N‐Bromoacetyl‐amino‐cyanopindolol: a highly potent beta‐adrenergic affinity label blocks irreversibly a non‐protein component tightly associated with the receptor

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Abstract
A new chemical affinity label for the .beta.-adrenergic receptor, based on the structure of pindolol, was synthesized and iodinated with 125I. The compound, N-bromoacetylamino-cyanopindolol (BAM-CYP), has an apparent Kd of 44 .+-. 7 pM towards the turkey erythrocyte membranes. This compound blocks irreversibly both the ability of .beta.-adrenergic receptors to bind 125I-cyanopindolol and the ability of .beta.-receptors to activate adenylate cyclase in the presene of .beta.-agonists. The irreversible binding of BAM-CYP to half of the .beta.-receptor sites abolishes the ligand binding activity of all the sites. These findings suggest that the .beta.-receptor is oligomeric in its native state. Although 125I-BAM-CYP blocks irreversibly and specifically the .beta.-adrenergic receptor, it does so by labeling a non-protein component, most probably a water-soluble lipid. The labeling is stereospecific since it is prevented by l-propranolol and not by d-propranolol. This lipid is apparently tightly associated with the receptor in close proximity to the binding site. This water-soluble lipid fraction may prove crucial for the optimal interaction between the .beta.-adrenergic receptor and the components of adenylate cyclase.