Scintillation proximity assay: a sensitive and continuous isotopic method for monitoring ligand/receptor and antigen/antibody interactions.

Scintillation proximity assay (SPA) makes it possible to use radioisotopes for monitoring binding reactions continuously without the need to separate free from bound components. As a result SPA can be carried out more rapidly than most other methods used to monitor binding reactions. The methodology also lends itself to automation. The sensitivities already achieved with SPA procedures are comparable to the sensitivities of other procedures in use today. Another feature of SPA is that the key reagents (beads, 125I labeling) are relatively inexpensive. The principles of SPA, utilizing 125I-labeled molecules, are discussed and some applications to immunology, receptor binding, and measurement of potential across membranes are presented. SPA should also be applicable to monitoring interactions involving nucleic acids, lipids, and carbohydrates. Characteristics of some radionuclides, other than tritium and 125I, that may be used in SPA are presented.