| Abstract
| - The achiral symmetric α-aminoisobutyric acid (Aib) replaced thecritical N-terminal residuesof the amphibian skin opioid deltorphin C(H-Tyr-d-Ala-Phe-Asp-Val-Val-Gly-NH2)withoutdetriment to the physicochemical requirements for δ opioid receptorrecognition. Substitutionsby the α,α-dialkyl amino acid in place ofd-Ala2 or Phe3, or both, exhibitedhigh δ receptoraffinity (Kiδ = 0.12−3.6 nM) and5−9-fold greater selectivity(Kiμ/Kiδ =5000−8500) than theparent compound. This is the first definitive demonstration thatthe d-chirality of alanineand the aromaticity of phenylalanine are replaceable by an achiralα,α-dialkylated residuewithout detrimental effects on ligand binding. Incorporation ofthe mono-α-alkyl amino acidl- or d-Ala at the third position also producedhighly selective δ ligands(Kiμ/Kiδ =2000−3500), albeit with reduced δ affinities (Kiδ= 6−15 nM). Replacement of the anionic residueAsp4 by Aib yielded an opioid peptide that fit two-sitebinding models for the δ receptor (η =0.763; P< 0.0001) and displayed dual high affinity forboth δ and μ receptors, emphasizingthe repulsive effect by a negative charge at μ receptor sites and theinsignificance of Asp for δaffinity. Molecular dynamics conformational analyses suggestedthat Aib residues causeddistinct changes in deltorphin C secondary structure when substitutedfor d-Ala2, Asp4, andsimultaneously d-Ala2 and Phe3 butnot when substituted for Phe3. Theseconformationalchanges might be critical factors for the proper orientation ofreactive constituents of residuesin the N-terminal region of deltorphin C. Disparities betweenbinding data and functionalbioassays of [Aib3] indicated that Phe3 wasrequired for bioactivity in mouse vas deferens butnot for interaction with δ opioid receptors in rat brainmembranes.
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