title: Substitution of aromatic and nonaromatic amino acids for the Phe3 residue in the delta-selective opioid peptide deltorphin I: effects on binding affinity and selectivity.
abstract: Deltorphins I and II (Tyr-D-Ala-Phe-Asp-Val-Val-Gly NH2 and Tyr-D-Ala-Phe-Glu-Val-Val-Gly NH2) display a high degree of delta-opioid receptor selectivity. Since they lack the intervening Gly3 residue found between the Tyr and Phe aromatic moieties in pentapeptide enkephalins, deltorphins I and II resemble a previously described series of cyclic tetrapeptides based on Tyr-c[D-Cys-Phe-D-Pen] (JOM-13). With the goal of development of structure-activity relationships for deltorphins and comparison with that of the cyclic tetrapeptides, ten analogs of deltorphin I were synthesized in which Phe3 was replaced with specific aromatic and nonaromatic amino acids with varying physicochemical properties. Results indicated that analogs containing the bicyclic aromatic amino acids 3-(1-naphthyl)-L-alanine [1-Nal; Ki(mu) = 767 nM, Ki(delta) = 7.70 nM], 3-(2-naphthyl)-L-alanine [2-Nal; Ki(mu) = 1910 nM, Ki(delta) = 49.2 nM], tryptophan [Ki(mu) = 1250 nM, Ki(delta) = 23.9 nM], and 3-(3-benzothienyl)-L-alanine [Bth; Ki(mu) = 112 nM, Ki(delta) = 3.36 nM] were fairly well tolerated at mu- and delta-receptors, though affinity was compromised to varying degrees relative to deltorphin I. Shortening the Phe side chain by incorporation of phenylglycine (Pgl) was detrimental to both mu (Ki = 4710 nM) and delta (Ki = 15.6 nM) binding, while extension of the side chain with homophenylalanine (Hfe) enhanced mu binding (Ki = 67.8 nM), leaving delta affinity unaffected (Ki = 2.64 nM). Substitution with nonaromatic amino acids valine and isoleucine led expectedly to poor opioid binding [Ki(mu) = > or = 10,000 nM for each, Ki(delta) = 160 and 94.7 nM, respectively], while peptides containing cyclohexylalanine (Cha) and leucine surprisingly retained affinity at both mu (Ki = 322 and 1240 nM, respectively) and delta (Ki = 10.5 and 12.4 nM, respectively) sites. In general, these trends mirror those observed for similar modification in Tyr-c[D-Cys-Phe-D-Pen].
authors.affiliation: Department of Chemistry, Eastern Michigan University, Ypsilanti.
authors.forename: D L
authors.initials: DL
authors.lastname: Heyl
authors.forename: S J
authors.initials: SJ
authors.lastname: Schmitter
authors.forename: H
authors.initials: H
authors.lastname: Bouzit
authors.forename: T W
authors.initials: TW
authors.lastname: Johnson
authors.forename: A M
authors.initials: AM
authors.lastname: Hepp
authors.forename: K R
authors.initials: KR
authors.lastname: Kurtz
authors.forename: C
authors.initials: C
authors.lastname: Mousigian
mesh_terms: D000595:Amino Acid Sequence; D000700:Analgesics / Q000378:metabolism; D004745:Enkephalins / Q000378:metabolism / Q000494:pharmacology; D007700:Kinetics; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000138:chemical synthesis* / Q000737:chemistry* / Q000378:metabolism* / Q000494:pharmacology; D010649:Phenylalanine / Q000378:metabolism*; D011487:Protein Conformation; D017465:Receptors, Opioid, delta / Q000378:metabolism*; D017473:Receptors, Opioid, kappa / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000378:metabolism; D013329:Structure-Activity Relationship; D014316:Tritium
chemical_list: D000700:Analgesics; D004745:Enkephalins; D009842:Oligopeptides; D017465:Receptors, Opioid, delta; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D014316:Tritium; C070386:H-tyrosyl-cyclo(cysteinyl-phenylalanyl-penicillaminyl)-OH; C060208:deltorphin I, Ala(2)-; C060209:deltorphin II, Ala(2)-; D010649:Phenylalanine