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

Abstract

α-melanotropin induces pigment dispersion in melanophores and pigment formation in melanocytes and melanoma cells. The pigmentdispersingeffect of MSH is completely reversible in vitro, as the melanosomes reaggregate readily upon removal of the hormone. The primary site of action of α-MSH appears to be a cell surface receptor, which can be stimulated by two different portions of the α-MSH molecule, namely the central (-Glu-His-Phe-Arg- Trp-) and the C-terminal (-Gly-Lys-Pro-Val . NH₂) message sequences. The C-terminal residue is essential for the interaction with the receptor, since its absence reduces the biological activity considerably (in preparation); the structure of its side-chain, however, is less important, and it may therefore contain an affinity label.