[24] Radioligand assays for insulin-like growth factor II

This chapter focuses on the methods of measurement of insulin-like growth factor II (IGF-II). There is assay evidence that chicken serum and turtle serum contain IGF-I and IGF-II, but toad and trout serum seem to contain only IGF-I-like peptide. Various radioligand displacement assays depend on the availability of highly purified peptides for iodination. Iodination of IGF-II has been achieved with various modifications of the chloramine-T and lactoperoxidase methods. The chapter presents a carefully controlled lactoperoxide method that provides unaggregated labeled IGF that retains immune and receptor binding properties. The chapter describes three IGF-II competitive binding assays that depend on membrane receptors or specific IGF binding sites of a serum-carrier protein. These methods, which were widely employed prior to the development of radioimmunoassays (RIAs), remain useful procedures.