Effect of phospholipid bilayer phase asymmetry on phospholipase D reaction-induced vesicle rupture

Spherical phospholipid bilayers, vesicles, were formed with respect to phase of each layer via a double emulsion technique. At the outer layer of the vesicles, phospholipase D catalyzed for the conversion of phosphatidylcholine (PC) to phosphatidic acid (PA). The reaction caused by phospholipase D (PLD) induced a curvature change in the vesicles, which eventually led them to rupture. Response time from the PLD injection to the rupture was monitored for the phase of each layer by using fluorescence intensity changes of pH-sensitive dye encapsulated in the vesicles. It was found that low ionic strength and asymmetric phase retarded response time. The retardation seems to be related to the stability of the vesicles, which is due to the interaction between the lipid molecules. In the liquid phases of the outer lipid layers, the unexpected slow response time may be attributed either to the fast lateral diffusion, which relieves the curvature change of the vesicles, or to the low concentration of PCs, which are less for the reaction compared to the solid phase of the outer lipid layer, rather than the stability.