αmelanocyte stimulating hormone: Chemical nature and mechanism of action

αMelanotropin (α-MSH) is a tridecapeptide, Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂, synthesized and secreted by the pars intermedia of the vertebrate pituitary. This peptide hormone is derived from pro-opiomelanocortin, a precursor protein which contains within its structure the sequences of other melanotropic peptides (γ- and rβ-MSH, corticotropin), and possibly other hormones. α-MSH is the physiologically relevant melanotropin secreted by the pituitary and in most vertebrates plays the essential role in adaptive color changes through its action on integumental chromatophores.The initial actions of α-MSH are mediated at the level of the melanocyte membrane and involve signal transduction from receptor to adenylate cyclase on the intracellular surface of the membrane. This results in elevated cytosolic cyclic AMP levels followed by melanosome dispersion within dermal melanocytes and melanogenesis within epidermal melanocytes. The action of α-MSH on dermal melanocytes requires calcium for transduction of signal and cyclic AMP production. Melanosome dispersion per se does not, however, require extracellular calcium. Structure-function studies of α-MSH analogues and fragments have provided important insights relative to the structural requirements of the hormone for receptor binding and transduction. Substitution of certain residues within α-MSH has led to the development of melanotropins that exhibit extraordinary potency and prolonged biological activity