbed oocytes was found to be between 0.1 and 0.2 microM. Fructose-6-phosphate,2-kinase levels measured in oocyte homogenates were between 0.02 and 0.06 mU per gram of ovary. After 60 min incubation, fructose-2,6-bisP microinjected into the oocytes was almost completely degraded, suggesting that fructose-2,6-bisphosphatase is active in vivo. The results presented in this paper indicate that fructose-2,6-bisP plays an important role in the in vivo regulation of glucose utilization in frog-grown oocytes.