| Abstract: | We previously reported that muscarinic acetylcholine receptors (mAChRs) from porcine brains are glycoproteins. When porcine brain membranes were solubilized with digitonin or 3-(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS), approximately 20% of the receptors were solubilized, most (90% or more) of which bound to Sepharose 4B conjugated with wheat germ agglutinin (WGA). In contrast, when membranes were solubilized with Lubrol PX, a much larger fraction (approximately 60%) of the receptors were solubilized. However, about a third of this solubilized receptor population remained unbound to WGA-Sepharose even in the presence of an excess amount of the lectin-Sepharose. These results suggested a structural heterogeneity of the mAChR in terms of its carbohydrate moiety. The effects of lectins on the ligand binding properties of mAChRs were also studied. WGA or concanavalin A (ConA) was found to cause a 2- to 3-fold increase in the affinity of membrane-bound receptors to an antagonist 3H]quinuclidinyl benzylate ( 3H]QNB) without affecting the maximum number of sites, whereas the lectins had no significant effects on the binding of the agonist 3H]cis-methyldioxolane. When the membranes were dissolved with detergents, lectin did not increase the 3H]QNB affinity: These lectins caused an approximately 2 fold decrease in the affinity of digitonin-solubilized receptors for 3H]QNB. Thus the lectins exert differential effects on agonist and antagonist binding to the brain membrane mAChRs, most likely by modulating some intermolecular interactions. |
