Beta-Adrenergic Receptor Interactions. Characterization of Iodohydroxybenzylpindolol As a Specific Ligand

Hydroxybenzylpindolol (HYP) is a specific and highly potent beta-adrenergic antagonist. Monoiodination of HYP produces an equally high affinity inhibitor of binding to and activation of the beta receptor-coupled adenylate cyclase in turkey erythrocyte membranes. Monoiodohydroxybenzylpindolol was isolated by high pressure liquid chromatography. Mass spectroscopy showed that the iodine was contained in the phenolic moiety of the molecule. 125I-HYP was purified in tracer amounts by ion exchange chromatography; specific activities were achieved (1500 to 2000 Ci/mmol) approaching theoretical for 1 mol of iodine/mol of HYP. 125I-HYP interacts with a single stereospecific site with affinity of 4 to 5 X 10(10) M-1 by Scatchard analysis. Maximal binding capacity was 0.2 to 0.3 pmol/mg of membrane protein. If recovery of receptor were complete, this would correspond to 400 to 600 receptor sites per cell. Kinetic analyses of the on and off reactions gave a kinetically derived KA in good agreement with that derived from thermodynamic methods both at 20 degrees and 37 degrees. No evidence is found in these experiments for cooperative interaction of ligands with the receptor system. Iodohydroxybenzylpindolol thus represents a high affinity, high specific activity ligand of established chemical structure which should prove useful in studying the interaction of other blockers and agonists with the beta-adrenergic receptor in this and other biological systems.