Iodination of beta-melanotropin. Time course analysis of reaction mixtures by high pressure liquid chromatography and characterization of biologically active mono- and diiodo-beta-melanotropin.

The production of highly purified, chemically well defined, iodinated beta-melanotropin possessing full biological activity is described. beta-Melanotropin, purified by high pressure liquid chromatography, was iodinated using Iodogen as the oxidizing agent. Biological activity was recovered, following iodination, by incubating the iodinated peptide mixture in 0.75 M dithiothreitol at 37 degrees C for 18-24 h. The qualitative and quantitative effects of dithiothreitol and the changes that occurred in the iodination mixture with time were examined using high pressure liquid chromatography. This separation technique has proved to be a useful tool both for optimizing the iodination procedure and for the rapid, efficient purification of mono- and diiodinated beta-melanotropins. Amino acid analysis of these two peptides revealed modifications only of the one tyrosine residue (Tyr 5): the expected incorporation of iodine to form mono- or diiodo tyrosine. Monoiodo-beta-melanotropin had full biological activity, as measured by tyrosinase stimulation in Cloudman S91 melanoma cells, while diiodo-beta-melanotropin was an order of magnitude less active.