title: Helix-inducing alpha-aminoisobutyric acid in opioid mimetic deltorphin C analogues.
abstract: The achiral symmetric alpha-aminoisobutyric acid (Aib) replaced the critical N-terminal residues of the amphibian skin opioid deltorphin C (H-Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2) without detriment to the physicochemical requirements for delta opioid receptor recognition. Substitutions by the alpha, alpha-dialkyl amino acid in place of D-Ala2 or Phe3, or both, exhibited high delta receptor affinity (Ki delta = 0.12-3.6 nM) and 5-9-fold greater selectivity (Ki mu/Ki delta = 5000-8500) than the parent compound. This is the first definitive demonstration that the D-chirality of alanine and the aromaticity of phenylalanine are replaceable by an achiral alpha, alpha-dialkylated residue without detrimental effects on ligand binding. Incorporation of the mono-alpha-alkyl amino acid L- or D-Ala at the third position also produced highly selective delta ligands (Ki mu/Ki delta = 2000-3500), albeit with reduced delta affinities (Ki delta = 6-15 nM). Replacement of the anionic residue Asp4 by Aib yielded an opioid peptide that fit two-site binding models for the delta receptor (eta = 0.763; P < 0.0001) and displayed dual high affinity for both delta and mu receptors, emphasizing the repulsive effect by a negative charge at mu receptor sites and the insignificance of Asp for delta affinity. Molecular dynamics conformation analyses suggested that Aib residues caused distinct changes in deltorphin C secondary structure when substituted for D-Ala2, Asp4, and simultaneously D-Ala2 and Phe3 but not when substituted for Phe3. These conformational changes might be critical factors for the proper orientation of reactive constituents of residues in the N-terminal region of deltorphin C. Disparities between binding data and functional bioassays of [Aib3] indicated that Phe3 was required for bioactivity in mouse vas deferens but not for interaction with delta opioid receptors in rat brain membranes.
authors.affiliation: Peptide Neurochemistry Group, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709, USA. bryantsd@opium.niehs.nih.gov
authors.forename: S D
authors.initials: SD
authors.lastname: Bryant
authors.forename: R
authors.initials: R
authors.lastname: Guerrini
authors.forename: S
authors.initials: S
authors.lastname: Salvadori
authors.forename: C
authors.initials: C
authors.lastname: Bianchi
authors.forename: R
authors.initials: R
authors.lastname: Tomatis
authors.forename: M
authors.initials: M
authors.lastname: Attila
authors.forename: L H
authors.initials: LH
authors.lastname: Lazarus
mesh_terms: D000595:Amino Acid Sequence; D000701:Analgesics, Opioid / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D001921:Brain / Q000166:cytology / Q000378:metabolism; D007700:Kinetics; D051379:Mice; D008956:Models, Chemical; D018716:Molecular Mimicry; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000737:chemistry / Q000378:metabolism*; D017433:Protein Structure, Secondary; D051381:Rats; D013574:Synaptosomes / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D000701:Analgesics, Opioid; D009842:Oligopeptides; C060208:deltorphin I, Ala(2)-; D005680:gamma-Aminobutyric Acid