Carboxylesterases metabolism

The metabolism of foreign compounds (xenobiotics) often takes place in two consecutive reactions, classically referred to as phases one and two. Phase I is a functionalization of the lipophilic compound that can be used to attach a conjugate in Phase II. The conjugated product is usually sufficiently water-soluble to be excretable into the urine. The most important biotransformations of Phase I are aromatic and aliphatic hydroxylations catalyzed by cytochromes P450. Other Phase I enzymes are for example epoxide hydrolases or carboxylesterases. Typical Phase II enzymes are UDP-glucuronosyltrans-ferases, sulfotransferases, N-acetyltransferases and methyltransferases e.g. thiopurin S-methyltransferase. 

Permethrin, a pyrethrin pesticide, decreased the inhibition of brain cholinesterase activity by methyl parathion, but methyl parathion decreased the LD50 of permethrin when the two pesticides were simultaneously administered to rats (Ortiz et al. 1995). The potentiation of permethrin lethality may be due to the inhibition by methyl parathion of carboxylesterase, which metabolizes permethrin. 

The microsomal fraction consists mainly of vesicles (microsomes) derived from the endoplasmic reticulum (smooth and rough). It contains cytochrome P450 and NADPH/cytochrome P450 reductase (collectively the microsomal monooxygenase system), carboxylesterases, A-esterases, epoxide hydrolases, glucuronyl transferases, and other enzymes that metabolize xenobiotics. The 105,000 g supernatant contains soluble enzymes such as glutathione-5-trans-ferases, sulfotransferases, and certain esterases. The 11,000 g supernatant contains all of the types of enzyme listed earlier. 

Keywords Carboxylesterase CYP Ester hydrolysis Metabolism Oxidation Pyrethroid... 

In addition, three types of lipophilic conjugates have been found in pyrethroid metabolism studies (Fig. 4). They are cholesterol ester (fenvalerate) [15], glyceride (3-PBacid, a common metabolite of several pyrethroids) [16], and bile acid conjugates (fluvalinate) [17]. It is noteworthy that one isomer out of the four chiral isomers of fenvalerate yields a cholesterol ester conjugate from its acid moiety [15]. This chiral-specific formation of the cholesterol ester has been demonstrated to be mediated by transesterification reactions of carboxylesterase(s) in microsomes, not by any of the three known biosynthetic pathways of endogenous cholesterol esters... 

Extensive metabolism studies carried out mainly in rats and mice show that pyrethroids are metabolized by oxidation and ester cleavage, which are mediated by CYP isoforms and carboxylesterases, respectively. CYP isozymes and carboxylesterases responsible for the metabolism are reviewed below. 

Table 1 Cytochrome P450s and carboxylesterases responsible for metabolism of pyrethroids... 

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]...

Ross MK, Crow JA (2007) Human carboxylesterases and their role in xenobiotic and endobi-otic metabolism. J Biochem Mol Toxicol 21 187-196... 

Ross MK, Borazjani A, Edwards CC, Potter PM (2006) Hydrolytic metabolism of pyrethroids by human and other mammalian carboxylesterases. Biochem Pharmacol 71 657-669... 

Wheelock CE, Shan G, Ottea J (2005) Overview of carboxylesterases and their role in the metabolism of insecticides. J Pestic Sci 30 75-83... 

Carboxylesterases and amidases catalyze hydrolysis of carboxy esters and carboxy amides to the corresponding carboxylic acids and alcohols or amines. In general those enzymes capable of catalyzing hydrolysis of carboxy esters are also amidases, and vice versa (110). The role of these enzymes in metabolsim of drugs and insecticides has been reviewed (111, 112). In addition to the interest in mammalian metabolism of drugs and environmental chemicals, microbial esterases have been used for enantioselective hydrolyses (113, 114). 

Cross-tolerance between disulfoton and another organophosphate, chlorpyrifos, was observed in mice (Costa and Murphy 1983b). Because of this cross-tolerance, a benefit is derived as a result of this interaction. In the same study, propoxur-tolerant mice were tolerant to disulfoton but not vice versa. Propoxur (a carbamate) is metabolized by carboxylesterases, and these enzymes are inhibited in disulfoton-tolerant animals disulfoton-tolerant animals are more susceptible to propoxur and/or carbamate insecticides than are nonpretreated animals. In another study, disulfoton-tolerant rats were tolerant to the cholinergic effects of octamethyl pyrophosphoramide (OMPA) but not parathion (McPhillips 1969a, 1969b). The authors were unable to explain why the insecticides OMPA and parathion caused different effects. 

