Neuromuscular junction acetylcholinesterase

Acetylcholine is a neurotransmitter that relays nerve impulses across the neuromuscular Junction. Acetylcholinesterase (AcChE) rapidly breaks dovm acetylcholine, thereby loweringits concentration in the synaptic cleft and ensuring that nerve impulses are of a finite length. As shown in Fig. 17.38, a nucleophilic serine residue reacts with the substrate to form an acetyl-serine intermediate (100) with concomitant release of choline. This intermediate is then rapidly hydrolyzed by wa-... 

There are three primary cholinesterases (ChE) in the body. Acetylcholinesterase is located in the vicinity of ACh receptors at neuronal and neuromuscular junctions. Acetylcholinesterase terminates ACh activity via hydrolysis into choline and acetic acid. The positively charged choline portion of the ACh molecule attaches to the anionic site and the acetyl region attaches to the esteratic site on the AChE molecule. Eollowing the attachment of the two regions, choline is rapidly released to be recycled back into the presynaptic nerve terminal, and the acetyl group reacts with... 

Acetylcholine serves as a neurotransmitter. Removal of acetylcholine within the time limits of the synaptic transmission is accomplished by acetylcholinesterase (AChE). The time required for hydrolysis of acetylcholine at the neuromuscular junction is less than a millisecond (turnover time is 150 ps) such that one molecule of AChE can hydrolyze 6 105 acetylcholine molecules per minute. The Km of AChE for acetylcholine is approximately 50-100 pM. AChE is one of the most efficient enzymes known. It works at a rate close to catalytic perfection where substrate diffusion becomes rate limiting. AChE is expressed in cholinergic neurons and muscle cells where it is found attached to the outer surface of the cell membrane. 

Arikawa-Hirasawa, E., Rossi, S. G., Rotundo, R. L. and Yamada, Y. Absence of acetylcholinesterase at the neuromuscular junctions of perlecan-null mice. Nat. Neurosci. 5, 119-123, 2002. 

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. 

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... 

F. 3 Comparison of the neuromuscular junction pattern and muscular fiber organization in control and clofibrate-treated larvae, using whole-mount acetylcholinesterase staining. 

ACh receptors are destroyed by endocytosis via coated pits and proteolysis in lysosomes. In myasthenia gravis, the receptors are crosslinked by antireceptor antibodies, which facilitate the rate-limiting endocytosis step receptor destmction occurs in less than half the normal time, resulting in net receptor loss. The chronic disease is characterized clinically by such muscular weakness and abnormal fatigue that patients cannot even keep their eyes open. Acetylcholinesterase inhibitors increase the ACh concentration and excitation of the neuromuscular junction, resulting in increased strength and endurance. As expected, AChE inhibitors are also potent curare antidotes because the increased ACh levels displace the blocker more readily. 

Schematic representation of the neuromuscular junction. ACh, acetylcholine AChE, acetylcholinesterase JF, junctional folds M, mitochondrion V, transmitter vesicle.

Diazinon toxicity results predominantly from the inhibition of acetylcholinesterase in the central and peripheral nervous system. The enzyme is responsible for terminating the action of the neurotransmitter, acetylcholine, in the synapse of the pre- and post-synaptic nerve endings or in the neuromuscular junction. However, the action of acetylcholine does not persist long as it is hydrolyzed by the enzyme, acetylcholinesterase, and rapidly removed. As an anticholinesterase organophosphate, diazinon inhibits acetylcholinesterase by reacting with the active site to form a stable phosphorylated complex which is incapable of destroying acetylcholine at the synaptic gutter between the pre- and post-synaptic nerve... 

One of the best-understood autoimmune diseases is myasthenia gravis, a condition associated with a decrease in the number of functional post-synaptic nicotinic acetylcholine receptors (Fig. 30-23) in neuromuscular junctions. e The resulting extreme muscular weakness can be fatal. Myasthenia gravis is not rare and affects about one in 10,000 peopled An interesting treatment consists of the administration of physostigmine, diisopropyl-phosphofluoridate (Chapter 12, Section C,l), or other acetylcholinesterase inhibitors (Box 12-E). These very toxic compounds, when administered in controlled amounts, permit accumulation of higher acetylcholine concentration with a resultant activation of muscular contraction. The same compounds... 

ACh, cholinomimetics and acetylcholinesterase inhibitors have been shown to inhibit the movement of flatworms and to cause flaccid paralysis (for reviews, see Fbx et a/., 1996 Halton et a/., 1997 Halton and Maule, 2004 Ribeiro et a/., 2005). This is in contrast to the mainly myoexcitatory effects of ACh at the vertebrate neuromuscular junction and in many other invertebrates including nematodes. Although ACh has predominantly inhibitory actions on flatworm muscle it has been shown to have inconsistent effects on the muscle of the monogenean, Diclidophora merlangi (Maule et a/., 1989), to induce the contraction of muscle fibres dispersed from planarians (Blair and Anderson, 1994 Moneypenny eta/., 2001) and, to cause increased muscle activity... 

Schematic representation of the neuromuscular junction. (V, transmitter vesicle M, mitochondrion ACh, acetylcholine AChE, acetylcholinesterase JF, junctional folds.) (Reproduced, with permission, from Drachman DB Myasthenia gravis. N Engl J Med 1978 298 135.)...

Acetylcholine is a neurotransmitter, a key substance involved with transmission of nerve impulses in the brain, skeletal muscles, and other areas where nerve impulses occur. An essential step in the proper function of any nerve impulse is its cessation (see Figure 6.9), which requires hydrolysis of acetylcholine as shown by Reaction 6.10.1. Some xenobiotics, such as organophosphate compounds (see Chapter 18) and carbamates (see Chapter 15) inhibit acetylcholinesterase, with the result that acetylcholine accumulates and nerves are overstimulated. Adverse effects may occur in the central nervous system, in the autonomic nervous system, and at neuromuscular junctions. Convulsions, paralysis, and finally death may result. 

Adverse effects The effects of physostigmine on the CNS may lead to convulsions when high doses are used. Bradycardia may also occur. Inhibition of acetylcholinesterase at the skeletal neuromuscular junction causes the accumulation of acetylcholine and ultimately results in paralysis of skeletal muscle. However, these effects are rarely seen with therapeutic doses. 

Martinez-Pena Y, Valenzuela I, Akaaboune M. 2007. Acetylcholinesterase mobility and stability at the neuromuscular junction of living mice. Mol Biol Cell 18 2904-2911. 

Cholinesterases, e.g., acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholi-nesterase (BChE, EC 3.1.1.8), are serine hydrolases that break down the neurotransmitter acetylcholine and other choline esters [5]. In the neurotransmission processes at the neuromuscular junction, the cationic neurotransmitter acetylcholine (ACh) is released from the presynaptic nerve, diffuses across the synapse and binds to the ACh receptor in the postsynaptic nerve (Fig. 1). Acetylcholinesterase is located between the synaptic nerves and functions as the terminator of impulse transmissions by hydrolysis of acetylcholine to acetic acid and choline as shown in Scheme 4. The process is very efficient, and the hydrolysis rate is close to diffusion controlled [6, 7]. 

---