Molecular Properties of Chloroplastic Thioredoxin f and the Photoregulation of the Activity of Fructose 1,6-Bisphosphatase.

Thioredoxin f has been isolated and purified to homogeneity from spinach chioroplasts. The protein is a dimer which dissociates in two apparently identical halves when ionic strength is raised. The dimer has a molecular weight of 16000. In its oxidized state, chloroplastic thioredoxin I has no sulihydry group directly available, even after denaturation of the protein. When reduced by dithiothreitol, four - SB groups become available on the dimer: Taken together, these results show that there exists one disulfide bridge per monomer. Reduction by dithiothreitol results in the breaking of two disulfide bridges and the appearance of four sulfliydryl groups. Monomeric thioredoxin f does not result in a significant activation of fructose 1 ,6-bisphosphatase. Full activation is obtained in presence of the dimeric protein. This is understandabJe since activation of fructose bisphosphatase necessitates reduction of two disulfide bridges of the enzyme. Full activation of fructose bisphosphatase is obtained in the presence of either an excess of dimeric thio- redoxin I, or in the presence of dithiothreitol. Moreover a large excess of oxidized thioredoxin deactivates fructose bisphosphatase. These results suggest that there exists an equilibrium where The and The stand for reduced and oxidized thioredoxin, oFruP2ase and rFruP2ase for oxidized and reduced fructose bisphosphatase. Moreover, this equilibrium must be strongly shifted towards the left, NADPH in presence of NADP-reductase may reduce thioredoxin, and conversely reduced thioredoxin may reduce NADP if the reductase is present. These results together with the existence of the above equilibrium provides a tentative scheme of fructose bisphosphatase inactivation in vivo in the dark. When the light has been turned off, electrons are transferred from thioredoxin to NADP via NADP-reductase. Thioredoxin becomes oxidized and then deactivates fructose bisphosphatase. [ABSTRACT FROM AUTHOR]