Activation and inactivation of acetylcholinesterase by metal ions

Abstract

The kinetic consequences of acetylcholinesterase peripheral site occupation by metal ions were examined using three substrates; acetylthiocholine, p-nitrophenylacetate, and 7-(dimethylcarbamoyloxy)-N-methylquinolinium iodide. Two classes of metal ion effects were noted: activation by a group including Mg2+, Ca2+, Mn2+, and Na+, and inactivation by a second group which to date includes Zn2+, Cd2+, Hg2+, Ni2+, Cu2+, and Pb2+. Activation is demonstrable only in solutions of low ionic strength whereas inactivation can be readily observed in solutions of both low and high ionic strength. Activation appears to be due to a combination of metal ion binding and ionic strength effects and involves binding to peripheral sites which are distinct from those which bind organic cationic activators such as gallamine, propidium, and 7-(dimethylcarbamoyloxy)-N-methylquinolinium. The principal activating effect is on the deacylation phase of the enzyme-substrate reaction. Inactivators effect a slow conversion of the enzyme to an unreactive form. The kinetics of inactivation are biphasic at low ionic strength but become essentially monophasic at high ionic strength. More than 80% of the enzyme activity can be recovered upon addition of EDTA provided the chelating agent is added immediately following completion of the inactivation process. Prolonged exposure to inactivators results in a progressive decrease in the amount of recoverable activity, Although peripheral ligand interactions may result in a variety of catalytic site conformations, the macroscopic properties can be accounted for in terms of three ligand-dependent states of the enzyme in which catalytic ability (actual or potential) is retained, and a fourth denatured state.

Therapeutic efficacy of chlorogenic acid on cadmium-induced oxidative neuropathy in a murine model

“…The modulation of AChE in the brain displayed by Cd is concentration dependent; at a low concentration (0.01 mM) activation of AChE occurs whilst at higher concentrations (>0.1 mM) AChE activity is inhibited (7). Further, Cd induces a conformational change in AChE that leads to the enzyme becoming unreactive (50). In the present study toxic doses (5 mg/kg) of Cd were reflected by diminished levels of AChE and treatment with CA restored the activity of AChE in the brain.…”

Fish Cholinesterases as Biomarkers of Organophosphorus and Carbamate Pesticides

“…a -60% inhibition; b -30% inhibition; c -30% inhibition. Ions can alter cholinesterase activity inhibiting or activating so that some authors even propose the enzymes as biomarkers of heavy metals and other pollutants (Abou-Donia and Menzel, 1967;Mukherjee and Bhattacharya, 1974;Olson and Christensen, 1980;Tomlinson et al, 1981;Hughes and Bennett, 1985;Gill et al, 1990;1991;Payne et al, 1996;Devi et al, 1996;Najimi, 1997;Reddy et al, 2003). This fact is not always taken into account during the use of cholinesterases as biomarkers of pesticides and can lead to false positives or negatives and misinterpretation of results.…”

“…This fact is not always taken into account during the use of cholinesterases as biomarkers of pesticides and can lead to false positives or negatives and misinterpretation of results. Tomlinson et al (1981) described that activation by ions is only observed in conditions of low ionic strength, while inhibition can be noted in both low and high ionic strength. Thus, heavy metals and ions can be present in samples of environmental matrices, as well as in food samples.…”

“…It could be concluded that phosphate is the best buffer for pesticide assays and that Tris is the best alternative for heavy metals assays. However, Tomlinson et al (1981) reported that EDTA has a protective action against divalent metallic cations which can cause some interference. Chatonnet and Lockridge (1989) reviewed cholinesterases and reported the different extracting strategies caused by ChEs molecular polymorphism: the globular forms G1, G2 and G4 are extractable in low ionic strength buffers (G2 glycophospholipid-linked is the form found in erythrocytes and in Torpedo electric organ, while G4 lipid-linked is present in vertebrates brain).…”

Ameliorative Effects of the Homeopathic Medicine Lycopodium 200c and Extract of <i>Phyllanthus emblica</i> in Cadmium-Induced Neurotoxicity in Mice

“…A significant reduction of intracellular thiols and antioxidants has been seen after the interaction of cadmium with mitochondrial sites 18 . It was also found that Cd acts as metal inactivator of the enzymes and induces a conformational change of the protein which leads to the formation of an 'unreactive' enzyme species 19 . Cadmium-induced free radical production in the brain of the mice interferes with the antioxidant defence system and leads to an alteration of the structural integrity of membrane lipids and membrane-bound enzymes, for e.g.…”