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The structure-function relationship of mitochondrial malate dehydrogenase was investigated at low enzyme concentration, as well as in the presence of polyethylene glycol (PEG 6000) and structure making ions. Previous reports claimed the dimeric enzyme to undergo dissociation in dilute solution, and PEG-induced pairing of dimers in the crystalline state. Sedimentation analysis and gel filtration in 0.1 M sodium phosphate pH 7.6 plus 1 mM EDTA and 1 mM dithioerythritol prove the enzyme to be a stable dimer at c greater than or equal to 0.2 microgram/ml (5 nM). In the presence of 8-20% (w/v) PEG 6000, association of the dimer to tetramers and higher aggregates is observed. At 20% (w/v) polyethylene glycol, ultracentrifugal analysis yields up to 50% tetramers; chemical cross-linking by glutaraldehyde confirms the association in a qualitative way. The enzymatic properties of mMDH (specific activity, Km for oxaloacetate and NADH) in the absence and in the presence of PEG 6000 are indistinguishable. At high polyethylene glycol concentrations (greater than or equal to 20%), the thermal stability of the enzyme is found to be increased. The fluorescence emission, as well as the far-UV and near-UV circular dichroism remain unaffected. Accumulated evidence from equilibrium experiments at low enzyme concentration and reconstitution kinetics (after dissociation at acid pH) proves the active species of mMDH to be the dimer.

Structure-Function Relationship of Mitochondrial Malate Dehydrogenase af High Dilution and in Multicomponent Systems