Interactions of insulin with sulfatide-containing vesicles of phosphatidylcholine at different pHs

Positively charged insulin is described to induce aggregation of phosphatidylcholine vesicles containing 10 mol% sulfatide at acidic pH. Techniques including light-scattering, Sepharose chromatography, centrifugation, trapped volume determination, circular dichroism and fluorescence polarization, demonstrate that large amounts of negatively charged insulin remain firmly associated to the vesicles upon raising the pH to 7. This is surprising, since only trace amounts of insulin associate to the sulfatide-containing vesicles upon direct incubation at pH 7. The possible molecular explanation of the phenomenon and the relevance of these findings to the actions of insulin in vivo are discussed.