α-melanotropin: The minimal active sequence in the lizard skin bioassay

α-Melanotropin (α-melanocyte-stimulating hormone, α-MSH) is a tridecapeptide, Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂. The minimal sequence of α-MSH required for agonism in the lizard (Anolis carolinensis) skin bioassay was determined to be Ac-His-Phe-Arg-Trp-NH₂ (Ac-α-MSH_6-9-NH₂). Smaller fragments of this sequence (Ac-α-MSH_6-8-NH₂, Ac-α-MSH_6-7-NH₂, Ac-α-MSH_7-9-NH₂, and Ac-α-MSH_7-8-NH₂) were devoid of melanotropic activity. The tetrapeptide, Ac-α-MSH_7-10-NH₂, was also inactive, thus again demonstrating the importance of His at position 6 for minimal activity. The important potentiating amino acids were found to be Met-4, Lys-11, and Pro-12, since Ac-α-MSH_4-10-NH₂ was about 100 times more potent than Ac-α-MSH_5-10-NH₂, and Ac-[Nle⁴]-α-MSH_4-11-NH₂ was about 40 times more potent than Ac-α-MSH_4-10-NH₂ or Ac-[Nle⁴]-α-MSH_4-10-NH₂. Ac-α-MSH_4-12-NH₂ and Ac-[Nle⁴]-α-MSH_4-12-NH₂ were equipotent and about six times more potent than α-MSH. Since [Nle⁴]-α-MSH and Ac-[Nle⁴]-α-MSH_4-13-NH₂ were both equipotent but about sixfold less active than Ac-[Nle⁴]-α-MSH_4-12-NH₂, it is clear that valine at position 13 does not contribute to the potency of α-MSH, except possibly in a negative way. The minimal message sequence for equipotency to α-MSH appears to be Ac-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-NH₂, since the analog, Ac-[Nle⁴]-α-MSH_4-11-NH₂, was as active as the native hormone. Ser-1, Tyr-2, Ser-3, Glu-5, and Val-13 are not important for melanotropic potency since Ac-α-MSH_4-12-NH₂ was more potent than α-MSH, and Ac-α-MSH_5-10-NH₂ and Ac-α-MSH_6-10-NH₂ were equipotent, being about 4,000 times less active than α-MSH.