Radiolabeling of antigens: procedures and assessment of properties.

The role of labeled tracers in radioimmunoassay procedures (RIA) and the advantages of the iodine-labeled antigens over 14C or tritium-labeled antigens are discussed. Different methods of labeling with iodine isotopes are presented. The ease with which microgram amounts of hormonal proteins are tagged, achieving high specific activity with the chloramine-T procedure of Hunter and Greenwood, is indicated in a detailed presentation of a typical procedure. More sophisticated methods that present none of the drawbacks of the chemical oxidation methods for iodination would seem to have greater versatility, resulting in labeled antigens with better immunologic properties. These are likely to come into vogue for many inhouse preparations of labeled antigens in the next few years. The iodination procedures developed for haptenic substances have extended the usefulness of the RIA method to many steroidal hormones and have helped to achieve a limit of detectability at picogram levels. Greater specificity has also resulted. The assessment of the properties of the labeled antigens is equally important, and methods to evaluate the parameters of purity, immunoreactivity, specific activity as well as storage, are discussed. This review is intended to serve as an introduction to the radiolabeling of antigens or haptens. A thorough appreciation of the problems and pitfalls in these procedures is essential for the development of a new RIA or adaptation of a well-studied method for routine use.