Abstract

Inhibition of acetylcholinesterase by sodium fluoride is reversible and depends on binding of one fluoride ion per active center. The inhibitor adds to the free enzyme, the enzyme-substrate complex (blocking acetylation), and the acetyl enzyme (blocking deacetylation). The fluoride binding site is probably in the active center, since cationic inhibitors compete with fluoride for the acetyl enzyme. Small cationic inhibitors do not compete with fluoride for the free enzyme, and fluoride can add to the enzyme-substrate complex whether the substrate is a cation or a neutral molecule. The pH dependence of fluoride binding to the free enzyme and the enzyme-substrate complex is practically identical, but a different pH dependence is observed for the acetyl enzyme. These findings indicate that the site of fluoride attachment is altered during the course of the enzyme reaction, suggesting a conformational change accompanying acetylation.