Substrate Specificity of Carboxylesterase Isozymes and Their Contribution to Hydrolase Activity in Human Liver and Small Intestine

• Published in: Drug metabolism and disposition Vol. 34; no. 10; pp. 1734 - 1741
• Main Authors: Imai, Teruko, Taketani, Megumi, Shii, Mayumi, Hosokawa, Masakiyo, Chiba, Kan
• Format: Journal Article
• Language: English
• Published: Bethesda, MD Elsevier Inc 01.10.2006; American Society for Pharmacology and Experimental Therapeutics
• Subjects: Animals; Benzoates - chemistry; Benzoates - metabolism; Biological and medical sciences; Carboxylesterase - genetics; Carboxylesterase - metabolism; Carboxylic Ester Hydrolases - genetics; Carboxylic Ester Hydrolases - metabolism; Cell Line; Flurbiprofen - chemistry; Flurbiprofen - metabolism; Humans; Hydrolases - metabolism; Hydrolysis; Intestine, Small - enzymology; Intestine, Small - metabolism; Isoenzymes - genetics; Isoenzymes - metabolism; Liver - enzymology; Liver - metabolism; Medical sciences; Microsomes - enzymology; Microsomes - metabolism; Microsomes, Liver - enzymology; Microsomes, Liver - metabolism; Molecular Structure; Pharmacology. Drug treatments; Propranolol - chemistry; Propranolol - metabolism; Substrate Specificity; Human; Digestive system; Enzyme; Isozyme; Liver; Substrate specificity; Hydrolases; Esterases; Carboxylesterase; Small intestine; Biological activity; Carboxylic ester hydrolases
• DOI: 10.1124/dmd.106.009381
• ISSN: 0090-9556

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.