2000. Activation of potato tuber ADP-glucose pyrophosphorylase by thioredoxin

cose pyrophosphorylase (ADP-GlcPPase) catalyzes the first committed step in starch biosynthesis. The main type of regulation of this enzyme is allosteric, and its activity is controlled by the ratio of activator, 3-phos-phoglycerate to inhibitor, Pi. It was reported (Fu, Y., Ballicora, M. A., Leykam, J. F., and Preiss, J. (1998) J. Biol. Chem. 273, 25045–25052) that the enzyme was activated by reduction of the Cys12 disulfide linkage present in the catalytic subunits. In this study, both reduced thioredoxin f and m from spinach (Spinacia oleracea) leaves reduced and activated the enzyme at low concentrations (10 mM) of activator (3-phosphoglyc-erate). Fifty percent activation was at 4.5 and 8.7 mM for reduced thioredoxin f and m, respectively, and 2 orders of magnitude lower than for dithiothreitol. The activa-tion was reversed by oxidized thioredoxin. Cys12 is con-served in the ADP-GlcPPases from plant leaves and other tissues except for the monocot endosperm en-zymes. We postulate that in photosynthetic tissues, re-duction could play a role in the fine regulation of the ADP-GlcPPase mediated by the ferredoxin-thioredoxin system. This is the first time that a covalent mechanism of regulation is postulated in the synthesis of starch. ADP-glucose pyrophosphorylase (ADP-GlcPPase)1 (ATP:a-D-glucose-1-phosphate adenylyltransferase, EC 2.7.7.27) cata-lyzes the first committed step toward the synthesis of starch in plants and bacterial glycogen (Reaction 1). a-Glc-1-P 1 ATP ¢O