Biochemical characterization of the beta-adrenergic receptor of the frog erythrocyte

The beta-adrenergic receptor which is coupled to adenylate cyclase in the frog erythrocyte plasma membrane provides a convenient model system for probing the molecular characteristics of an adenylate cyclase coupled hormone receptor. Direct radioligand binding studies with beta-adrenergic agonists and antagonists such as [3H]hydroxybenzylisoproterenol and [3H]dihydroalprenolol have shed new light on the biochemical properties of the receptor as well as on its mode of interaction with other components of the adenylate cyclase system. Agonist binding to the receptor induces a high affinity state of the receptor which can be selectively reverted to a low agonist affinity state by guanyl nucleotides. This agonist-induced high affinity state of the receptor appears to correspond to a receptor moiety which has larger apparent molecular weight and which is probably a complex of the beta-adrenergic receptor and nucleotide regulatory binding protein. Antagonists do not appear capable of inducing or stabilizing the formation of this high affinity receptor-nucleotide site complex. The beta-adrenergic receptors have been solubilized using the plant glycoside digitonin as the detergent and have been highly purified by biospecific affinity chromatography on an alprenolol-agarose affinity support. These highly purified receptor preparations retain all of the binding characteristics observed in the unpurified soluble receptor preparations. Remarkably, antibodies raised in rabbits against affinity chromatography purified preparations of the receptor, themselves bind beta-adrenergic ligands with typical beta-adrenergic specificity. Such antibodies which possess binding sites similar to those of physiological receptors provide useful model systems for further probing the molecular characteristics of beta-adrenergic binding sites.