The physiological functions of carboxylesterases are still partly obscure but these enzymes are probably essential, since their genetic codes have been preserved throughout evolution [84] [96], There is some evidence that microsomal carboxylesterases play an important role in lipid metabolism in the endoplasmic reticulum. Indeed, they are able to hydrolyze acylcamitines, pal-mitoyl-CoA, and mono- and diacylglycerols [74a] [77] [97]. It has been speculated that these hydrolytic activities may facilitate the transfer of fatty acids across the endoplasmic reticulum and/or prevent the accumulation of mem-branolytic natural detergents such as carnitine esters and lysophospholipids. Plasma esterases are possibly also involved in fat absorption. In the rat, an increase in dietary fats was associated with a pronounced increase in the activity of ESI. In the mouse, the infusion of lipids into the duodenum decreased ESI levels in both lymph and serum, whereas an increase in ES2 levels was observed. In the lymph, the levels of ES2 paralleled triglyceride concentrations [92] [98],... 

T. Satoh, Role of Carboxylesterases in Xenobiotic Metabolism , in Reviews in Biochemical Toxicology , Eds. E. Hodgson, J. R. Bend, R. M. Philpot, Elsevier, New York,... 

Amidases can be found in all kinds of organisms, including insects and plants [24], The distinct activities of these enzymes in different organisms can be exploited for the development of selective insecticides and herbicides that exhibit minimal toxicity for mammals. Thus, the low toxicity in mammals of the malathion derivative dimethoate (4.44) can be attributed to a specific metabolic route that transforms this compound into the nontoxic acid (4.45) [25-27]. However, there are cases in which toxicity is not species-selective. Indeed, in the preparation of these organophosphates, some contaminants that are inhibitors of mammalian carboxylesterase/am-idase may be present [28]. Sometimes the compound itself, and not simply one of its impurities, is toxic. For example, an insecticide such as phos-phamidon (4.46) cannot be detoxified by deamination since it is an amidase inhibitor [24],... 

The in vivo metabolism of a homologous series of alkyl carbamates (7.2, Fig. 7.3) has yielded some informative results [13]. The hydrolysis of these esters liberates carbamic acid (7.3, Fig. 7.3), which breaks down spontaneously to C02 and NH3, allowing the extent of hydrolysis to be determined conveniently and specifically by monitoring C02 production. When such substrates were administered to rats, there was an inverse relationship between side-chain hydroxylation and ester-bond hydrolysis. Thus, for compounds 12 the contribution of hydrolysis to total metabolism (90 - 95% of dose) decreased in the series R=Et (ca. 85-90%), Bu (ca. 60-65%), hexyl (ca. 45 - 50%), and octyl (ca. 30%). Ethyl carbamate (urethane) is of particular toxicological interest, being a well-established carcinogen in experimental animals. In vitro studies of adduct formation have confirmed the competition between oxidative toxification mediated by CYP2E1 and hydrolytic detoxification mediated by carboxylesterases [14]. 

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... 

The rifamycins are a group of antibiotics of large size (Mr close to 1,000), which, among other chemical similarities, have an acetyl group at position 25. Deacetylation by carboxylesterases is a major metabolic pathway in animals and humans, as seen for example with rifampin, rifabutin, and rifalazil [97] [98],... 

Natural (-)-cocaine (7.57, Fig. 7.8), which has the (2/ ,3S)-configuration, is a relatively poor substrate for hepatic carboxylesterases and plasma cholinesterase (EC 3.1.1.8), and also a potent competitive inhibitor of the latter enzyme [116][121], In contrast, the unnatural enantiomer, (+)-(2S,3/ )-cocaine, is a good substrate for carboxylesterases and cholinesterase. Because hydrolysis is a route of detoxification for cocaine and its stereoisomers, such metabolic differences have a major import on their monooxygenase-catalyzed toxification, a reaction of particular effectiveness for (-)-cocaine. 

Thioesters play a paramount biochemical role in the metabolism of fatty acids and lipids. Indeed, fatty acyl-coenzyme A thioesters are pivotal in fatty acid anabolism and catabolism, in protein acylation, and in the synthesis of triacylglycerols, phospholipids and cholesterol esters [145], It is in these reactions that the peculiar reactivity of thioesters is of such significance. Many hydrolases, and mainly mitochondrial thiolester hydrolases (EC 3.1.2), are able to cleave thioesters. In addition, cholinesterases and carboxylesterases show some activity, but this is not a constant property of these enzymes since, for example, carboxylesterases from human monocytes were found to be inactive toward some endogenous thioesters [35] [146], In contrast, allococaine benzoyl thioester was found to be a good substrate of pig liver esterase, human and mouse butyrylcholinesterase, and mouse acetylcholinesterase [147],... 

