[24] Preparation of Stable Radioiodinated Polypeptide Hormones and Proteins Using Polyacrylamide Gel Electrophoresis

This chapter discusses the preparation of stable radioiodinated polypeptide hormones and proteins using polyacrylamide gel electrophoresis. Radioactively labeled polypeptide hormones and proteins are widely used as tracers in radioimmunoassays and receptor studies. The peptide or protein is most easily labeled using iodination with ¹²⁵I or ¹³¹I. The radioactive iodine is substituted in the tyrosine groups of the peptide as monoiodotyrosine or di-iodotyrosine, resulting in a heterogeneous mixture of iodine-substituted molecules plus native peptide and unreacted iodide. The iodination mixture has to be fractionated to obtain a well-characterized iodinated product to be used as a tracer. An ideal tracer should retain the full biological activity or immunological reactivity of the native peptide hormone or protein and have a high specific activity and a long shelf life. Studies in the laboratory have shown that it is possible to fractionate iodinated insulin preparations into homogeneous monoiodoinsulin derivatives by polyacrylamide gel electrophoresis using long rods. The same fractionation technique has been useful for the preparation of a number of radioiodinated polypeptide hormones and proteins.