A carboxylesterases

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. 

Ecroyd, H., Belghazi, M., Dacheux, J.L., Miyazaki, M., Yamashita, T. and Gatti, J.L. (2006) An epididymal form of cauxin, a carboxylesterase-like enzyme, is present and active in mammalian male reproductive fluids. Biol. Reprod. 74, 439-447. 

A more complex pathway of activation is seen in N-amino acid derivative of phosphoramidic acid diesters of antiviral nucleosides, as exemplified by prodrugs of stavudine (9.79, Fig. 9.14) [153 -155], The activation begins with a carboxylesterase-mediated hydrolysis of the terminal carboxylate. This is followed by a spontaneous nucleophilic cyclization-elimination, which forms a mixed-anhydride pentacycle (9.80, Fig. 9.14). The latter hydrolyzes spontaneously and rapidly to the corresponding phosphoramidic acid monoester (9.81, Fig. 9.14), which can then be processed by phosphodiesterase to the nucleoside 5 -monophosphate, and by possible further hydrolysis to the nucleoside. 

Another important problem is the development of insects resistant to insecticides. This often arises as a result of increased levels of carboxylesterases which hydrolyze both organophosphates and car-baryl.h/1 A mutation that changed a single active site glycine to aspartate in a carboxylesterase of a blowfly changed the esterase to an organophosphorus hydrolase which protected the fly against insecticides.)... 

Upadhya, G., Govardhan, L., and Veerabhadrappa, P. S. (1985). Purification and properties of a carboxylesterase from germinated finger millet (Eleusine coracana Gaertn.).. Biosci. 7, 289-301. 

The term potentiation is then reserved for those cases where both compounds have appreciable intrinsic toxicity, such as in the case of malathion and EPN. Malathion has a low mammalian toxicity due primarily to its rapid hydrolysis by a carboxylesterase. EPN (Figure 9.6) another organophosphate insecticide, causes a dramatic increase in malathion toxicity to mammals at dose levels, which, given alone, cause essentially no inhibition of acetylcholinesterase. The increase in toxicity as a result of coadministration of these two toxicants is the result of the ability of EPN, at low concentrations, to inhibit the carboxylesterase responsible for malathion degradation. 

Carboxylesterase activity is elevated in mastitic milk and colostrum (Fitz-Gerald et al., 1981) and may correspond to that of the reported lipases from somatic cells (Gaffney and Harper, 1965 Azzara and Dimick, 1985a) and colostrum (Driessen, 1976), respectively. The retinyl esterase activity that co-purifies with, but can be separated from, LPL may also be due to a carboxylesterase (Goldberg et al., 1986). It is of interest that the BSSL in human milk that has been shown to be identical with pancreatic carboxylesterase, has retinyl esterase activity (O Connor and Cleverly, 1989). 

There is little doubt that lipases claimed to be non-regioselective display activity towards the secondary alcoholic mid-position of TAGs, but this may well be different when it comes to re-esterification of that position with fatty acids or their derivatives. Interestingly enough, there is a recent report on conversion of a carboxylesterase into a triacylglycerol lipase by random mutation (Reyes-Duarto et al., 2005). The hydrolytic enzyme was observed to display preference for the sn-2 position of triacylglycerols in an ethanolysis reaction. 

Reyes-Duarto, D., Polaina, J., L6pez-Cortes, N., Alcalde, M., Plou, F. J., Elborough, K., Ballesteros, A., Timmis, K. N., Golyshin, P. N., and Ferrer, M. 2005. Conversion of a carboxylesterase into a triacylglycerol lipase by a random mutation. Angew. Chem., 117,1-5. 

Several human carboxylesterases have been cloned, sequenced and expressed. These human carboxylesterases are important in the hydrolysis of certain pesticides such as the pyrethroids. In certain strains of insects that are resistant to malathion, the resistance mechanism is associated with a higher level of a carboxylesterase, which detoxifies malathion (Figure 10.10D). 

Devonshire, A.L., The properties of a carboxylesterase from the peach-potato aphid, Myzuz persicae (Sulz.), and its role in conferring insecticide resistance, Biochem. J., 161, 675,1977. 

Newcomb, R.D., Campbell, P.M., Ollis, D.L., Cheah, E., Russell, R.J., and Oakeshott, J.G., A single amino acid substitution converts a carboxylesterase to an organophosphorus hydrolase and confers insecticide resistance on a blow fly, Proc. Natl. Acad. Sci. USA, 94, 7464,1997. 

Pretreatment of rats with a carboxylesterase inhibitor enhances the respiratory irritation and lethality produced by the inhalation of ethyl acrylate. This and other observations suggest that the toxicity of ethyl acrylate becomes manifest when local detoxi-fication/defense mechanisms become overwhelmed. 

Metabolism also plays a critical role in the pharmacology of cocaine. The rapid hydrolysis of cocaine via two different pathways leads to its rapid inactivation/detoxification. This rapid metabolism has been a major determinant in the methods and modes of cocaine abuse. Identification and characterization of these hydrolytic enzymes would be useful in that selective induction of these enzymes offers a potential treatment strategy for dealing with cocaine overdose. It is conceivable that long-term elevation of the enzyme or enzymatic activity could be used in conjunction with maintenance therapy for cocaine addicts. Hydrolases or esterases are also responsible for the transesterfication of cocaine. The pharmacological effect of cocaine is prolonged and enhanced when cocaine is used in conjunction with ethanol. A carboxylesterase catalyzes an ethyl transeterification of cocaine to cocaethylene, which is biologically active. 

