Carboxylesterase kinetics

In human blood, only 6-acetylmorphine was formed from heroin, with no 3-acetylmorphine or morphine being detected. Four kinetically distinct enzyme activities were seen, namely one in plasma, one in the cytosol of red blood cells, and two in red blood cell membranes [92], In human plasma at 37°, hydrolysis to 6-acetylmorphine occurs with a tm value of some minutes, the enzyme responsible being cholinesterase [90] [93]. These and other results tend to indicate that the formation of morphine from 6-acetylmorphine is due to tissue carboxylesterases, in particular cerebral enzymes [94],... 

The L-dopa esters were also examined for their enzymatic hydrolysis in human plasma and/or by purified pig liver carboxylesterase (EC 3.ELI Table 8.1). In human plasma under the conditions of study, hydrolysis again followed pseudo-first-order kinetics. In all but two cases examined, enzymatic hydrolysis was slightly faster than chemical hydrolysis. For the methyl ester, rates of chemical and enzymatic hydrolysis were comparable, whereas the /erf-butyl ester was not hydrolyzed in plasma and was protected from chemical hydrolysis presumably by becoming bound to plasma proteins. 

Saboori, A.M., and D.S. Newcombe. 1990. Human monocyte carboxylesterase. Purification and kinetics. J Biol Chem 265 19792. 

Bogdanffy MS, Taylor ML. 1993. Kinetics of carboxylesterase-mediated metabolism of vinyl acetate. DrugMetab. Dis-pos. 21 1107-11... 

Age-related differences in kinetic parameters of detoxification can partially explain the increased sensitivity of the young to acute exposure to chlorpyrifos and other OPs (Mortensen et ai, 1996 Moser et ai, 1998 Padilla et ai, 2000, 2004). Thus, in vitro assays show that differences in vivo are not due to intrinsic differences in sensitivity of the target enzyme (Mortensen el ai, 1998). Furthermore, differences in liver microsomal metabolism, which mediates activation and/or inactivation of some pesticides, do not adequately explain the increased sensitivity (Benke and Murphy, 1975 Brodeur and DuBoks, 1%7). Differences in detoxification pathways correlate better with age sensitivity. B-esterases (e.g., carboxylesterases) and A-esterase.s bind to and/or hydrolyze, and thus detoxify, some cholinesterase-inhibiting pesticides (Jokanovic et al., 1996 Maxwell, 1992). These pathways are much less well developed in the young, and maturation of these systems tracks the decreasing sensitivity to acute exposure to chlorpyrifos and other OPs (Attcrberry el ai, 1997 Benke and Murphy, 1975 Brodeur and DuBois, 1967 Chanda et ai, 1997, 2002 Mendoza, 1976 Mortensen et ai, 1996, 1998 ... 

Chahinian H, Ali YB, AbousaUiam A, Petty S, Mandrich L, Manco G, Canaan S, Sarda L (2005) Substrate specificity and kinetic properties of enzymes belonging to the hormone-sensitive lipase family comparison with non-lipolytic and lipolytic carboxylesterases. Biochim Biophys Acta 1738 29-36... 

Ross et al. (2006) studied the hydrolytic metabolism of Type 1 pyrethroids (bioresmethrin, IRS fraws-permethrin, and IRS c/s-permethrin) and several Type II pyrethroids (alpha-cypermethrin and deltamethrin) by pure human CEs (hCE-1 and hCE-2), a rabbit CE (rCE), and two rat CEs (Hydrolases A and B). Hydrolysis rates were based on the formation of 3-phenoxybenzyl alcohol (PBAlc) (CAS no. 13826-36-2) for the cis and trans isomers of permethrin. For bioresmethrin, hydrolysis was based on the production of the trans-chrysanthemic acid (CPCA) (CAS no. 10453-89-1). For alpha-cypermethrin and deltamethrin, hydrolysis was based on the formation of c/s-permethrinic acid (DCCA) (CAS no. 57112-16-0) and 3-phenoxybenzyl aldehyde (PBAld CAS no. 39515-51-0), respectively. Human CE-1 and hCE-2 hydrolyzed trans-permethrin 8- and 28-fold more efficiently (based on kcat/Km values) than did c/s-permethrin, respectively. The kinetic parameters (Fmax> for the hydrolysis of trans- and c/s-permethrin, bioresmethrin and alpha-cypermethrin by rat, mouse, and human hepatic microsomes are given in Table 7. The trans- isomer of permethrin is more readily hydrolyzed by rat, mouse and human hepatic microsomal carboxylesterase than c/s-permethrin (13.4, 85.5 and 56.0 times, respectively). However, the lower for hydrolysis of cis-permethrin in human microsomes suggests that there are both stereoisomer and species-specific differences in metabolism kinetics. 

Human intestinal microsomes effectively hydrolyzed frans-permethrin however, bioresmethrin, and deltamethrin were not metabolized in the intestine to any appreciable extent. Human hepatic microsomes and cytosol contained both hCE-1 and hCE-2 when examined by native PAGE (polyacrylamide gel electrophoresis) human intestine contained only hCE-2. Table 8 gives the kinetic parameters obtained by Crow et al. (2(X)7) with tranr-permethrin and liver and intestinal carboxylesterases. 

Table 8 Kinetic parameters of trani-permethiin hydrolysis catalyzed by human and rat hepatic subcellular fractions (microsomes and cytosols) and intestinal carboxylesterases...

Table 11 Hydrolysis of deltamethrin kinetic parameters of human and rat carboxylesterases...

Very few kinetic data on fungal biodegradation are available, but Chen et al. showed that the half-lives (Tm) of fenvalerate, fenpropathrin, p-cypermethrin, deltamethrin, bifenthrin and permethrin were 0.99-1.54 days, when degraded by carboxylesterase from Cladosporium sp. For Trichoderma viride the Tih of p-cyfluthrin was 7 days, while half-lives for Aspergillus nidulans and Sepedonium maheswarium were 11 and 19 days, respectively (Mukherjee and Mittal 2007 Chen et al. 201 Id Saikia and Gopal 2004). 

The enzymatic activity of lipases is very comparable to that of esterases, with the main difference being the chain length and hydrophobicity of the acid moiety of the substrate. Therefore in fine chemical applications, lipases and esterases are being used as alternatives for several conversions. For instance, for the kinetic resolution of 2-arylpropionic acids such as naproxen and ibu-profen, both a lipase and an esterase have been found that can perform a stereoselective hydrolysis yielding the pharmaceutically preferred enantiomer S-naproxen (Bertola et al. 1992 Hedstrom et al. 1993). High activity and ease of production have made the carboxylesterase from Bacillus subtilis Thai 1-8 the prime choice of industry (Quax and Broekhuizen 1994). 

Profens are an important group of NSAIDs. The biological activity of these drugs resides exclusively in the (S)-enantiomer, so considerable effort has been invested in developing efficient routes for their preparation. For instance, (S)-naproxen has been prepared via recrystallization of diastereomeric mixtures. The carboxylesterase-catalyzed kinetic resolution of R/S)-naproxen methyl ester achieves excellent optical purity of the product. Nevertheless, the AMDase-catalyzed as5unmetrizahon of prochiral a-aryl-a-methylmalonates gives rise to a 100% theoretical 5deld of profens, a clear improvement from kinetic resolution (50%). Unfortunately, wild-type AMDase produces only the undesirable (R)-enantiomers. 

Koller, E. Wolfbeis, O. S. Syntheses and spectral properties of longwave absorbing and fluorescing substrates for the direct and continuous kinetic assay of carboxylesterases, phosphatases, and sulfatases. Monatsh. Chem. 1985,116, 65-75. 

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