Synthesis and biological properties of p‐azidophenylalanine13‐β‐melanotropin, a potent photoaffinity label for MSH receptors

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Abstract
p‐Azidophenylalanine13‐α‐melantropin ([Pap13]‐α‐MSH) was synthesized in homogeneous solution by the fragment condensation method, and its biological activity was determined in three different assay systems. The pigment‐dispersing activity relative to α‐MSH was 65%, measured with melanophores of Rana pipiens or of Xenopus laevis tadpoles. The tyrosinase‐stimulating activity was 50%, determined with cultured mouse melanoma cells. UV. irradiation of solutions containing ≤10−4M[Pap13]‐α‐MSH at 338 nm (intensity: 10−3 W · cm−2) led to complete photolysis of the photolabel within 13]‐α‐MSH was covalently inserted into MSH‐receptors which produced a longlasting pigment dispersion in Xenopus melanphores (see [3]). The extent of this prolonged stimulation depended on the hormone concentration used during photolysis. 1.8·10−9M [Pap13]‐α‐MSH which produced a full initial response failed to prolong the effect, whereas 1.2·10−8M hormone caused irreversible stimulation. It appears that only about 10% of the initially occupied receptors were covalently labelled because the log dose response curve was shifted to ∼ 10x higher concentration after a 200 min wash period: EC50 immediately after photolysis was 6 · 10−10M; after 200 min EC50 increased to ∼8·10−9M.

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