Regulation of Ribulose Bisphosphate Carboxylase Activity in Vivo by a Light-modulated Inhibitor of Catalysis

The activity of ribulose 1,5-bisphosphate carboxylase [RuBPCase; 3-phospho-D-glycerate carboxylyase (dimerizing), EC 4.1.1.39] in leaf extracts of a number of species kept in the dark overnight was found to be very low. This was not the result of a change in the activation state or in the amount of enzyme that could be extracted from "dark" leaves. Rather, in Phaseolus vulgaris it was due to an inhibitor of catalysis that occupied the catalytic site of the enzyme. This inhibitor was compartmentalized in the chloroplast and its maximum concentration in both dark leaves and in intact chloroplasts made from such leaves was slightly in excess of the RuBPCase catalytic site concentration. The inhibitor (a phosphate ester) was bound preferentially to the activated form of the enzyme, apparently functioning as a positive effector of activation. Treatment of the enzyme-inhibitor complex in vitro with alkaline phosphatase could restore RuBPCase activity. In vivo, both the initial rate of disappearance and the final concentration of inhibitor in intact leaves was found to vary with light intensity, and these changes could account for observed light-dependent changes in RuBPCase activity, indicating that light modulation of inhibitor concentration controlled RuBPCase activity. Recovery of activity in vivo could be inhibited by 3-(3',4',4-dichlorophenyl)-1,1-dimethylurea.