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 alpha,beta-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 as mall 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.

Figure 1

CES-UGT interaction in the luminal side of the ER membrane and CES- transporter interaction in the cell. Two hydrolyzed products from ester-substrate are formed by CES; alcohol or phenol, which are substrates for UGT, and organic anions, which are substrates for organic anion transporter such as multidrug resistance-associated protein 2 (MRP2) or breast cancer resistance protein (BCRP).

Figure 2

Phylogenic tree and nomenclature of CES families. CES isozymes are classified into five families, CES1, CES2, CES3, CES4 and CES5. Each families also divided into subfamilies.

Figure 3

Proposed mechanism for the action of CES. Conformation of the Ser-His-Glu catalytic triad in CES.

Figure 4

Structure of the 5’ flanking region of CES 1A1 and CES 1A2 genes. Sp1 and C/EBPα could bind to each responsive element of the CES1A1 promoter but that Sp1 and C/EBP could not bind to the 5’ flanking region of the CES1A2 promoter. NF-Y, nuclear factor Y, CBF, CCAAT-binding factor