Calcium-induced aggregation of phosphatidylcholine vesicles containing free oleic acid

Small unilamellar vesicles (SUV) formed by egg yolk phosphatidylcholine (PC) and free oleic acid (OA) undergo aggregation induced by Ca2+ at pH greater than 7.0. The rate of the process, as monitored by turbidity changes, presents a linear dependence on phospholipid concentration and a hyperbolic dependence on Ca2+ concentration. The aggregation curves show a lag period which is tentatively attributed to an activation step induced by Ca2+. The incorporation of either cholesterol, alpha-tocopherol or egg yolk phosphatidylethanolamine (PE) produced a decrease in the aggregation rate and an increase in the lag period. Fusion was not detected in any of the different experiments, either by the assay of mixing the membrane phospholipids or by the assay of mixing the aqueous contents. A possible mechanism, explaining the aggregation process is proposed.