Regulation of NADP-malate dehydrogenase in C4 plants: relationship among enzyme activity, NADPH to NADP ratios, and thioredoxin redox states in intact maize mesophyll chloroplasts.

NADP-malate dehydrogenase activity, the ratio of NADPH to NADP, and thioredoxin redox state in Zea mays chloroplasts were determined after various treatments. Following transfer from dark to light, NADP-malate dehydrogenase was activated more than 20-fold within 10 min while the proportion of pyridine nucleotide as NADPH increased from about 25 to 90%, and the proportion of thioredoxin in the reduced form increased from 20 to more than 90%, in less than 1 min. After transfer back to the dark, NADPH levels dropped very rapidly to the initial values recorded before illumination, while enzyme activity and reduced thioredoxin levels decreased more slowly. Addition of oxaloacetate or 3-phosphoglycerate to illuminated chloroplasts results in a decrease of about 70% in the activity of NADP-malate dehydrogenase, a 30% decrease in the level of NADPH, and a 25% decrease in the reduced thioredoxin content. Adding dihydroxyacetone phosphate and pyruvate had no effect. These results are considered in relation to the hypothesis that NADP-malate dehydrogenase activity in chloroplasts may be determined by factors regulating the ratio of NADPH to NADP as well as those influencing the redox state of thioredoxin.