Control of Chloroplastic NADP-malate dehydrogenase activity by thioredoxins

Oxidized (inactive) chloroplastic NADP-malate dehydrogenase is 56-k Da homodimer, the two subunits of which are bound by a disulfide bridge. The enzyme is activated by three distinct chloroplast thioredoxins, thioredoxin m, f_A and f_B. When reduced by these proteins, the active enzyme is stable, whereas when reduced by dithiothreitol, activity is unstable. This result is apparently the consequence of aggregation of the reduced protein during prolonged incubation with dithiothreitol. At pH-values similar to those in the chloroplast stroma in the dark (pH 7), the reduced enzyme is still active.