Activation of the Potato Tuber ADP-glucose Pyrophosphorylase by Thioredoxin

The potato tuber (<i>Solanum tuberosum</i> L.) ADP-glucose 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-phosphoglycerate to inhibitor, P_i. It was reported (Fu, Y., Ballicora, M. A., Leykam, J. F., and Preiss, J. (1998) <i>J. Biol. Chem. </i> 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 <i>f</i> and <i>m</i> from spinach (<i>Spinacia oleracea</i>) leaves reduced and activated the enzyme at low concentrations (10 µM) of activator (3-phosphoglycerate). Fifty percent activation was at 4.5 and 8.7 µM for reduced thioredoxin <i>f</i> and <i>m</i>, respectively, and 2 orders of magnitude lower than for dithiothreitol. The activation was reversed by oxidized thioredoxin. Cys¹² is conserved in the ADP-GlcPPases from plant leaves and other tissues except for the monocot endosperm enzymes. We postulate that in photosynthetic tissues, reduction 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.