The effects of ethanol on the structural stability of acetylcholine receptor and the activity of various molecular forms of acetylcholinesterase

The actions of ethanol on the structural stability of acetylcholine receptor (AchR)-enriched membrane vesicles and the activity of various molecular forms of acetylcholinesterase (AchE) were investigated, using the receptor and the enzyme isolated from the electric organ of Torpedo californica. In the presence of ethanol up to 200 mM, the thermogram of AchR-enriched membranes exhibited no significant decrease in the temperature (td) of receptor transition at 57 degrees C, but a decrease in the enthalpy change (delta Hd) indicated a slight ethanol-induced structural perturbation. The presence of 12.5 nmol alpha-bungarotoxin also caused a decrease in delta Hd. A complete loss of the receptor transition was observed at a higher concentration 500 nmol of alpha-bungarotoxin and no recovery of the transition was found with the addition of 200 mM ethanol. The results suggested a noncompetitive interaction of ethanol with the receptor. In the presence of 200-1000 mM ethanol, the activity of two soluble forms of AchE, a higher (117 S) aggregate and a lower (10 S) aggregate was not significantly affected. Comparing the activity of these two aggregates over a wide concentration range of ethanol (200-2000 mM) revealed no obvious difference in the level of ethanol effect between them. However, after removal of ethanol, the higher aggregate form of AchE exhibited a greater recoverability of the activity, suggesting a possible slightly greater structure-functional stability for it. Studies of soluble AchE and membrane-bound AchE showed that the presence of 200 or 600 mM ethanol caused a greater level of inhibition in membrane-bound enzyme than in soluble enzyme, possible due to a disruption of protein-lipid interaction needed to maintain the conformation of membrane-bound AchE. Interestingly, at a much higher concentration of ethanol (2.0 M), membrane-bound AchE became more resistant to ethanol than did the soluble forms of AchE. In this case, the effective concentration of ethanol felt by the enzyme was expected to be less for membrane-bound AchE, owing to ethanol's solubility in lipids.