Acetylcholinesterase fluoride

Even more reactive towards acetylcholinesterase are the organophosphorus derivatives developed as chemical warfare nerve agents, e.g. sarin. Such compounds react readily with the enzyme and form very stable addition intermediates. It is unusual to see fluoride as a leaving group, as in sarin, but its presence provides a huge inductive effect, thus accelerating the initial nucleophilic addition step (see also Section 13.7). 

Diisopropylfluorophosphate (DFP) and phenylmethanesulfonyl fluoride (PMSF), both organophosphorus inhibitors, are potent irreversible inhibitors of serine proteases. However, because of their additional inhibition of acetylcholinesterase these compounds are highly toxic [26]. Another toxic but potent trypsin inhibitor is (4-aminophenyl)-methanesulfonyl... 

Table 1. The effect of 1 mM NaF + 20 pM A1C13 on the acetylcholinesterase activity (AChE) in freshly prepared intact RBC and in hemolysate of patients with AD (mean age 72.5 5.1 years), age-matched healthy controls (AM-HS) (72.1 1.6 years), and the group of young healthy subjects (YS) (35.9 8.5 years). Whole venous blood samples were drawn from each subject after overnight fasting., always at 07 30 AM. Red blood cells (RBC) were isolated from the blood of patients with AD, AM-HS, and YS by centrifugation [68], RBC AChE activity was evaluated in intact freshly prepared RBC or hemolyzate following the spectrophotometric method [45] with modifications. Buffer was Tris-HCl, pH 7.5 in the solution of 154 mmol L 1 NaCl, acetylthiocholine iodide was a substrate. Measurement of enzymatic activity was performed in fluorimeter polystyrene cuvettes for 3 min (UV/VIS spectrophotometer Shimadzu, Japan). The effects of 1 mmol L-1 NaF in the presence of 20 pmol L 1 A1C13 were measured. Data are expressed in percentage of the AChE activity in the absence of aluminum and fluoride ions. No differences between the AChE activity were found between the investigated groups...

Organophosphates are active not only against acetylcholinesterase but also serine proteases - which is obviously due to the shared catalytic mechanism. DFP is actually being used as a protease inhibitor in biotechnology. Another inhibitor that shares its mode of action but is less dangerous (because it is not volatile, and the enzyme adducts it forms are less stable) is PMSF (phenylmethylsulfone fluoride). You may have encountered it in one or the other research lab it is commonly added to crude cell extracts in order to minimize enzymatic breakdown of proteins during purification. 

Dahisch, P.A., Davis, E.A., Renner, J.A., Jakuhowski, E.M., Mioduszewski, R.J., Thomson, S.A. (2008h). Biomarkers oflow-level exposure to soman vapor comparison of fluoride regeneration to acetylcholinesterase inhibition. Inhal. Toxicol. 20 149-56. 

Milatovic, D., Johnson, M.K. (1993). Reactivation of phosphor-amidated acetylcholinesterase and neuropathy target esterase hy treatment of inhibited enzyme with potassium fluoride. Chem. Biol. Interact. 87 425-30. 

Sarin is reported to be a weaker inhibitor of acetylcholinesterase (AChE) than is the enantiomer (Boter et al., 1966). Racemic sarin is reported to have an LD50 that is twice that of the (—) enantiomer. The kinetics of the fluoride-induced racemization of sarin is available (Christen and Van Den Muysenberg, 1965). 

Organophosphate IA class of toxic organic molecules containing phosphate, and often fluoride, that are used as insecticides and nerve gases (such as sarin) many of these molecules block the action of an enzyme (acetylcholinesterase) that recycles an important brain chemical called acetylcholine. 

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 .

Acetylcholinesterase has also been shown to be Increased by a de novo Induced synthesis by actlnomycln D. This synthesis of acetylcholinesterase is Inhibited by puronycln and 5-fluororotlc acid. Acetylcholinesterase activity can be accelerated with acetyl fluoride In the presence of acetylcholine. 

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