Synthesis and biological properties of p ‐azidophenylalanine 13 ‐β‐melanotropin, a potent photoaffinity label for MSH receptors

p ‐Azidophenylalanine 13 ‐α‐melantropin ([Pap 13 ]‐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 −4 M [Pap 13 ]‐α‐MSH at 338 nm (intensity: 10 −3 W · cm −2 ) led to complete photolysis of the photolabel within <20 min. Under these conditions [Pap 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 −9 M [Pap 13 ]‐α‐MSH which produced a full initial response failed to prolong the effect, whereas 1.2·10 −8 M 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: EC 50 immediately after photolysis was 6 · 10 −10 M ; after 200 min EC 50 increased to ∼8·10 −9 M .