Mechanisms underlying agonist efficacy

Agonist efficacy is a measure of how well an agonist can stimulate a response system linked to a receptor. Efficacy can be assessed in functional assays and various parameters (E_max, K_A/EC₅₀, E_max·K_A/EC₅₀) determined. The E_max·K_A/EC₅₀ parameter provides a good estimate of efficacy across the full range of efficacy. A convenient assay for the efficacy of agonists for some receptors is provided by the [³⁵S]GTP[S] (guanosine 5′-[γ-[³⁵S]thio]triphosphate)-binding assay. In this assay, the normal GTP-binding event in GPCR (G-protein-coupled receptor) activation is replaced by the binding of the non-hydrolysable analogue [³⁵S]GTP[S]. This assay may be used to profile ligands for their efficacy, and an example here is the D₂ dopamine receptor where an efficacy scale has been set up using this assay. The mechanisms underlying the assay have been probed. The time course of [³⁵S]GTP[S] binding follows a pseudo-first-order reaction with [³⁵S]GTP[S] binding reaching equilibrium after approx. 3 h. The [³⁵S]GTP[S]-binding event is the rate-determining step in the assay. Agonists regulate the maximal level of [³⁵S]GTP[S] bound, rather than the rate constant for binding. The [³⁵S]GTP[S]-binding assay therefore determines agonist efficacy on the basis of the amount of [³⁵S]GTP[S] bound rather than the rate of binding.