[Identification of the ligand binding sites and novel drug target molecules by photoaffinity labeling]

Photoaffinity labeling is a useful and reliable method for 1) the identification of the ligand-target receptor and 2) the structural investigation of its binding site. Using photoaffinity labeling techniques, the binding sites of four typical calcium antagonists, 1,4-dihydropyridines, benzothiazepines, phenylalkylamines, and benzothiazines, were successfully identified within the primary structure of the skeletal muscle calcium channels. The results confirm pharmacological observations of the four antagonists, which had been proposed to interact allosterically with each other. Secondarily we demonstrated that human glutathione S-transferase class pi (GST pi) is specifically photolabeled by the antidiabetic agent sulfonylurea glibenclamide (GB) and it also inhibits the enzyme activities of glutathione conjugation by GB in a competitive manner for glutathione. These results indicate that GST pi is another target molecule of sulfonylurea since a subunit of ATP-sensitive potassium channels is well known to be a sulfonylurea receptor. This review focuses on photoaffinity labeling techniques as a useful tool for drug discovery and development.