Hydrolyzable choline esters

ChEs hydrolyze choline esters and carboxylic esters (1, 2 respectively) ... 

AChEs and BuChEs are specialized carboxylic ester hydrolases that preferentially hydrolyze choline esters. They are classed among the B-esterases, enzymes that are inhibited by OPs. Another B-esterase is neuropathy target esterase (NTE), an enzyme associated with organophosphate-induced delayed neuropathy (OPIDN). Enzymes that actively hydrolyze OPs are known as A-esterases. They provide an important route of detoxification. Examples are par-aoxonase and DEPase (Table 1). The tertiary structure and amino acid sequences of several AChEs and BuChEs have been elucidated. 

The contribution of pseudocholinesterase, also known simply as cholinesterase, to drug metabolism is much greater as it possesses considerably broader substrate selectivity. In addition to acetylcholine, it will hydrolyze other choline esters like the muscle relaxant succinylcholine. It will also hydrolyze non-choline-containing drugs like the local anesthetic procaine and the anti-inflammatory agent aspirin (Fig. 6.5). Cholinesterases, particularly... 

The choline ester, carbachol, activates M-cholinoceptors, but is not hydrolyzed by AChE. Carbachol can thus be effectively employed for local application to the eye (glaucoma) and systemic administration (bowel atonia, bladder ato-nia). The alkaloids, pilocarpine (from Pilocarpus jaborandi) and arecoline (from Areca catechu betel nut) also act as direct parasympathomimetics. As tertiary amines, they moreover exert central effects. The central effect of muscarinelike substances consists of an enlivening, mild stimulation that is probably the effect desired in betel chewing, a widespread habit in South Asia. Of this group, only pilocarpine enjoys therapeutic use, which is limited to local application to the eye in glaucoma... 

Acetylcholinesterase (AChE) In pesticides, an enzyme that will most rapidly hydrolyze acetylcholine as substrate, will not hydrolyze most non-choline esters, is inhibited by excess substrate, and is derived primarily from nervous tissue. 

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... 

This type of modified lipopeptide cannot be deblocked under basic conditions because the labile palmitic acid thioester group would be preferentially hydrolyzed. The C-terminus of the peptide chain was selectively deprotected by removing the choline ester with choline esterase without affecting the palmitic acid thioester bond. The observed chemoselectivity here is exactly opposite to that found in nonenzymatic conversions. Some of the synthesized N-terminally deprotected lipopeptides, have been labeled with biotinand are expected to serve as anchors for a protein moiety in an artificial membrane. Butyrylcholine esterase mediated cleavage of the choline ester has been utilized l as the key step in the synthesis of S-palmitoylated peptides such as Myr-Gly-Cys(Plm)-Thr-Leu-Ser-Ala-OH, which represents the characteristic N-terminus of the a-subunit of human G o protein. 

The two enzymes differ in specificity toward some substrates while behaving similarly toward others. The serum enzyme acts on benzoylcholine but cannot hydrolyze acetyl-p-methylcholine the red cell enzyme acts on the latter but not on the former. The red cell enzyme splits only choline esters aryl or alkyl esters are not attacked. The red cell enzyme is inhibited by its substrate, acetylcholine, if present at about 10 mol/L the serum enzyme is not inhibited by this substrate. 

Use Biochemical research, determination of phosphorus in insecticides and poisons. (2) Pseudo or nonspecific cholinesterase prepared from horse serum. This esterase hydrolyzes other esters, as well as choline esters. It occurs in blood serum, the pancreas, and the liver. 

In later reports of work using purified enzymes, it was suggested that the erythrocyte enzyme was specific for choline esters and should be called true cholinesterase, while the serum enzyme, which could also hydrolyze noncholine esters, should be called pseudocholinesterase. In fact, both enzymes are to some degree nonspecific, and these names are not recommended by the Commission on Biochemical Nomenclature of the International Union of Pure and Applied Chemistry and the International Union of Biochemistry (12). Instead, the trivial names acetylcholinesterase and cholinesterase should be used for the eiythroe) e and serum enzymes (EC 3.1.1.7 and EC 3.1.1.8, respectively). ... 

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... 

The introduetion of a double bond at the a, 3 position in the alkyl group of choline esters reduces the rate of hydrolysis of the ester compared with that of the corresponding saturated alkyl ester. A double bond at any other position appears to have a less predictable effect, e.g., 4-pentenoylchoIine—CH2 = CH(CH2)2COO(CH2)2N+(0113)3—is hydrolyzed faster than is vinylacetylcholine—CH2 = CHCH2COO... 

