Influence of α-Tocopherol Incorporation on Ca2+-Induced Fusion of Phosphatidylserine Vesicles

The effect of α-tocopherol on the Ca2+-induced fusion of large unilamellar phosphatidylserine vesicles has been investigated. Mixing of aqueous vesicle contents was followed continually with the terbium–dipicolinic acid (Tb–DPA) assay, while the dissociation of preencapsulated Tb–DPA complex was taken as a measure of the release of vesicle contents. Vesicles consisting of pure phosphatidylserine and phosphatidylserine containing 2, 5, and 10 mol % of α-tocopherol were employed at different Ca2+concentrations. The presence of low amounts of α-tocopherol decreased the initial rate of fusion without changing the Ca2+threshold concentration. The reduction of the initial rate of fusion was proportional to the amount of α-tocopherol present in the bilayer. An α-tocopherol concentration-dependent decrease of both the initial rate and the final extent of release of vesicle contents was also observed. This effect was more evident as more α-tocopherol was incorporated in the bilayer, so that in the presence of 10 mol % of α-tocopherol no significant release was observed after 5 min. The stabilization of the vesicular structure exerted by α-tocopherol was responsible for the apparent increase of the fluorescence intensity of the Tb–DPA complex at later stages of the process. The results reflect a perturbation of the membrane by low concentrations of α-tocopherol which may account for a number of biological effects of this vitamin, not related to its antioxidant role.