Radioligand saturation binding for quantitative analysis of ligand-receptor interactions

The reversible combination of a ligand with specific sites on the surface of a receptor is one of the most important processes in biochemistry. A classic equation with a useful simple graphical method was introduced to obtain the equilibrium constant, K_d, and the maximum density of receptors, B_max. The entire ¹²⁵I-labeled ligand binding experiment includes three parts: the radiolabeling, cell saturation binding assays and the data analysis. The assay format described here is quick, simple, inexpensive, and effective, and provides a gold standard for the quantification of ligand-receptor interactions. Although the binding assays and quantitative analysis have not changed dramatically compared to the original methods, we integrate all the parts to calculate the parameters in one concise protocol and adjust many details according to our experience. In every step, several optional methods are provided to accommodate different experimental conditions. All these refinements make the whole protocol more understandable and user-friendly. In general, the experiment takes one person less than 8 h to complete, and the data analysis could be accomplished within 2 h.