Divergent Melanophore‐Dispersing and Tyrosinase‐Stimulating Activity of Synthetic Leucine 9 ‐α‐melanotropin

Leucine 9 ‐α‐melanotropin ([Leu 9 ]α‐MSH) was synthesized in homogeneous solution by a fragment condensation approach, and it was assayed for its melanophore‐dispersing and its tyrosinase‐stimulating activity with a reflectometric in vitro frog skin assay and with cultured mouse melanoma cells, respectively. In both assay systems, parallel log dose‐response curves were obtained for ([Leu 9 )]α‐MSH and α‐MSH; however, in the frog skin assay the activity of the title compound was 1 middot; 10 10 Units/mmol, i.e. 25% of the activity of α‐MSH, whereas its tyrosinasestimulating potency was only 1% compared to α‐MSH (EC 50 = 2.5 · 10 −7 M ). This indicates a major difference in the recognition/stimulation process of the receptors of the two cell types.