Iodoazidobenzylpindolol, a photoaffinity probe for the beta-adrenergic receptor.

Abstract
A high-affinity pindolol derivative, (.+-.)-1-(indol-4-yloxy)-3-[1-(p-azido-m-iodophenyl)-2-isobutylamine]-2-propanol (IABP), was prepared; it contains an iodide and an azide functional group and acts as a photoaffinity label for the .beta.-adrenergic receptor. When [125I]IABP (specific activity 1300 Ci/mmol) was photolyzed with crude duck erythrocyte membrane preparations, which contain .beta.-adrenergic receptor binding sites, highly specific labeling of 2 polypeptides was observed upon electrophoresis on sodium dodecyl sulfate/polyacrylamide gels. These 2 polypeptides, A (MW .apprx. 45,000) and B (MW .apprx. 48,500), were photolabeled in a ratio of .apprx. 4:1 A/B. Binding of [125I]IABP and covalent derivatization of the .beta. receptor was at least 70% specific. [125I]IABP is a very effective compound for identification of polypeptides containing the .beta.-receptor binding site even in crude membrane preparations. The .beta. receptor of the duck erythrocyte plasma membrane may be composed of more than 1 subunit.