Examination of the requirement for an amphiphilic helical structure in beta-endorphin through the design, synthesis, and study of model peptides.

In our approach to beta-endorphin modeling, we have proposed that the biological properties of the natural peptide are determined by the combination of three basic structural units: a highly specific opiate recognition sequence at the NH2 terminus (residues 1-5) connected via a hydrophilic peptide link (residues 6-12) to a potential amphiphilic helix in the COOH-terminal residues 13-31. In the alpha-helical conformation the hydrophobic domain twists around the length of the helix and covers almost one-half of its surface. The other distinctive features of the helix include its basicity and the two aromatic residues Phe18 and Tyr27. In contrast to previous models we have studied, peptide 4 is a "negative" model in the sense that it was designed and examined in order to determine how the lack of a well defined amphiphilic structure affects the biological properties of beta-endorphin. For this purpose, peptide 4 retains the three structural units previously postulated for beta-endorphin, but the amino acids of the 13-31 region are arranged in such a way that no definite continuous hydrophobic zone could be formed in an alpha- or pi-helical conformation of this region. In aqueous buffered solutions, peptide 4 showed almost the same amount of alpha-helical structure as beta-endorphin, with a slight tendency toward less helicity in 50% aqueous 2,2,2-trifluoroethanol. In rat brain homogenate, peptide 4 was degraded slightly slower than beta-endorphin, in contrast to the apparently much higher stability of previous models under the same conditions. With regard to opiate receptor binding, peptide 4 was twice as potent as beta-endorphin in mu-receptor assays but half as potent in delta-receptor assays. The opiate potency of peptide 4 on the guinea pig ileum was higher than that of beta-endorphin. In contrast, in the rat vas deferens assay, which is very specific for beta-endorphin, the potency of peptide 4 was very low and could be shown not to be mediated by the same opiate mechanism or by the same opiate receptor. A comparison of these results with those of previous model peptides provides further evidence for the importance of an amphiphilic helical structure in beta-endorphin residues 13-31, which determines the resistance to proteolysis of the natural molecule and contributes to the delta- and mu-opiate receptor interaction. The amphiphilicity of this helical structure must also be essential for high opiate activity on the rat vas deferens (epsilon-receptors), whereas no such structural requirement appears to be necessary for interaction with the opiate receptors on the guinea pig ileum.