Evidence for tyrosine at the ligand binding center of beta-adrenergic receptors

Beta 1- and beta 2-adrenergic receptor radioligand antagonist binding activities in plasma membrane preparations from mammalian lung, as well as amphibian and avian red blood cells, have been shown to be inactivated by agents which specifically alkylate tyrosine. In membrane preparations, protection against inactivation was afforded by both agonists and antagonists. In soluble purified preparations, antagonists but not agonists protected against inactivation. These results suggest that tyrosine is located at or near the ligand binding site of both beta 1- and beta 2-adrenergic receptors.