Interactions of Retinol with Lipid Bilayers: Studies with Vesicles of Different Radii

The interactions of retinol with vesicles of dioleoylphosphatidylcholine of varying radii were studied. The rate constants of dissociation of retinol from bilayers (k(off)) and the equilibrium partition constants (Keq) of retinol into bilayers of different sized vesicles were measured. The rate constants for association of retinol with vesicles were calculated from the expression Keq = k(on)/k(off). k(off) was 10-fold faster in the smallest versus the largest vesicles tested. K(on) was also somewhat faster in vesicles with small radii, but the effect on k(off) was more pronounced, leading to an overall higher affinity for retinol of bilayers in large vesicles. The thermodynamic parameters of the dissociation reaction were studied in vesicles with 0.025, 0.1, and 0.4 microns diameter. The enthalpy of activation decreased while the entropy of activation of the dissociation of retinol from bilayers increased as the vesicles become larger. It is suggested that restructuring of lipid-lipid interactions within the bilayer play a role in determining the rate by which retinol is solvated off bilayers. Overall, the data indicate that the rates by which retinol moves between different cell types in vivo may depend on the geometry of cellular surfaces.