Chloroplast Sedoheptulose 1,7-Bisphosphatase: Evidence for Regulation by the Ferredoxin/Thioredoxin System

Sed-P2ase, Ferredoxin, Thioredoxin, Enzyme Regulation 1. A substrate-specific sedoheptulose-l,7-bisphosphatase has been found in chloroplasts and separated from its fructose-1,6-bisphosphatase counterpart. Experiments with antibodies indicate that the two enzymes are structurally different. 2. Activity of the sedoheptulose-l,7-bisphosphatase enzyme was dependent on M gi+ and a reductant. The most effective reductant tested was thioredoxin that was reduced either photochemically via ferredoxin with chloroplasts or chemically with dithiothreitol. Dithiothreitol added alone also activated the enzyme, but reduced glutathione or 2-mercaptoethanol did not. The thioredoxinactivated enzyme was deactivated by oxidized glutathione. 3. The results suggest that the new substrate-specific sedoheptulose-l,7-bisphosphatase depends on light for activity and resembles certain other regulatory enzymes of the reductive pentose phos­ phate cycle in its mode of regulation.