Identification of the serotonin-S2 receptor ligand binding site by photoaffinity labelling with 7-azido-8-[125I]ketanserin ([125I]AZIK).

7-Azido-8-[125I]ketanserin ([125I]AZIK) was characterized as a potent photoaffinity probe for serotonin-S2 receptors. In reversible binding experiments, [125I]AZIK bound with high affinity (Kd = 0.69 nM) to rat frontal cortex membranes. When incubation with [125I]AZIK was followed by UV irradiation, the binding was found to be irreversible. Protection experiments with various drugs demonstrated the serotonin-S2 nature of the photoaffinity labelling. SDS-polyacrylamide gel electrophoresis of the photolabelled membranes allowed one to identify the serotonin-S2 receptor ligand binding site as a single polypeptide with a molecular mass of approx. 67,500 Da. [125I]AZIK will be a valuable tool for the elucidation of the serotonin-S2 receptor structure.