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SUMMARYThymopentin (TP5, Arg-Lys-Asp-Val-Tyr), a synthetic pentapeptide corresponding to amino acids 32-36 of the thymic hormone thymopoietin, has the biological activity of the parent compound. The tyrosyl residue was iodinated using the chloramine-T method and the reaction mixture separated by HPLC into peaks corresponding to free TPS, mono-iodo TP5 and di-iodo TP5, identified by mass spectrometry. Biological activity was retained in the iodinated peptides but, surprisingly, free TP5 had lost biological activity during the separation process. The biologically active monoiodinated TP5 had a specific activity of 28 pCi/Pg and will be useful in studies of receptor binding.

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