Acetylcholinesterase AChE reactivation

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... 

Metabolites that are less reactive than suicide inhibitors may impact more distant enzymes, within the same cell, adjacent cells, or even in other tissues and organs, far removed from the original site of primary metabolism. For example, organopho-sphates (OPs), an ingredient in many pesticides, are metabolized by hepatic CYPs to intermediates, which, when transported to the nervous system, inhibit esterases that are critical for neural function. Acetylcholinesterase (AChE) catalyzes the hydrolysis of the ester bond in the neurotransmitter, acetylcholine, allowing choline to be recycled by the presynaptic neurons. If AChE is not effectively hydrolyzed by AChE in this manner, it builds up in the synapse and causes hyperexcitation of the postsynaptic receptors. The metabolites of certain insecticides, such as the phos-phorothionates (e.g., parathion and malathion) inhibit AChE-mediated hydrolysis. Phosphorothionates contain a sulfur atom that is double-bonded to the central phosphorus. However, in a CYP-catalyzed desulfuration reaction, the S atom is... 

The toxicity of organophosphoric esters for insects and mammals is associated with inhibition of cholinesterases. Investigations on the relation between chemical structure of organophosphoric esters and the inactivation of acetylcholinesterase (AChE) have revealed that anticholinesterase activity depends to a large extent on the chemical reactivity of the esters. As a rule, the chemical reactivity of the phosphorus atom is the single most important property which determines the anticholinesterase activity of an organophosphoric ester. 

In recent studies, it was shown that eel acetylcholinesterase (AChE, EC 3.1.1.7) previously inhibited with 1,3,2-dioxaphosphor-inane 2-oxides (I) undergoes spontaneous reactivation with tj, =... 

FIGURE 69.2. Reaction scheme of acetylcholinesterase inhibition, reactivation, and protection activities. OPH, 2-PAM, and paraoxon are used in the example. Reaction (I) - cholinesterase AChE reaction with ASCh Reaction (II) - inhibition of AChE inhibition by paraoxon Reaction (III) - aging of AChE associated with OP exposure Reaction (TV) -reactivation of AChE by 2-PAM Reaction (V) - hydrolysis of paraoxon by OPH Reaction (VI) - reaction of 2-PAM oxime with ASCh and Reaction (VII) - DTNB reaction with SCh. The indicates a photometrically detectable metabolite. 

Acetylcholinesterase (AChE) catalyses the hydrolysis of the ester bond of acetylcholine to yield choline and acetate (Sussman et al., 1991). This is a critical reaction for the termination of impulses transmitted through cholinergic synapses. It is a highly efficient catalyst, with reaction rates approaching the diffusion limit. Its overall structure resembles the lipases with an active site gorge. Above the base of the gorge is the reactive serine to be activated by the classical (Ser-200...His-440...Glu-327) catalytic triad. 

The fundamental processes involved in acetylcholinesterase (AChE) inhibition, reactivation and ageing , are similar for both nerve agents... 

Figure 1. Reaction of soman (GD) with acetylcholinesterase (AChE). (a) Soman and the active site of AChE shown together but not having undergone any reaction, (b) Soman combined with AChE to form an inhibitor-enzyme intermediate, (c) The leaving group (fluoride) has been lost, leaving a complex of soman with AChE. (d) The ester link in the phosphonylated AChE has been hydrolyzed, the enzyme has reactivated and an alkylphosphate has been formed, (e) The link between the large pinacolyl group and phosphorus has been cleaved with the formation of a stable monoalkylphosphonylated complex with AChE and pinacolyl alcohol. The process is known as ageing .

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