Phosphorylated acetylcholinesterase reactivators

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. 

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

FIGURE 65.2. Reactivation of phosphorylated acetylcholinesterase with pralidoxime and formation of reactivated enzyme and phosphorylated oxime. 

L35. Loomis, T. A., Welsh, M. J., Jr., and Miller, G. T., A comparative study of some pyridinium oximes as reactivators of phosphorylated acetylcholinesterase and as antidotes in sarin poisoning. Toxicol. Appl. Pharmacol. 5, 588-598 (1963). 

H12. Hobbiger, F., and Vojvodic, Z., The reactivation by pyridinium aldoximes of phosphorylated acetylcholinesterase in the central nervous system. Biochem. Pharmacol. 16, 455-462 (1967). 

Hobbiger, P.W., Vojvodic, V., 1966. The reactivating and antidotal actions of N, N-trimethylenbis (pyridinium-4-aldoxime) (TMB-4) and N,N-oxydimethylenbis (pyridinium-4-aldoxime) (toxogonin), with particularly reference to their effect on phosphorylated acetylcholinesterase in the brain. Biochem. Pharmacol. 15, 1677-1690. 

Saint-Andre, G., Kliachyna, M., Kodepelly, S., et al, 2011. Design, synthesis and evaluation of new a-nucleophiles for the hydrolysis of organophoshorus nerve agents application to the reactivation of phosphorylated acetylcholinesterase. Tetrahedron 67, 6352-6361. 

Antidotes. Because the acute toxicity to man of many of the phosphorus insecticides is high, first-aid remedies are kept on hand. The most useful of these is an injection of atropine (which acts primarily on muscarinic sites) followed by an oxime specific for nicotinic sites. These oximes reactivate phosphorylated acetylcholinesterase in patients (Holmes and Robins, 1955) just as with the isolated enzyme (Wilson and Meislich, 1953). This reaction involves a competition, between the hydroxyl-groups of serine and of the hydroxylamine, for the phsphoryl-group. The covalent bond with the serine is broken, and simultaneously a new covalent bond formed with the hydroxylamine (see Scheme 12.1). One of the best reactivators is pralidoxime 12.27) (2-PAM), which is the anti form of pyridine-2-aldoxime methochloride. Widely differing doses of antidote are required, depending on the strength of the phosphate-enzyme bond, which varies with the nature of the insecticide. 

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). 

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). 

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. 

Luo, C., Saxena, A., Smith, M., Garcia, G., Radic, Z., Taylor, P., Doctor, B.P. (1999). Phosphoryl oxime inhibition of acetylcholinesterase during oxime reactivation is prevented by edrophonium. Biochemistry 38 9937-47. 

A dramatic increase in the brain delivery of 2-PAM by the use of Pro-2-PAM is thus achieved, resulting in a reactivation of phosphorylated brain acetylcholinesterase... 

Wallace. K., and Herzberg, U. (1988). Reactivation and aging of phosphorylated brain acetylcholinesterase from fish and rodents. Toxicol, Appl. Pharmacol. 92, 307-314. 

All three of these molecules contain highly reactive phosphoryl groups that readily react with the active-site serine of acetylcholinesterase to form a stable derivative. Without active acetylcholinesterase, synaptic transmission at the cholinergic synapses is impossible, resulting in respiratory paralysis. 

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