Dermorphin interaction with rat brain opioid receptors: involvement of hydrophobic sites in the binding domain

Dermorphin structural analogues were utilized to determine the nature of opioid receptor subsite specificity, affinity, and selectivity in rat brain membranes. The data suggest that these parameters are influenced by the amino acid composition and sequence and the known solution conformation of dermorphin, in addition to the conformation of the membrane receptor. Two hydrophobic components of dermorphin are required for optimal binding. One component encompasses the stacked phenol groups in Tyr1 and Tyr5; the second involves the phenyl group of Phe3. Evidence to support this proposal includes the following results: (a) removal of aromaticity, as occurs in [des-Tyr5]- and [Gly5]dermorphin, drastically reduced binding to both mu and delta sites; (b) inversion of the Phe3-Gly4 sequence in dermorphin to the Gly3-Phe4 in enkephalin enhanced binding to delta receptor sites, yet the peptide remained mu-selective; (c) substitution of Pro4 for Gly4 disrupted the solution conformation of dermorphin and decreased affinities at both receptor subsites, substantiating the requirement for the Phe3-Gly4-Tyr5 sequence in dermorphin to interact with mu sites; and (d) modification of the serine residue, as occurs in [Ser(Bzl7)] dermorphin and [Ser-NHNHZ7]dermorphin, enhanced interaction with delta opioid receptors; the apparent delta affinity increased over 50-fold with [Ser(Bzl7)]dermorphin, although it retained a weak mu-selectivity. However, both [Ser(Bzl7)]- and [Ser-NHNHZ7]dermorphin exhibited high affinity for mu receptor sites. Furthermore, the D-configuration about the alpha-carbon of residue 2 and the alpha-amine function and hydroxyl group on Tyr1 are essential for receptor binding. We conclude that mu-opioid receptors contain distinct regions that accomodate the stacked phenol groups of Tyr1 and Tyr5 residues and the phenyl group of Phe3.