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The activity of NADP+‐dependent isocitrate dehydrogenase (ICDH, EC 1.1.1.42) was investigated during the post‐germinative growth of cucumber (Cucumis sativus L. cv. Marketmore) seedlings. Isoelectric focusing showed the presence of several isoenzymes, two of which represented 70–80% of the total NADP+‐ICDH activity in cotyledons of seedlings grown 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 purified isoenzyme is a dimeric protein, consisting of two apparently identical 43‐kDa subunits. It is specific for NADP+, inhibited by ATP and 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 peculiar biochemical characteristics.

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