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Articles

Light-Dark Regulation of Starch Metabolism in Chloroplasts

II. Effect of Chloroplastic Metabolite Levels on the Formation of ADP-Glucose by Chloroplast Extracts ¹

^a Laboratory of Chemical Biodynamics, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720

The rate of ADP-glucose formation from [¹⁴C]glucose 6-phosphate and ATP by the soluble fraction of lysed chloroplasts is studied as a function of the levels of metabolites (3-phosphoglycerate, orthophosphate, hexose monophosphate, and ATP) as determined in whole chloroplasts of Spinacia oleracea in light and dark.

A change in 3-phosphoglycerate concentration (from 4 to 1.4 millimolar, as in whole chloroplasts during light-dark transition) decreases the rate of ADP-glucose formation 6- to 7-fold. An increase in hexose monophosphate concentration from 2 to 6 millimolar, which occurs at the same time in whole chloroplasts, stimulates ADP-glucose formation only slightly.

At constant levels of orthophosphate (4 millimolar) and 3-phosphoglycerate (4 millimolar), a change in ATP concentration from 0.2 to 1 millimolar causes an immediate 4- to 5-fold increase in the rate of ADP-glucose formation. Another significant stimulation of ADP-glucose formation (about 4- to 6-fold) is obtained after addition of dithiothreitol at high concentrations (50 millimolar). A simultaneous increase in the concentrations of 3-phosphoglycerate, ATP, and dithiothreitol, with orthophosphate and Mg²⁺ being constant at 4 and 5 millimolar, respectively, causes a 130-fold increase in the rate of ADP-glucose formation (from 0.042 to 5.49 microgram atoms carbon per milligram chlorophyll per hour).

The role of these and other factors is discussed with respect to light-dark regulation of starch formation in intact chloroplasts.

¹ This work was supported in part by the Biomedical and Environmental Research Division of the United States Department of Energy.

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