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Article

Biological and Conformational Examination of Stereochemical Modifications Using the Template Melanotropin Peptide, Ac-Nle-c[Asp-His-Phe-Arg-Trp- Ala-Lys]-NH₂, on Human Melanocortin Receptors

Carrie Haskell-Luevano,^†^‡ Gregory Nikiforovich,^§ Shubh D. Sharma, Ying-Kui Yang,^† Chris Dickinson, Victor J. Hruby,* and Ira Gantz^#

Departments of Internal Medicine, Pediatrics, and Surgery, University of Michigan Medical Center, Ann Arbor, Michigan 48109, Center for Molecular Design, Washington University, St. Louis, Missouri 63110, and Department of Chemistry, University of Arizona, Tucson, Arizona 85721

J. Med. Chem., 1997, 40 (11), pp 1738–1748

DOI: 10.1021/jm960845e

Publication Date (Web): May 23, 1997

†

Department of Internal Medicine, University of Michigan Medical Center.

‡

Present address: Vollum Institute for Advanced Biomedical Research, Oregon Health Sciences University L-474, 3181 S. W. Sam Jackson Park Road, Portland, OR 97201.

Center for Molecular Design, Washington University.

To whom reprint requests should be addressed at the University of Arizona.

Department of Chemistry, University of Arizona.

Department of Surgery, University of Michigan Medical Center.

Abstract

Examination of conformationally constrained melanotropin peptides (Ac-Nle⁴-c[Asp⁵-His-Phe⁷-Arg-Trp⁹-Ala-Lys]-NH₂) on four human melanotropin receptors (hMC1R, hMC3R, hMC4R, and hMC5R) resulted in identifying the importance of ligand stereochemistry at positions 5, 7, and 9 for agonist binding affinity and receptor selectivity. A trend in ligand structure−activity relationships emerged for these peptides, with the hMC1R and hMC4R possessing similar tendencies, as did the hMC3R and hMC5R. α-MSH (Ac-Ser-Tyr-Ser-Met⁴-Glu-His-Phe⁷-Arg-Trp-Gly-Lys-Pro-Val-NH₂), NDP-MSH (Ac-Ser-Tyr-Ser-Nle⁴-Glu-His-d-Phe⁷-Arg-Trp-Gly-Lys-Pro-Val-NH₂), and MTII (Ac-Nle⁴-c[Asp⁵,d-Phe⁷,Lys¹⁰]-α-MSH(4−10)-NH₂) were also examined at each of these melanocortin receptors. Interestingly, the linear NDP-MSH possessed greater binding affinity for the hMC3R and hMC5R than did the cyclic analogue MTII. The peptide Ac-Nle-c[Asp-His-Phe-Arg-d-Trp⁹-Ala-Lys]-NH₂ demonstrated the greatest differentiation in binding affinity between the hMC1R and hMC4R (78-fold). Analogue Ac-Nle-c[Asp-His-Phe⁷-Arg-Trp-Ala-Lys]-NH₂ resulted in micromolar binding affinity (or greater) at the hMC3R and hMC5R, demonstrating the importance of d-Phe⁷ for ligand binding potency at these receptors. Ac-c[Asp-His-Phe-Arg-Trp-Ala-Lys]-NH₂ resulted in loss of binding affinity at the hMC5R, implicating the importance of Nle⁴ (or a hydrophobic residue in this position) for binding to this receptor. Ac-Nle-c[d-Asp⁵-His-Phe-Arg-Trp-Ala-Lys]-NH₂ was unable to competitively displace [¹²⁵I]NDP-MSH binding at micromolar concentrations on the hMC3R and hMC5R, suggesting the importance of chirality of Asp⁵ either for ligand−receptor interactions or for orientation of the side chain lactam bridge and the structural integrity of the peptide conformation. Energy calculations performed for these peptides resulted in the identification of a low-energy ligand conformer family that is common to all the ligands. The differences in ligand binding affinities observed in this study are postulated to be a result of different ligand−receptor complexed interactions and not solely to the ligand structure.