Radioligand binding assays are a relatively simple but at the same time a very important and efficient tool to study target-ligand interactions. Since they are very widely used in drug discovery and have been described extensively, this section only discusses their fundamental aspects and those aspects that are important for the description of the MS binding assays below. More detailed information can be found in the relevant literature [6, 7, 16-21].
Radioligand binding assays depend on the use of a radioligand addressing the target of interest. Depending on the assay type performed (e.g. saturation, competition or kinetic experiments) different information is obtained. All types have in common that a radioligand as a marker is first incubated with a target. After a period of time, that is dependent on the aim of the experiment, the target and the radioligand bound to it are separated from the rest of the incubation mixture and, as a last step, the amount of bound radioligand is quantified by measuring the radioactivity. As a result the total binding, that is the specific binding (i.e. binding of the marker to the target) and the nonspecific binding (i.e. binding of the marker to different binding sites independent from the target) is obtained. To ascertain the specific binding, being relevant for the binding assay, the nonspecific binding has to be determined in a separate experiment [7, 16, 17, 21].
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