ABSTRACT
In this chapter, we look at ways in which the binding of ligands to macromolecules can be directly investigated. Although most interest centers on the interaction of drugs and hormones with receptors, the approach taken here can be applied to any similar process — for example, the combination of drugs with ion channels or membrane transport systems. The binding of ligands, including drugs, to plasma proteins has been studied for more than 50 years, but the study of binding to the much smaller amounts of protein (e.g., receptors) in cell membranes is more recent, having become feasible only when suitable radioactively labeled ligands became available. The first rigorous study of drug binding to receptors was that of Paton and Rang (1965), who investigated
H-atropine binding to muscarinic receptors in smooth muscle. The use of radiolabeled drugs in radioligandbinding studies is now common and for many pharmaceutical manufacturers forms an essential part of the screening process, providing a rapid means of determining the affinity of new drugs for a wide range of receptors. Labeling of drugs with radioisotopes is attractive because very small quantities, often as low as 1 fmol, can be readily and accurately measured. Receptor pharmacologists are also interested in the measurement of ligand concentration by fluorescence, but this, of course, requires the availability or novel synthesis of ligands with suitable fluorescent moieties, and currently this method requires substantially higher ligand concentrations. Fluorescent probes do, however, have a particular utility in kinetic experiments, where the changes in fluorescence that occur on binding are immediate, allowing the binding to be continuously monitored.
