Acetylcholine ester hydrolysis

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 special case of the endogenous transmitter acetylcholine illustrates well the high velocity of ester hydrolysis. Acetylcholine is broken down at its sites of release and action by acetylcholinesterase (pp. 100,102) so rapidly as to negate its therapeutic use. Hydrolysis of other esters catalyzed by various esterases is slower, though relatively fast in comparison with other biotransformations. The local anesthetic, procaine, is a case in point it exerts its action at the site of application while being largely devoid of undesirable effects at other locations because it is inactivated by hydrolysis during absorption from its site of application. 

In addition to hydrogen ions, other species can also affect the enzymatic catalytic activity. This phenomenon is called inhibition it may be specific, nonspecific, reversible, or irreversible. The inhibition reactions can also be used for the sensing of inhibitors. The best-known example is the sensor for detection of nerve gases. These compounds inhibit the hydrolysis of the acetylcholine ester which is catalyzed by the enzyme acetylcholine esterase. Acetylcholine ester is a key component in the neurotransmission mechanism. 

The mechanism for the hydrolysis that is catalyzed by the enzyme involves the hydroxy group of a serine amino acid residue in the protein acting as a nucleophile and attacking the carbonyl carbon of the acetylcholine ester. The ester is cleaved, and the acetyl group becomes bonded to the enzyme. Then the acetyl group is hydrolyzed off the enzyme. enabling it to perform another catalytic cycle. This hydrolysis is very facile, so a single enzyme molecule can catalyze the hydrolysis of many acetylcholine molecules ... 

Diphenoxylate HCI (2.5 mg) and atropine (0.025 mg) are combined in tablets or 5 mL liquid and are used effectively as symptomatic treatment for diarrhea. The typical dose is two tablets or 10 mL every 3 to 4 hours. The combination with atropine enhances the block of acetylcholine-stimulated peristalsis, and the adverse effects of atropine helps to limit the abuse of the opioid. The combination is Schedule V under the Controlled Substances Act. Diphenoxylate itself has low p opioid agonist activity. It is metabolized rapidly by ester hydrolysis to the zwitterionic free carboxylate (difenoxin), which is five times more potent after oral dosing. The zwitterionic properties of difenoxin probably limits its penetration into the CNS and explains the low abuse potential of this agent. High doses of diphenoxylate (40-60 mg) will cause euphoria and addiction. 

There are at least two cholinesterases acetylcholinestarase (AChE), a specific cholinesterase hydrolyzing predominantly the choline esters and occurring in high concentrations in brain, nerve and red blood cells and the other, butyrylcholinesterase (BChE), a nonspecific ( pseudo ) cholinesterase, hydrolyzing other esters as well, and found in the blood, serum, pancreas and liver. These enzymes manifest maximum catalytic activity around neutral pH and at the low levels of acetylcholine. The hydrolysis reaction of acetylcholine catalyzed by AChE is shown below ... 

An elegant example for a dynamic resolution of an allylic alcohol via enantiose-lective ester hydrolysis is depicted in Scheme 2.66 [463]. Thus, Pseudomonas sp. lipase hydrolyzed the acetate ester with high specificity, while the in-situ racemiza-tion of the substrate enantiomers was effected by a catalytic amount of Pd leading to the product alcohol in 96% e.e. and 81% yield. However, the lipase has to be chosen with great care, since other hydrolytic enzymes such as acetylcholine esterase and lipases from Penicillium roqueforti, Rhizopus niveus, and Chromobacterium viscosum were incompatible with the metal catalyst. 

FIGURE 5.46 Interaction of the serine hydroxyl residue in the catalytically active site of acetylcholinesterase enzyme with esters of organophosphates or carbamates. The interaction leads to binding of the chemical with the enzyme, inhibition of the enzyme, inhibition of acetylcholine hydrolysis, and thus accumulation of acetylcholine in the synapses. 

