Esterases and Amidases

This is a rather large and heterogeneous family of proteins, from multiple gene families. Collectively they constitute the third largest group of reactions on 

One group of esterases has an a,p-fold and is prominent in the liver cytosol (Quinn, 1997). Acetylcholinesterase, butyl cholinesterase, and lipases have been used as models for these esterases. Generally esterases also have amidase activity (and vice versa, due to the basic mechanisms). All esterases appear to use a catalytic triad to activate a nucleophile, which is used to form an enzyme-acyl intermediate. The triad consists of a nucleophile, a general base catalyst, and an acidic residue. 

Some esterases are loosely bound to the endoplasmic reticulum and constitute a separate family, using ester and amide substrates. At least six of these have been reported in humans (Sone and Wang, 1997). Some evidence exists for induction and other regulation of this group. In general, the soluble esterases do not seem to be inducible. 

Al-Waiz M, Ayesh R, Mitchell SC, Idle JR, Smith RL. A genetic polymorphism of the N-oxidation of trimethylamine in humans. Clin Pharmacol Therap 1987 42 588-594. 

Baker MT, Van Dyke RA. Reductive halothane metabolite formation and halothane binding in rat hepatic microsomes. Chem-Biol Interact 1984 49 121-132. 

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Although hydrolytic enzymes, esterases and amidases, are named after their major substrates, the same enzyme can often hydrolyze esters, thioesters, and amides therefore, the differentiation between esterases and amidases is sometimes artificial. The highest hydrolytic activity is in the liver, but the enzyme pseudocholinesterase is found in the serum. Gut bacteria also contain hydrolytic enzymes. 

Fig. 9 Three haptens, [15]—[17], containing a 1,2-aminoalcohol functionality were investigated as alternatives for esterase and amidase induction. Of antibodies raised against hapten [15], 50% were shown to catalyse the hydrolysis of ester [18], thereby establishing the necessity for a compact haptenic structure. Hapten [19] along with [16] was employed in a heterologous immunization programme to elicit both a general and acid/base function in the antibody binding site.

Esterases and amidases E. coli, P. vulgaris, B.. subtilis, B. mycoides Cleavage of esters or amidases of carboxylic acids... 

The findings that, both in ester and amide cleavage, an alkaline-earth metal ion is still catalytically active when complexed with a crown ether, and that a fraction of the binding energy made available by coordinative interactions with the polyether chain can be translated into catalysis, provide the basis for the construction of supramolecular catalysts capable of esterase and amidase activity. 

Esters, amides, hydrazides, and carbamates can all be metabolized by hydrolysis. The enzymes, which catalyze these hydrolytic reactions, carboxylesterases and amidases, are usually found in the cytosol, but microsomal esterases and amidases have been described and some are also found in the plasma. The various enzymes have different substrate specificities, but carboxylesterases have amidase activity and amidases have esterase activity. The two apparently different activities may therefore be part of the same overall activity. 

The hydrolysis of esters by esterases and of amides by amidases constitutes one of the most common enzymatic reactions of xenobiotics in humans and other animal species. Because both the number of enzymes involved in hydrolytic attack and the number of substrates for them is large, it is not surprising to observe interspecific differences in the disposition of xenobiotics due to variations in these enzymes. In mammals the presence of carboxylesterase that hydrolyzes malathion but is generally absent in insects explains the remarkable selectivity of this insecticide. As with esters, wide differences exist between species in the rates of hydrolysis of various amides in vivo. Fluoracetamide is less toxic to mice than to the American cockroach. This is explained by the faster release of the toxic fluoroacetate in insects as compared with mice. The insecticide dimethoate is susceptible to the attack of both esterases and amidases, yielding nontoxic products. In the rat and mouse, both reactions occur, whereas sheep liver contains only the amidases and that of guinea pig only the esterase. The relative rates of these degradative enzymes in insects are very low as compared with those of mammals, however, and this correlates well with the high selectivity of dimethoate. 

Despite the presence of water throughout the body, hydrolysis reactions of esters and amides require enzymes to proceed at an appreciable rate. Numerous enzymes throughout the body carry out hydrolysis reactions. The enzymes, both esterases and amidases, are found in the digestive system, individual cells, and plasma. The exact site of hydrolysis of a specific drug depends on the drug s structure and functionality. The ester in aspirin... 

Plettner E., DeSantis G., Stabile M. and Jones J. B. (1999) Modulation of esterase and amidase activity of subtilisin Bacillus lentus by chemical modification of cysteine mutants. J. Am. Chem. Soc. 121, 4977-4981. 

