[half-Cys⁴,half-Cys¹⁰]-α-Melanocyte-stimulating hormone: A cyclic α-melanotropin exhibiting superagonist biological activity

α-Melanocyte-stimulating hormone (α-melanotropin; α-MSH) is a linear tridecapeptide (Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂) that reversibily darkens amphibian skins by stimulating melanosome (pigment granule) dispersion within melanophores. By using a number of in vitro melanocyte assays, we have examined the conformational requirements for α-MSH activity. Synthesis of [half-Cys⁴,half-Cys¹⁰]-α-MSH, a cyclic, conformationally restricted, 'isosteric' analogue of α-MSH, provided a melanotropin with a potency > 10,000 times that of the native hormone in stimulating frog (Rana pipiens) skin darkening. The cyclic analogue also showed substantially prolonged activity relative to the native hormone. [half-Cys⁴,half-Cys¹⁰]-α-MSH was ≃30 times more potent than α-MSH in stimulating lizard (Anolis carolinensis) skin melanophores in vitro. By using a cell-free Cloudman S-91 mouse melanoma plasma membrane preparation, we found the cyclic analogue to be ≃3 times as potent as the native hormone in stimulating adenylate cyclase activity. These results provide insight into the conformational requirements for biological activity of α-MSH, and the comparative conformational requirements of α-MSH at a number of pigment cell receptors.