Activation of the potato tuber ADP-glucose pyrophosphorylase by thioredoxin

The potato tuber (Solanum tuberosum 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) J, Biol, Chem, 273, 25045-25052) that the enzyme was activated by reduction of the Cys(12) 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 mu M) of activator (3-phosphoglycerate), Fifty percent activation was at 4.5 and 8.7 mu M for reduced thioredoxin f and m, respectively, and 2 orders of magnitude lower than for dithiothreitol, The activation was reversed by oxidized thioredoxin. Cys(12) 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.