Molecular Pharmacology, Vol 19, 509-512, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Alternative Explanation for the Apparent "Two-Step" Binding Kinetics of High-Affinity Racemic Antagonist Radioligands

¹ Howard Hughes Medical Institute Laboratory, Departments of Medicine (Cardiology) and Biochemistry, Duke University Medical Center, Durham, North Carolina 27710

Recent studies of agonist and antagonist binding to the beta-adrenergic receptor and to other receptors have established the notion of agonist specific binding properties unshared by antagonists and reflecting the activation of the effector. However, previous reports on the dissociation kinetics of the widely used high-affinity beta-adrenergic antagonist (±)-[¹²⁵I]hydroxybenzylpindolol (HYP) have indicated complex binding kinetics which led to the proposal of a receptor isomerization model involving antagonist promoted transitions. We report here that the binding properties of two high-affinity beta-adrenergic antagonists can be fully explained by their racemic nature. (a) Binding data for the association and the dissociation kinetics of (±)-[¹²⁵I]HYP to frog erythrocyte membranes can be adequately fitted by computer modeling assuming different rate constants for each enantiomer. (b) In contrast, purified (+)-[¹²⁵I]HYP shows only uniphasic fast-dissociation kinetics. (c) Similarly, the antagonists (-)-[³H]carazolol and (±)-[³H]carazolol show uniphasic slow-dissociation kinetics and biphasic dissociation kinetics, respectively. These results demonstrate that the complex binding kinetics of these high-affinity racemic radioligands are not due to receptor state transitions but rather to simultaneous binding of both isomers.