Liver carboxylesterases

Types of Carboxylesterase Isolated from Rat Liver Microsomes... 

Rivory LP, Bowles MR, Robert J et al. Conversion of irinotecan (CPT-11) to its active metabolite, 7-ethyl-10-hydroxy-camptothecin (SN-38), by human liver carboxylesterase. Biochem Pharmacol 1996 52 1103-1111. 

Carboxylesterases (CESs) catalyze hydrolysis of pyrethroids. The expression of CESs is ubiquitous in mammals. The highest hydrolase activity is present in liver. 

Fig. 6 Seven-compartment PBPK model for deltamethrin Km represents metabolic rate constant (Kml carboxylesterase in blood, Km2 cytochrome P450 in liver, Km3 carboxylesterase in liver, KmFcc rate constant in feces) [49]...

Munger JS, Shi GP, Mark EA, Chin DT, Gerard C, Chapman HA (1991) A serine esterase released by human alveolar macrophages is closely related to liver microsomal carboxylesterases. J Biol Chem 266 18832-18838... 

Crow JA, Borazjani A, Potter PM, Ross MK (2007) Hydrolysis of pyrethroids by human and rat tissues examination of intestinal, liver and serum carboxylesterases. Toxicol Appl Pharmacol 221 1-12... 

Huang H, Fleming CD, Nishi K, Redinbo MR, Hammock BD (2005) Stereoselective hydrolysis of pyrethroid-like fluorescent substrates by human and other mammalian liver carboxylesterases. ChemRes Toxicol 18 1371-1377... 

Cauxin is markedly different from previously reported mammalian CESs in term of urinary excretion. Other mammalian CESs comprise multigene families, and CES isozymes are highly and ubiquitously expressed in tissues such as the brain, liver, kidney, lung, and small intestine (Satoh and Hosokawa 1998). Our work on cauxin was the first description of a carboxylesterase excreted in urine. 

First generation of topi inhibitors were developed as drugs from camptothecins, a family of compounds derived from wood and bark of the Chinese tree Camptotheca acuminata) [9, 10], Many of these are already in clinical use or clinical trials, including irinotecan, topotecan, exatecan, rubitecan, and lurtotecan. Irinotecan (CPT-11) is bioactivated in liver by carboxylesterase to the active metabolite SN-38, 1000-fold more active [11]. Irinotecan received in 1998 FDA approval for treatment of metastatic colorectal cancer after failure of treatment with 5FU [12],... 

Fourth, most esterases are highly polymorphic enzymes. Many of the purified carboxylesterases are mixtures of isoenzymes that have different substrate specificities [60][61]. For example, the substrate(s) used to isolate pig liver carboxylesterase influences the isoenzyme composition, and, hence, the substrate specificity of the resulting esterase preparation. 

A number of rat liver carboxylesterases identified by their pI values are listed in Table 2.6 [73] five nonspecific carboxylesterases were purified from rat liver and were characterized according to their p/ values [61]. They appeared to be isoenzymes, since they had similar substrate specificities toward phenyl and naphthyl esters and monooleylglycerol. Subsequent studies, however, revealed different specificities with respect to their physiological substrates. The pI 5.2 and 5.6 enzymes were shown to be acylcamitine hydrolases (EC 3.1.1.28), and a p/ 6.0 enzyme an octanoylglycerol lipase. The p/... 

Table 2.6. Classification of Rat Liver Carboxylesterases Based on pi Value (taken from [73])...

Three isoenzymes of carboxylesterase were purified from rat liver micro-somes and were named RL1, RL2, and RH1. These differ from each other in their response to hormone treatment, inducibility, substrate specificity, and immunological properties [75], It was shown that RL1, RL2, and RH1 resemble hydrolases p/ 6.2/6.4, pI 6.0, and pI 5.6, respectively. Enzyme RL2 was found to be identical to egasyn, a protein with esterase activity found in the endoplasmic reticulum [76], The role of egasyn is to stabilize glucuronidase (EC 3.2.1.31) by noncovalent binding to the microsomal membrane. 

