NADP‐Linked Isocitrate Dehydrogenase from Beef Liver

NADP‐dependent isocitrate dehydrogenase has been 500‐fold purified from beef liver; the protein is homogeneous in the ultracentrifuge as well as in electrophoresis. From beef liver sub‐cellular fractionation, the enzyme is found to be cytoplasmic. Some physical parameters of the enzyme have been determined. The enzyme consists of two subunits having the same molecular weight of 48000 ± 5000. From equilibrium sedimentation studies, it is shown that the dimer dissociates at low concentration, and that the dimeric form is induced by the substrate metal 2+ isocitrate complex; the reassociation is a low phenomenon, which results in a lag time preceding the establishment of the catalytic activity at the steady state; this lag decreases when the enzyme is preincubated with the substrate. A study of the number of –SH groups shows that 2‐SH among 5 in the protomer, are implicated in the catalytic activity and protected by metal 2+ ‐isocitrate against the attack of the classical sulfhydryl reagents.