Photoaffinity labeling of mammalian α₁-adrenergic receptors. Identification of the ligand binding subunit with a high affinity radioiodinated probe

We have synthesized and characterized a novel high affinity radioiodinated α₁-adrenergic receptor photoaffinity probe, 4-amino-6,7-dimethoxy-2-[4-[5-(4-azido-3-[¹²⁵I]iodophenyl)pentanoyl]-1-p iperazinyl] quinazoline. In the absence of light, this ligand binds with high affinity (K(D) = 130 pM) in a reversible and saturable manner to sites in rat hepatic plasma membranes. The binding is stereoselective and competitively inhibited by adrenergic agonists and antagonists with an α₁-adrenergic specificity. Upon photolysis, this ligand incorporates irreversibly into plasma membranes prepared from several mammalian tissues including rat liver, rat, guinea pig, and rabbit spleen, rabbit lung, and rabbit aorta vascular smooth muscle cells, also with typical α₁-adrenergic specificity. Autoradiograms of such membrane samples subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveal a major specifically labeled polypeptide at M(r) = 78,000-85,000, depending on the tissue used, in addition to some lower molecular weight peptides. Protease inhibitors, in particular EDTA, a metalloprotease inhibitor, dramatically increases the predominance of the M(r) = 78,000-85,000 polypeptide while attenuating the labeling of the lower molecular weight bands. This new high affinity radioiodinated photoaffinity probe should be of great value for the molecular characterization of the α₁-adrenergic receptor.