Regulation of NADP-malate dehydrogenase in C4 plants: effect of varying NADPH to NADP ratios and thioredoxin redox state on enzyme activity in reconstituted systems.

Activation and inactivation of NADP-malate dehydrogenase purified from Zea mays leaves were followed in a reconstituted system provided with thioredoxin poised in various redox states with dithiothreitol. The initial rate of activation or inactivation of NADP-malate dehydrogenase was proportional to the concentration of reduced or oxidized thioredoxin, respectively. The rate of inactivation was about 16 times that for activation at pH 7.4. Both activities increased when the pH was increased from 7.4 to 8.0. The redox potentials (E'0, pH 7) for the dithiol-disulfide systems of thioredoxin and NADP-malate dehydrogenase were estimated to be about -0.30 and -0.33 V, respectively. As would be predicted from these values, high proportions of active malate dehydrogenase were developed only in the presence of very high ratios of reduced to oxidized thioredoxin. Similarly, when pyridine nucleotide was included, a high degree of activation of malate dehydrogenase was only observed with high NADPH/NADP ratios. These results confirm predictions based on models developed in earlier studies that the NADPH to NADP ratio as well as the thioredoxin redox state may be critical in determining the level of NADPH-malate dehydrogenase activity in vivo.