Acetylcholinesterase inhibition kinetics

Ali HM, Sharaf EHA, Hikal MS. Selectivity, acetylcholinesterase inhibition kinetics and quantitative structure-activity relationships of a series of N-(2-oxido-l,3,2-benzodioxa-phosphol-2-yl) amino acid ethyl or diethyl esters. Pest Biochem Physiol 2005 83 58-65. 

Although OPPs and carbamates exhibit very similar modes of action in various animal species, i.e, acetylcholinesterase inhibition in the CNS with resulting paralysis—there is an important difference between the two classes of pesticides. Carbamates do not require metabolic conversion prior to exhibiting their toxicity. Furthermore the enzyme activity may at times be rapidly regenerated by reversal of inhibition. The kinetics of the inhibition (carbamoylation) reaction have been well studied in it electrophilic carbamoyl moieties form covalent bonds with enzyme esteratic sites. This is followed by carbamate transfer of an acidic group to the site to yield the acetylated enzyme complex (ref. 176). 

Weinstock M, et al. Acetylcholinesterase inhibition by novel carbamates a kinetic and nuclear magnetic resonance study. In Multidisciplinary Approaches to Cholinesterase Functions. Velan B (Ed.). Plenum Press, New York, 1992,... 

Worek, F., Tbiermarm, H., Szinicz, L. (2004). Reactivation and aging kinetics of human acetylcholinesterase inhibited by organophosphonylcholines. Toxicol. 78 212-17. 

Patocka, J., Cabal, J., Kuca, K., Jun, D. (2005). Oxime reactivation of acetylcholinesterase inhibited by toxic phosphorus esters in vitro kinetics and thermodynamics. J. Appl Biomed. 2 91-9. 

Su C-T, Wang P-H, Liu R-F et al. (1986). Kinetic studies and structure-activity relationships of bispyridinium oximes as reactivators of acetylcholinesterase inhibited by organophosphorus compounds. Fund Appl Toxicol, 6, 506-524. 

Wcinstock, M Razin, M, Ringcl, I., Tashma, Z., and Chorev, M. (1992). Acetylcholinesterase inhibition by novel carbamatc.s A kinetic and nuclear magnetic rc.sonancc study. In Multidisciplinary Approaches to Choline.stera.se FunctUms (A. Shafferman and E. Vclan, Eds.), pp, 251-259. Plenum. New York. 

Kamal MA, Greig NH, Alhomida AS, Al-Jafari AA (2000) Kinetics of human acetylcholinesterase inhibition by the novel experimental Alzheimer therapeutic agort, tolserine. Biochem Pharmacol 60 561-570... 

Jianmongkol, S., Marable, B.R., Berkman, C.W, et al., 1999. Kinetic evidence for different mechanisms of acetylcholinesterase inhibition by (IR)- and (IS)-stereoisomers of isomalathion. Toxicol. Appl. Pharmacol. 155, 43-53. 

An enzymatic assay can also be used for detecting anatoxin-a(s). " This toxin inhibits acetylcholinesterase, which can be measured by a colorimetric reaction, i.e. reaction of the acetyl group, liberated enzymatically from acetylcholine, with dithiobisnitrobenzoic acid. The assay is performed in microtitre plates, and the presence of toxin detected by a reduction in absorbance at 410 nm when read in a plate reader in kinetic mode over a 5 minute period. The assay is not specific for anatoxin-a(s) since it responds to other acetylcholinesterase inhibitors, e.g. organophosphoriis pesticides, and would need to be followed by confirmatory tests for the cyanobacterial toxin. 

Antibody 15C5 was able to catalyse the hydrolysis of the triester [105] with cat 2.65 x 10 3 min 1 whilst a second antibody from the same immunization programme was later found to hydrolyse the acetylcholinesterase inhibitor Paraoxon [106] with kcat = 1.95 x 10 3min-1 at 25°C (Appendix entry 6.2) (Lavey and Janda, 1996b). Antibody 3H5 showed Michaelis-Menten kinetics and was strongly inhibited by the hapten [104]. It exhibited a linear dependence of the rate of hydrolysis on hydroxide ion concentration, suggesting that 3H5 effects catalysis by transition state stabilization rather than by general acid/base catalysis. 

Acetylcholinesterase is subject to substrate Inhibition at high concentrations, but Mlchaells kinetics are observed at lower concentrations, because the substrate constant and the Mlchaells constant differ by a factor of 100. Turnover numbers run about 2-9 x 10 min l, and (Mlchaells constant) values are about 0.2 mM.76,116 Whatever the source, the enzyme is subject to inhibition by the same reversible and irreversible inhibitors. Most of the kinetic work has been done with the saline-extracted 11S enzyme from electric eel and the detergent-extracted 6S enzyme from erythrocytes. The former Is a tetramer derived from the native enzyme by the action of proteases the latter is a dimer. 