In humans, erythrocytes contain an esterase that displays genetic polymorphism [86], This esterase has been called esterase D (ES-D), a name without connection to the above-presented A-, B-, and C-classification. Three carboxylesterases named HU1, HU2, and HU3 have been found in human liver microsomes. Other tissues where esterases have been found include brain, plasma, stomach, small intestine, and colon [79]. 

Carboxylesterases (EC 3.1.1.1) can be detected in most mammalian tissues. Besides organs with high carboxylesterase activity such as liver, kidney, and small intestine, esterase activity is present, e.g., in the brain, nasal mucosa, lung, testicle, and saliva. Compared to rat plasma, human plasma contains little carboxylesterase, its esterase activity being essentially due to cholinesterase [61][73][79][89-91],... 

The intracellular localization of carboxylesterases is predominantly microsomal, the esterases being localized in the endoplasmic reticulum [73] [79] [93], They are either free in the lumen or loosely bound to the inner aspect of the membrane. The carboxylesterases in liver mitochondria are essentially identical to those of the microsomal fraction. In contrast, carboxylesterases of liver lysosomes are different, their isoelectric point being in the acidic range. Carboxylesterase activity is also found in the cytosolic fraction of liver and kidney. It has been suggested that cytosolic carboxylesterases are mere contaminants of the microsomal enzymes, but there is evidence that soluble esterases do not necessarily originate from the endoplasmic reticulum [94], In guinea pig liver, a specific cytosolic esterase has been identified that is capable of hydrolyzing acetylsalicylate and that differs from the microsomal enzyme. Also, microsomal and cytosolic enzymes have different electrophoretic properties [77]. Cytosolic and microsomal esterases in rat small intestinal mucosa are clearly different enzymes, since they hydrolyze rac-oxazepam acetate with opposite enantioselectivity [95], Consequently, studies of hydrolysis in hepatocytes reflect more closely the in vivo hepatic hydrolysis than subcellular fractions, since cytosolic and microsomal esterases can act in parallel. 

Seven carboxylesterase isoenzymes were purified from liver microsomes of mouse, hamster, guinea pig, rabbit, and monkey, and found to be glycoproteins hydrolyzing long-chain monoglycerides. Marked physical, enzymatic, and immunological similarities were found among these carboxylesterases, except for the monkey isoenzyme MK2 [99]. 

Thiolester hydrolases are present in most tissues and cell compartments. High concentrations are found in liver microsomes and in brown adipose tissue mitochondria and peroxisomes. Several acyl-CoA hydrolases have shown a close relationship to the nonspecific carboxylesterases EC 3.1.1.1. Thus, palmitoyl-CoA hydrolase purified from rat liver microsomes was found to be identical to esterase pI 6.2I6A (ES4 type). An acyl-CoA hydrolase was isolated that showed high similarity to esterase pI 6.1 [74a] [129] [130]. These few examples are further illustrations of the unsatisfying situation of the traditional classification of esterases. 

R. Mentlein, A. Ronai, M. Robbi, E. Heymann, O. von Deimling, Genetic Identification of Rat Liver Carboxylesterases Isolated in Different Laboratories , Biochim. Biophys. Acta 1987, 913, 27-38. 

M. Robbi, H. Beaufay, Cloning and Sequencing of Liver Carboxylesterase ES3 (Egasyn) , Biochem. Biophys. Res. Commun. 1994, 203, 1404-1411 M. Robbi, E. Van Schaftingen, H. Beaufay, Cloning and Sequencing of Rat Liver Carboxylesterase ES-4 (Microsomal Palmitoyl CoA Hydrolase) , Biochem. J. 1996, 313(Part 3), 821-826. 

B. F. Yan, D. F. Yang, M. Brady, A. Parkinson, Rat Kidney Carboxylesterase - Cloning, Sequencing, Cellular Localization, and Relationship to Rat Liver Hydrolase , J. Biol. Chem. 1994, 269, 29688-29696 B. F. Yan, D. F. Yang, M. Brady, A. Parkinson, Rat Testicular Carboxylesterase Cloning, Cellular Localization, and Relationship to Liver Hydrolase A , Arch. Biochem. Biophys. 1995, 316, 899-908. 

