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Graduate School of Pharmaceutical Sciences, Kumamoto University, Kumamoto, Japan (T.I., M.T., M.S.); Faculty of Pharmaceutical Sciences, Chiba Institute of Sciences, Chiba, Japan (M.H.); and Graduate School of Pharmaceutical Sciences, Chiba University, Chiba, Japan (K.C.)
Hydrolase activity from human liver and small intestine microsomes was compared with that of recombinant human carboxylesterases, hCE-1 and hCE-2. Although both hCE-1 and hCE-2 are present in human liver, the dominant component was found to be hCE-1, whereas the hydrolase activity of the human small intestine was found to be predominantly hCE-2. hCE-2 has a limited ability to hydrolyze large acyl compound substrates. Interestingly, propranolol derivatives, good substrates for hCE-2, were easily hydrolyzed by substitution of the methyl group on the 2-position of the acyl moiety, but were barely hydrolyzed when the methyl group was substituted on the 3-position. These findings suggest that hCE-2 does not easily form acylated intermediates because of conformational interference in its active site. In contrast, hCE-1 could hydrolyze a variety of substrates. The hydrolytic activity of hCE-2 increased with increasing alcohol chain length in benzoic acid derivative substrates, whereas hCE-1 preferentially catalyzed the hydrolysis of substrates with short alcohol chains. Kinetic data showed that the determining factor for the rate of hydrolysis of p-aminobenzoic acid esters was Vmax for hCE-1 and Km for hCE-2. Furthermore, the addition of hydrophobic alcohols to the reaction mixture with p-aminobenzoic acid propyl ester induced high and low levels of transesterification by hCE-1 and hCE-2, respectively. When considering the substrate specificities of hCE-1, it is necessary to consider the transesterification ability of hCE-1, in addition to the binding structure of the substrate in the active site of the enzyme.
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