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

Abstract

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-MSH6-9-NH2). Smaller fragments of this sequence (Ac-α-MSH6-8-NH2, Ac-α-MSH6-7-NH2, Ac-α-MSH7-9-NH2, and Ac-α-MSH7-8-NH2) were devoid of melanotropic activity. The tetrapeptide, Ac-α-MSH7-10-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-MSH4-10-NH2 was about 100 times more potent than Ac-α-MSH5-10-NH2, and Ac-[Nle4]-α-MSH4-11-NH2 was about 40 times more potent than Ac-α-MSH4-10-NH2 or Ac-[Nle4]-α-MSH4-10-NH2. Ac-α-MSH4-12-NH2 and Ac-[Nle4]-α-MSH4-12-NH2 were equipotent and about six times more potent than α-MSH. Since [Nle4]-α-MSH and Ac-[Nle4]-α-MSH4-13-NH2 were both equipotent but about sixfold less active than Ac-[Nle4]-α-MSH4-12-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-MSH4-11-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-MSH4-12-NH2 was more potent than α-MSH, and Ac-α-MSH5-10-NH2 and Ac-α-MSH6-10-NH2 were equipotent, being about 4,000 times less active than α-MSH. © 1989.