Hydrolysis by Cholinesterases

The chemical modes of bimolecular ester hydrolysis, as represented by Ingold (26) and his school, are related to either acidic or basic catalysts. They involve attack at the carbonyl portion of the ester group, giving as intermediates or transition states derivatives of the ortho acid, which may undergo reversible (4) or practically irreversible (B) changes  

In contrast to these reaction schemes, enzymatic hydrolysis shows optimal rates at pH 7-8, at least as far as animal esterases are concerned. Increasing concentrations of either H+ or 0H slow the hydrolytic reactions down. Therefore, a different catalytic mechanism, not included in Ingold s reaction schemes, must be operative in enzymatic systems. 

An ester can be split either at point A (acyl oxygen fission) or at point B (alkyl oxygen fission)  

In both schemes of V,l, the bond A between the carbonyl carbon and the ethereal oxygen is broken i.e., acyl fission is involved. That the same should apply to the reactions catalyzed by ChE s had been derived from theoretical considerations by Burgen (27) and by Wilson and Bergmann (28). Direct proof was advanced by experiments which were designed in analogy to those which had served in the elucidation of the pathway of chemical hydrolysis. 

The above results lead to the conclusion that hydrolysis of carboxylic and phosphoric acid derivatives by ChE s proceeds in the same fashion. Therefore, we can now subdivide ester fission into two steps  

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Unlike acetylcholine and methacholine, carbachol contains a carbamino functional group instead of an acetyl group, which is not responsive to hydrolysis by cholinesterase. In vitro studies have shown that the rate of hydrolysis is at least twice as slow as that of acetylcholine. 

Following the administration of succinylcholine, 0.75-1.5 mg/kg IV, transient muscle fasciculations occur over the chest and abdomen within 30 seconds, although general anesthesia and the prior administration of a small dose of a nondepolarizing muscle relaxant tends to attenuate them. As paralysis develops rapidly (< 90 seconds), the arm, neck, and leg muscles are initially relaxed followed by the respiratory muscles. As a result of succinylcholine s rapid hydrolysis by cholinesterase in the plasma (and liver), the duration of neuromuscular block typically lasts less than 10 minutes (Table 27-1). 

Succinylcholine is a neuromuscular blocking agent, which is used clinically to cause muscle relaxation. Its duration of action is short due to rapid metabolism—hydrolysis by cholinesterases (pseudocholinesterase or acylcholine acyl hydrolase)—in the plasma and liver to yield inactive products (Fig. 7.55). Thus, the pharmacological action is terminated by the metabolism. However, in some patients, the effect is excessive, with prolonged muscle relaxation and apnea lasting as long as two hours compared with the normal duration of a few minutes. 

In view of the high rate of hydrolysis by cholinesterases, it is usually possible to work with tissue homogenates, whole cells (erythrocytes), or partly purified enzymes. Only in a few cases have methods for the preparation of highly purified or crystalline ChE s been developed. 

The combination of process a and b explains the occurrence of a pH-optimum for oxygen ester hydrolysis by cholinesterases around pH 7-8 and thus sheds light on the problem, raised in V, 1. 

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... 

Increases acetylcholine in CNS through reversible inhibition of its hydrolysis by cholinesterase APOE, APP, CHAT... 

The enzymatic process is remarkably efficient due to the close proximity of the serine nucleophile and the histidine acid/base catalyst. As a result, enzymatic hydrolysis by cholinesterase is one hundred million times faster than chemical hydrolysis. The process is so efficient that acetylcholine is hydrolysed within a hundred microseconds of reaching the enzyme. 

Carbachol is a potent direct-acting miotic agent its duration of action is longer than that of pilocarpine (8 to 10 hours) because of resistance to hydrolysis by cholinesterases. This drug also may act as a weak inhibitor of cholinesterase. Patients with an inadequate response to or intolerance of pilocarpine as a result of ocular hritation or allergy frequently do well on carbachol. The ocular and systemic adverse effects of carbachol are similar to butmore frequent, constant, and severe than those of pilocarpine. " ... 

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. 

Its depolarizing neuromuscular blocking effect is very transient because of its rapid hydrolysis by cholinesterases. It does not cause histamine liberation and hence it is well tolerated. Single-dose therapy of suxamethonium chloride is generally used to relax the skeletal muscle for orthopedic manipulation, endotracheal intubation, in laryngospasm and also to check the intensity of convulsions in patients receiving electroshock treatment (electroconvulsive therapy). 

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