Radioligand Binding Assay

Quantitative receptor assay measuring saturation and competition binding of radioisotope-labeled ligands at biological targets.

Specifications & Metrics

type: Receptor Binding
readout: Scintillation counting (β or γ decay)
measures: Equilibrium dissociation constant (Kd), receptor density (Bmax), inhibition constant (Ki)
format: Filtration or scintillation proximity assay (SPA) in 96- or 384-well plates
pioneer: Robert J. Lefkowitz and colleagues (first radio-receptor assay using radiolabeled hormone, 1970)

Saturation binding experiments titrate increasing concentrations of a radiolabeled ligand (commonly tritium ³H or iodine-125 ¹²⁵I) to determine receptor affinity (Kd) and maximum binding site density (Bmax).
Competition binding assays measure how an unlabeled test compound displaces a fixed concentration of radioligand, yielding an IC50 value that converts to an equilibrium inhibition constant (Ki) via the Cheng–Prusoff equation.
Scintillation proximity assays (SPA) immobilize receptors on microbeads containing scintillant so only bound radioligand within beta-decay range emits light, enabling homogeneous mix-and-measure screening without filtration.