title: beta-Methylation of the Phe7 and Trp9 melanotropin side chain pharmacophores affects ligand-receptor interactions and prolonged biological activity.
abstract: Topographically modified melanotropin side chain pharmacophore residues Phe7 and Trp9 in a cyclic peptide template (Ac-Nle4-c[Asp-His-Xaa7-Arg-Yaa9-Lys]-NH2) and Phe7 in a linear peptide template (Ac-Ser-Tyr-Ser-Nle4-Glu-His-Xaa7-Arg-Trp-Gly-Lys-Pro-Val-NH2) result in differences in potency and prolonged biological activity in the frog and lizard skin bioassays. These topographic modifications included the four isomers of beta-methylphenylalanine (beta-MePhe)7 and beta-methyltryptophan (beta-MeTrp)9 and the two isomers of 1,2,3,4-tetrahydro-beta-carboline (Tca)9 Modifications in the cyclic template resulted in up to a 1000-fold difference in potency for the beta-MePhe7 stereoisomeric peptides; up to a 476-fold difference in potency resulted for the beta-MeTrp9 peptides, and about a 50-fold difference between the Tca9-containing peptides. Up to a 40-fold difference in potency resulted for the beta-MePhe7 stereoisomeric peptides using the linear template in these assays. The relative potency ranking for modifications in the cyclic template of beta-MePhe7 were 2R,3S > 2S,3S = 2S,3R > 2R,3R in the frog assay and 2S,3R > 2R,3S > 2S,3S > 2R,3R in the lizard assay. The relative potencies for modifications in the cyclic template of beta-MeTrp9 were 2R,3S > 2R,3R > 2S,3S > > 2S,3R in the frog assay and 2S,3S = 2R,3R > 2R,3S > 2S,3R in the lizard assay. The relative potencies for modifications in the cyclic template of Tca9 were DTca > LTca in both assays. Significant differences in prolonged (residual) activities were also observed for these modified peptides and were dependent upon stereochemistry of the beta-methyl amino acid, peptide template, and bioassay system. Furthermore, comparisons of beta-MeTrp9 stereoisomeric peptides on the frog, lizard, and human MC1 receptors suggest that structure-activity relationships on both the classical frog and lizard skin bioassays do not necessarily predict corresponding SAR profiles for the human melanocortin receptors, indicating a remarkable species specificity of the MC1 receptor requirements.
authors.affiliation: Department of Chemistry, University of Arizona, Tucson 85721, USA.
authors.forename: C
authors.initials: C
authors.lastname: Haskell-Luevano
authors.forename: K
authors.initials: K
authors.lastname: Toth
authors.forename: L
authors.initials: L
authors.lastname: Boteju
authors.forename: C
authors.initials: C
authors.lastname: Job
authors.forename: A M
authors.initials: AM
authors.lastname: Castrucci
authors.forename: M E
authors.initials: ME
authors.lastname: Hadley
authors.forename: V J
authors.initials: VJ
authors.lastname: Hruby
mesh_terms: D000818:Animals; D006801:Humans; D008024:Ligands; D008116:Lizards; D009074:Melanocyte-Stimulating Hormones / Q000737:chemistry* / Q000378:metabolism; D008745:Methylation; D008956:Models, Chemical; D008958:Models, Molecular; D010649:Phenylalanine / Q000378:metabolism*; D011894:Rana pipiens; D011979:Receptors, Pituitary Hormone / Q000378:metabolism*; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D014364:Tryptophan / Q000378:metabolism*
chemical_list: D008024:Ligands; D011979:Receptors, Pituitary Hormone; D010649:Phenylalanine; C029446:MSH receptor; D014364:Tryptophan; D009074:Melanocyte-Stimulating Hormones