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Key Event Title
Acetylcholinesterase (AchE) Inhibition
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP||Point of Contact||Author Status||OECD Status|
|AChE inhibition - acute mortality||MolecularInitiatingEvent||Dan Villeneuve (send email)||Under Development: Contributions and Comments Welcome||Under Development|
|AChE Inhibition Leading to Neurodegeneration||MolecularInitiatingEvent||Karen Watanabe (send email)||Under development: Not open for comment. Do not cite|
|AChE inhibition - acute mortality via predation||MolecularInitiatingEvent||Kristie Sullivan (send email)||Under development: Not open for comment. Do not cite|
|Organo-Phosphate Chemicals leading to impaired cognitive function||MolecularInitiatingEvent||SAROJ AMAR (send email)||Under development: Not open for comment. Do not cite|
|Organo-Phosphate Chemicals leading to sensory axonal peripheral neuropathy and mortality||MolecularInitiatingEvent||SAROJ AMAR (send email)||Under development: Not open for comment. Do not cite|
|All life stages||High|
Key Event Description
"Acetylcholinesterase is found primarily in blood, brain, and muscle, and regulates the level of the neurotransmitter ACh [acetylcholine] at cholinergic synapses of muscarinic and nicotinic receptors. Acetylcholinesterase features an anionic site (glutamate residue), and an esteratic site (serine hydroxyl group) (Wilson, 2010; Soreq, 2001). In response to a stimulus, ACh is released into the synaptic cleft and binds to the receptor protein, resulting in changes to the flow of ions across the cell, thereby signaling nerve and muscle activity. The signal is stopped when the amine of ACh binds at the anionic site of AChE, and aligns the ester of ACh to the serine hydroxyl group of the enzyme. Acetylcholine is subsequently hydrolyzed, resulting in a covalent bond with the serine hydroxyl group and the subsequent release of choline, followed by a rapid hydrolysis of the enzyme to form free AChE and acetic acid (Wilson, 2010; Soreq, 2001)." [From Russom et al. 2014. Environ. Toxicol. Chem. 33: 2157-2169]
Molecular target gene symbol: ACHE
KEGG enzyme: EC 220.127.116.11
How It Is Measured or Detected
- Direct measures of AChE activity levels can be made using the modified Ellman method, although selective inhibitors that remove other cholinesterases not directly related to cholinergic responses (e.g., butyrylcholinesterase) are required [45,46].
- Radiometric methods have been identified as better for measuring inhibition because of carbamylation (carbamate exposure) [20,46,47].
- TOXCAST: NVS_ENZ_hAChE
- A direct measure of cholinesterase activity levels can be made within the relevant tissues after in vivo exposure, specifically the brain as well as red blood cells in mammals. Some analytical methods used to measure cholinesterase activity may not distinguish between butyrylcholinesterase, which is found with AChE in plasma and some skeletal and muscle tissues. Although the structure of butyrylcholinesterase is very similar to AChE, its biological function is not clear, and its activity is not associated with cholinergic response covered under this AOP (Lushington et al., 2006). Therefore experimental procedures used to measure cholinesterase as well as the tissue analyzed should be considered when evaluating studies reporting AChE inhibition (Wilson 2010; Wilson and Henderson 2007). For measuring AChE levels, the Ellman method is recommended with some modifications (Ellman et al., 1961; Wilson et al., 1996) while radiometric methods have been identified as better for measuring inhibition due to carbamylation (carbamate exposure) (see Wilson 2010; Wilson et al., 1996; Johnson and Russell 1975).
- In order to effectively bind to the AChE enzyme, thion forms of OPs (i.e., RO)3P=S) must first undergo a metabolic activation via mixed function oxidases to yield the active, oxon form (Fukuto 1990). Estimating the potential toxicity in whole organisms based on in vitro data may be problematic since metabolic activation may be required (e.g., phosphorothionates) and may not be reflected in the in vitro test result (Guo et al. 2006; Lushington et al. 2006).
- Typically, carbamates do not require metabolic activation in order to bind to the enzyme, although some procarbamates (e.g., carbosulfan) have been developed that are not direct inhibitors of AChE, but take advantage of metabolic distinctions between taxa, resulting in a toxic form in invertebrates (e.g., carbofuran) but not vertebrate species (Stenersen 2004). Therefore in vitro assays measuring AChE inhibition for procarbamates in invertebrate species will not account for metabolic activation and therefore may not represent the actual enzyme activity.
