An evaluation of the inhibition of human butyrylcholinesterase and acetylcholinesterase by the organophosphate chlorpyrifos oxon

Abstract

Acetylcholinesterase (EC 3.1.1.7) and butyrylcholinesterase (EC 3.1.1.8) are enzymes that belong to the superfamily of α/β-hydrolase fold proteins. While they share many characteristics, they also possess many important differences. For example, whereas they have about 54% amino acid sequence identity, the active site gorge of acetylcholinesterase is considerably smaller than that of butyrylcholinesterase. Moreover, both have been shown to display simple and complex kinetic mechanisms, depending on the particular substrate examined, the substrate concentration, and incubation conditions. In the current study, incubation of butyrylthiocholine in a concentration range of 0.005 mM – 3.0 mM, with 317 pM human butyrylcholinesterase in vitro resulted in rates of production of thiocholine that were accurately described by simple Michaelis-Menten kinetics, with a Km of 0.10 mM. Similarly, the inhibition of butyrylcholinesterase in vitro by the organophosphate chlorpyrifos oxon was described by simple Michaelis-Menten kinetics, with a ki of 3,048 nM−1h−1, and a KD of 2.02 nM. In contrast to inhibition of butyrylcholinesterase, inhibition of human acetylcholinesterase by chlorpyrifos oxon in vitro followed concentration-dependent inhibition kinetics, with the ki increasing as the inhibitor concentration decreased. Chlorpyrifos oxon concentrations of 10 nM and 0.3 nM gave kis of 1.2 nM−1h−1 and 19.3 nM−1h−1, respectively. Although the mechanism of concentration-dependent inhibition kinetics is not known, the much smaller, more restrictive active site gorge of acetylcholinesterase almost certainly plays a role. Similarly, the much larger active site gorge of butyrylcholinesterase likely contributes to its much greater reactivity towards chlorpyrifos oxon, compared to acetylcholinesterase.

Safety assessment and behavioral effects of Solanum guaraniticum leaf extract in rats

“…The enzyme acetylcholinesterase (AChE, EC 3.1.1.7) plays a major role in the regulation of several physiological events, and the activity of the cholinergic system is vital to normal behavior and muscular function (Schetinger et al, 2000;Payne et al, 1996). As such, the crucial role of this enzyme in neural transmission makes it a primary target for a large number of cholinesterase-inhibiting drugs and toxins (Pohanka, 2011), and a valuable diagnostic tool for verifying exposure to chemical agents (Shenouda, Green, Sultatos, 2009). The heme pathway enzyme deltaaminolevulinate dehydratase (δ-ALA-D, EC 4.2.1.24) presents high sensitivity to pro-oxidant situations, and to impairment of metabolic processes, thus being used to evaluate toxicity as well (Nogueira et al, 2003;Souza et al, 2009).…”

Activity of superoxide dysmutase, catalase and glutathione peroxidase in rats exposed to chlorpyrifos and erofloxacin

“…The mechanism of toxic action, similarly to other phosphoroorganic substaces, mainly results from inhibition of enzymes from the group of esterases i.e. : acetylcholinesterases and butyrylcholinesterases (Amitai et al 1998, Shenouda et al 2009). One to the action of enzymes, mainly cytochrome P450, chlorpyrifos is converted to an oxygen analoque -chlorpyrifos oxone, which acts more strongly in comparison with maternal substance -chlorpyrifos (Sams et al 2004, Mutch andWilliams 2006).…”

Semipreparative enantioseparation of methamidophos by HPLC‐UV and preliminary in vitro study of butyrylcholinesterase inhibition

“…Surprisingly, in the present study, methamidophos enantiomers showed a different behavior for plasma BChE in hens, because the (þ)-methamidophos presented an IC50 lower than (À)-methamidophos. This inversion between inhibition of two a/b hydrolyses with 54% of identical amino acid sequence [36] may be explained by the fact that the active site gorge of BChE is higher than that of AChE [37]. Thus, it is possible to say that the results of enantiomers inhibition on esterases are not easily extrapolated from one hydrolyse to another.…”