α‐Melanotropin Labelled at its Tyrosine2 Residue: Synthesis and Biological Activities of 3′‐Iodotyrosine2‐,3′‐125Iodotyrosine2‐,3′,5′‐Diiodotyrosine2‐, and (3′,5′‐3H2)tyrosine2‐α‐Melanotropin, and of Related Peptides

Abstract α‐MSH was labelled at its tyrosine2 residue with tritium and iodine. Several synthetic routes were investigated by preparing 13 precursor or mode compounds and 4 different labelled products (via about 40 intermediates). Their melanotropic activity was determined with an in vitro frog skin assay and, for some of the compounds, with a tyrosinase assay. The tritiation was performed on [Tyr(I2)2]α‐MSH by catalytic halogen/tritium exchange, yielding α‐MSH of high specific radioactivity (34 Ci/mmol) and full biological activity. Iodination was studied in detail using five different techniques. An equimolar chloramine T procedure proved to be the most convenient and reproducible method, resulting in monoiodinated α‐MSH containing 99% of the label in position 2. The biological activity was 50% that of α‐MSH; the specific radioactivity, determined in a competitive binding assay with a highly specific α‐MSH antiserum and [Tyr(I)2]α‐MSH as competitor, was 1530 Ci/mmol. The labelling techniques and the bioligical results are discussed.