What is Bmax in radioligand binding assays?

• A. Bmax is the total number of binding sites; it indicates receptor density available for binding.
• B. Bmax is the affinity of ligand for receptor.
• C. Bmax is the rate of ligand internalization.
• D. Bmax is the maximal signal produced.

Answer: (B)

In radioligand binding assays, Bmax represents the total number of binding sites available for the ligand, i.e., the receptor density. It’s derived from a saturation binding curve, where specific binding approaches a maximum as ligand concentration becomes very high. The relationship is typically described by a form of the binding equation: specific binding equals (Bmax times ligand concentration) divided by (Kd plus ligand concentration). The Kd in this context reflects affinity—the ligand concentration needed to occupy half of the binding sites. Because of that, Bmax is about how many sites exist, not how tightly they bind. Bmax also isn’t about the rate of internalization or the maximal signal produced by the assay; those are different measures.