Acetylcholine degradation

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. 

The actions of acetylcholine are terminated in the synaptic cleft primarily by acetylcholinesterase, which cleaves the transmitter at the ester bond. The agents in Table 2.6 inhibit acetylcholinesterase, thus preventing transmitter (acetylcholine) degradation. 

Myasthenia gravis is an autoimmune disease in which antibodies bind to nicotinic receptors at the neuromuscular junction. Cholinesterase inhibitors prevent acetylcholine degradation, which increases the probability that remaining receptors will bind acetylcholine. 

Figure 2.6 Acetylcholinesterase inhibitors (ACHI) prevent acetylcholine degradation by inhibiting the enzyme acetylcholinesterase. The mechanisms of enzyme inhibition are described in the text (left).

Reversal Cholinesterase inhibitors inhibit acetylcholine degradation (Table 2.6). The resulting increased acetylcholine concentration competes with neuromuscular blockers, reversing the blockade. 

Acetylcholinesterase (AChE) is responsible for acetylcholine degradation in the synaptic deft. Therefore, inhibitors of AChE are increasing cholinergic... 

While these functions can be a carried out by a single transporter isoform (e.g., the serotonin transporter, SERT) they may be split into separate processes carried out by distinct transporter subtypes, or in the case of acetylcholine, by a degrading enzyme. Termination of cholinergic neurotransmission is due to acetylcholinesterase which hydrolyses the ester bond to release choline and acetic acid. Reuptake of choline into the nerve cell is afforded by a high affinity transporter (CHT of the SLC5 gene family). 

Extracellular degradation removes acetylcholine, the neuropeptides and ATP. Acetylcholine is rapidly hydrolyzed to choline and acetate by acetylcholinesterase. The enzyme is localized in both the presynaptic and the postsynaptic cell membrane and splits about 10,000 molecules of acetylcholine per second. 

Compounds that affect activities of hepatic microsomal enzymes can antagonize the effects of methyl parathion, presumably by decreasing metabolism of methyl parathion to methyl paraoxon or enhancing degradation to relatively nontoxic metabolites. For example, pretreatment with phenobarbital protected rats from methyl parathion s cholinergic effects (Murphy 1980) and reduced inhibition of acetylcholinesterase activity in the rat brain (Tvede et al. 1989). Phenobarbital pretreatment prevented lethality from methyl parathion in mice compared to saline-pretreated controls (Sultatos 1987). Pretreatment of rats with two other pesticides, chlordecone or mirex, also reduced inhibition of brain acetylcholinesterase activity in rats dosed with methyl parathion (2.5 mg/kg intraperitoneally), while pretreatment with the herbicide linuron decreased acetylcholine brain levels below those found with methyl parathion treatment alone (Tvede et al. 1989). 

Galantamine is a ChE inhibitor, which elevates acetylcholine in the cerebral cortex by slowing the degradation of acetylcholine.37 It also modulates the nicotinic acetylcholine receptors to increase acetylcholine from surviving presynaptic nerve terminals. In addition, it may increase glutamate and serotonin levels. The clinical benefit of action of these additional neurotransmitters is unknown. 

The primary mechanism used by cholinergic synapses is enzymatic degradation. Acetylcholinesterase hydrolyzes acetylcholine to its components choline and acetate it is one of the fastest acting enzymes in the body and acetylcholine removal occurs in less than 1 msec. The most important mechanism for removal of norepinephrine from the neuroeffector junction is the reuptake of this neurotransmitter into the sympathetic neuron that released it. Norepinephrine may then be metabolized intraneuronally by monoamine oxidase (MAO). The circulating catecholamines — epinephrine and norepinephrine — are inactivated by catechol-O-methyltransferase (COMT) in the liver. 

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). 

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. 

Through its structural similarity to acetylcholine (Figure 3.7b), muscarine binds to the acetylcholine receptor on the synapses of nerve endings of smooth muscles and endocrine glands, causing the well-known parasympaticomimetic effects. Because muscarine is not an ester like acetylcholine, and hence resists esterase activity, it is not degraded and so can cause continuous stimulation of the affected neurons. 

All botulin neurotoxins act in a similar way. They only differ in the amino-acid sequence of some protein parts (Prabakaran et al., 2001). Botulism symptoms are provoked both by oral ingestion and parenteral injection. Botulin toxin is not inactivated by enzymes present in the gastrointestinal tracts. Foodborne BoNT penetrates the intestinal barrier, presumably due to transcytosis. It is then transported to neuromuscular junctions within the bloodstream and blocks the secretion of the neurotransmitter acetylcholine. This results in muscle limpness and palsy caused by selective hydrolysis of soluble A-ethylmalemide-sensitive factor activating (SNARE) proteins which participate in fusion of synaptic vesicles with presynaptic plasma membrane. SNARE proteins include vesicle-associated membrane protein (VAMP), synaptobrevin, syntaxin, and synaptosomal associated protein of 25 kDa (SNAP-25). Their degradation is responsible for neuromuscular palsy due to blocks in acetylcholine transmission from synaptic terminals. In humans, palsy caused by BoNT/A lasts four to six months. 

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). 

We noted above that too much acetylcholine in the synapse or at a neuromuscular junction can be a problem black widow spider venom works that way by causing massive release of this neurotransmitter. There is another way to accomplish the same thing inhibit the normal route by which acetylcholine once released is subsequently removed. That route is degradation by acetylcholinesterase, an enzyme that catalyzes... 

Figure 14.9 Axonal transport of enzymes, neurotransmitter synthesis, storage in vesicles, release and uptake by presynaptic neurone or enzymic degradation. The neurotransmitter in the synaptic cleft may be removed by the presynaptic neurone (i.e. recycling), by the postsynaptic neurone or by glial cells (not shown). Alternatively, the neurotransmitter may be degraded, and therefore inactivated, by enzyme action. For example, acetylcholine is degraded by acetylcholinesterase in the synaptic cleft (Chapter 3). One of the products, choline, is transported back into the neurone to be reacted with acetyl-CoA to re-form acetylcholine. The vesicle, once empty, may also be recycled for re-packaging (Figure 14.8).

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). 

Keller SH, Lindstrom J, Taylor P (1998) Inhibition of glucose trimming with castano-spermine reduces calnexin assodation and promotes proteasome degradation of the a-subunit of the nicotinic acetylcholine receptor. J Biol Chem 273 17064-17072... 

Orga nophosphate insecticides Increases the neurotransmitter acetylcholine by blocking its degradation Stimulant... 

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