Photoaffinity labeling of human platelet and rabbit kidney α₂-adrenoceptors with [³H]SKF 102229

A newly developed α₂-adrenergic photoaffinity ligand, 3-methyl-6-chloro-9-azido-1H-2,3,4,5-tetrahydro-3-benzazepine (SKF 102229), has been radiolabeled with tritium to a specific activity of ∼ 80 Ci/mmol. Using membranes prepared from human platelets and from rabbit kidney, α₂-adrenoceptors have been covalently labeled following photolysis in the presence of [³H]SKF 102229. As determined by SDS-PAGE, the apparent molecular weight of α₂-adrenoceptors from both of these tissues was 64,000. The yield of covalent insertion of [³H]SKF 102229 into the α₂-adrenoceptor was very good. Thus, following photolysis up to 90% of the α₂-adrenoceptors could be irreversibly labeled with [³H]SKF 102229.