An evaluation of phospholipids as regulators of monoamine oxidase A and monoamine oxidase B activities.

When rat liver mitochondria are treated with the phospholipase A2 isolated from Naja naja venom in amounts sufficient to cause about 70% loss of mitochondrial phospholipids, both of the monoamine oxidase isozymes monoamine oxidase A and monoamine oxidase B are reduced in activity. With both monoamine oxidases, phospholipase treatment affects the enzyme’s affinity for the amine substrates rather than the maximum velocity, but monoamine oxidase A retains sensitivity to the irreversible monoamine oxidase A inhibitor, clorgyline, while monoamine oxidase B is still resistant. Reconstitution and analysis of the monoamine oxidases in the presence of high concentrations of phospholipids suggested that enzymatic activity could be restored by phosphatidylinositol, phosphatidylserine, or phosphatidylcholine but not by phosphatidylenthanolamine. On the other hand, these same phospholipids also could activate the monoamine oxidase in mitochondria that had not been treated with the phospholipase. The restoration of activity by the phospholipids was abolished if the excess phospholipids which were not incorporated into the mitochondria were removed. Our data indicate that, while phospholipase A2 treatment of mitochondria inactivates both monoamine oxidase A and monoamine oxidase B, there is no evidence that reconstitution of membrane phospholipids will selectively or collectively activate monoamine oxidase A or monoamine oxidase B. However, high concentrations of unreconstituted phospholipids in the incubation medium will apparently stimulate both monoamine oxidase A and monoamine oxidase B activities.