Aging of acetylcholinesterase

Mason, H.J., Sams, C., Stevenson, A.J., Rawbome, R. (2000). Rates of spontaneous reactivation and aging of acetylcholinesterase in human erythrocytes after inhibition by organo-phosphorus pesticides. Plum. Exp. Toxicol. 19 511-16. 

Dc Jong. L. P, A, and Wolring, G. Z. (1978). Effect of l-(AR)alkyI-2-hydroxyimino mcihyl-pyridiuni salts on reactivation and aging of acetylcholinesterase inhibited by ethyl dimethylphosphor-amidocyanidate (tabun), Biochem. Pharmacol. 27, 2229-22.35. 

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). 

In dogs poisoned with soman (Intravenously at 30 pg/kg) and treated with I at 104 mg/kg (Intravenously 31/2 min after soman), the large dose of I stopped aging of Inhibited cholinesterase and reactivated 24.0% and 35.6% of the red-cell and diaphragm cholinesterase activities, respectively. It failed to reactivate brain cholinesterase. Indeed, the brain acetylcholinesterase activity after the treatment with 1 was lower than that just before the injection of I. The last finding indicates the inability of I to cross the blood-brain barrier in significant quantities. 

Crone, H.D. 1974. Can allosteric effectors of acetylcholinesterase control the rate of ageing of the phosphonylated enzyme Blochem. Pharmacol. 23 460-463. 

Pralidoxime is administered by intravenous infusion, 1-2 g given over 15-30 minutes. In spite of the likelihood of aging of the phosphate-enzyme complex, recent reports suggest that administration of multiple doses of pralidoxime over several days may be useful in severe poisoning. In excessive doses, pralidoxime can induce neuromuscular weakness and other adverse effects. Pralidoxime is not recommended for the reversal of inhibition of acetylcholinesterase by carbamate inhibitors. Further details of treatment of anticholinesterase toxicity are given in Chapter 58. 

It is also important to mention the use of the reactivation of the acetylcholinesterase by pyridine-2-aldoxime methochloride to discriminate between the toxin and potential insecticides [96]. Once phos-phorylated, the active site serine of the enzyme can be reactivated by powerful nucleophilic agents such as oximes. However, this reactivation is not possible if attempted too late due to the stable adduct formed by the dealkylation (aging) of the inhibitor s remaining group. When acetylcholinesterase is inhibited by anatoxin-a(s), it shows immediately the characteristics of an aged enzyme and cannot be reactivated. In this way, it is possible to distinguish between the inhibition caused by anatoxin-a(s) and the one provoked by other insecticides. 

OPs bound to tyrosine do not age. Aging of OP adducts on acetylcholinesterase and butyrylcholinesterase is defined as the loss of an alkoxy group from the phosphorus atom. Every OP-tyrosine adduet in Table 56.1 has been examined... 

FIGURE 57.9. Inhibition and aging of serine esterases by diisopropylphosphorofluoridate (DFP). The active site serine is organophosphorylated in the inhibition step. Aging results in net loss of an isopropyl group to yield the monoisopropylphosphoryl esterase. This mode of inhibition and aging has been established for acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and neuropathy target esterase catalytic domain (NEST) (Kropp and Richardson, 2007). 

Kropp, T.J., Richardson, R.J. (2006). Aging of mipafox-inhibited human acetylcholinesterase proceeds by displacement of both isopropylamine groups to yield a phosphate adduct. Chem. Res. Toxicol. 19 334—9. 

Millard, C.B., Kryger, G., Ordentlich, A., Greenblatt, H.M., Harel, M., Raves, M.L., Segall, Y., Barak, D., Shafferman, A., Silman, I., Sussman, J.L. (1999a). Crystal structures of aged phosphonylated acetylcholinesterase nerve agent reaction products at the atomic level. Biochemistry 38 7032-9. 

Shafferman, A., Ordentlich, A., Barak, D., Stein, D., Ariel, N., Velan, B. (1996). Aging of phosphylated human acetylcholinesterase catal dic processes mediated by aromatic and polar residues of the active site. Biochem. J. 318 833 0. 

Schoene, K., Wulf, K. (1972). Retarding effect of pyridinium salts on ageing of soman-inhibited acetylcholinesterase. Arzm. Forsch/Dmg Res. 22 1802. 

Van Dongen, C.J., Elskamp, R.M., De Jong, L.P.A. (1987). The influence of atropine upon reactivation and ageing of rat and human erythrocyte acetylcholinesterase inhibited by soman. Biochem. Pharmacol. 36 1167-9. 

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. 

---