Structure and Catalytic Properties of Carboxylesterase Isozymes Involved in Metabolic Activation of Prodrugs

Abstract

Mammalian carboxylesterases (CESs) comprise a multigene family whose gene products play important roles in biotransformation of ester- or amide-type prodrugs. They are members of an α,β-hydrolase-fold family and are found in various mammals. It has been suggested that CESs can be classified into five major groups denominated CES1-CES5, according to the homology of the amino acid sequence, and the majority of CESs that have been identified belong to the CES1 or CES2 family. The substrate specificities of CES1 and CES2 are significantly different. The CES1 isozyme mainly hydrolyzes a substrate with a small alcohol group and large acyl group, but its wide active pocket sometimes allows it to act on structurally distinct compounds of either a large or small alcohol moiety. In contrast, the CES2 isozyme recognizes a substrate with a large alcohol group and small acyl group, and its substrate specificity may be restricted by the capability of acyl-enzyme conjugate formation due to the presence of conformational interference in the active pocket. Since pharmacokinetic and pharmacological data for prodrugs obtained from preclinical experiments using various animals are generally used as references for human studies, it is important to clarify the biochemical properties of CES isozymes. Further experimentation for an understanding of detailed substrate specificity of prodrugs for CES isozymes and its hydrolysates will help us to design the ideal prodrugs.

Hyperpolarized [1,3‐13C2]ethyl acetoacetate is a novel diagnostic metabolic marker of liver cancer

“…Unregulated cellular uptake as well as cosubstrate independent enzymatic conversion of EAA, made this substrate highly useful as a hyperpolarized 13 C-MR marker. This could be appreciated by the signal-tonoise (SNR) obtained from EAA, which was comparable to the SNR reported in a literature liver cancer study with state-of-the-art hyperpolarized substrate, [1][2][3][4][5][6][7][8][9][10][11][12][13] C]pyruvate. Also, the contrast-to-noise (CNR) in the EAA based metabolic ratio images was significantly improved compared with the CNR in equivalent images reported using [1- Hepatocellular carcinoma (HCC) is the most common form of primary liver cancer.…”

“…This work led to the identification of a class of substrates, low molecular weight ethyl-esters, which showed high specificity for carboxyl esterases and proved in many cases to possess good properties for signal enhancement. In particular, hyperpolarized [1,[3][4][5][6][7][8][9][10][11][12][13] C 2 ]ethyl acetoacetate (EAA) was shown to provide a metabolic fingerprint of HCC. Using this substrate a liver cancer implanted in rats was diagnosed as a consequence of an~4 times higher metabolic substrate-to-product ratio than in the surrounding healthy tissue, (p 5 0.009).…”

“…Carboxyl esterases are ubiquitously expressed in mammalian tissues, 11 being especially common in mammalian liver. 12 CE concentrations and activities are generally lower in cancer cells compared to the corresponding healthy cell types. 13 Dependent on the carboxyl esterase isoform, the activities in healthy cells are reported up to 3-4 times higher than in cancer cells.…”

“…15 In the reported study, we evaluate the potential for using the d-DNP-MR method to diagnose liver cancer with carboxyl esterase activity. [1,[3][4][5][6][7][8][9][10][11][12][13] C 2 ]ethyl acetoacetate was developed into a hyperpolarized substrate for the liver isoform of carboxyl esterase (CE-1). With this hyperpolarized substrate it was shown how a liver cancer implanted in rat could be detected due to a higher substrate-to-product ratio in the cancer tissue compared with the surrounding healthy tissue.…”

Multicolor Imaging of Endoplasmic Reticulum-Located Esterase As a Prodrug Activation Enzyme

“…3 The CEs are classified into five families (CES1, CES2, CES3, CES4, and CES5) according to the homology of their amino acid sequence. 4 The hydrolytic abilities of the CES1 and CES2 families are important for the bioconversion of ester-containing prodrugs. Both families contain the HXEL (His-X-Glu-Leu) sequence at the C-terminal, which can bind with the KDEL (Lys-Asp-Glu-Leu) sequence in the endoplasmic reticulum (ER) protein retention receptor.…”