Enzyme carboxyl esterase

Irinotecan is a prodmg, and hydrolysis of irinotecan by the high-affinity carboxyl-esterase-2 enzyme in many normal tissues and tumors is responsible for activa-... 

We must stress that organo-phosphorus compounds are not specific inhibitors for the cholinesterases, but are rather inhibitors for enzymes possessing carboxylic esterase activity. All the enzymes mentioned below will hydrolyse carboxylic esters. However, not all esterases are inhibited, for example, A-esterase which hydrolyses phenyl acetate is not inhibited by organo-phosphorus compounds. 

Figure 5 Production of S-ibuprofen with a racemization step and an enzymatic conversion using the carboxyl esterase enzyme.

The search for intestinal cholesterol transporters extended for many years, beginning with a debate about whether or not it was even a protein-facilitated process (4, 5). The pancreatic enzyme carboxyl ester lipase (CEL, also called cholesterol esterase) was believed to be important to this process (6,7) and several companies devoted considerable resources to the development and testing of compounds to inhibit CEL, with mixed results (8-10). These efforts were abandoned in the mid-1990s, however, after studies with gene-knockout mice demonstrated that the enzyme was important only for absorption of cholesteryl ester (11, 12), which is a minor component of dietary cholesterol and is present at very low levels in bile. Interestingly, CEL is also found in liver where it has been shown to affect HDL metabolism (13). Thus, it may ultimately play an important role in cholesterol metabolism and may yet prove to be a useful drug target for CVD treatment (Camarota and Howies, unpublished). 

There emerges a very complex picture of three hormones synthesized and secreted at variable rates, competing for carrier binding proteins, presumed receptor proteins, epoxide hydratase and carboxyl esterase enzymes (35,36). It is possible experimentally to measure tEe timing of critical periods for larval determination and to measure total levels of JH at these critical periods although both measurements involve extreme difficulty. Approaches to this were described recently by G.B. Staal (3 7) using third instar larvae of the tobacco hornworm moth, Manduca sexta, which were allatectomized and raised on JH impregnated diets as an experimentally reproducible method of JH therapy. 

During natural evolution, a broad variety of enzymes has been developed, which are classified according to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (IUBMB). Thus, for each type of characterized enzyme an EC (Enzyme Commission) number has been provided (see http // www.expasy.ch/enzyme/). For instance, all hydrolases have EC number 3 and further subdivisions are provided by three additional digits, e.g. all lipases (official name triacylglycerol lipases) have the EC number 3.1.1.3 and are thus distinguished from esterases (official name carboxyl esterases) having the EC number 3.1.1.1. This classification is based on the substrate (and cofactor) specificity of an enzyme only, however often very similar amino acid sequences and also related three-dimensional structures can be observed. 

Fig. 11. Electrophoretic distribution in agar of gastric mucosal extract protein (A) protease activity at pH 2.2 (B) carboxylic esterase activity (C) and immuno-electrophoretic pattern (D). The relative mobility is shown at the bottom (UR) with 0 representing the location of the uncharged dextran, levan and 1 the migration of human serum albumin. The zones of mobility (Z), arbitrarily defined on the basis of protein distribution, are indicated at the top. Each antigen and enzyme is designated by the zone in which it is found. The antigens are alsc designated by a letter. From Kushner et al. (K32).

Metabolism of the local anaesthetic procaine provides an example of esterase action, as shown in figure 4.42. This hydrolysis may be carried out by both a plasma esterase and a microsomal enzyme. The insecticide malathion is metabolized by a carboxyl esterase in mammals, rather than undergoing oxidative desulphuration as in insects (figure 5,10). 

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. 

The racemic resolution of this molecule is very important because the 5 -enantiomer is 28-fold more active than the R-enantiomer. Sakaki and co-workers [4.69] realized the production of (5)-naproxen from the racemic naproxen methyl ester using lipase immobilized in hollow fibers. Their results showed that the MBR had good enzyme stability and enantiomeric excess of up to 0.92. The stereoselective hydrolysis of racemic 2-substituted propionates catalyzed by carboxyl esterase has been performed by Cretich and coworkers... 

Carboxylic esterases 1. a subgroup of esterases ading on carboxylic esters, e.g. lipases. 2. Carboxyles-terase (EC 3.1.1.1), a class of enzymes with wide specificity, usually for a short-chain acid and an alcohol... 

Another acid-base reaction of 79 with water, which is in the cellular medium, generates 80, which regenerates 75 by loss of a carboxylate unit, RC02. Note that both 77 and 80 are known as tetrahedral intermediates, which will be important in Chapter 20. While the focus of the reaction is conversion of a cholesterol ester to cholesterol and regeneration of the carboxylate unit derived from 76, the reaction is driven by acid-base reactions that occur on the enzyme, cholesterol esterase. Cholesterol is a part of mammalian cell membranes and it is very important for membrane permeability and fluidity. Cholesterol is an important precursor for the biosynthesis of bile acids and some fat-soluble vitamins. 

Williams reported the enzymatic hydrolysis of poly(L-lactic acid) (PLLA) in the presence of enzymes such as pronase, proteinase K, and bromelain. However, these enzymes are proteases and not PLLA depolymerases [3]. Regarding PLA stereocopolymers, Fukuzaki et al. reported that the hydrolysis of PLA was accelerated in the presence of specific enzymes and the most rapid enzymatic degradation of the stereocopolymers was observed on the poly(DL-lactic acid) (PDLLA) sample containing 50% L-lactic acid (LLA) [4]. Furthermore, Makino et al. showed that the addition of a carboxylic esterase accelerated a decrease in the weight-average molecular weight of PDLLA [5]. Reeve et al. reported the effects of stereochemical composition on enzymatic degradability of PLA films by proteinase K from Tritirachium album [6]. 

Typical examples of enzymes involved in food applications are cholinesterase for organophosphorous and carbamate pesticide analysis tyrosinase or laccase for analysis of phenols, quinones, and related compounds glucose oxidase for sugar content analysis, carboxyl esterase, alcohol oxidase, carboxypeptidase, L-aspartase, peptidase, aspartate... 

Chirazymes. These are commercially available enzymes e.g. lipases, esterases, that can be used for the preparation of a variety of optically active carboxylic acids, alcohols and amines. They can cause regio and stereospecific hydrolysis and do not require cofactors. Some can be used also for esterification or transesterification in neat organic solvents. The proteases, amidases and oxidases are obtained from bacteria or fungi, whereas esterases are from pig liver and thermophilic bacteria. For preparative work the enzymes are covalently bound to a carrier and do not therefore contaminate the reaction products. Chirazymes are available form Roche Molecular Biochemicals and are used without further purification. 

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