Thus a distinction was provided between simple esterases, such as fiver esterase, which catalysed the hydrolysis of simple aliphatic esters but were ineffective towards choline esters. The term 1 cholinesterase was extended to other enzymes, present in blood sera and erythrocytes of other animals, including man, and in nervous tissue, which catalysed the hydrolysis of acetylcholine. It was assumed that only one enzyme was involved until Alles and Hawes2 found that the enzyme present in human erythrocytes readily catalysed the hydrolysis of acetylcholine, but was inactive towards butyrylcholine. Human-serum enzyme, on the other hand, hydrolyses butyrylcholine more rapidly than acetylcholine. The erythrocyte enzyme is sometimes called true cholinesterase, whereas the serum enzyme is sometimes called pseudo-cholinesterase. Stedman,3 however, prefers the names a-cholinesterase for the enzyme more active towards acetylcholine, and / -cholinesterase for the one preferentially hydrolysing butyrylcholine. Enzymes of the first type play a fundamental part in acetylcholine metabolism in vivo. The function of the second type in vivo is obscure. Not everyone agrees with the designation suggested by Stedman. It must also be stressed that enzymes of one type from different species are not always identical in every respect.4 Furthermore,... 

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]. 

Hydrolysis involves nucleophilic attack by the serine hydroxyl onto the ester carbonyl (see Box 7.26). This leads to transfer of the acetyl group from acetylcholine to the enzyme s serine hydroxyl, i.e. formation of a transient acetylated enzyme, and release of choline. We have met this type of reaction before under transesterification (see Section 7.9.1). Hydrolysis of the acetylated enzyme then occurs rapidly, releasing acetate and regenerating the free enzyme. 

This enzyme [EC 3.1.1.7], also known as true cholinesterase, choline esterase I, and cholinesterase, catalyzes the hydrolysis of acetylcholine to produce choline and acetate. The enzyme will also act on a number of acetate esters as well as catalyze some transacetylations. 

The first suggestion of a practical form of antidotal therapy came in 1949 from Hestrin, who found that acetylcholinesterase (AChE) catalyzed the formation of acetohydroxamlc acid when incubated with sodium acetate and hydroxylamine. Critical in vitro studies in the next decade led to the development of a practical approach to therapy. The crucial concept in these studies was the recognition that the compound formed when AChE reacted with a phosphorus ester was a covalent phosphoryl-enzyme Intermediate similar to that formed in the hydrolysis of acetylcholine. 3 Wilson and colleagues, beginning in 1951, demonstrated that AChE inhibited by alkyl phosphate esters (tetraethyl pyrophosphate, TEPP) could be reactivated by water, but that free enzyme formed much more rapidly in the presence of hydroxylamine. 0 21 Similar results... 

Acetylcholine (ACh) is an ester of choline and acetic acid available in powder form as chloride or bromide salt. It is extremely hygroscopic and rapidly undergoes hydrolysis in a neutral or alkaline medium. 

Hydrolysis of activated esters of a-amino acids or acetylcholine analogs catalysed by Cu or Zn modified with the tenside ligands 118,119 and 120 (Table... 

Metabolites that are less reactive than suicide inhibitors may impact more distant enzymes, within the same cell, adjacent cells, or even in other tissues and organs, far removed from the original site of primary metabolism. For example, organopho-sphates (OPs), an ingredient in many pesticides, are metabolized by hepatic CYPs to intermediates, which, when transported to the nervous system, inhibit esterases that are critical for neural function. Acetylcholinesterase (AChE) catalyzes the hydrolysis of the ester bond in the neurotransmitter, acetylcholine, allowing choline to be recycled by the presynaptic neurons. If AChE is not effectively hydrolyzed by AChE in this manner, it builds up in the synapse and causes hyperexcitation of the postsynaptic receptors. The metabolites of certain insecticides, such as the phos-phorothionates (e.g., parathion and malathion) inhibit AChE-mediated hydrolysis. Phosphorothionates contain a sulfur atom that is double-bonded to the central phosphorus. However, in a CYP-catalyzed desulfuration reaction, the S atom is... 