W. Junge, K. Krisch, The Carboxylesterases/Amidases of Mammalian Liver and Their Possible Significance , CRC Crit. Rev. Toxicol. 1975, 3, 371-434. 

R. Mentlein, H. Rix-Matzen, E. Heymann, Subcellular Localization of Nonspecific Carboxylesterases, Acylcarnitine Hydrolase, Monoacylglycerol Lipase and Palmitoyl-CoA Hydrolase in Rat Liver , Biochim. Biophys. Acta 1988, 964, 319-328. 

M. Hosokawa, T. Maki, T. Satoh, Characterization of Molecular Species of Liver Microsomal Carboxylesterases of Several Animal Species and Humans , Arch. Biochem. Biophys. 1990, 277, 219-227. 

S. E. H. Alexson, R. Mentlein, C. Wernstedt, U. Hellmann, Isolation and Characterization of Microsomal Acyl-CoA Thioesterase - A Member of the Liver Microsomal Carboxylesterase Multi-Gene Family , Eur. J. Biochem. 1993, 214, 719-727. 

The hydrolysis of the amide bond in chloramphenicol (4.26), which liberates dichloroacetic acid (4.27) and the primary amine (4.28), has been shown in bacteria, rodents, and humans [13-15]. In the microsomal fraction of guinea pig liver, moreover, the enzyme responsible for hydrolysis has been identified as one of the B-type carboxylesterase isoenzymes [16]. 

The indomethacin-hydrolyzing enzyme from pig liver microsomes was purified and partially characterized [60]. The enzyme was found to be different from known pig liver esterases, since it did not hydrolyze naphth-l-yl-acetate and (4-nitrophenyl)acetate, which are typical substrates for these car-boxy lesterases. The amino acid sequence of the enzyme showed high homology with the mouse carboxylesterase isoenzyme ES-male. Human liver car-... 

R. Arndt, H. E. Schlaak, D. Uschtrin, D. Sudi, K. Michelssen, W. Jungle, Investigations on the Functional Role of Rat Liver Carboxylesterase Isoenzymes , Hoppe-Seyler s Z Physiol. Chem. 1978, 359, 641-651. 

L. Luan, T. Sugiyama, S. Takai, Y. Usami, T. Adachi, Y. Katagiri, K. Hirano, Purification and Characterisation of Pranlukast Hydrolase from Rat Liver Microsomes The Hydrolase Is Identical to Carboxylesterase p/ 6.2 , Biol. Pharm. Bull. 1997, 20, 71-75. 

For example, the hydrolysis of phenyl acetate (7.15) by carboxylesterase isozymes was investigated over a broad pH range, allowing many insights into their catalytic mechanisms [30] (see Chapt. 3). This substrate was also used together with various inhibitors to characterize esterases in human and rat tissues [31], Thus, the approximate values of Km (in pM) and Vmax (in pmol min 1 (g tissue)-1) in human tissues were 300 and 60 in liver micro-somes, 200 and 40 in liver cytosol, and 1500 and 250 in plasma, respective-... 

Series of homologous esters have been investigated to try to establish structure-metabolism relationships, however partial and limited the latter may be. This aspect will be discussed again in the context of prodrugs (Chapt. 8). Here, we mention a few representative studies in which model substrates were used. Table 7.2 documents the substrate specificity of a rabbit liver carboxylesterase (ES-1A) toward homologous series of methyl, 4-nitrophenyl, a-naphthyl, /1-naphthyl, and 4-methylumbelliferyl esters [41]. In... 

In comprehensive studies, the hydrolysis of some 30 naphthyl esters by human, rat, and mouse liver carboxylesterases was investigated [43], A general trend that was apparent was that the rate of hydrolysis of a- and /3-naphthyl carbonates (7.21, R = alkyl or arylalkyl) catalyzed by human microsomes or rat hydrolases showed a tendency to decrease with increasing lipophilic-ity (range ca. 2 to 5). A similar trend was not seen with naphthyl aryl carbonates nor with a-naphthyl carboxylates. These results tell us that, even with purified enzymes and large series of substrates, it is very difficult indeed to discern sound structure-hydrolysis relationships due to the complexity of the structural and enzymatic factors involved. 

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