Acetylcholinesterase sources

Alternatively, some studies used expert opinion to extrapolate the effectiveness of donepezil over a longer period (Neumann et al, 1999 O Brien et al, 1999). However, it is recognized that expert opinion can be the weakest source of evidence, which introduces considerable uncertainty into the analysis and interpretation of the results. In addition, the cost-effectiveness of acetylcholinesterase inhibitors depends heavily on the distribution of the cohort of patients across different severity states. O Brien s team found that the results of their model were very sensitive to this variable. In this context, the correct... 

P.R.B. de O Marques, G.S. Nunes, T.C.R. dos Santos, S. Andreescu, and J.L. Marty, Comparative investigation between acetylcholinesterase obtained from commercial sources and genetically modified Drosophila melanogaster application in amperometric biosensors for methamidophos pesticide detection. Biosens. Bioelectron. 20, 825-832 (2004). 

Acetylcholinesterase is subject to substrate Inhibition at high concentrations, but Mlchaells kinetics are observed at lower concentrations, because the substrate constant and the Mlchaells constant differ by a factor of 100. Turnover numbers run about 2-9 x 10 min l, and (Mlchaells constant) values are about 0.2 mM.76,116 Whatever the source, the enzyme is subject to inhibition by the same reversible and irreversible inhibitors. Most of the kinetic work has been done with the saline-extracted 11S enzyme from electric eel and the detergent-extracted 6S enzyme from erythrocytes. The former Is a tetramer derived from the native enzyme by the action of proteases the latter is a dimer. 

At the physiological level it is well established that vital dyes such as nile blue, neutral red and methylene blue retard larval development under normal lighting conditions (12L/12D with source unspecified) (25 27). Female but not male pupal weights are also reduced. Unfortunately experiments were conducted without dark controls so that it is difficult to evaluate the role of photosensitization in these effects. As house flies and fire ants succumb to photosensitization, they lose motor control and become more excitable (28). This suggested a neurotoxic effect and investigation of fire ant acetylcholinesterase vitro revealed that this enzyme was sensitive to photo-oxidation. vivo results, however, revealed no effect on the enzyme which suggests another mode of action. Epoxldatlon of cholesterol and membrane lysis may be alternative primary sites. If this were the case ecdysone metabolism of insects would probably also be effected. 

There are problems with this approach since enzymes isolated from natural sources such as the electric organ of electric eels often display low sensitivity and selectivity to the wide range of potential pesticide targets [21]. A possible solution to this is the development of a multisensor array where a variety of genetically modified acetylcholinesterases are immobilised on an array of electrochemical sensors and the responses from these are then processed via a neural network. 

Earlier work in this field [28] indicated that acetylcholinesterase enzymes would be suitable biomolecules for the purpose of pesticide detection, however, it was found that the sensitivity of the method varied with the type and source of cholinesterase used. Therefore the initial thrust of this work was the development of a range of enzymes via selective mutations of the Drosophila melanogaster acetylcholinesterase Dm. AChE. For example mutations of the (Dm. AChE) were made by site-directed mutagenesis expressed within baculovirus [29]. The acetylcholinesterases were then purified by affinity chromatography [30]. Different strategies were used to obtain these mutants, namely (i) substitution of amino acids at positions found mutated in AChE from insects resistant to insecticide, (ii) mutations of amino acids at positions suggested by 3-D structural analysis of the active site,... 

FIGURE 4.2 Mortality of fathead minnows in relation to acetylcholinesterase activity.3 a As measured in the brain tissue following (A) a 14-day exposure to a ternary mixture of azinphos-methyl, diazinon, and chlorpyrifos in a concentration-response study and (B) a 7-day exposure to a ternary mixture of azinphos-methyl, diazinon, and chlorpyrifos applied as equipotent (toxic equivalent [TE]) mixtures. Note Dashed lines correspond to 50% reduction in AChE activity the dashed-dotted line corresponds to 50% mortality. Source Redrawn from Sibley et al. (2000). 

Substrates acetylthiocholine for ChE, butyrylthiocholine for BuChE, and acetylthiocholine and 0.1 mM iso-OMPA. Abbreviations AChE, acetylcholinesterase BuChE, butyrylcholinesterase ChE, cholinesterase iso-OMPA, tetraisopropylpyrophosphoramide Source Trainaand Serpietri (1984). 

Thiocarbonate derivatives of choline (262a, 262b) competitively inhibit acetylcholinesterase from various sources (332). 

Clement J., Lockwood, P, and Tomson, A. The acetylcholinesterase reactivator HI-6 comparative study of HI-6 samples from various sources. Arch.Toxicology 62,2-3,220-223,1988. 

Table 5.2 shows some velocity constants for the hydrolysis of acylated acetylcholinesterases and the source of the enzyme. Most of the data are taken from Aldridge and Reiner (1972). 

Table 5.2 Velocity Constants for the Hydrolysis of Acylated Acetylcholinesterases and the Source of Enzymes...

For proper nerve activity, the change in membrane permeability caused by the opening of acetylcholine receptor channels must be short-lived. In order to close the receptor channels, it is important to remove the source of the stimulation, the acetylcholine. Once initiated, the nerve impulse moves on and the postsynaptic membrane must return to its resting state in order to be ready to receive and propagate another signal. (Some notable nerve poisons such as DIPF—see problem 15—act by inhibiting the acetylcholinesterase.)... 

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