Choline esters are poorly absorbed and poorly distributed into the central nervous system because they are hydrophilic. Although all are hydrolyzed in the gastrointestinal tract (and less active by the oral route), they differ markedly in their susceptibility to hydrolysis by cholinesterase in the body. Acetylcholine is very rapidly hydrolyzed (see Chapter 6 Introduction to Autonomic Pharmacology) large amounts must be infused intravenously to achieve concentrations high enough to produce detectable effects. A large intravenous bolus injection has a brief effect, typically... 

Question. The acid hydrolysis of a positively charged ester HE+ (acetylcholine, (CH3)3N+CH2CH2OCOCH3) is first order in ester and first order in H30+. It is catalysed by SO] ions. A possible mechanism is... 

Unlike the acetate ester, the phosphate ester is not readily hydrolyzed off the enzyme. Therefore, the enzyme can no longer catalyze the hydrolysis of acetylcholine, and the muscle remains paralyzed. Paralysis of the muscles that are involved in breathing results quickly in asphyxiation and death. 

Quon et al. (1985) investigated the stability of esmolol in blood, plasma, red blood cells, and purified enzymes (human serum pseudocholinesterase, human and dog serum albumin, acetyl choline esterase, carbonic anhydrases A and B and human haemoglobin). Udata et al. (1999) studied the hydrolysis of propranolol ester prodrugs in purified acetylcholine esterase. 

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]. 

To avoid repetitive excitation of the receiving cell caused by the presence of acetylcholine, excessive acetylcholine has to be removed by the enzyme acetylcholinesterase (AChE), which catalyzes hydrolysis of the ester bond as follows ... 

Acyl Cholinesterases. Acetylcholinesterase (AChE EC 3.1.1.7 CAS 9000-81-1) is the serine esterase which catalyzes the hydrolysis of acetylcholine and possesses an esteratic site, and which is responsible for unspecific hydrolyses of several substrates. Also, butyrylcholinesterase (EC 3.1.1.8 CAS 9001-08-5) has been sometimes used for asymmetric hydrolysis of esters. Acetylcholinesterase has been used for... 

Fig. 2 Seeps involved in Che hydrolysis of acetylcholine (ACh) by acetylcholinesterase (AChE) (I), and in the inhibition of AChE by reversible (II)> carbamyl ester (1I1) and organophosphorus (IV) agents. Heavy, light, and dashed arrows represent extremely rapid. Incecmedlace. and extremely slow or insignificant reactions, respectively. Reproduced from Koelle. G.B.. In The acmacologlc Basis of Therapeutics (Goodman. L.S. and Gilman. A., eds.). Sth ed. Macmillan Publ. Co.. 1975, pg. 448.

Acetylcholinesterase (AChE) catalyses the hydrolysis of the ester bond of acetylcholine to yield choline and acetate (Sussman et al., 1991). This is a critical reaction for the termination of impulses transmitted through cholinergic synapses. It is a highly efficient catalyst, with reaction rates approaching the diffusion limit. Its overall structure resembles the lipases with an active site gorge. Above the base of the gorge is the reactive serine to be activated by the classical (Ser-200...His-440...Glu-327) catalytic triad. 

Acetylcholinesterase (AChE) (also termed true cholinesterase ) is found in the synaptic cleft of cholinergic synapses, and is of undoubted importance in regulation of neurotransmission by rapid hydrolysis of released endogenous acetylcholine (ACh). AChE is also found in erythrocytes and in the CSF, and can be present in soluble form in cholinergic nerve terminals, but its function at these sites is not clear, AChE is specific for substrates that include acetylcholine and the agents methacholine and acetylthiocholine. but it has little activity with other esters. It has a maximum turnover rate at very low concentrations of AChE (and is inhibited by high concentrations). 

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