A solid-phase receptor binding assay for ¹²⁵I-hCG

A solid-phase radioligand-receptor assay (RRA) to measure the binding of ¹²⁵I-labelled human chorionic gonadotropin (¹²⁵I-hCG) to target cell membranes has been developed. The binding of ¹²⁵I-hCG to membranes immobilized on the wells of microtitration plates reached a maximum at about 3 hours at 37°C, was saturable, displayed a high affinity (Ka = 2.4 × 10⁹M⁻¹) and was specifically inhibited by unlabelled hCG. In comparison with RRAs carried out with membranes in suspension, the solid-phase RRA is significantly simpler and much faster to perform as it avoids centrifugation or filtration procedures. The solid-phase RRA was adapted profitably to process large numbers of samples at the same time. It proved particularly useful as a screening assay to detect anti-hCG monoclonal antibodies with high inhibitory activity for binding of ¹²⁵I-hCG to its receptors.