Preparation of an iodinated radioligand.

Radioactive tracers are useful for receptor measurement and for radioimmunoassays because of the ease with which radioactive ligands are detected. Typical receptor measurement methods include whole tissue receptor assays, autoradiography, and cell and membrane binding assays. The isotopes most commonly used in receptor measurement studies are tritium, carbon-14, and iodine-125, with the choice of isotope most often based on the specific activity required and the sensitivity and type of detection system available. Its high specific activity and easy detection make I-125 ideal for labeling large peptides and proteins. While the short half-life (60 days) of iodine requires frequent replacement with freshly iodinated compounds, this is also an advantage, as unused portions of radioligand can be held for decay and disposed of as nonradioactive waste. This unit describes the iodination of proteins or peptides using the 125I-labeled Bolton Hunter reagent, a convenient, easy-to-use, nonoxidizing labeling reagent.