Acetylcholinesterase enzyme

A two-site immunometric assay of undecapeptide substance P (SP) has been developed. This assay is based on the use of two different antibodies specifically directed against the N- and C-terminal parts of the peptide (95). Affinity-purified polyclonal antibodies raised against the six amino-terminal residues of the molecule were used as capture antibodies. A monoclonal antibody directed against the carboxy terminal part of substance P (SP), covalently coupled to the enzyme acetylcholinesterase, was used as the tracer antibody. The assay is very sensitive, having a detection limit close to 3 pg/mL. The assay is fiiUy specific for SP because cross-reactivity coefficients between 0.01% were observed with other tachykinins, SP derivatives, and SP fragments. The assay can be used to measure the SP content of rat brain extracts. 

In the body, metrifonate converts to the active metaboUte dichlorvos, (2,2-dichlorovinyl dimethyl phosphate), which is responsible for the inhibition of the enzyme acetylcholinesterase in the susceptible worm. This effect alone is unlikely to explain the antischistosomal properties of metrifonate (19). Clinically, metrifonate is effective only against infection caused by S. haematobium. Metrifonate is administered in three doses at 2-wk intervals (17). The dmg is well tolerated. Side effects such as mild vertigo, nausea, and cramps are dose-related. This product is not available in the United States. The only manufacturer of metrifonate is Bayer A.G. of Leverkusen, Germany. 

ACh is metabolised extraneuronally by the enzyme acetylcholinesterase, to reform precursor choline and acetate. Blocking its activity with various anticholinesterases has been widely investigated and some improvement in memory noted. Such studies have invariably used reversible inhibition because of the toxicity associated with long-term irreversible inhibition of the enzyme. Physostigmine was the pilot drug. It is known to improve memory in animals and some small effects have been seen in humans (reduces number of mistakes in word-recall tests rather than number of words recalled), but it really needs to be given intravenously and has a very short half-life (30 min). 

Both the G- and V-agents have the same physiological action on humans. They are potent inhibitors of the enzyme acetylcholinesterase (AChE), which is required for the function of many nerves and muscles in nearly every multicellular animal. Normally, AChE prevents the accumulation of acetylcholine after its release in the nervous system. Acetylcholine plays a vital role in stimulating voluntary muscles and nerve endings of the autonomic nervous system and many structures within the CNS. Thus, nerve agents that are cholinesterase inhibitors permit acetylcholine to accumulate at those sites, mimicking the effects of a massive release of acetylcholine. The major effects will be on skeletal muscles, parasympathetic end organs, and the CNS. 

Acetylcholine, which is set free from vesicles present in the neighbourhood of the presynaptic membrane, is transferred into the recipient cell through this channel (Fig. 6.25). Once transferred it stimulates generation of a spike at the membrane of the recipient cell. The action of acetylcholine is inhibited by the enzyme, acetylcholinesterase, which splits acetylcholine to choline and acetic acid. 

Acetylcholine is formed from acetyl CoA (produced as a byproduct of the citric acid and glycolytic pathways) and choline (component of membrane lipids) by the enzyme choline acetyltransferase (ChAT). Following release it is degraded in the extracellular space by the enzyme acetylcholinesterase (AChE) to acetate and choline. The formation of acetylcholine is limited by the intracellular concentration of choline, which is determined by the (re)uptake of choline into the nerve ending (Taylor Brown, 1994). 

Finally, some neurotransmitters, like acetylcholine, are inactivated solely by a catabolic enzyme. Acetylcholinesterase rapidly breaks down the neurotransmitter to acetate and choline, and the choline is then actively transported into the presynaptic... 

Anticholinesterase A drug that inhibits the enzyme acetylcholinesterase, which normally inactivates acetylcholine at the synapse. The effect of an anticholinesterase (or cholinesterase inhibitor) is thus to prolong the duration of action of the neurotransmitter. An example is rivastigmine, used in the treatment of Alzheimer s disease. 

Diazinon exerts its toxic effects by binding to the neuronal enzyme acetylcholinesterase (AChE) for long periods after exposure. Diazinon, in turn, is converted to diazoxon, which has a higher affinity for AChE (and thus greater toxicity) than the parent compound. There is a latent period in... 

Nerve Agent Substances that interfere with the central nervous system. Organic esters of phosphoric acid used as a chemical warfare agent because of their extreme toxicity (tabun-GA, sarin-GB, soman-GD, GF, and VX). All are potent inhibitors of the enzyme, acetylcholinesterase, which is responsible for the degradation of the neurotransmitter, acetylcholine in neuronal synapses or myoneural junctions. Nerve agents are readily absorbed by inhalation and/or through intact skin. 