Domain of Applicability
AChE is present in all life stages of both vertebrate and invertebrate species (Lu et al 2012).
Acetylcholinesterase associated with cholinergic responses in most insects is coded by the ace1 gene and in vertebrates by the ace gene (Lu et al 2012; Taylor 2011.
Plants have AChE but it is most likely involved in regulation of membrane permeability and the ability of a leaf to unroll (Tretyn and Kendrick 1991).
The primary amino acid sequence of the AChE enzyme is relatively well conserved across vertebrate and invertebrate species, suggesting that chemicals are likely to interact with the enzyme in a similar manner across a wide range of animals. From the sequence similarity analyses, the taxonomic domain of applicability of this MIE likely includes species belonging to many lineages, including branchiopoda (crustaceans, e.g., daphnids), insecta (insects), arachnida (arachnids, e.g., spiders, ticks, scorpions), cephalopoda (molluscans, e.g., octopods, squids), lepidosauria (reptiles, e.g., snakes, lizards), chondrichthyes (cartilaginous fishes, e.g., sharks), amphibia (amphibians), mammalian (mammals), aves (birds), actinopterygii (bony fish), ascidiacea (sac-like marine invertebrates), trematoda (platyhelminthes, e.g., flatworms), and gastropoda (gastropods, e.g., snails and slugs) Species within these taxonomic lineages and others are predicted to be intrinsically susceptible to chemicals that target functional orthologs of the daphnid AChE (Russom, 2014).
Advanced computational approaches such as crystal structures of the enzyme and transcriptomics have provided empirical evidence of the enzyme structure, relevant binding sites, and function across species (Lushington et al., 2006; Lu et al., 2012; Wallace 1992).
Studies have found that AChE activity increases as the organism develops.
Prakesh and Kaur 1982 looked at AChE inhibition across three insect species; controls and those exposed to DDVP. They saw little difference in the larval stages but did see increased inhibition in pupal and adult stages (greatest inhibition).
Karanth and Pope 2003 looked at AChE and acetylcholine synthesis in rat striatum in controls and animals exposed to 0.3 and 1 times the maximum tolerated dose. Although these doses are below the lethal concentrations and they mention that not observed cholinergic responses were observed, they do provide differences related to life stages of the rodents.
Grue et al 1981 present baseline (no toxicity exposure) in wild starlings (both sexes) of brain cholinesterase and found activity increased as birds aged from 1-20 days until it reached a steady state at adulthood.
A study with Red Flour Beetle found that the gene associated with cholinergic functions (Ace1) was expressed at all life-stages, with increases as the organism developed from egg to larva to pupa to adult. (Lu et al., 2012 cited in Russom et al 2014.)
In mammals and birds, studies have determined that skeletal muscles of immature birds and mammals contain both butyrylcholinesterase and AChE, with butyrylcholinesterase decreasing and AChE increasing as the animal develops (Tsim et al. 1988; Berman et al, 1987).
Another study found that changes in AChE within the developing pig brain were dependent on the area of the brain, and life stage of the animal, with significant decreases in activity within the pons and hippocampus from birth to 36 months, and no significant change in activity in the cerebellum, where activity increased up to four months of age, leveling off thereafter (Adejumo and Egbunike, 2004).
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Moser, Virginia C. 2011. “Age-Related Differences in Acute Neurotoxicity Produced by Mevinphos, Monocrotophos, Dicrotophos, and Phosphamidon.” Neurotoxicology and Teratology 33 (4): 451–57. https://doi.org/10.1016/j.ntt.2011.05.012.
Monserrat, J.M. and A. Bianchini. 2001. Anticholinesterase effect of eserine (physostigmine) in fish and crustacean species. Braz. Arch. Biol. Technol. 44(1): 63-68.
Russom, Christine L., Carlie A. LaLone, Daniel L. Villeneuve, and Gerald T. Ankley. 2014. “Development of an Adverse Outcome Pathway for Acetylcholinesterase Inhibition Leading to Acute Mortality.” Environmental Toxicology and Chemistry 33 (10): 2157–69. https://doi.org/10.1002/etc.2662.
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Stenersen, J. 2004. Specific enzyme inhibitors. In: Chemical Pesticides: Mode of action and toxicology. (41 p). CRC Press, Boca Raton, FL.
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