In our approach to β-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 α-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 β-endorphin. For this purpose, peptide 4 retains the three structural units previously postulated for β-endorphin, but the amino acids of the 13-31 region are arranged in such a way that no definite continuous hydrophobe zone could be formed in an α- or π-helical conformation of this region. In aqueous buffered solutions, peptide 4 showed almost the same amount of α-helical structure as β-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 β-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 β-endorphin in μ-receptor assays but half as potent in δ-receptor assays. The opiate potency of peptide 4 on the guinea pig ileum was higher than that of β-endorphin. In contrast, in the rat vas deferens assay, which is very specific for β-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 β-endorphin residues 13-31, which determines the resistance to proteolysis of the natural molecule and contributes to the δ- and μ-opiate receptor interaction. The amphiphilicity of this helical structure must also be essential for high opiate activity on the rat vas deferens (ε-receptors), whereas no such structural requirement appears to be necessary for interaction with the opiate receptors on the guinea pig ileum.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Biological Chemistry|
|State||Published - 1983|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology