Identification of the regulatory steps in gluconeogenesis in cotyledons of Cucurbita pepo.

1. The aim of this work was to discover the steps at which the conversion of oxaloacetate to glucose 6-phosphate during gluconeogenesis is regulated in the cotyledons of 5-day-old seedlings of Cucurbita pepo. 2. We estimated the maximum catalytic activities of all the enzymes in the above sequence and also the amounts of their substrates present in vivo. The results show that the reactions catalysed by fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase are the only ones in the sequence that are substantially displaced from equilibrium in vivo. 3. We also determined the effects of 3-mercaptopicolinic acid, an inhibitor of gluconeogenesis, on the amounts of the gluconeogenic intermediates present in vivo. The results show that the enzyme system, fructose-1,6-bisphosphatase: phosphofructokinase, and the system phosphoenolpyruvate carboxykinase: phosphoenolpyruvate carboxylase make major contributions to the regulation of gluconeogenesis in the cotyledons. 4. Possible mechanisms for the above regulation are discussed.