YY Falah, WE Hammers. Kinetics of the inhibition of electric eel and honeybee head acetylcholinesterase by organophosphorus pesticides and A -mcthylcarbamatcs using JV-methylindoxyl acetate as the substrate. Toxicol Environ Chem 42 19-34, 1994. 

A reaction looked at earlier simulates borate inhibition of serine proteinases.33 Resorufin acetate (234) is proposed as an attractive substrate to use with chymotrypsin since the absorbance of the product is several times more intense than that formed when the more usual p-nitrophcnyl acetate is used as a substrate. The steady-state values are the same for the two substrates, which is expected if the slow deacylation step involves a common intermediate. Experiments show that the acetate can bind to chymotrypsin other than at the active site.210 Brownian dynamics simulations of the encounter kinetics between the active site of an acetylcholinesterase and a charged substrate together with ah initio quantum chemical calculations using the 3-21G set to probe the transformation of the Michaelis complex into a covalently bound tetrahedral intermediate have been carried out.211 The Glu 199 residue located near the enzyme active triad boosts acetylcholinesterase activity by increasing the encounter rate due to the favourable modification of the electric field inside the enzyme and by stabilization of the TS for the first chemical step of catalysis.211... 

J.A. Doom, D.A. Gage, M. Schall, T.T. Talley, C.M. Thompson and R.J. Richardson, Inhibition of acetylcholinesterase by (1S, 3,Y)-isomalalhion proceeds with loss of thiomethyl Kinetic and Mass Spectral Evidence for an Unexpected Primary Leaving Group, Chem. Res. Toxicol., 13, 1313-1320 (2000). 

Worek, F., Aurbek, N., Wetherell, J., Pearce, P., Mann, T., Thiermann, H. (2008). Inhibition, reactivation and aging kinetics of highly toxic OP compounds pig versus minipig acetylcholinesterase. Toxicology 244 35-41. 

Doom, J.A., Gage, D.A., Schall, M., Talley, T.T., Thompson, C.M., Richardson, R.J. (2000). Inhibition of acetylcholinesterase by (15,35)-isomalathion proceeds with loss of thio-methyl kinetic and mass spectral evidence for an unexpected primary leaving group. Chem. Res. Toxicol. 13 1313-20. 

Stojiljkovic, M.P., Pantelic, D., Maksimovic, M. (2001). Tabun, sarin, soman and VX poisoning in rats kinetics of inhibition of central and peripheral acetylcholinesterase, ageing, spontaneous and oxime-facilitated reactivation. Vll International Symposium on Protection against Chemical and Biological Agents. Stockholm, Sweden, June 15-19,... 

Worek, F., Reiter, G., Eyer, P., Szinicz, L. (2002). Reactivation kinetics of acetylcholinesterase from different species inhibited by highly toxic organophosphates. Arch. Toxicol. 76 523-9. Yang, G.Y., Yoon, J.H., Seong, C.M., Park, N.S., Jung, Y.S. (2003). Synthesis of bis-pyridinium oxime antidotes using... 

Gray, P.J. and Dawson, R. 1987. Kinetic constants for the inhibition of eel and rabbit brain acetylcholinesterase by some organophosphates and carbamates of mihtary significance. Toxicol. Appl. Pharmacol., 91 140-144. 

Worek, E. et al. Reactivation kinetics of acetylcholinesterase from different species inhibited by highly toxic... 

Activation at high substrate concentrations not only explains the failure of butyrylcholinesterase to follow simple Michaelis-Menton kinetics, but also explains the enigma of substrate inhibition of the enzyme using either benzoylcholine (A21, T7) or acetyl- or butyryl-salicylcholine as substrates. The proposal made by Hastings is analogous to that of Myers (M24, M25) for the inhibition of acetylcholinesterase by excess substrate, in this case acetylcholine. 

Richardson, R. J., Moore, T.B., Kayyali, U.S., Fowke, J.H., and Randall, J.C. Inhibition of hen brain acetylcholinesterase and neurotoxic esterase by chlorpyrifos in vivo and kinetics of inhibition by chlorpyrifos oxon in vitro. Application to assessment of neuropathic risk, Fundam. Appl. Toxicol. 20,273-279,1993. 

Carr. L. R., and Chambers, J. E. (1996). Kinetic analysis of in vitro inhibition, aging and reactivation of brain acetylcholinesterase from rat and channel cat fish by paraoxon and chlorpyrifo.s-oxon, Toxicol. Appl. Pharmacol. 139, 365-373,... 

---