A protein is characterized by binding to its receptor or ligand. The protein is bound to a solid support (ELISA plate, beads, column), incubated with labeled ligand, and the amount of bound ligand is determined. Equilibrium dialysis can be performed for precise Kd affinity values when applicable. Alternatively, the receptor may be on a cell surface. The affinity of the interaction, measured by the dissociation constant, Kd, is determined. These analyses may involve a UV spectophotometric, colorimetric, fluorescence or radioactivity method of detection.

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