Perhaps the most prominent and well-studied class of synthetic poisons are so-called cholinesterase inhibitors. Cholinesterases are important enzymes that act on compounds involved in nerve impulse transmission - the neurotransmitters (see the later section on neurotoxicity for more details). A compound called acetylcholine is one such neurotransmitter, and its concentration at certain junctions in the nervous system, and between the nervous system and the muscles, is controlled by the enzyme acetylcholinesterase the enzyme causes its conversion, by hydrolysis, to inactive products. Any chemical that can interact with acetylcholinesterase and inhibit its enzymatic activity can cause the level of acetylcholine at these critical junctions to increase, and lead to excessive neurological stimulation at these cholinergic junctions. Typical early symptoms of cholinergic poisoning are bradycardia (slowing of heart rate), diarrhea, excessive urination, lacrimation, and salivation (all symptoms of an effect on the parasympathetic nervous system). When overstimulation occurs at the so-called neuromuscular junctions the results are tremors and, at sufficiently high doses, paralysis and death. 

Once synthesized, acetylcholine is stored in synaptic vesicles until time for its use. Once liberated into the synapse, acetylcholine diffuses across the synaptic cleft in about 100 microseconds (10 " seconds one ten-thousandth of a second), where it interacts with its receptor, and then dissociates from it in the next 1 or 2 milliseconds. Once liberated, acetylcholine is degraded by a second enzyme, acetylcholinesterase, a target for drug discovery (as I develop a bit later). 

Figure 3.3 (a) Covalent catalysis the catalytic mechanism of a serine protease. The enzyme acetylcholinesterase is chosen to illustrate the mechanism because it is an important enzyme in the nervous system. Catalysis occurs in three stages (i) binding of acetyl choline (ii) release of choline (iii) hydrolysis of acetyl group from the enzyme to produce acetate, (b) Mechanism of inhibition of serine proteases by diisopropylfluorophosphonate. See text for details. 

The action of acetylcholine is brought to an end by the enzyme acetylcholinesterase, which catalyses the hydrolysis... 

We shall meet another important example of transesterification in the action of the enzyme acetylcholinesterase (see Box 13.4). 

Tlie neurotransmitter acetylcholine is both a quaternary ammonium compound (see Box 6.7) and an ester. After interaction with its receptor, acetylcholine is normally degraded by hydrolysis in a reaction catalysed by the enzyme acetylcholinesterase. This enzyme contains a serine residue that acts as the nucleophile, hydrolysing the ester linkage in acetylcholine (see Box 13.4). This effectively acetylates the serine hydroxyl, and is an example of transesterification (see Section 7.9.1). For continuation of acetylcholine degradation, the original form of the enzyme must be regenerated by a further ester hydrolysis reaction. 

Acetylcholine is a relatively small molecule that is responsible for nerve-impulse transmission in animals. As soon as it has interacted with its receptor and triggered the nerve response, it must be degraded and released before any further interaction at the receptor is possible. Degradation is achieved by hydrolysis to acetate and choline by the action of the enzyme acetylcholinesterase, which is located in the synaptic cleft. Acetylcholinesterase is a serine esterase that has a mechanism similar to that of chymotrypsin (see Box 13.5). 

A reaction whose rate is limited (or controlled) only by the speed with which reactants diffuse to each other. For a ligand binding to a protein, the bimolecular rate constant for diffusion-limited association is around 10 M s. The enzyme acetylcholinesterase has an apparent on-rate constant of 1.6 x 10 M s with its natural cationic substrate acetylcholine, and the on-rate constant of about 6 X 10 with acetylselenoylcholine and about... 

Parathion is one of a class of phosphorothionate triesters widely used as insecticides. These compounds exert their toxic effects in insects and mammals by inhibiting the enzyme acetylcholinesterase. The phosphorothionates, in general, are relatively poor inhibitors of acetylcholinesterase but are converted by the cytochrome P-450-containing monooxygenase enzyme systems in insects and mammals to the corresponding phosphate triesters that are potent inhibitors of this enzyme. 

Neurotoxic venoms of cobras, mambas, and coral snakes Inhibit the enzyme acetylcholinesterase. - This hydrolase normally breaks down the neurotransmitter acetylcholine within nerve synapses. 

Mechanism of Action A cholinesterase inhibitor that inhibits the enzyme acetylcholinesterase, thus increasing the concentration of acetylcholine at cholinergic synapses and enhancing cholinergic function in the CNS. Therapeutic Effect Slows the progression of Alzheimer s disease. 

Mechanism of Action A cholinergic drug that prevents destruction of acetylcholine by inhibiting the enzyme acetylcholinesterase, thus enhancing impulse transmission across the myoneural junction. Therapeutic Effect Improves intestinal and skeletal muscle tone stimulates salivary and sweat gland secretions. 

Acetylcholine is removed from the synapse through hydrolysis into acetylCoA and choline by the enzyme acetylcholinesterase (AChE). Removing ACh from the synapse can be blocked irreversibly by organophosphorous compounds and in a reversible fashion by drugs such as physostigmine. 

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