Acetylcholinesterase phosphorylation

Table IV. Reduced Rate of Acetylcholinesterase Phosphorylation in Resistant Tobacco Budworm Larvae, Heliothis virescens...

This process of aging is believed to be critical in the development of delayed neuropathy, after NTE has been phosphorylated by an OP (see Chapter 10, Section 10.2.4). It is believed that most, if not all, of the B-esterases are sensitive to inhibition by OPs because they, too, have reactive serine at their active sites. It is important to emphasize that the interaction shown in Fignre 2.11 occurs with OPs that contain an oxon group. Phosphorothionates, which contain instead a thion group, do not readily interact in this way. Many OP insecticides are phosphorothionates, but these need to be converted to phosphate (oxon) forms by oxidative desulfuration before inhibition of acetylcholinesterase can proceed to any significant extent (see Section 2.3.2.2). 

Fish rapidly absorb, metabolize, and excrete chlorpyrifos from the diet (Barron etal. 1991). The mechanism of action of chlorpyrifos occurs via phosphorylation of the active site of acetylcholinesterase after initial formation of chlorpyrifos oxon by oxidative desulfuration. In studies with channel catfish (Ictalurus punctatus), the oral bioavailability of chlorpyrifos was 41%, substantially higher than in mammals. Catfish muscle contained less than 5% of the oral dose with an... 

G. B. Quistad, N. Zhang, S. E. Sparks, J. E. Casida, Phosphoacetylcholinesterase Toxicity of Phosphorus Oxychloride to Mammals and Insects That Can be Attributed to Selective Phosphorylation of Acetylcholinesterase by Phosphorodichloridic Acid , Chem. Res. Toxicol. 2000, 13, 652-657. 

Obidoxime is an antidote used to treat poisoning with insecticides of the organophosphate type (p. 102). Phosphorylation of acetylcholinesterase causes an irreversible inhibition of ace-Ltillmann, Color Atlas of Pharmacology 2000 Thieme All rights reserved. Usage subject to terms and conditions of license. 

Hobbiger, F. Reactivation of phosphorylated acetylcholinesterase. In Koelle, G.B., ed. Cholinesterases and Anticholinesterase Agents. Handb. Exp. Pharmakol. 15 921-988, 1963. Berlin Springer-Verlag. 

The first suggestion of a practical form of antidotal therapy came in 1949 from Hestrin, who found that acetylcholinesterase (AChE) catalyzed the formation of acetohydroxamlc acid when incubated with sodium acetate and hydroxylamine. Critical in vitro studies in the next decade led to the development of a practical approach to therapy. The crucial concept in these studies was the recognition that the compound formed when AChE reacted with a phosphorus ester was a covalent phosphoryl-enzyme Intermediate similar to that formed in the hydrolysis of acetylcholine. 3 Wilson and colleagues, beginning in 1951, demonstrated that AChE inhibited by alkyl phosphate esters (tetraethyl pyrophosphate, TEPP) could be reactivated by water, but that free enzyme formed much more rapidly in the presence of hydroxylamine. 0 21 Similar results... 

Thus, the phosphorylated oxime can itself be a potent cholinesterase inhibitor. Wilson and Ginsburg advanced this concept when they noticed incomplete reactivation of acetylcholinesterase by 2-PAM and suggested that the phosphorylated oxime reinhibited the enzyme (according to Reaction 2). 

It causes reactivation of the phosphorylated acetylcholinesterase enzyme. After administration, it is metabolised in liver. 

In mammals as well as insects, the major effect of these agents is inhibition of acetylcholinesterase through phosphorylation of... 

Diazinon toxicity results predominantly from the inhibition of acetylcholinesterase in the central and peripheral nervous system. The enzyme is responsible for terminating the action of the neurotransmitter, acetylcholine, in the synapse of the pre- and post-synaptic nerve endings or in the neuromuscular junction. However, the action of acetylcholine does not persist long as it is hydrolyzed by the enzyme, acetylcholinesterase, and rapidly removed. As an anticholinesterase organophosphate, diazinon inhibits acetylcholinesterase by reacting with the active site to form a stable phosphorylated complex which is incapable of destroying acetylcholine at the synaptic gutter between the pre- and post-synaptic nerve... 

Figure 7.50 The mechanism of reaction of the antidote pralidoxime with phosphorylated acetylcholinesterase. The original acetylcholinesterase is thereby regenerated.

The mode of action, which is now well authenticated and understood, involves the irreversible inhibition of the enzyme acetylcholinesterase, which is essential to nervous conduction in insects, by phosphorylation of a hydroxy group at the active site. The detailed mechanisms have been reviewed by O Brien (B-67MI10700, B-76MI10701). 

The characteristic high group transfer potential of a phospho group in pyrophosphate linkage, which makes ATP so useful in cells, also permits tetraethyl pyrophosphate to phosphorylate active sites of acetylcholinesterases. While TEPP is very toxic, it is rapidly hydrolyzed all harmful residues are gone within a few hours after use. 

Carbary , a widely used methyl carbamate, is a pseudosubstrate of acetylcholinesterase that reacts 105 to 106 times more slowly than do normal substrates. The carbamoylated enzyme formed is not as stable as the phosphorylated enzymes and the inhibition is reversible. 

The OPPs inhibit acetylcholinesterase (AChE) by phosphorylating the esteratic site of the enzyme. As a result of AChE inhibition, ACh accumulates and binds to muscarinic and nicotinic receptors throughout the nervous system. Transformation of OPPs in the organisms takes place by conversion of the phosphorothioate (P=S) group to oxon (P=0) analogs. These oxo compounds are of concern because they are the activated forms of the OPPs, with a considerably stronger inhibition of acetylcholinesterase activity (27). 

The ketoxime derivative fluvoxamine (12) is a newer antidepressant thought to potentiate the action of 5-hydroxytryptamine76. Oxacillin (13), cefuroxime (14) as well as the monobactam aztreonam (15) represent potent antibacterial agents of the beta-lactam type77. The aldoxime pralidoxime (16) and a number of bi.v-quarternary oximes, such as obidoxime (17), can be used as reactivators of the phosphorylated esteratic site of acetylcholinesterase that occurs in the presence of organophosphate inhibitors78,79. 

Reversible Masking of Acetylcholinesterase by Covalent Phosphorylation in the Presence of a Novel Cyclic Phosphate Ester... 

Quistad, G.B. et al., Toxicity of phosphorus oxychloride to mammals and insects that can be attributed to selective phosphorylation of acetylcholinesterase by phosphorodichloridic acid, Chem. Res. Toxicol., 13, 652-657, 2000. 

Answer A hormone can be degraded by extracellular enzymes (such as acetylcholinesterase). The GTP bound to a G protein can be hydrolyzed to GDP. A second messenger can be degraded (cAMP, cGMP), further metabolized (IP3), or resequestered (Ca2 +, in the endoplasmic reticulum). A receptor can be desensitized (acetylcholine receptor/channel), phosphorylated/inactivated, bound to an arrestin, or removed from the surface ( -adrenergic receptor, rhodopsin). 

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