Binding characteristics of 3H-dihydroalprenolol to beta-adrenergic receptors of rat brain: influence of exo- and endo-glycosidases and glycopeptidase.

Abstract

The significance of carbohydrate moieties containing the beta-adrenoceptor molecule in the rat brain was examined using radioligand binding assay methods. Thus, this experiment was designed to assess the effects of exoglycosidase (alpha-D-mannosidase and neuraminidase), endoglycosidase (endoglycosidase D and endoglycosidase H), and glycopeptidase A on the affinity of beta-adrenoceptor. The main reason why five kinds of enzymes were used in the present study is that they can hydrolyze different carbohydrate molecules from cell membranes. Rat brain was used and beta-adrenoceptor binding assay was carried out using 3H-dihydroalprenolol (3H-DHA) as a ligand. 3H-DHA binding to beta-adrenoceptors was sensitive to very low concentration of endoglycosidase H and glycopeptidase A, thus indicating that the treatments with these enzymes of rat brain membrane appear to decrease the number of beta-receptor binding sites. On the other hand, the treatment with neuraminidase, endoglycosidase H, and glycopeptidase A of the membrane induced lower values of the dissociation constant (Kd) than those of the control. alpha-D-mannosidase and endoglycosidase D are without effect in spite of the removal of hexose contents and total carbohydrate contents with these treatments, respectively. These results imply that complex type N-linked acidic carbohydrate chains containing neuraminic acid and high mannose type N-linked carbohydrate chains, which are hydrolyzed with endoglycosidase H and glycopeptidase A, of the rat brain membrane containing beta-adrenoceptor molecules play a crucial role in the drug-receptor interaction.