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 (Emax, KA/EC50, Emax·KA/EC50) determined. The Emax·KA/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.

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