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.

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ABSTRACT:To evaluate the first-pass hydrolysis of O-isovaleryl-propranolol (isovaleryl-PL), which was used as a model ester-compound, rat intestinal jejunum and blood vessels were perfused simultaneously. The membrane permeability of isovaleryl-PL was greater than that of PL because it was more lipophilic. Isovaleryl-PL was almost completely hydrolyzed to PL and isovaleric acid (IVA) in epithelial cells at a rate limited by its uptake. Based on pH partitioning, PL and IVA were transported into both vascular (pH 7.4) and luminal sides (pH 6.5). Therefore, when isovaleryl-PL was perfused into the jejunal lumen, more than 90% permeated into the blood vessel as PL. In addition, PL appeared in the lumen at a rate 6-fold greater than that in blood vessels. When isovaleryl-PL was perfused, its disappearance (50.5 ؎ 1.95 nmol/min) was the sum of the absorption and secretion rates of PL. In contrast, IVA was transported into blood vessels rather than the jejunal lumen. In addition, the calculated degradation clearance from in vitro hydrolysis (K m 13.7 ؎ 1.71 M, V max 29.1 ؎ 3.81 nmol/min/mg protein) was 3.42 ml/min/10 cm jejunum, which was 24-fold greater than the observed degradation clearance (CL deg ) (0.14 ؎ 0.02 ml/min/10 cm jejunum). These findings indicate that in addition to the liver, the intestine markedly contributes to first-pass hydrolysis.

An inhibition study showed that the stereoselective hydrolysis of butyryl propranolol (butyryl PL) in rat liver microsomes and plasma involves carboxylesterase. The hydrolysis of (S)‐butyryl PL in plasma was specifically inhibited by eserine and bis‐nitrophenyl phosphate (BNPP), compared to the (R)‐isomer, despite the non‐stereoselective hydrolysis of butyryl PL in plasma. In addition, inhibition of hydroloysis by eserine and BNPP showed little stereoselectivity for butyryl PL in liver, although liver microsomes showed an (S)‐preferential hydrolysis for butyryl PL (R/S ratio of Vmax/Km: 2.1 ± 0.2). The hydrolysis of butyryl PL was not inhibited by a polyclonal antibody against a high affinity carboxylesterase (hydrolase A, RH1). Moreover, the high Km value and the high IC50 for phenylmethylsulfonyl fluoride (PMSF) against the hydrolysis of butyryl PL in rat liver microsomes suggest that a low affinity carboxylesterase (perhaps hydrolase B) might be involved in this hydrolysis in rat liver. Chirality 11:10–13, 1999. © 1999 Wiley‐Liss, Inc.