Induction of Pyrophosphate-Dependent Phosphofructokinase in Watermelon (Citrullus lanatus) Cotyledons Coincides with Insufficient Cytosolic D-Fructose-1,6-Bisphosphate 1-Phosphohydrolase to Sustain Gluconeogenesis

During germination of Citrullus lanatus, pyrophosphate-dependent phosphofructokinase (PFP) activity is induced. The peak of PFP activity coincides with the maximum gluconeogenic flux and high fructose-2,6-bisphosphate (Fru-2,6-P2) concentrations. Determination of cytosolic fructose-1,6 bisphosphatase (FBPase) activity in crude extracts is unreliable because of the high PFP activity. The FBPase activity, after correction for the contaminating PFP, is only one-third of the PFP activity. Purified cytosolic FBPase is inhibited by Fru-2,6-P2. The low cytosolic FBPase activity and high Fru-2,6-P2 most probably result in inadequate in vivo activity to catalyze the observed gluconeogenic flux. The total PFP activity is sufficient to catalyze the required carbon flux.