An α-MSH peptide analogue, named MTII (Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-
NH2), is one of the most important ligands of melanotropic receptors but are relatively nonselective. In
order to improve the melanotropic activities of the well-characterized MTII analogues, we report here
a new analogue by modifying the core structure as well as the size of the cyclic region of MTII peptide. The analogue
peptide, Ac-Nle-c[Asp-His-D-Phe-Lys-Trp-Gly-Lys]-OH (F Peptide), in which we replaced Arg at position 8 with Lys
and added a Gly to position 10 of the MTII peptide sequence, was synthesized and used as a new melanotropic hormone
in controlling rapid color changes in frogs by its actions on mobilizing pigment granule movements within chromatophores.
The in vivo responses of chromatophores to MTII and the related analogue F Peptide were studied in frogs. The results
show that the F Peptide was a superpotent agonist with similar melanotropic activity to the MTII peptide according
to MTII peptide by in vivo studies. The analogue also exhibited ultraprolonged melanotropic activity. The F peptide can
be useful in the study of numerous physiological processes, particularly when superpotent and prolonged melanotropic activity
Keywords: Antagonist, F peptide, frog skin, Melanophore Index, melanotropic activity, MTII peptide.
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