Evidence for essential disulfide bonds in beta1-adrenergic receptors of turkey erythrocyte membranes. Inactivation by dithiothreitol.

Abstract

The radiolabeled β-adrenergic antagonists (-)-[3H]-dihydroalprenolol binds to a single class of noncooperative sites on turkey erythrocyte membranes. These sites have previously been identified as the functional β-adrenergic receptors. Treatment of the membranes with the reducing agent dithiothreitol causes a decrease in the number of binding sites, without affecting the affinity of (-)-[3H]dihydroalprenolol for the remaining sites. The binding activity is partially restored by extensive washing of the dithiothreitol-treated membranes. No restoration occurs when the wash buffer contains 2 mM N-ethylmaleimide or 10 mM reduced glutathione. The effect of dithiohreitol is mimicked by a hundredfold higher concentration of the monosulfydryl derivatives: reduced glutathione, cysteine, and mercaptoethanol. In contrast, treatment of the membranes with the metal chelators ethylenediamine tetraacetate and ethylene glycol bis(β-amino-ethyl ether)N,N'-tetraacetic acid (10 mM) does not affect (-)-[3H]dihydroalprenolol binding. Kinetic data indicate that dithiothreitol inactivates the β-receptors according to a bimolecular reaction mechanism, with a second order rate constant (k2) of approximately 1.27 M-1 x S-1 at 30°C. The data suggest that dithiothreitol inactivates the β-receptors by reducing one or more disulfide bonds. Both β-adrenergic agonists and antagonists cause an effective protection of the (-)-[3H]dihydroalprenolol binding sites against inactivation by dithiothreitol. The protection is dose-dependent, and linearly related to the fraction of receptor sites occupied by the tracer. The protection is stereospecific for both agonists (epinephrine) and antagonists (propranolol) and reflects, for the same concentration of agonists, the order of affinities for the receptor. The α-adrenergic agents clonidine (agonist) and phentolamine (antagonist), and the nonbioactive compound pyrocatechol do not confer an appreciable protection at concentrations as high as 100 μM. Receptor protection by β-adrenergic agonists and antagonists proceed by causing a conformational change of the receptors as to bury the disulfide bonds or by shielding bonds located near or at the binding site of the receptor.