Interaction of glucagon with sphingomyelins

In the presence of either egg or bovine brain sphingomyelin, the spectral properties of glucagon undergo changes which are similar to those which occur in the presence of synthetic phosphatidylcholines. The fluorescence emission spectra are blue shifted about 10 nm in the presence of lipid and the peptide acquires an increased helical content, determined by circular dichroism. As with phosphatidylcholines, the changes in spectral properties do not occur above the phase transition temperature of the glucagon-lipid mixture. Freeze-fracture electron microscopy indicates that glucagon forms an ellipsoidal complex with bovine brain sphingomyelin, similar to the glucagon-dimyristoylphosphatidylcholine complex. However, the sphingomyelin complexes break down to vesicular structures both above and below the region of the phase transition. These results indicate that the dissociation of glucagon from the lipid at higher temperatures results from changes in the phase of the lipid rather than from a thermal denaturation of glucagon. The effect of glucagon on the phase transition behaviour of palmitoyl sphingosine phosphorylcholine was measured by differential scanning calorimetry. The major effect of glucagon on both this lipid and on dimyristoylphosphatidylcholine is to broaden the phase transition and to shift it to higher temperatures. Similar results are obtained for the effects of glucagon on an equimolar mixture of dimyristoylphosphatidylcholine and palmitoyl sphingosine phosphorylcholine. Glucagon is able to solubilize mixtures of bovine brain sphingomyelin with either dimyristoylphosphatidylcholine or egg lecithin. The lipid composition of the solubilized material is similar to that of the starting lipid film. These results together with those from the differential scanning calorimetry on the synthetic mixtures indicate that glucagon can bind to sphingomyelin-phosphatidylcholine mixtures and that it does not induce extensive lateral phase separation between the components. The maximal stability of the glucagon-lipid complex at the phase transition of the lipids indicates that the glucagon-lipid interaction is highly dependent on the structural organization of the lipid.