Physical evidence for the dimerization of the triphosphopyridine-specific isocitrate dehydrogenase from pig heart

The TPN-linked isocitrate dehydrogenase from pig heart has been purified to homogeneity using blue Sepharose CL-6B as the only column chromatographic step and subsequently crystallized. Molecular sieve chromatography and analytical ultracentrifugation showed that the enzyme dimerizes in response to magnesium DL-isocitrate. TPNH or EDTA also brought about this effect, whereas TPN+ did not. The molecular weight of the monomer was 60,000 +/- 2,500 with an s20,w of 4.8 +/- 0.1 and a Stokes radius of 29 A. The molecular weight of the dimer was 115,000 +/- 5,000 with an s20,w of 6.7 +/- 0.2 and a Stokes radius of 39 A. Active enzyme centrifugation showed that the dimer was the active species in the oxidative decarboxylation of isocitrate.