Another therapeutic class to be briefly discussed is that of the lipid-lowering agents known as fibrates, e.g., clofibrate and fenofibrate (8.5). Here also, the acidic metabolite is the active form clofibrate (an ethyl ester) is rapidly hydrolyzed to clofibric acid by liver carboxylesterases and blood esterases [11], Human metabolic studies of fenofibrate (8.5), the isopropyl ester of fenofibric acid, showed incomplete absorption after oral administration, while hydrolysis of the absorbed fraction was quantitative [12], This was followed by other reactions of biotransformation, mainly glucuronidation of the carboxylic acid group. 

Fig. 9.14. Metabolic activation of phosphoramidic acid diester prodrugs 9.79 of stavudine (and analogous nucleosides). Carboxylesterase-mediated hydrolysis of the terminal carboxy-late is followed by spontaneous cyclization-elimination with formation of a pentacyclic mixed-anhydride species. The latter hydrolyzes rapidly to the corresponding phosphoramidic acid monoester, which can then be processed to stavudine 5 -monophosphate.

Differences among species in distribution patterns of histological changes may be caused by species variations in the distribution in the nasal epithelium of chemical-metabolizing enzymes. For example, in rats exposed to methyl methacrylate, nasal lesions were shown to be caused by the carboxylesterase mediated metabolism of methyl methacrylate to methacrylic acid, an irritant and corrosive metabohte. The distribution of these enzymes in the nasal tissues of man, rat, and hamster indicated a lower rate of metabolism in man compared to rat and hamster, suggesting a lower sensitivity to methacrylate in humans (Mainwaring et al. 2001). 

Metabolism 5-FU must be converted into a nucleotide precursor (5FdUMP or 5FdUTP to interfere with thymidylate synthase or RNA metabolism) Yes, must be metabolized to SN-38 by carboxylesterase... 

By analogy with malathlon, derivatives of this type were expected to be metabollcally degraded to nontoxic products In mammals by carboxylesterase action but to the parent methylcarbamate In Insects by phosphatase action (8). Thus, these derivatives were expected and found to be toxic to Insects and safe to mammals. Subsequent Investigations on the comparative metabolism of the N-dlmethoxyphosphlnothloyl derivative of carbofuran In Insects and rodents provided direct support for the rationale used In the design of these derivatives (9). 

Irinotecan has demonstrated a broad spectrum of activity in vitro and in vivo, and synergistic effects have been observed when it is administered in combination with other antineoplastic agents. Clinically irinotecan is now an active agent in patients with colorectal carcinoma. Irinotecan is metabolized by carboxylesterase to an active metabolite. It is cleared by hepatic metabolism and biliary excretion with a terminal elimination half-life of approximately 15 hours. The principal toxicities associated with irinotecan are diarrhoea and leucopenia. 

Capecitabine is a fluoropyrimidine carbamate prodrug with 70-80% oral bioavailability. It undergoes extensive metabolism in the liver by the enzyme carboxylesterase to an intermediate, 5 -deoxy-5-fluorocytidine. This is converted to 5 -deoxy-5-fluorouridine by the enzyme cytidine deaminase. These two initial steps occur mainly in the liver. The 5 -deoxy-5-fluorouridine metabolite is then hydrolyzed by thymidine phosphorylase to 5-FU directly in the tumor. The expression of thymidine phosphorylase has been shown to be significantly higher in a broad range of solid tumors than in corresponding normal tissue, particularly in breast cancer and colorectal cancer. 

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. 

Various esterases exist in mammalian tissues, hydrolyzing different types of esters. They have been classified as type A, B, or C on the basis of activity toward phosphate triesters. A-esterases, which include arylesterases, are not inhibited by phosphotriesters and will metabolize them by hydrolysis. Paraoxonase is a type A esterase (an organophosphatase). B-esterases are inhibited by paraoxon and have a serine group in the active site (see chap. 7). Within this group are carboxylesterases, cholinesterases, and arylamidases. C-esterases are also not inhibited by paraoxon, and the preferred substrates are acetyl esters, hence these are acetylesterases. Carboxythioesters are also hydrolyzed by esterases. Other enzymes such as trypsin and chymotrypsin may also hydrolyze certain carboxyl esters. 

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