264 Biochimica etBiophvsicaActa, 772 (1984) 264-272 Elsevier BBA72089 EFFECTS OF LIPID STRUCTURE ON PEPTIDE-LIPID INTERACTIONS COMPLEXES OF SALMON CALCITONIN WITH PHOSPHATIDYLGLYCEROL AND WITH PHOSPHATIDIC ACID s.w. HUI a R.M. EPAND b.,, K.R. DELLb, R.F. EPAND b and R.C. ORLOWSKIc Department of Biophysics, Roswell Park Memorial Institute, Buffalo, N Y 14263 (U.S.A.) bDepartment of Biochemistry, McMaster University, Hamilton, Ontario LSN 3Z5 (Canada) and c Armour Pharmaceutical Company, Kankakee, IL 60901 (U.S.A.) (Received September18th, 1983) (Revised manuscriptreceivedFebruary 7th, 1984) Key words: Calcitonin," Phosphatidic acia~"Phosphatidylglycerol; Lipid- peptide interaction," Electron microscopy; Circular dichroism The interactions of salmon calcitonin with a number of phospholipids are studied by electron microscopy, circular dichroism and the leakage of carboxyfluorescein. At room temperature, calcitonin reacts strongly with dimyristoylphosphatidylglyceroi and egg phosphntidic acid, while only moderate or no interaction is observed with several other phospholipids. The interaction is judged by the dissolution of the pbospholipid dispersion and by electron microscopic observation and is in general concomitant with an increase in the helical content of the peptide. The electrostatic charge and the transition temperature of each of the phospholipids are important factors in determining the extent of reaction with salmon calcitonin. An exception is the sulphatide from bovine brain. The resulting morphology of the complex formed between salmon calcitonin and phosphatidic acid is quite different from that formed with phosphatidyiglycerol. In the case of phosphatidylglycerol and most other negatively charged phospholipids, disc-shaped complexes are observed under the electron microscope by negative staining. The caicitonin-DMPG complexes are about 7 nm thick and their diameter increases with an increasing lipid-to-peptide ratio. In contrast, phosphatidic acids form spherical complexes with salmon calcitonin causing large multilamellar structures to spontaneously break-up into smaller particles of about 10 to 20 nm in diameter independent of the lipid-to-peptide ratio. The contrasting effects of salmon calcitonin on the morphology of these two phosphulipids is explicable by consideration of the size of the lipid headgroup. Phosphatidic acid can accommodate the peptide without rupture of the bilayer, while the larger headgroup of phosphatidylglyceroi requires the bHayer to rupture. This model is supported by studies of calcitonin-induced leakage of carboxyfluorescein from sonicated vesicles of 75% egg phosphatidylcholine and 25% either egg phosphatidic acid, egg phosphatidyiglycerol or di- myristoylphosphatidylglyceroL There was a much greater increase in carboxyfluorescein leakage from phosphatidylglycerol-containingvesicles induced by salmon calcitonin demonstrating the greater ability of the peptide to rupture bilayers containing this phospholipid. * To whomrequests for reprints shouldbe addressed. Abbreviations: DMPG, dimyristoylphosphatidylglycerol; DMPA, dimyristoylphosphatidic acid; PC, egg phosphatidyl- choline; PG, phosphatidylglycerol from PC; PA, phosphatidic acid from PC; PS, phosphatidylserine from bovine brain; PI, phosphatidylinositol from yeast. Pipes, 1,4-piperazinediethane- sulfonicacid. Introduction Calcitonin is one of a number of peptide hormones which contain resulady spaced hydro- phobic amino acid residues at every third or fourth position along the chain [1]. Such sequences can 0005-2736/84/$03.00 © 1984 Elsevier SciencePublishersB.V.