The interaction of insulin with phospholipids.

1. A simple two-phase chloroform-aqueous buffer system was used to investigate the interaction of insulin with phospholipids and other amphipathic substances. 2. The distribution of (125)I-labelled insulin in this system was determined after incubation at 37 degrees C. Phosphatidic acid, dicetylphosphoric acid and, to a lesser extent, phosphatidylcholine and cetyltrimethylammonium bromide solubilized (125)I-labelled insulin in the chloroform phase, indicating the formation of chloroform-soluble insulin-phospholipid or insulin-amphipath complexes. Phosphatidylethanolamine, sphingomyelin, cholesterol, stearylamine and Triton X-100 were without effect. 3. Formation of insulin-phospholipid complex was confirmed by paper chromatography. 4. The two-phase system was adapted to act as a simple functional system with which to investigate possible effects of insulin on the structural and functional properties of phospholipid micelles in chloroform, by using the distribution of [(14)C]glucose between the two phases as a monitor of phospholipid-insulin interactions. The ability of phospholipids to solubilize [(14)C]glucose in chloroform increased in the order phosphatidylcholine<sphingomyelin<phosphatidylethanolamine<phosphatidic acid. Insulin decreased the [(14)C]glucose solubilized by phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid, but not by sphingomyelin. 5. The significance of these results and the molecular requirements for the formation of insulin-phospholipid complexes in chloroform are discussed.