On the above view, NLSD is primarily a disease of defective phospholipid metabohsm and the TAG-derived route of phospholipid biosynthesis is essential for the normal functioning of skin, muscle, liver, and the central nervous system. Further studies on the fatty hver dystrophic mouse which bears a close resemblance to human NLSD [103] may identify fhe defective gene. Defective neutral phospholipid metabolism in this mouse model is reflected by a decreased phosphohpid content of peripheral nerve myelin [109]. The nature of the hpase(s) involved in normal TAG-to-phospholipid acyltransfer has not been characterized, but may be similar to a previously described microsomal neutral hpase [110] or to a carboxylesterase [111]. 

Hydrolysis. Carboxylesterases are frequently one of the major factors in OP resistance. In some insects, for instance the house fly (28), there are highly substrate specific esterases which attack only one or a very few molecules. "Malathionase", the prominent esterase responsible for many cases of malathion resistance, is highly specific for malathion. It cleaves one or both of the ethyl ester groups leaving malathion mono- or diacid (29). This enzyme is a true serine carboxylesterase that is inhibited by malaoxon (28) and does not hydrolyze any of the phosphoester bonds. In Anopheles stephensi from Pakistan, the malathion resistance decreased with adult age, but there was no concommittant decrease in general esterase activity as measured with 1- and 2-naphthylace-tate as model substrates (301. other mosquitoes have a carboxylesterase with broad substrate specificity that is associated with resistance (31-331. As mentioned above, the green peach aphid has a carboxylesterase, E4, with broad substrate specificity that sequesters toxicants (24). 

Figure 9.2 Devonshire and Sawicki (1979) at Rothamstead Experimental Station, Hertfordshire, U.K., found seven variants of the aphid Myzus persicae with different resistances to parathion. Excess production of a carboxylesterase as a result of one or several gene duplications was found to be the resistance mechanism. High levels of carboxylesterases take paraoxon away from acetylcholinesterase so that aphids become resistant.

Robbi, M., Beaufay, H., and Octave, J.N., Nucleotide sequence of cDNA coding for rat liver pi 6.1 esterase (ES-10), a carboxylesterase located in the lumen of the endoplasmic reticulum. Biochemistry, 269, 451, 1990. 

Figure 19.9 Metabolic possibilities for model compounds having representative functionality. Selected phase 1 reactions (1) Hydrolysis of various types of esters, in this case mediated by a carboxylesterase (2) N-dealkylation mediated by certain of the Cytochrome P-450 (CYP) enzymes (3) O-dealkylation mediated by certain of the CYPs and (4) Aromatic hydroxylation also mediated by certain of the CYPs. Depending upon the subtleties of their electronic and steric environments, the relative competitive biotransformation rates for these processes will generally be (1) (2) > (3) (4). Selected phase 2 reactions (5) Formation of a glucuronic acid conjugate (or in some cases a sulfate conjugate) and (6) N-acetylation. In terms of relative biotransformation rates in general (5) >> (6).

C. Hydrolysis A wide range of esterases exist for the hydrolysis of compounds. Examples include (a) carboxylesterases (EC 3.1.1.1) (b) amidases (EC 3.5.1.4 and others) (c) phosphatases (EC 3.1.3.1, 3.1.3.2 and others) Esters, amides, hydrazides and carbonates Thio-orga nophosphate Single or multiple end-products can be produced by hydrolysis depending on the enzymes involved and the substrate of the reaction, ... 

A carboxylesterase (EC 3.1.1.1) from T. fusca [8, 86] and a steryl esterase (EC 3.1.1.13) from Melanocarpus albomyces [59] have also shown activity with PET oligomers and fabrics. The enzyme from M. albomyces with high specificity for fatty acid esters of sterols increased the hydrophilicity of PET fabrics. The highly hydrophobic serine hydrolase from T. fusca with a catalytic triad composed of serine, glutamic acid, and histidine hydrolyzed CTR and PET nanoparticles. The esterase showed high specificity towards short and middle chain-length fatty acyl esters of p-nitrophenol. In addition, p-nitrobenzyl esterases from Bacillus subtilis and B. licheniformis that hydrolyzed short chain dialkylphthalates and PET nanoparticles have been reported [74, 87]. 

Billig S, Oeser T, Birkemeyer C, Zimmermann W (2010) Hydrolysis of cyclic poly(ethylene terephthalate) trimers by a carboxylesterase from Thermobifida fusca KW3. Appl Microbiol Biotech 87 1753-1764... 

Jewell, W.T., Miller, M.G., 1998. Identification of a carboxylesterase as the major protein bound by molinate. Toxicol. Appl. Pharmacol. 149, 226-234. 

Jackson, C.J., Liu, J.W., Carr, P.D., et al, 2013. Structure and function of an insect a-carboxylesterase (aEsterase7) associated with insecticide resistance. Proc. Natl. Acad. Sci. USA 110, 10177-10182. 

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