Acetyl choline cholinesterase

A number of substituted p-aminoacetates inhibit the enzyme cholinesterase. The main function of this enzyme is to hydrolyze acetyl choline and thereby terminate the action of that substrate as a neurotransmitter. Such inhibition is functionally equivalent to the administration of exogenous acetylcholine. Direct administration of the neurotransmitter substance itself is not a useful therapeutic procedure due to rapid drug destruction and unacceptable side... 

The cholinesterases, acetylcholinesterase and butyrylcholinesterase, are serine hydrolase enzymes. The biological role of acetylcholinesterase (AChE, EC 3.1.1.7) is to hydrolyze the neurotransmitter acetylcholine (ACh) to acetate and choline (Scheme 6.1). This plays a role in impulse termination of transmissions at cholinergic synapses within the nervous system (Fig. 6.7) [12,13]. Butyrylcholinesterase (BChE, EC 3.1.1.8), on the other hand, has yet not been ascribed a function. It tolerates a large variety of esters and is more active with butyryl and propio-nyl choline than with acetyl choline [14]. Structure-activity relationship studies have shown that different steric restrictions in the acyl pockets of AChE and BChE cause the difference in their specificity with respect to the acyl moiety of the substrate [15]. AChE hydrolyzes ACh at a very high rate. The maximal rate for hydrolysis of ACh and its thio analog acetyl-thiocholine are around 10 M s , approaching the diffusion-controlled limit [16]. 

In addition to battlefield trauma, there is also the risk of exposure to chemical weapons such as the nerve agents, notably the organophosphorus gases (soman, sarin, VX, tabun) [6]. Organophosphorus toxicity arises largely from their ability to irreversibly inhibit acetyl-cholinesterases, leading to effects associated with peripheral acetyl-choline accumulation (muscarinic syndrome) such as meiosis, profuse sweating, bradychardia, bronchioconstriction, hypotension, and diarrhoea. Central nervous system effects include anxiety, restlessness, confusion, ataxia, tremors. 

The medicinal chemistry of Alzheimers is complicated by the fact that the etiology of this disease is still far from clear. Evidence points to an association with decreased levels of acetyl choline in the brain. Many of the drugs that have been introduced to date for treating this disease thus comprise agents intended to raise the deficient levels of that neurotransmitter by inhibiting the loss of existing acetylcholine due to the action of cholinesterase. A compound based on an indene that, perhaps surprisingly, inhibits that enzyme has been proposed for the treatment of Alzheimer s. Aldol condensation of piperidine aldehyde (4-2) with the indanone (4-1) from cyclization of 3,4-dimethoxycinnamic acid leads to the olefin (4-3). Catalytic reduction removes the double bond to afford donepezil (4-4) [3]. 

Acetyl cholinesterase (from electric eel) Esterase, acetyl choline (9) (9000-81-1)... 

Cholinesterases (ChEs) are a ubiquitous group of enzymes that hydrolyze esters of choline. A well-known example is acetylcholinesterase (AChE, acetyl choline hydrolase, EC 3.1.1.7), the enzyme responsible for hydrolyzing the important neurotransmitter acetylcholine (ACh). Another ChE is butyrylcholi-nesterase (BuChE, acylcholine acylhydrolase, EC 3.1.1.8), also known as nonspecific cholinesterase. The preferred substrate for AChEs is ACh BuChEs prefer to hydrolyze esters like butyrylcholine and propionylcholine. Both AChE and BuChE are inhibited by some organophosphate (OP) and carbamate (CB) esters and also by other chemicals. 

The spleen in some species contains relatively large amounts of propionyl-choline . This fact, together with the presence of butyrocholinesterase in the central nervous system and the pharmacological activity of the choline esters, suggests the possibility that substances related to acetylcholine may also participate in the processes of synaptic transmission. Although choline esters other than acetylcholine, as well as other substances with acetyl-choline-like activity, have been reported to occur in the brain " , pharmacological and biochemical analyses of brain extracts have failed to confirm these claims . Similar reports of the existence of acetylcholine-like substances in peripheral nerve extracts have abo proved impossible to confirm . The physiological substrate of pseudocholinesterase in the nervous system—if, indeed, it is not acetylcholine—remains unknown. Attempts to solve the problem by differential inhibition of the two cholinesterases in the brain have so far proved unsuccessful. 

Figure 6.1 Synthesis and metabolism of acetylcholine. Choline is acetylated by reacting with acetyl-CoA in the presence of choline acetyltransferase to form acetylcholine (1). The acetylcholine binds to the anionic site of cholinesterase and reacts with the hydroxy group of serine on the esteratic site of the enzyme (2). The cholinesterase thus becomes acetylated and choline splits off to be taken back into the nerve terminal for further ACh synthesis (3). The acetylated enzyme is then rapidly hydrolised back to its active state with the formation of acetic acid (4)...

Some agonists, such as methacholine, carbachol and bethanecol are structurally very similar to ACh (Fig. 6.6). They are all more resistant to attack by cholinesterase than ACh and so longer acting, especially the non-acetylated carbamyl derivatives carbachol and bethanecol. Carbachol retains both nicotinic and muscarinic effects but the presence of a methyl (CH3) group on the p carbon of choline, as in methacholine and bethanecol, restricts activity to muscarinic receptors. Being charged lipophobic compounds they do not enter the CNS but produce powerful peripheral parasympathetic effects which are occasionally used clinically, i.e. to stimulate the gut or bladder. 

