Effect of lipids on enzymatic activity of pig heart mitochondrial malate dehydrogenase monomolecular films.

The effect of various lipids on the enzymatic activity of pig heart mitochondrial malate dehydrogenase monomolecular films was studied using the subphase exchange technique described previously. Surface pressure-surface area (pi-A) curves of mixed films of enzyme with dipalmitoyllecithin, egg lecithin, cholesterol, and phospholipids extracted from pig hearts showed that the enzyme interacted with all of the lipids and that the enzyme remained in the film at pressures well above the collapse pressure of malate dehydrogenase in the absence of lipid. The surface enzyme activity was dependent on surface pressure for each lipid; in all cases, the lipids greatly broadened the range of surface pressures where surface enzyme activity was observed. The pi-A and enzyme activity data showed good correlation. Although the simple model system employed does not simulate the complexity of the biological membrane, it gives some evidence for the role of lipids in the stability of membrane-bound enzymes.