Bmax

Bmax is a pharmacological term that denotes the maximum number of ligand-binding sites available in a biological preparation, typically reflecting receptor density rather than ligand affinity. In a saturation binding experiment, a preparation such as a cell suspension or tissue homogenate is exposed to increasing concentrations of a labeled ligand. After equilibrium, total binding is measured, non-specific binding is determined in the presence of excess unlabeled ligand, and specific binding is obtained by subtraction. The resulting curve saturates at a plateau, whose value is Bmax. Nonlinear regression using the binding equation B = (Bmax [L])/(Kd + [L]) yields Bmax and the dissociation constant Kd, a measure of affinity.

Bmax is usually reported in units such as femtomoles of bound ligand per milligram of protein, or sites per cell, depending on the assay. It is proportional to the number of receptors or binding sites and is independent of ligand affinity; thus changes in Bmax across samples indicate alterations in receptor expression or availability, such as upregulation or downregulation.

Interpretation must consider experimental factors. Bmax can be influenced by assay conditions, the presence of multiple receptor subtypes, receptor internalization, or allosteric effects, and some systems exhibit spare receptors that affect occupancy without changing Bmax. In practice, Bmax is used to compare receptor density between tissues or treatment groups and to estimate receptor occupancy by drugs.

Related concepts include Kd, the Scatchard plot, and saturation binding assays.