Inhibition kinetics of acetylcholinesterase with fluoromethyl ketones

A series of trifluoromethyl ketones that reversibly inhibit acetylcholinesterase and pseudocholinesterase were synthesized. By analogy to chymotrypsin and on the basis of data reported here, we propose that the active-site serine adds to the ketone to form an ionized hemiketal. The compound (5,5,5-trifluoro-4-oxopentyl)trimethylammonium bicarbonate (1) inhibits acetylcholinesterase with Ki = 0.06 X 10(-9)M and pseudocholinesterase with Ki = 70 X 10(-9)M. Replacement of the nitrogen of 1 by carbon (compound 2) increases Ki for 1 200-fold for acetylcholinesterase but does not significantly alter Ki for pseudocholinesterase. The Ki for the methyl ketone corresponding to 2 is 2 X 10(-4)M for both enzymes, as compared with 12 X 10(-9)M for the trifluoromethyl ketone (acetylcholinesterase). For both enzymes, a linear decrease in log Ki with decreasing pK of the inhibitor hydrate was observed with ketones containing from 0 to 3 fluorines. We attribute this effect to the stabilization of the hemiketal oxyanion. The reduction of the pK of the hemiketal by the trifluoromethyl group is an important contributing factor to the low Ki of trifluoromethyl ketones. The inhibition of acetylcholinesterase by tetramethylammonium chloride and trifluoroacetone was compared to the inhibition by 1, which is a composite of the two smaller inhibitors. The entropic advantage of combining the smaller inhibitors into one molecule is 1.1 X 10(3)M. Inhibitors with Ki less than or equal to 70 X 10(-9) M are slow binding (Morrison, 1982; Morrison & Walsh, 1988). The kinetic data do not require formation of a noncovalent complex prior to formation of the ketal, although such a complex(es) cannot be excluded.(ABSTRACT TRUNCATED AT 250 WORDS)