NADP-malate dehydrogenase, a L;ght-modulated enzyme of C4 photosynthesis, was prHifed to homogeneity from leaves of corn The pure enzyme was activated by thloredoxin m that was reduced either photochemicafly (with ferredoxin and ferredoxin-thioredoxin reductase) or chemicafly (with dithiothreitol). Unactivated corn leaf NADP-malate dehydrogenase had a molecular weight of 50,000 to 60,000 and was chromophorefree. The enzyme appeared to have a high content of serine and glycine and to contain both S-S and SH groups. Consequently, NADP-malate dehydrogenase seems to be capable of undergoing reversible oxidation/ reduction during its photoreguladon.NADP-MDH5 is a light-modulated chloroplast enzyme that is widely distributed in green plants (1,9,13,21). NADP-MDH is particularly abundant in C4 plants where it functions photosynthetically in the NADP-dependent reduction of oxaloacetate to malate. Prior to effecting this catalytic reaction, NADP-MDH requires an activation that can be achieved in vivo by light (13) or in vitro by DTT added in the presence of a soluble protein factor (11,14,22,23) that has been identified as a specific chloroplast thioredoxin (thioredoxin m) (12,19,24,25). NADP-MDH can also be activated photochemically in vitro by thioredoxin m reduced via ferredoxin and ferredoxin-thioredoxin reductase (19,24,25) or by a soluble protein factor that appears to be independent of the ferredoxin/thioredoxin system (2).Although NADP-MDH has been partially purified from leaves of several different C3 and C4 plants (13,19,(22)(23)(24), a procedure for obtaining homogeneous preparations has not been reported and consequently the enzyme has not been characterized. We have therefore focused our attention on NADP-MDH and now describe a purification procedure that yields homogeneous preparations of the enzyme from corn leaves. Certain properties of