Acetylcholine, hydrolysis

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. 

Experiment 1. The effects of alkaloids and ozone on the rate of acetylcholine hydrolysis... 

FIGURE 68.1. CarbE and ChE activities in liver versus blood plasma of thiouracil fed rats. The animals received rat chow supplemented with 0.05% thiouracil for 22, 30, or 38 days. Methyl butyrate hydrolysis (A A) and 4-nitrophenyl butyrate hydrolysis ( O) were measured in liver homogenate and plasma in accordance with Sterri et al. (1985b), and acetylcholine hydrolysis ( ) by the method of Sterri and Fonnum (1978). The activities are percent of corresponding control (=100%) in male (closed symbols) and female (open symbols) animals fed standard rat chow. 

Figure 8-8. Mechanism of acetylcholine hydrolysis (see text for explanation).

Physostigmine competitively blocks acetylcholine hydrolysis by cholinesterase, resulting in acetylcholine accumulation at cholinergic synapses that antagonizes the muscarinic effects of overdose with antidepressants and anticholinergics. With ophthalmic use, miosis and cihary-muscle contraction increases aqueous humor outflow and decreases lOP. 

Butyrylcholinesterase (BChE), also known as serum cholinesterase, catalyzes the hydrolysis of butyrylcholine at rates similar to those of acetylcholine hydrolysis by AChE. Though the function of BChE is poorly understood, it shows 73% similarity and 53% amino acid sequence identity with AChE from electric eel. There is some evidence that BChE is responsible for the activation or inactivation of compounds such as heroine and cocaine and that it is involved in defense against cholinesterase inhibitors... 

One of the attractive features of the aminoacid-substituted macrocycles is the possibility to provide strong and chiral binding elements for transition metal ions. Potentiometric titrations with 4 showed a stability increase from Zn " " to Cu " " to Fe " ", similar to the parent aminoacid complexes [27]. However, the stabilities themselves are considerably increased, e.g. with Cu " " by six units (Scheme 13). This is the result of the additional Coulomb attraction by the phenolate (half)-anions in the macrocycle, leading to four independent strong tridentate binding sites. Attempts to use transition metal ions implemented in the macrocycle for the catalysis of acetylcholine hydrolysis showed moderate success, consisting essentially in the removal of the inhibition discussed above for bound acetylcholine. 

Mode of Action. All of the insecticidal carbamates are cholinergic, and poisoned insects and mammals exhibit violent convulsions and other neuromuscular disturbances. The insecticides are strong carbamylating inhibitors of acetylcholinesterase and may also have a direct action on the acetylcholine receptors because of their pronounced stmctural resemblance to acetylcholine. The overall mechanism for carbamate interaction with acetylcholinesterase is analogous to the normal three-step hydrolysis of acetylcholine however, is much slower than with the acetylated enzyme. 

The use of mutant 34486 of Neurospora crassa for the microbiological assay of ch oline has been described (8). A physiological method has also been used in which the ch oline is extracted after hydrolysis from a sample of biological material and acetylated. The acetylcholine is then assayed by a kymographic procedure, in which its effect in causing contraction of a piece of isolated rabbit intestine is measured (33). 

Acetylcholine bromide [66-23-9] M 226.1, m 143 , 146 . Hygroscopic solid but less than the hydrochloride salt. It crystd from EtOH as prisms. Some hydrolysis occurs in boiling EtOH particularly if it contains some H2O. It can also be recryst from EtOH or MeOH by adding dry Et20. [Acta Chem Scand 12 1492, 1497, 1502 1958.]... 

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. 

Acetylcholinesterase is a component of the postsynaptic membrane of cholinergic synapses of the nervous system in both vertebrates and invertebrates. Its structure and function has been described in Chapter 10, Section 10.2.4. Its essential role in the postsynaptic membrane is hydrolysis of the neurotransmitter acetylcholine in order to terminate the stimulation of nicotinic and muscarinic receptors (Figure 16.2). Thus, inhibitors of the enzyme cause a buildup of acetylcholine in the synaptic cleft and consequent overstimulation of the receptors, leading to depolarization of the postsynaptic membrane and synaptic block. 

Absorbance- and reflectance-based measurements are widespread, as there are many enzymatic reaction products or intermediates that are colored or if not, can react with the appropriate indicator. Sensors using acetylcholinesterase for carbamate pesticides detection are an example of indirect optical fiber biosensors. This enzyme catalyses the hydrolysis of acetylcholine with concomitant decrease in pH41 ... 

The synaptic action of acetylcholine is unique among neurotransmitters in that it is terminated by hydrolysis rather than by transport (Ch. 11). Consequently, cholinergic neurons recover choline, rather than acetylcholine, via... 

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],... 

The primary function of acetylcholinesterase is to terminate the activity of the neurotransmitter, acetylcholine (Fig. 6.4), through hydrolysis at the various cholinergic nerve endings. In this regard, it is probably the most highly efficient enzyme that operates in the human. It is capable of hydrolyzing 300,000 molecules of acetylcholine per molecule of enzyme... 

There is some confusion in the literature regarding the substances designated as anti-choline-esterases (usually shortened to anticholinesterases). The term cholinesterase was first used1 in connexion with an enzyme present in the blood serum of the horse which catalysed the hydrolysis of acetylcholine and of butyrylcholine, but exhibited little activity towards methyl butyrate,... 

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 most outstanding example illustrating this strategy came from the team of Alain Friboulet and Daniel Thomas, who produced anti-idiotype antibodies against a monoclonal antibody AE2 that was a competitive inhibitor of acetylcholine esterase. One of the selected antibodies, 9A8, catalyzes the hydrolysis of acetylthio-choline with a pseudo first-order rate constant /feat = 81 and a factor of acceleration of 4.2 x 10 . These remarkable parameters, which are only two orders of magnitude lower when compared to those of the enzyme, make abzyme 9A8 the most powerful abzyme known until now. 

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