Comparison of Tryptic Peptides of Benzodiazepine Binding Proteins Photolabeled with [ 3 H]Flunitrazepam or [ 3 H]Ro 15–4513

When rat brain membranes were incubated with the benzodiazepine agonist [ 3 H]flunitrazepam or the partial inverse benzodiazepine agonist [ 3 H]Ro 15–4513 in the presence of ultraviolet light one protein (P 51 ) was specifically and irreversibly labeled in cerebellum and at least two proteins (P 51 and P 55 ) were labeled in hippocampus. After digestion of the membranes with trypsin, protein P 51 was degraded into several peptides. When P 51 was photolabeled with [ 3 H]Ro 15–4513, four peptides with apparent molecular weights of 39,000, 29,000, 21,000, and 17,000 were observed. When P 51 was labeled with [ 3 H]flunitrazepam, only two peptides with apparent molecular weights of 39,000 and 25,000 were obtained. Protein P 55 was only partially degraded by trypsin, and whether it was labeled with [ 3 H]‐flunitrazepam or [ 3 H]Ro 15–4513 it yielded the same two proteolytic peptides with apparent molecular weights of 42,000 and 45,000. These results support the existence of at least two different benzodiazepine receptor subtypes associated with proteins P 51 and P 55 . The different receptors seem to be differentially protected against treatment with trypsin. In addition, these results indicate that in the benzodiazepine receptor subtype associated with P 51 benzodiazepine agonists and partial inverse benzodiazepine agonists irreversibly bind to different parts of the molecule.