NADP‐Specific Isocitrate Dehydrogenase from the Simian Malaria Parasite Plasmodium knowlesi: Partial Purification and Characterization

ABSTRACT. Cell‐free schizonts of Plasmodium knowlesi, a simian malaria parasite, possess significant isocitrate dehydrogenase (IDH) activity, about 90% of which is contributed by the NADP‐specific enzyme that is localized in the cytosolic fraction. The enzyme has been partially purified by affinity chromatography using Blue sepharose CL‐6B. Although unstable in nature, it is stabilized by citrate and glycerol. Kinetic studies with dl‐isocitrate and NADP yielded hyperbolic curves with Michaelis constants of 0.210 and 0.038 mM, respectively. Manganous or magnesium ions are essential for activity. The enzyme is thermosensitive, shows maximum activity at pH 8.0, and has a molecular mass of about 48.5 kDa. It is strongly inhibited by thiolblocking agents but protected against them by thiol‐providing agents. Cupric and argentic ions also have a marked inhibitory effect on its activity. The enzyme is significantly inhibited by chloroquine and oxytetracycline in vitro, but to a lesser degree by tetracycline.