[Binding and entrapment of insulin by lecithin-phosphatidic acid liposomes in acid solutions]

Microvesicles made of mixtures of lecithin and phosphatidic acid were precipitated by insulin in acid solution. Insulin was bound by the precipitates up to saturation, that was governed by the portion of phosphatidic acid in the liposomal matrix. The amount of bound insulin was described by a binding isotherm for a bivalent ligand. In neutral solutions the precipitates were repeptised thereby releasing bound insulin. An increase of the particle size, observed by gel filtration analysis, was explained by liposome fusion. If the liposomes were generated in acid solutions of insulin by sonication, up to 50% of the insulin remained entrapped even in the neutralized solution. Especially with cholesterol as a lipid component the entrapped insulin was highly protected from the proteolytic attack by pronase.