Re-evaluation of molecular weight of pig heart NAD-specific isocitrate dehydrogenase

NAD-specific pig heart isocitrate dehydrogenase was earlier reported, on the basis of gel filtration experiments, to have a molecular weight of approximately 340,000. In the present study, the enzyme is shown by equilibrium ultracentrifugation to have a weight average molecular weight of approximately 224,000 which can be attributed to a rapidly associating-dissociating protein system. The results of light-scattering measurements are consistent with the lower value of molecular weight. The enzyme exhibits an average frictional ratio, f/f0, of 1.39 as determined from ultracentrifuge experiments, and this deviation from typical proteins may account for the previous high molecular weight estimates. An average Stokes radius of 6.0 nm was calculated from the present gel filtration experiments. By use of this value and a sedimentation coefficient of 9.1 S, an average molecular weight of 245,000 has been calculated. Previous studies (Ramachandran, N., and Colman, R. F. (1980) J. Biol. Chem. 255, 8859-8864) have indicated that the enzyme is composed of three different subunits, present in the ratio 2:1:1, each of which has a molecular weight of about 40,000. These results, together with the present observations, lead to the conclusion that, under stabilizing conditions in solution, the NAD-dependent isocitrate dehydrogenase predominantly exhibits a minimum, molecular weight of 160,000 but behaves as a mixture of oligomeric species with an average Mr of about 224,000.