Light-Dark Regulation of Starch Metabolism in Chloroplasts

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Abstract
The rate of ADP-glucose formation from [14C]G-6-P 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 4-1.4 mM, 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 2-6 mM, which occurs at the same time in whole chloroplasts, stimulates ADP-glucose formation only slightly. At constant levels of orthophosphate (4 mM) and 3-phosphoglycerate (4 mM), a change in ATP concentrarom from 0.2-1 mM 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 mM). A simultaneous increase in the concentration of 3-phosphoglycerate, ATP and dithiothreitol, with orthophosphate and Mg2+ being constant at 4 and 5 mM, respectively, causes a 130-fold increase in the rate of ADP-glucose formation (from 0.042-5.49 .mu.g atoms C/mg chlorophyll per h). The role of these and other factors is discussed with respect to light-dark regulation of starch formation in intact chloroplasts.