Ethanol-acetylcholinesterase-inhibitor interactions: inhibitor hydrophobicity and site specificity dependence

1. Depending on the hydrophobicity and the site specificity of an inhibitor, striking differences were found in ethanol-acetylcholinesterase (AChE)-inhibitor interactions. 2. AChE used was from electric eel and was purified by affinity chromatography. 3. Ethanol at 10-200 mM reduced the inhibitory ability of tetrabutylammonium bromide (Bu4NBr). 4. The observed reduction might be a result of Bu4NBr inhibition being partially compensated for by an ethanol activation effect. 5. In contrast to Bu4NBr, propidium and edrophonium are not involved in hydrophobic interaction with AChE. 6. Their abilities to inhibit AChE activity were enhanced by ethanol. 7. Such an enhancement could not result from combining individual perturbations from ethanol and propidium or edrophonium, since ethanol itself increased the AChE activity. 8. In the presence of ethanol, propidium which binds to the peripheral site of the enzyme remained as an uncompetitive inhibitor, while edrophonium which binds to the active site was changed from a competitive inhibitor to a mixed one. 9. The effect of ethanol was therefore greater in the inhibitor which is involved with the active-site binding. 10. Fluorescence quenching studies of propidium-bound enzyme and edrophonium-bound enzyme revealed that ethanol in the concentration less than or equal to 400 mM did not cause significant conformational change at both the peripheral and the active sites of the enzyme.