Irreversible stimulation of Xenopus melanophores by photoaffinity labelling with p‐azidophenylalanine¹³‐α‐melanotropin

α-melanotropin induces pigment dispersion in and pigment formation in melanocytes melanoma cells. The pigmentdispersingeffect MSH is completely reversible in vitro, as the reaggregate readily upon removal of hormone. The primary site of action of α-MSH to be a cell surface receptor, which can be by two different portions of the α-MSH , namely the central (-Glu-His-Phe-Arg- -) and the C-terminal (-Gly-Lys-Pro-Val . ₂) message sequences. The C-terminal residue essential for the interaction with the receptor, since absence reduces the biological activity considerably (in preparation); the structure of its side-chain, however, less important, and it may therefore contain affinity label.