Figure 13.3. An overview of the chemical events at a cholinergic synapse and agents commonly used to alter cholinergic transmission acetyl CoA, acetyl coenzyme A Ch, choline. Nicotine and scopolamine bind to nicotinic and muscarinic receptors, respectively (nicotine is an agonist while scopolamine is an antagonist). Most anti-Alzheimer drugs inhibit the action of the enzyme cholinesterase.

The effect of Li+ upon the synthesis and release of acetylcholine in the brain is equivocal Li+ is reported to both inhibit and stimulate the synthesis of acetylcholine (reviewed by Wood et al. [162]). Li+ appears to have no effect on acetyl cholinesterase, the enzyme which catalyzes the hydrolysis of acetylcholine [163]. It has also been observed that the number of acetylcholine receptors in skeletal muscle is decreased by Li+ [164]. In the erythrocytes of patients on Li+, the concentration of choline is at least 10-fold higher than normal and the transport of choline is reduced [165] the effect of Li+ on choline transport in other cells is not known. A Li+-induced inhibition of either choline transport and/or the synthesis of acetylcholine could be responsible for the observed accumulation of choline in erythrocytes. This choline is probably derived from membrane phosphatidylcholine which is reportedly decreased in patients on Li+ [166],... 

True and pseudo-cholinesterase. The above serum preparations contained both the true and pseudo- cholinesterases of Mendel and Rudney.1 The effect of di-isopropyl phosphorofluoridate on these components was examined separately by means of the specific substrates described by Mendel, Mundel and Rudney,2 using the titration method described above. Phosphorofluoridate (5 x 10 8m) gave an inhibition of 57 per cent of the activity towards 00045m acetylcholine, 30 per cent of the activity towards 0-0005 m acetyl-/ methyl-choline, and 40 per cent of that towards 0-005 m benzoylcholine, after incubating the enzyme with the poison for 5 min. Thus in these experiments there appeared to be no appreciable difference in sensitivity of the true and pseudo-cholinesterases of horse serum to phosphorofluoridates. 

H-4) Nerve gas. It is not necessary to cripple the Main Powerhouse in order to poison someone. Nerve gas, as well as organophosphate insecticides, inhibit acetyl cholinesterase (AOiE), an enzyme important in the degradation of acetylcholine to choline. This can cause paralysis through inhibiting AChE at the junction between... 

Acetyl-chohnesterases (AChE, EC 3.1.1.7, also called true cholinesterase) and acylcholine acylhydrolases (or pseudocholinesterases including butyryl-cholinesterase, BuChE, EC 3.1.1.8) are commercially available enz5mies from different biological sources that catalyse the hydrolysis of acetyl- or butyryl-choline into choline and acetate or butyrate, respectively, according to the following reaction ... 

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... 

The cholinesterases, as indicated by the name, hydrolyze the esters of choline. However, the specificity for choline esters is not absolute, as evidenced by the hydrolysis of other esters, albeit at a slower rate. While acetylcholinesterase has high specificity not only for acetylcholine— CH3C00(CH2)2N+(CH3)3—but also for acetylthiocholine and other acetyl esters, most plasma cholinesterases catalyze the hydrolysis of butyrylcholine, butyrylthiocholine, and other butyryl esters at faster rates than the corresponding acetyl derivatives. With some homologous esters of acetylcholine, it was shown early on (DIO) that butyrylcholine... 

At high substrate concentrations, neither human nor horse plasma cholinesterase shows substrate inhibition with either acetyl- or butyryl-choline, but substrate inhibition is observed with halogenoacetylcholines... 

Choline esters are simply choline bound to an acetyl derivative by an ester bond. The ester bond of acetylcholine and related drugs is hydrolyzed by enzymes known as cholinesterases (e.g., acetylcholinesterase). Choline esters are more or less sensitive to cholinesterase deactivation depending on their chemical structure. 

Urease/glutamic dehydrogenase Acetyl cholinesterase/choline oxidase... 

According to the affinity to natural substrates choline esters cholinesterases are divided into AChE and BuChE. AChE, specific or true cholinesterase, the e type of cholinesterase (EC 3.1.1.7) has a higher affinity to acetylcholine than to butyrylcholine, and splits acetyl-beta methylcholine. 

One of the simplest possible applications of the LAPS/microflow chamber combination is the measurement of enzyme activity. One way to demonstrate this is to immobilize an enzyme in a chamber, and provide it with its substrate in the flow medium. As an example, let s consider acetyl cholinesterase, which catalyzes the hydrolysis of acetylcholine to acetate and choline, liberating protons in the process. Acetyl cholinesterase-coated agarose beads (Sigma) were immobilized between two thin polycarbonate membranes in a Cytosensor [4] chamber, and the pH response measured when an acetylcholine-containing medium is flowed through. Figure 5 shows the data from this experiment. Rates of about 120 pV/s are obtained. Note that the presence of the membranes slows down the time constant of the return to baseline of the pH during the flow-on periods. This demonstrates the measurement of enzyme activity, with possible applications to immunoassays. 

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