Changes in vesicle morphology induced by lateral phase separation modulate phospholipase A(2) activity

The action of phospholipase A(2) (PLA(2)) toward zwitterionic bilayers is modulated by lateral phase separation of reaction products and substrate. The experiments here address the mechanism of this modulation. PLA(2) is particularly active toward lipid dispersions containing reaction products and substrates in which lateral phase separation has occurred. Here, we study PLA(2) activity in two related model systems: first in a system in which lateral phase separation can be produced a priori, and second in a system in which the action of PLA(2) produces sufficient reaction product in situ such that lateral phase separation occurs. The dispersions in which lateral phase separation occurs a priori form either disk micelles or disk vesicles, not canonical vesicles. When lateral phase separation occurs due to in situ PLA(2) activity, there is an abrupt change in vesicle structure and a simultaneous profound increase in catalytic rate. This observation is surprising in light of several reports that vesicles remain intact even when the entire outer monolayer has been hydrolyzed. Membrane curvature and the associated structural defects and dynamic fluctuations in membrane structure have been proposed to modulate PLA(2) activity. The mechanism by which lateral phase separation modulates PLA(2) activity has been unclear. The data presented here indicate that lateral phase separation affects PLA(2) activity by altering membrane curvature and/or inducing defects in the membrane structure.