Excised dorsal skin of Anolis carolinensis was exposed to high intensity UV-irradiation in the presence of different photoreactive alpha-MSH derivatives. The resulting covalent binding of the hormone to its receptor induced irreversible pigment dispersion. The duration of the longlasting response depended on the type and length of irradiation; it was maximal after two 5 min irradiation phases with a light intensity of approximately 180 mW/cm2 and a spectrum from 310 to 550 nm, fresh hormone being added after the first phase. [N alpha-(4-Azidophenylacetyl-serine1]-alpha-MSH (I), [2'-(2-nitro-4-azidophenylsulphenyl)-tryptophan9]-alpha-MSH (II) and [p-azidophenylalanine13]-alpha-MSH (III) all inserted into the receptor to about the same extent, as judged from the persistence of the longlasting signal. In contrast, [D-alanine1, p-azidophenylalanine2, norvaline4]-alpha-MSH (IV) and [N alpha-(4-azidophenylacetyl)-serine1, leucine9]-alpha-MSH (V) gave much less insertion and [leucine9, p-azidophenylalanine13]-alpha-MSH (VI) hardly any insertion when applied in the same relative excess (5-fold the concentration inducing a maximal response). Covalent attachment of the cleavable photolabel [N alpha-(4-azidophenyl)-1, 3'-dithio-propionyl-serine1]-alpha-MSH (VII) and subsequent washing of the skin in buffer containing 1% beta-mercaptoethanol released the peptide from the receptor. Insertion of the C-terminal photolabel [p-azidophenylalanine13]-alpha-MSH was reduced by the weak antagonist H-Phe-Ala-Trp-Gly-Gly-Pro-Val-NH2. These experiments prove that hormone receptors can be covalently labelled in tissue with very limited light transparency.
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