NADP(-ISOCITRATE DEHYDROGENASE IN GERMINATING CUCUMBER COTYLEDONS - PURIFICATION AND CHARACTERIZATION OF A CYTOSOLIC ISOENZYME())

The activity of NADP(+)-dependent isocitrate dehydrogenase (ICDH, EC 1 .1.1.42) was investigated during the post-germinative growth of cucumb er (Cucumis sativus L. cv. Marketmore) seedlings. Isoelectric focusing showed the presence of several isoenzymes, two of which represented 7 0-80% of the total NADP(+)-ICDH activity in cotyledons of seedlings gr own in the dark. They had pi values between 4.8 and 5.8. The isoenzyme with higher pi was purified to homogeneity by hydrophobic interaction , affinity, hydroxylapatite and anion exchange chromatography The puri fied isoenzyme is a dimeric protein, consisting of two apparently iden tical 43-kDa subunits. It is specific for NADP(+), inhibited by ATP an d by 2-oxoglutarate, whereas it is not inhibited by citrate, succinate , and glyoxylate. The data indicate that NADP(+)-ICDH from cucumber is structurally similar to ICDHs from other plants, but it shows some pe culiar biochemical characteristics.