In soils nearly 50% of the total sulphur is found in the hydroiodic acid reducible fraction , consisting of sulphate esters, which are probably in part polysaccharide sulphates (e.g. condroitin sulphate), keratin sulphates, choline sulphate, and arylsulphates (Zinder and Brock, 1978b). The heteropolysaccharide sulphates, which are also reported from msirine species (e.g. Torres-Pombo et al., 1969 Abdel-Fattah et al., 1973 Batey and Turvey, 1975), serve mainly as structural materials reaccessible to the marine sulphur cycle as SO ", depending on the ability of microorganisms, plants, and mammals to produce enzymes (sulphhydrolases) to hydrolyze these esters (Fitzgerald, 1978). 

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. 

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 ... 

BuChE is synthesized in the liver and has a replacement time of about 50 days. Its activity is decreased in parenchymal liver disease, acute infections, malnutrition, and chronic debilitating diseases, and is increased in the nephrotic syndrome.20 This enzyme has no known physiological function in blood, but may assist in hydrolyzing certain choline esters. 

Enzyme assays based on inhibition effects are not as commonly employed as substrate determinations. but one or two are very important. Preeminent is the determination of organophosphorus compounds by using their inhibitory effect on cholinesterase enzymes (E.C. 3.1.1.8—the first digit signifies a hydrolase enzyme, the second that the compounds hydrolyzed are esters, and the third that they are phosphoric monoesters). The latter catalyze the conversion of acylcholines to choline and the corresponding acid ... 

Simple esters are split by extracts of all tissues. The number of different esterases is not known. The digestive esterases produced by the pancreas are called lipases because they hydrolyze the triglycerides that are the most prominent lipids. They also hydrolyze simple esters. A special group of enzymes hydrolyze phospholipids. So-called lecithinase A removes one acyl group from lecithin to form lysolecithin, which causes hemolysis of erythrocytes. This enzyme has been crystallized from snake venom. Other animal toxins and bacteria also form lysolecithin by hydrolysis of lecithin. The removal of the second acyl group is catalyzed by phospholipase B, which has been studied in plant and animal extracts and also occurs in bacteria. Other enzymes, phospholipase C and D, specifically remove phosphorylcholine and choline, respectively, from lecithin. 

Two types of enzyme that hydrolyze acetylcholine rapidly have been identified. One is represented by an enzyme present in the serum of many animals. This esterase attacks many other esters at faster rates than acetylcholine, and has been called nonspecific or pseudocholinesterase, in distinction to the so-called specific or true cholinesterase found in erythrocytes, nervous and electrical tissue. The latter hydrolyzes acetylcholine more rapidly than other choline esters. Extensive kinetic studies have been made with both types of enzyme. Studies with highly purified true cholinesterase have been particularly revealing in explorations of the chemical nature of enzymatic action. 

Acetylcholine is a neurotransmitter released at a synapse as a means for one neuron to communicate with a neighboring neuron. The enzyme acetylcholinesterase rapidly hydrolyzes the ester to produce choline, terminating the signal. 

Ethanol and choline glycerolipids were isolated from calf brain and beef heart lipids by PTLC using silica gel H plates. Pure ethanol amine and choline plasmalogens were obtained with a yield of 80% [74]. Four phosphohpid components in the purple membrane (Bacteriorhodopsin) of Halobacterium halobium were isolated and identified by PTLC. Separated phosphohpids were add-hydrolyzed and further analyzed by GC. Silica gel G pates were used to fractionate alkylglycerol according to the number of carbon atoms in the aliphatic moiety [24]. Sterol esters, wax esters, free sterols, and polar lipids in dogskin hpids were separated by PTLC. The fatty acid composition of each group was determined by GC. 

A number of enzymes known as sulfuric ester hydrolases (EC 3.1.6) are able to hydrolyze sulfuric acid esters. They comprise arylsulfatase (sulfatase, EC 3.1.6.1), steryl-sulfatase (steroid sulfatase, steryl-sulfate sulfohydrolase, arylsulfatase C, EC 3.1.6.2), choline-sulfatase (choline-sulfate sulfohydrolase, EC 3.1.6.6), and monomethyl-sulfatase (EC 3.1.6.16). Whereas mono-methyl-sulfatase is highly specific and does not act on higher homologues, arylsulfatase has a broad substrate specificity and is of particular significance in the hydrolysis of sulfate conjugates of phenols, be they endogenous compounds, drugs, or their metabolites [167-169],... 

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]. 

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... 

Fate of the remaining chylomicron components After most of tt triacylglycerol has been removed, the chylomicron remnan (which contain cholesteryl esters, phospholipids, apolipoprotein and some triacylglycerol) bind to receptors on the liver (seej 228) and are then endocytosed. The remnants are the hydrolyzed to their component parts. Cholesterol and the nitrogf nous bases of phopholipids (for example, choline) can be req cled by the body. [Note If removal of chylomicron remnants by th liver is defective, they accumulate in the plasma. This is seen i type III hyperlipoproteinemia (also called familial dysbetalipopro teinemia, see p. 229). 

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