Kinetics of 13 new cholinesterase inhibitors

Why is the hydrolytic activity of acetylcholinesterase pH dependent? Kinetic study of acetylcholine and acetylthiocholine hydrolysis catalyzed by acetylcholinesterase from electric eel

The dependence of the activity of acetylcholinesterase from electric eel at a pH value range of 4.8–9.8 (phosphate buffer), regarding acetylcholine and acetylthiocholine hydrolysis, was determined at 25 °C, ionic strength of 0.11 M, and initial substrate concentration of 4 mM. At a pH range of 4.8–9.8, the dependences A(pH) form a sigmoid increasing curve with the maximum catalytic activity at a pH range 8–9.5. For acetylcholine hydrolysis, the kinetic reason for such an increase in A consists mainly of an increase in the rate constant k 2 (Michaelis-Menten) model with increasing pH of the reaction mixture. For acetylthiocholine hydrolysis, the kinetic explication of the determined dependence A(pH) is more complicated.

Inhibition of acetylcholinesterase by 14 achiral and five chiral imidazole derivates

Homological series of 14 achiral derivates and series of five chiral derivates of imidazole were tested in vitro as inhibitors of hydrolysis of acetylcholine using enzyme preparation of acetylcholinesterase from electric eel. The batch stirred reactor at 25 degrees C, pH 8 (phosphate buffer), ionic strength 0.11 M and catalytic activity of the enzyme preparation 0.14 U ml(-1) of the reaction mixture were used. The temporal dependences of actual concentrations of acetylcholine, choline and acetic acid were determined by an original HPLC method. For all used inhibitors, these time dependences conform with the probability of more than 90% to the model of competitive irreversible inhibition. All kinetic constants including k(3) defining the rate of inhibition (0.38-5.3M(-1)s(-1)) and qualified estimation of the absolute acetylcholinesterase concentration in the reaction mixture (40-110 nM) were determined.

Irreversible inhibition of the thermophilic esterase EST2 from Alicyclobacillus acidocaldarius

Kinetic studies of irreversible inhibition in recent years have received growing attention owing to their relevance to problems of basic scientific interest as well as to their practical importance. Our studies have been devoted to the characterization of the effects that well-known acetylcholinesterase irreversible inhibitors exert on a carboxylesterase (EST2) from the thermophilic eubacterium Alicyclobacillus acidocaldarius. In particular, sulfonyl inhibitors and the organophosphorous insecticide diethyl-p-nitrophenyl phosphate (paraoxon) have been studied. The incubation of EST2 with sulfonyl inhibitors resulted in a time-dependent inactivation according to a pseudo-first-order kinetics. On the other hand, the EST2 inactivation process elicited by paraoxon, being the inhibition reaction completed immediately after the inhibitor addition, cannot be described as a pseudo-first-order kinetics but is better considered as a high affinity inhibition. The values of apparent rate constants for paraoxon inactivation were determined by monitoring the enzyme/substrate reaction in the presence of the inhibitor, and were compared with those of the sulfonyl inhibitors. The protective effect afforded by a competitive inhibitor on the EST2 irreversible inhibition, and the reactivation of a complex enzyme/irreversible-inhibitor by hydroxylamine and 2-PAM, were also investigated. The data have been discussed in the light of the recently described dual substrate binding mode of EST2, considering that the irreversible inhibitors employed were able to discriminate between the two different binding sites.