Alternative explanation for the apparent "two-step" binding kinetics of high-affinity racemic antagonist radioligands
Bürgisser, E.; Lefkowitz, R.J.; DeLean, A.
Molecular Pharmacology 19(3): 509-512
1981
ISSN/ISBN: 0026-895X PMID: 6267448 Document Number: 168538
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. 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. The binding properties of 2 high-affinity .beta.-adrenergic antagonists can be fully explained by their racemic nature. 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. In contrast, purified (+)-[125I]HYP shows only uniphasic fast-dissociation kinetics. The antagonists (-)-[3H]carazolol and .+-.-[3H]carazolol show uniphasic slow-dissociation kinetics and biphasic dissociation kinetics, respectively. The complex binding kinetics of the high-affinity racemic radioligands are not due to receptor state transitions but rather to simultaneous binding of both isomers.