Design of Potent Linear ±-Melanotropin 4–10 Analogues Modified in Positions 5 and 10

a-Melanocyte stimulating hormone (a-MSH) is a linear tridecapeptide (Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂) that has diverse physiological functions in addition to its reversible darkening of amphibian skins by stimulating melanosome dispersion within melanophores. On the basis of theoretical and experimental results from our laboratory and others, we have designed a group of 1–13, 4–13, and especially 4–10 analogues related to the superpotent analogue [Nle⁴,D-Phe⁷] α-MSH in which the Glu⁵has been replaced with Asp⁵, and the Gly¹⁰ has been replaced with Lys¹⁰ and other basic amino acid residues in the 4–10 analogues, and in which Gly¹⁰and Lys¹¹were interchanged in the longer peptide analogues. In the 1–13 and 4–13 series the Lys¹⁰, Gly¹¹ analogues generally retained superpotency for the D-Phe⁷-containing analogues. Most interestingly, synthesis of Ac-[Nle⁴,Xxx⁵,Yyy⁷,Zzz¹⁰]a-MSH4_₁₀-NH₂ analogues where Xxx = Asp or Glu, Yyy = Phe or D-Phe, and Zzz = basic amino acids (Lys, Orn, α,γ-diaminobutyric acid (Dab), and α,β-diaminopropionic acid (Dpr)) provided melanotropins with potencies up to 10 times that of the native hormone in stimulating frog (Rana pipiens) skin darkening and 8–50 times more potent than a-MSH in stimulating lizard (Anolis carolinensis) skin melanophores in vitro. To our knowledge, Ac- [Nle⁴,Asp⁵,D-Phe⁷,Dab¹⁰] α-MSH₄_₁₀-NH₂, the most potent analogue, is the most potent melanotropin obtained thus far for the Anolis assay system. These results provide new insights into the structural and conformational requirements for biological potency of a-MSH and the differential structural and conformational requirements of a-MSH and its analogues at two different types of pigment cell receptors.