The role of acidic phospholipids in the binding of monoamine oxidase to the mitochondrial structure

It has previously been shown that when pig liver mitochondria are extracted with methyl ethyl ketone in the presence of 0.05 m ammonium sulfate, approximately one-fourth of their monoamine oxidase can subsequently be extracted with buffer. To investigate the binding of the enzyme to the mitochondrial structure, the liberation of enzyme was compared with the extraction of individual phospholipids under various extraction conditions. Phosphatidylethanolamine and phosphatidylcholine could be largely extracted without liberation of monoamine oxidase, whereas there was a correlation between the yield of monoamine oxidase soluble in buffer and the extraction of anionic phospholipids, cardiolipin being the major constituent. When a dispersion of phospholipids from an extraction step effective in liberating monoamine oxidase was added to the buffer used to extract soluble enzyme, less enzyme was liberated from the lipid-depleted mitochondria. Addition of phospholipids from a noneffective extraction step had no effect. It is suggested that the binding of the enzyme to mitochondria depends on the presence of anionic phospholipids.