PARTITIONING OF VASOACTIVE INTESTINAL POLYPEPTIDE INTO LIPID BILAYERS

Incubation of radiolabeled vasoactive intestinal polypeptide (VIP) wit h preformed lipid vesicles composed of phosphatidylcholine, phosphatid ylglycerol and cholesterol resulted in reversible and saturable associ ation of the peptide with the lipid bilayer. The pH-optimum for the re action was in the physiological range. The vesicle-associated peptide displayed enhanced stability to proteolytic digestion, it was efficien tly released into the supernatant by detergent-solubilization of the v esicles but remained vesicle-associated during treatment with agents t hat disrupt ionic interactions. Peptide binding by electrically neutra l vesicles was lower than that by negative vesicles. Electron spin res onance studies with 5-doxylstearic acid or 16-doxylstearic acid labele d vesicles suggested that VIP decreased the fluidity close to the surf ace of the bilayer and increased the fluidity in its hydrophobic core. These observations suggest that VIP can bind and penetrate lipid bila yers.