Regulation of Starch Biosynthesis in Plant Leaves: Activation and Inhibition of ADPglucose Pyrophosphorylase

The ADPglucose pyrophosphorylases of 7 plant-leaf tissues were partially purified and characterized. In all cases the enzymes showed stability to heat treatment at 65° for 5 minutes in the presence of 0.02 m phosphate buffer, pH 7.0. The leaf ADPglucose pyrophosphorylases were activated 5 to 15-fold by 3-phosphoglycerate. Fructose-6-phosphate and fructose 1, 6-diphosphate stimulated ADPglucose pyrophosphorylase to lesser extents. The A 0.5 (conc of activator required to give 50% of the observed maximal activation) of 3-phosphoglycerate for the barley enzyme was 7 × 10 −6 m while for the sorghum enzyme it was 3.7 × 10 −4 m. Inorganic phosphate proved to be an effective inhibitor of ADPglucose synthesis. The I 0.5 (conc of inhibitor that gave 50% inhibition of activity for the various leaf enzymes varied from 2 × 10 −5 m (barley) to 1.9 × 10 −4 m (sorghum). This inhibition was reversed or antagonized by the activator 3-phosphoglycerate. These results form the basis for an hypothesis of the regulation of leaf starch biosynthesis.