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 degrees 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 x 10(-6)m while for the sorghum enzyme it was 3.7 x 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 x 10(-5)m (barley) to 1.9 x 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.