Selective properties of C- and N-terminals and core residues of the melanocyte-stimulating hormone on binding to the human melanocortin receptor subtypes

We synthesised nine analogues of [Nle⁴,D-Phe⁷]α-MSH (melanocyte-stimulating hormone) (NDP) where (1) the N- or C-terminals were deleted or exchanged by those of β- or γ-MSH and (2) the core residues His⁶, Phe⁷, Arg⁸ and Trp⁹ were individually substituted by Glu⁶ β-(2-naphthyl)-D-alanine (D-Nal⁷), Lys⁸ and His⁹, respectively. We tested these analogues in ligand binding assays with cells transiently expressing the human melanocortin MC₁, MC₃, MC₄ and MC₅ receptors. The results show that the N-terminal segment (Ser¹-Tyr²-Ser³) of NDP was not important for binding to melanocortin MC₁ and MC₄ receptors whereas it affects binding to melanocortin MC₃ and MC₅, receptors. The C-terminal segment (Gly¹⁰-Lys¹¹-Pro¹²-Val¹³) of NDP was clearly important for binding to all the four melanocortin receptor subtypes. The data indicate that the low affinity of γ-MSH for the melanocortin MC₄ receptor is due to its C terminal (Asp¹⁰-Arg¹¹-Phe¹²). Substitution of D-Phe⁷ by D-Nal⁷ increased the affinity for the melanocortin MC₄ receptor but not for the other melanocortin receptor subtypes. The other core residue substitutions lowered the affinity in a differentiated manner for each of the melanocortin receptors. These results are valuable for the molecular modelling and design of selective drugs for the melanocortin receptors.