Gluconeogenesis in the castor bean endosperm: I. Changes in glycolytic intermediates

Abstract

The control points of the Embden-Meyerhof-Parnas pathway in germinating castor bean (Ricinus communis) endosperms are sought in two ways: (a) by measuring the amounts of various glycolytic intermediates at intervals during the germination; (b) by determining the crossover points appearing during anoxia.A significant departure from thermodynamic equilibrium between substrates and products is found at the level of fructose 1,6-diphosphatase and phosphofructokinase. A definite shift of this ratio is observed at the onset of active gluconeogenesis. The concentrations of phosphoenolpyruvate and 3-phosphoglyceric acid increase at the same time. Another departure from the expected equilibrium is also observed at the level of the pyruvate kinase.The imposition of anoxia on 5-day-old endosperms reveals two crossover points, at the level of the same enzymes. It is therefore concluded that they regulate the glycolytic flow.The maximal glycolytic flow, however, is only 1/10 of the gluconeogenic one. To account for this high gluconeogenic efficiency, it is postulated that gluconeogenesis and glycolysis occur in separate intracellular regions. The consistent departure from equilibrium between adenylates observed during the early stages of anoxia supports the concepts that the pools of glycolytic and gluconeogenic intermediates are indeed compartmented and that the two processes are independently regulated.