Acetylcholinesterase aging

The only other information regarding the potential for age-related differences in susceptibility to methyl parathion came from a study by Garcia-Lopez and Monteoliva (1988). The investigators showed increasing mean erythrocyte acetylcholinesterase activity levels with increasing age range, starting at birth (in 10-year increments and >60 years of age) in both males and females. However, it is not known whether increased erythrocyte acetylcholinesterase activity indicates a decreased susceptibility to methyl parathion toxicity. 

Several studies in animals suggest that age may affect susceptibility to methyl parathion toxicity, and that children may be more susceptible than adults, but the data are limited. (See Section 3.7 for more information on Children s susceptibility.) A study in humans showed that mean erythrocyte acetylcholinesterase activity levels increase with increasing age from birth through old age in both sexes (Garcia-Lopez ad Monteoliva 1988), but it is not known whether increased erythrocyte acetylcholinesterase activity indicates decreased susceptibility to methyl parathion. 

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 cases where the mode of action is the strong or irreversible inhibition of an enzyme system, the assay may measure the extent of inhibition of this enzyme. This may be accomplished by first measuring the activity of the inhibited enzyme and then making comparison with the uninhibited enzyme. This practice is followed when studying acetylcholinesterase inhibition by organophosphates (OP). Acetylcholinesterase activity is measured in a sample of tissue of brain from an animal that has been exposed to an OP. Activity is measured in the same way in tissue samples from untreated controls of the same species, sex, age, etc. Comparison is then made between the two activity measurements, and the percentage inhibition is estimated. 

Organophosphorus esters are known to react with a serine hydroxyl group in the active site of the acetylcholinesterase protein (Ecobichon 1991 Murphy 1986). Some organophosphorus esters (e.g., diisopropyl fluorophosphate, [DFP]) bind irreversibly, while others bind in a slowly reversible fashion, thereby leading to a slow reactivation (dephosphorylation) of the enzyme. A process known as "aging" has also been described in which reversibly bound compounds are changed with time to moieties that are essentially irreversibly... 

Two types of OPIDN have been described in animals (Abou-Donia and Lapadula 1990). Type I is produced by compounds with a pentavalent phosphorus (like TOCP), and Type II is produced by compounds with a trivalent phosphorus. Characteristics used to differentiate between the types of OPIDN include species selectivity, age sensitivity, length of latent period, and morphology of neuropathologic lesions. Thus, at doses that did not produce death due to acetylcholinesterase inhibition, TOCP (a Type I compound) produced lesions in the spinal cord of rats without producing ataxia. In contrast, triphenyl phosphite (a Type II compound) produced delayed (1 week) ataxia in the rat and a distribution of spinal cord lesions distinct from those produced by TOCP (Abou-Donia and Lapadula 1990). 

Herzog CD, Nowak KA, Sarter M, Bruno JP. 2003. Microdialysis without acetylcholinesterase inhibition reveals an age-related attenuation in stimulated cortical acetylcholine release. Neurobiol Aging 24(6) 861-863. 

K. A. Frey, M.R. Kilbourn, In vivo mapping of cerebral acetylcholinesterase activity in aging and Alzheimer s disease. Neurology 52 (1999) 691-699. 

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. 

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...

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