The serine hydrolase family is one of the largest and most diverse classes of enzymes. They include proteases, peptidases, lipases, esterases, and amidases and play important roles in numerous physiological and pathological process including inflammation [53], angiogenesis [54], cancer [55], and diabetes [56]. This enzyme family catalyzes the hydrolysis of ester, thioester, and amide bonds in a variety of protein and nonprotein substrates. This hydrolysis chemistry is accomplished by the activation of a conserved serine residue, which then attacks the substrate carbonyl. The resulting covalent adduct is then cleaved by a water molecule, restoring the serine to its active state [57] (Scheme 1). 

Although the transition state analog approach is suitable for enzymes that bind their transition state noncovalendy, many natural enzymes achieve rate accelerations through covalent catalysis. For example, in the mechanism of most esterases and amidases, a functional group (e.g., a serine hydroxyl) of the protein covalently interacts with the substrate to form a protein bound intermediate. Furthermore, nature s most fundamental carbon-carbon bond-forming enzymes, class I aldolases, use... 

Dehydrogenation A-Demethylation Hydroxylation Epoxidation Sulfoxidation Oxidations Acetaminophen, benzidine, DES, epinephrine Dimethylaniline, benzphetamine, aminocarb Benzo[a]pyrene, 2-aminofluorene, phenylbutazone 7,8-Dihydrobenzo[a]pyrene Methylphenylsulfide FANFT, ANFT, bilirubin Esterases and Amidases Paraoxon, dimethoate, phenyl acetate Epoxide Hydrolase Benzo(a)pyrene epoxide, styrene oxide DDT-Dehydrochlorinase p,p- DDT Glutathione Reductase Disulfiram... 

The term carboxylesterase refers to a wide variety of enzymes with both esterase and amidase activity. They cleave carboxylesters, carboxylamides, and car-boxylthioesters, producing a carboxylic acid and an alcohol or phenol (Figure 8), amine, or mercaptan, respectively. There are many different esterases, some of which are important for the hydrolysis and detoxication of toxic organophosphate esters. In general, esterases are present in almost all mammalian tissues, occur as multiple isozymes, and are concentrated in the liver. The esterase activity present in plasma is normally due to the release of these enzymes from liver. 

This consideration is significant for hydrolytic reactions with hydrolases such as lipases, esterases, and amidases. These include penicillin amidases (synonymous with penicillin acylases) and cephalosporin acylases which are used for hydrolytic cleavage of penicillins and cephalosporins in thousands of tons per year [98]. These hydrolyses have to be performed at a pH-value of 8 which is close to the optimum pH of the enzyme. Lower pH-values lead to lower reaction rates and reversibility of the reaction, and hence to a significant loss in product formation. Higher pH-values are not advisable owing to the instability of the reaction partners. Moreover, addition of buffers is not accepted because of the costly removal of the buffer components. 

Many pesticides are esters or amides that can be activated or inactivated by hydrolysis. The enzymes that catalyze the hydrolysis of pesticides that are esters or amides are esterases and amidases. These enzymes have the amino acid serine or cysteine in the active site. The catalytic process involves a transient acylation of the OH or SH group in serin or cystein. The organo-phosphorus and carbamate insecticides acylate OH groups irreversibly and thus inhibit a number of hydrolases, although many phosphorylated or carbamoylated esterases are deacylated very quickly, and so serve as hydrolytic enzymes for these compounds. An enzyme called arylesterase splits paraoxon into 4-nitrophenol and diethyl-phosphate. This enzyme has cysteine in the active site and is inhibited by mercury(ll) salts. Arylesterase is present in human plasma and is important to reduce the toxicity of paraoxon that nevertheless is very toxic. A paraoxon-splitting enzyme is also abundant in earthworms and probably contributes to paraoxon s low earthworm toxicity. Malathion has low mammalian toxicity because a carboxyl esterase that can use malathion as a substrate is abundant in the mammalian liver. It is not present in insects, and this is the reason for the favorable selectivity index of this pesticide. 

Hydrolysis occurs during the metabolism of esters and amides. The reactions are catalysed by esterases and amidases occurring in the liver and... 

Other enzymes involved in phase I reactions are hydrolases (e.g., esterases and amidases) and the nonmicrosomal oxidases (e.g., monoamine oxidase and alcohol and aldehyde dehydrogenase). 

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