Photoaffinity labelling of Ca2+ channels with [3H]azidopine

A 1,4-dihydroypyridine arylazide photoaffinity ligand, 3H]azidopine (50.6 Ci/mmol), has been synthesized. 3H]Azidopine binds reversibly with a Kd of 350 pM to guinea-pig skeletal muscle membranes in the absence of ultraviolet light. The reversible 3H]azidopine binding is inhibited steroselectively by 1,4-dihydropyridines, phenylalkylamine Ca2+ channel blockers and La3+. Covalent incorporation into membrane proteins after photolysis was investigated by sodium dodecyl sulfate polyacrylamide slab gel electrophoresis. 3H]Azidopine is photoincorporated specifically into a protein of Mr approximately 145 000. The covalent labelling of the Mr approximately 145 000 band is inhibited stereoselectively by drugs and cations which block the reversible 3H]azidopine binding. It is suggested that 3H]azidopine is photoincorporated into a subunit of the putative Ca2+ channel.