Lipid transfer between vesicles: Effect of high vesicle concentration

The problem of the desorption of a lipid molecule from a lipid vesicle (don or) and its incorporation into another vesicle (acceptor) at high acceptor concentrations, which has been investigated experimentally (Jones, J. D. an d Thompson, T. E., 1990. Biochemistry, 29:1593-1600), is analyzed here from a theoretical point of view, formulated in terms of the diffusion equation with appropriate boundary conditions. The goal is to determine whether or not the observed acceleration of the off-rate from a donor is caused by int eraction with an acceptor vesicle at short range, or is simply the result o f statistical effects due the proximity of the acceptor and its influence o n the probability of the test lipid returning to the donor. We establish a correspondence between the theoretical parameters and the experimental, the rmodynamic and dynamic variables entering the problem. The solution shows t hat, because of the extremely high Gibbs activation energy for desorption o f a phospholipid, the process would always be first-order, even at very hig h vesicle concentrations. This means that acceleration of the off-rate must be due to donor-acceptor interactions at short distances, as proposed in t he experimental work.