Amphipathic helix and its relationship to the interaction of calcitonin with phospholipids

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Abstract
Salmon, porcine and human calcitonins interact with phosphatidylglycerol to form water-soluble complexes, but these peptides do not interact with the zwitterionic lipids phosphatidylcholine or sphingomyelin. The calcitonins are more helical in the presence of dimyristoylphosphatidylglycerol than in its absence, but human calcitonin is considerably less helical than the other 2, particularly in the presence of the lipid. This may explain the previously reported faster rate of degradation of human compared with salmon calcitonin in vivo. The ability of human calcitonin to solubilize dimyristoylphosphatidylglycerol and to alter the phase transition properties of this phospholipid while maintaining a low content of helix indicates that the presence of an amphipathic helix is not a requirement for these effects. The binding of salmon calcitonin to dimyristoylphosphatidylglycerol has been studied by determining the dependence of the circular dichroism properties of the peptide on the concentration of lipid. At 25.degree. C, salmon calcitonin binds to 5 molecules of dimyristoylphosphatidylglycerol with an affinity constant of 1 .times. 105 M-1. Little change in these parameters is observed at 38.degree. C, and the complex is stable over a wide range of temperatures both above and below the phase transition temperature. The rate of reaction of salmon calcitonin with dimyristoylphosphatidylglycerol is rapid at or above the phase transition temperature of the lipid but not at low temperatures. Salmon calcitonin also interacts with egg phosphatidylglycerol. Salmon calcitonin evidently can react with phosphatidylglycerol at or above its phase transition temperature to form complexes which are at least kinetically stable both above and below the phase transition temperature. Salmon calcitonin can solubilize mixtures of dimyristoylphosphatidylglycerol and dimyristoylphosphatidylcholine containing .gtoreq. 25% of the former phospholipid. The helical content of the peptide in the presence of these lipid mixtures is dependent on the fraction of the lipid which is phosphatidylglycerol, with larger fractions of this lipid leading to the formation of a higher helical content. At 25% phosphatidylglycerol, salmon calcitonin can solubilize the lipid mixture without much increase in the helix content of the peptide, again demonstrating that an amphipathic helical structure is not required for the solubilization of phospholipids. Ionic bonding appears to be an important component in the binding of the cationic calcitonins to phospholipids. Salmon calcitonin binds to the acidic phospholipids phosphatidylinostitol and phosphatidic acid, but not to zwitterionic phospholipids. High concentrations of NaCl cause the dissociation of the complex between salmon calcitonin and dimyristoylphosphatidylglycerol. Some specificity is exhibited by the interaction of salmon calcitonin and lipids, with only small effects on the conformation of the peptide being observed with the negatively charged sulfatide, cardiolipin, or phosphatidylserine. Calcitonin has no effect on the phase transition properties of dimyristoylphosphatidylcholine, but it greatly broadens the phase transition and lowers the transition enthalpy of dimyristoylphosphatidylglycerol. The effect of salmon calcitonin on the phase transition properties of mixtures of phosphatidylglycerol and phosphatidylcholine indicates that if the peptide induces any lateral phase separation of the lipids it is not very extensive.