ABSTRACT
Fructose-2,6-bisphosphate was discovered 1 during the study of the action of glucagon on the fructose 6-phosphate/fructose-1,6-bisphosphate cycle. In the liver, glucagon inhibits glycolysis and stimulates gluconeogenesis. This hormone has been shown to change the kinetic properties of phosphofructokinase resulting in a decreased affinity for fructose 6-phosphate and an increased inhibition by ATP and citrate without a change in Vmax, 2 – 5 These modifications suggested that, like pyruvate kinase, phosphofructokinase could be regulated by covalent modification of the protein. Glucagon, through the second messenger cyclic AMP, activates cyclic AMP-dependent protein kinase which, in turn, catalyzes the phosphorylation of various proteins. Phosphofructokinase could conceivably be one of these proteins. However, subsequent studies showed that the difference in phosphofructokinase activity could also be obtained by gel filtration of an extract from control hepatocytes. 6 Moreover, the effect could be abolished by adding a low-molecular-weight fraction obtained from control cells, but not from glucagon-treated cells to the purified preparation. 6 This stimulatory factor was purified and its properties indicated that it was a nonnucleotidic-diphosphoric ester, which was extremely acid labile. Hydrolysis of the partially purified stimulator yielded equimolar amounts of fructose 6-phosphate, Pi, and a reducing group suggesting that phosphate was bound to the second carbon of fructose 6-phosphate as in fructose-2,6-bisphosphate. 1
