Kinetic parameters of the interactions of retinol with lipid bilayers

The process of transfer of vitamin A alcohol (retinol) between unilamellar vesicles of phosphatidylcholine was studied. The transfer was found to proceed spontaneously by hydration from the bilayer and diffusion through the aqueous phase. The rate-limiting step for transfer was the dissociation from the bilayer, a step that was characterized in bilayers of egg phosphatidylcholine (PC) by a rate constant koff = 0.64 s-1. The rate constant for association of retinol with bilayers of egg PC was also determined: kon = 2.9 x 10(6) s-1. The relative avidities for retinol of vesicles comprised of PC lipids with the various fatty acyl chains were measured. It was found that the binding affinity was determined by the composition of the lipids, such that PC with symmetric acyl chains had a lower affinity for retinol vs those with mixed chains. To clarify the mechanism underlying this observation, the rates of dissociation and association of retinol bound to vesicles of dioleoyl-PC were determined. The rate of association of retinol with bilayers strongly depended on the composition of the fatty acyl chains of the lipids. The rate of dissociation of retinol from the bilayers of PC was found to be independent of that composition. The implications of the observations for the interactions of hydrophobic ligands with lipid bilayers are discussed.