PURIFICATION AND PROPERTIES OF NAD-DEPENDENT MALATE-DEHYDROGENASE FROM MESEMBRYANTHEMUM-CRYSTALLINUM L EXHIBITING CRASSULACEAN ACID METABOLISM

By DEAD-cellulose anion exchange column chromatography, three differen t forms of NAD-dependent malate dehydrogenase (EC 1.1.1.37) (NAD-MDH) were isolated from leaves of Mesembryanthemum crystallinum L. in the c rassulacean acid metabolism mode. NAD-MDH that eluted Last from an ani on exchange column was purified to a specific activity of 1,096 units . (mg protein)(-1) using Blue Sepharose CL-6B chromatography. Citrate and adenosine 5'-triphosphate effectively inhibited the activity of NA D-MDH. The inhibition of the enzymatic activity by citrate was reverse d by inorganic phosphate and the degree of reversal increased with inc reasing the concentration of oxaloacetate, the substrate of the reacti on. The optimal pH for NAD-MDH activity was around 7.5. Citrate inhibi ted the enzymatic activity over a wide range of pH and caused a shift in the optimal pH. The enzymatic activity in the presence of citrate w as increased by adding inorganic phosphate below pH 7.5. On the basic side, higher than pH 7.5, however, the inhibition by citrate was enhan ced by adding inorganic phosphate.