Biological and Physical Properties of a @ - Endorphin Analog Containing Only D - Amino Acids in the Amphiphilic Helical Segment

Our approach to the modeling of B-endorphin has been based on the proposal that three basic structural units can be distinguished in the natural peptide hormone: a highly specific opiate recognition sequence at the N terminus (residues 1-5) connected via a hydrophilic link (residues 6-12) to a potential amphiphilic helix in the C-terminal residues 13-31. Our previous studies showed the validity of this approach and have demonstrated the importance of the amphiphilic helical structure in the C terminus of &endorphin. The present model, peptide 5, has been designed in order to evaluate further the requirements of the amphiphilic secondary structure as well as to determine the importance of this basic structural element as compared to more specific structural features which might occur in the C-terminal segment. For these reasons, peptide 5 retains the three structural units previously postulated for ,%endorphin; the major difference with regard to previous models is that the whole C-terminal segment, residues 13-31, has been built using only D-amino acids. In aqueous buffered solutions as well as in 2,2,2trifluoroethanol-containing solutions, the CD spectra of peptide 5 show the presence of a considerable amount of left-handed helical structure. Enzymatic degradation studies employing rat brain homogenate indicate that peptide 5 is stable in this milieu. In 6and b-opiate receptor-binding assays, peptide 5 shows a slightly higher affinity than &endorphin for both receptors while retaining the same 6 / p selectivity. In opiate assays on the guinea pig ileum, the potency of peptide 5 is twice that of &endorphin. In the rat vas deferens assay, which is very specific for &endorphin, peptide 5 displays mixed agonist-antagonist activity. Most remarkably, peptide 5 displays a potent opiate analgesic effect when injected intracerebroventricularly into mice. At equal doses, the analgesic effect of peptide 5 is less than that of &endorphin (10-15%) but longer lasting. In conjunction with our previous model studies, these results clearly demonstrate that the amphiphilic helical structure in the C terminus of &endorphin is of predominant importance with regard to activity in rat vas deferens and analgesic assays. The similarity between the in vitro and in vivo opiate activities of &endorphin and peptide 5, when compared