Covalent Labeling of the Cerebral Cortex Alpha 1-adrenergic Receptor with a New High Affinity Radioiodinated Photoaffinity Probe

A novel high affinity radioiodinated photoaffinity probe, 4-amino-6,7-dimethoxy-2[4-[5(3-[125I]iodo-4-azidophenyl)pentanoyl]-1- piperazinyl]-quinazoline, structurally related to the potent alpha 1-adrenergic antagonist prazosin, was developed and used to covalently label the rat cerebral cortex alpha 1-adrenergic receptor. In the absence of light, this ligand binds to cortex plasma membranes with a dissociation constant of 308 pM and with a maximal number of binding sites of 200 fmol/mg protein. Upon photolysis, the ligand incorporates irreversibly into plasma membrane proteins. Autoradiograms of such membrane samples subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveal a major specifically labeled polypeptide at Mr = 79,000. The covalent incorporation into the peptide at Mr = 79,000 can be inhibited by several adrenergic receptor ligands with a typical alpha 1-adrenergic receptor specificity and stereoselectivity.