The effect of ionic environment on pig heart mitochondrial malate dehydrogenase.

1. The effect of ionic environment on pig heart mitochondrial malate dehydrogenase was investigated by means of two low resolution conformational probes, thermal stability and proteolytic liability. 2. High ionic strength and high enzyme concentrations stabilize the enzyme towards thermal inactivation, probably by maintaining the enzyme in the dimeric form. 3. Proteolysis of malate dehydrogenase under conditions where the enzyme is thermally stable indicates a secondary ionic effect on structure. 4. Attempts to demonstrate dissociation of native malate dehydrogenase dimer into its constituent subunits using gel filtration on Sephacryl S-200 proved inconclusive. At low ionic strength malate dehydrogenase does not exhibit normal gel filtration behaviour.