Mass action kinetics of phosphatidylserine vesicle fusion monitored by coalescence of internal vesicle volumes

Abstract

The kinetics of Ca2+-induced fusion of sonicated phosphatidylserine vesicles is analyzed by means of the mass action model. The results of calculations simulate the experimental results for the mixing of aqueous vesicle volumes, release of vesicle contents and for the observed increase in light scattering. The calculations give the distribution of vesicle sizes during the initial stages of the fusion process and an estimate for the occurrence of multiple fusion events. During the first few seconds from the beginning of the fusion process in the above systems apparently only a small fraction of the material trapped will leak during each fusion event. The fraction of material which leaks per fusion event is further reduced with increased Ca2+ concentrations. The values of the rates of fusion which describe the above experiments suggest that the rate limiting step of the overall fusion reaction is the aggregation and close approach of vesicles to each other rather than the fusion event per se.