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

Abstract

ABSTRACT: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 V max for hCE-1 and K m 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.

Carboxylesterases: structure, function and polymorphism in mammals

“…It includes human intestinal CE (CES2A1), 34,[40][41][42][43][44] rat CES2 (CES2A10), 22) rat intestinal CE RL4 (rCES2) (CES2A6), 45) rabbit form 2 46) and hamster AT51 (CES2A11). 47) CES3 includes ES-male (CES3A2) and human CES3 (CES3A1).…”

“…Most of the CES1 families, except CES1G, are mainly expressed in the liver. The CES 1A subfamily includes the major forms of human CEs, 18,19,34,[36][37][38] and the major isoforms of rat, 9) dog, 19) rabbit 37) and mouse 35) CE. The CES1B subfamily includes the major isoforms of rat, 9) mouse 20) and hamster 47) CE, and CES1C includes the major isoforms of dog, 18,19) cat 50) and human 38) CE.…”

Carboxylesterases: Structure, Function and Polymorphism

“…4). Actually, it has been shown that dog CES1 isozyme was involved in a pulmonary first-pass effect in the disposition of a propranolol ester prodrug (Imai, 2006;Imai et al, 2006). It has also been shown that the expression level of the human CarbE isozyme was correlated with the conversion ratio of CPT-11 to SN-38, the active metabolite, which is thought to be a key step for the chemotherapeutic action of this anti-tumor drug (Pindel et al, 1997;Zhang et al, 2002;Ohtsuka et al, 2003;Sanghani et al, 2003).…”

“…It includes human intestinal CarbE (CES2A1) (Schwer et al, 1997;Humerickhouse et al, 2000;Imai et al, 2006;Taketani et al, 2007;Yang et al, 2007;Shi et al, 2008), rCES2 (CES2A10) (Furihata et al, 2005), rat intestinal CarbE RL4 (rCES2) (CES2A6) (Furihata et al, 2003), rabbit form 2 (Ozols, 1989) and hamster AT51 (CES2A11) (Sone et al, 1994). CES3 includes ES-male (CES3A2) and human CES3 (CES3A1) (Aida et al, 1993;Sanghani et al, 2004).…”

“…For example, Inoue et al (Inoue et al, 1979a) showed that esterase activity in the dog intestine is very weak and produced no appreciable active band in a disk electrophoresis coupled with staining of esterase activity. On the other hand, esterase activities were observed in the intestines of other species (human, rat, mouse, guinea pig and rabbit) (Inoue et al, 1979b;Prueksaritanont et al, 1996;Mansbach and Nevin, 1998;Imai et al, 2006;Satoh and Hosokawa, 2006) and found to produce a few active bands in an electrophoretic assay.…”