[half-Cys4,half-Cys10]-alpha-Melanocyte-stimulating hormone: a cyclic alpha-melanotropin exhibiting superagonist biological activity.

alpha-Melanocyte-stimulating hormone (alpha-melanotropin; alpha-MSH) is a linear tridecapeptide (Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2) that reversibly darkens amphibian skins by stimulating melanomsome (pigment granule) dispersion within melanophores. By using a number of in vitro melanocyte assays, we have examined the conformational requirements for alpha-MSH activity. Synthesis of [half-Cys4,half-Cys10]-alpha-MSH, a cyclic, conformationally restricted, "isosteric" analogue of alpha-MSH, provided a melanotropin with a potency greater than 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]-alpha-MSH was approximately 30 times more potent than alpha-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 approximately 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 alpha-MSH, and the comparative conformational requirements of alpha-MSH at a number of pigment cell receptors.