THE Jo”nNaL OF BIOLOGIC*L CHEMISTEY

The dissociations of porcine heart mitochondrial, bovine heart mitochondrial, and porcine heart cytoplasmic malate dehydrogenase dimers (I,-malate: NAI)’ oxidoreductase, EC 1.1.1.37) have been examined by Sephadex G-100 gel filtration chromatography and sedimentation velocity ultracentrifugation. The porcine mitochondrial enzyme was found to chromatoyraph as subunits when applied to a gel filtration column at a concentration of 0.2 PM or less at pH 7.0. The presence of coenzymes shifted the dissociation equilibrium at low enzyme concentrations in favor of dimer formation. Monomer formation was also favored when porcine mitochondrial enzyme was incubated at pH .5.0 even at concentrations as high as 120 pM. This shift in equilibrium has been correlated with the increased rate and specificity of sulfhydryl residue modification with N-ethylmaleimide at pH 5.0 (Gregory, E. M., Yost, F. J., .Jr., Kohrbach, M. S., and Harrison, J. H. (19’71) .I. Bid. Chem. 246, 5491-5497). Bovine mitochondrial enzyme did not exhibit a concentration-dependent dissociation under the conditions examined. However, at pH j.0 monomer formation was favored, and correlations could again be drawn with sulfhydryl residue modification (Gregory, JC. M. (1975) J. Zliol. Chem. 250, .5470-5474). In both mitochondrial enzymes, coenzyme binding was found capable of overcoming the effects of pH on the dissociation equilibrium, and dimer formation was favored. Unlike either of the above mentioned enzymes, porcine cytoplasmic malate dehydrogenase did not dissociate into its monomeric form under any conditions investigated.