Drug Metab. Pharmacokinet. 27 (5): 466­477 (2012). Copyright © 2012 by the Japanese Society for the Study of Xenobiotics (JSSX) Review TheEmergingRoleofHumanEsterases Tatsuki FUKAMI* and Tsuyoshi YOKOI Drug Metabolism and Toxicology, Faculty of Pharmaceutical Sciences, Kanazawa University, Kanazawa, Japan Full text of this paper is available at http://www.jstage.jst.go.jp/browse/dmpk Summary: In this review, novel aspects of the role of esterases, which contribute to the metabolism of 10% of therapeutic drugs, are described. Esterases hydrolyze the compounds that contain ester, amide, and thioester bonds, which cause prodrug activation or detoxification. Among esterases, carboxylesterases are well known to be involved in the hydrolysis of a variety of drugs. Additionally, other esterases have recently received attention for their pharmacological and toxicological roles. Arylacetamide deacetylase (AADAC) is involved in the hydrolysis of flutamide, phenacetin, and rifamycins. AADAC is associated with adverse drug reactions because the hydrolytic metabolites of flutamide and phenacetin appear to be associated with hepatotoxicity and nephrotoxicity/hematotoxicity, respectively. Paraoxonase and butyrylcholinesterase hydrolyze pirocarpine/simvastatin and succinylcholine/bambuterol, respectively. Although the esterases that hydrolyze the acyl-glucuronides of drugs have largely been unknown, we recently found that ¡/¢ hydrolase domain containing 10 (ABHD10) is responsible for the hydrolysis of mycophenolic acid acyl- glucuronide in human liver. Because acyl-glucuronides are associated with toxicity, ABHD10 might function as a detoxification enzyme. Thus, various esterases, which include enzymes that have not been known to hydrolyze drugs, are involved in drug metabolism with different substrate specificity. Further esterase studies should be conducted to promote our understanding in clinical pharmacotherapy and drug development. Keywords: esterases; drug hydrolysis; activation of prodrugs; drug detoxification; drug adverse reaction Esterases are responsible for prodrug activation and drug Introduction detoxification and have been divided into 3 categories ¤A-, Drug metabolism refers to the biochemical transformation B-, and C-esterases¥ based on substrate specificity and sen- of a compound into its more polar chemical form. Drug- sitivity to various inhibitors ¤i.e., organophosphates and metabolizing enzymes are responsible for the detoxification sulfhydryl reagents¥. A-esterases rapidly hydrolyze aromatic of many drugs and xenobiotics to facilitate their excretion, esters, which include organophosphate diisopropyl fluoro- which is an important determinant of drug action. Drug phosphates ¤DFP¥. The representative A-esterase enzyme is metabolism is classified into phase I and phase II reactions. paraoxonase ¤PON¥. B-esterases are inhibited by organo- Phase I metabolism usually results in a change in molecular phosphates, carbamate, and organosulfur compounds. In weight or water solubility of the substrate, and its reactions humans, most esterases, such as carboxylesterases ¤CESs¥ confer the sites at which phase II metabolism occurs. Phase II and cholinesterases, are B-esterases and members of the conjugation causes an appreciable increase in substrate serine esterase superfamily that possess a serine residue in molecular weight and water solubility. Of the enzymes in- the conserved -Gly-X-Ser-X-Gly- motif at the active site. C- volved in phase I reactions, cytochrome P450 enzymes play esterases neither hydrolyze organophosphate esters nor are a pivotal role in drug metabolism ¤i.e., approximately 75% of inhibited by them. To our knowledge, the C-esterases that clinically used drugs¥, followed by esterases, which contrib- likely hydrolyze drugs have not been identified. Nowadays, ute to the metabolism of 10% of the clinical, therapeutic the above classification is not generally applicable because drugs that contain ester, amide, and thioester bonds.1¥ the function of each esterase has been clarified. Received April 26, 2012; Accepted July 8, 2012 J-STAGE Advance Published Date: July 17, 2012, doi:10.2133/dmpk.DMPK-12-RV-042 *To whom correspondence should be addressed: Tatsuki FUKAMI, Ph.D., Drug Metabolism and Toxicology, Faculty of Pharmaceutical Sciences, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan. Tel. +81-76-234-4438, Fax. +81-76-234-4407, E-mail: [email protected]. ac.jp The authors’ studies were supported by a Grant-in-Aid for Young Scientists (B) and a Grant-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science. 466 Human Esterases Responsible for Drug Metabolism 467 Table 1. Summary of esterases responsible for drug metabolism Enzyme EC number Expressed tissue Representative substrate drug CES1 Carboxylesterase 1 EC3.1.1.1 Liver, lung Capecitabine, clopidogrel, imidapril, methylphenidate, oseltamivir EC3.1.1.56 CES2 Carboxylesterase 2 EC3.1.1.1 Liver, small intestine, kidney CPT-11, flutamide, heroin, prasugrel EC3.1.1.56 EC3.1.1.84 AADAC Arylacetamide deacetylase EC3.1.1.3 Liver, small intestine Flutamide, phenacetin, rifampicin, rifabutin, rifapentine BCHE Butyrylcholinesterase EC3.1.1.8 Liver, plasma Bambuterol, cocaine, CPT-11, mivacurium, succinylcholine PON1 Paraoxonase 1 EC3.1.1.2 Liver, plasma Olmesartan medoxomil, pilocarpine, prulifloxacin EC3.1.1.81 EC3.1.8.1 PON3 Paraoxonase 3 EC3.1.1.2 Liver, plasma Lovastatin, simvastatin, spironolactone EC3.1.1.81 EC3.1.8.1 ABHD10 Æ/Ç Hydrolase domain containing 10 EC3.4.-.- Liver, small intestine Mycophenolic acid acyl-glucuronide APEH Acylpeptide hydrolase EC3.4.19.1 Liver Valproic acid acyl-glucuronide SIAE Sialic acid 9-O-acetylesterase EC3.1.1.53 Liver Alacepril CMBL Carboxymethylenebutenolidase homolog EC3.1.-.- Liver, kidney, small intestine Olmesartan medoxomil, faropenem medoxomil, lenampicillin EC3.1.1.45 BPHL Biphenyl hydrolase-like protein EC.3.1.-.- Small intestine, liver, lung Valacyclovir, valganciclovir PAFAH Platelet-activating factor acetylhydrolase EC3.1.1.47 Erythrocytes Aspirin ALDH Aldehyde dehydrogenase EC1.2.1.3 Liver Nitroglycerin Albumin EC6.1.1.16 Liver, plasma Aspirin, ketoprofen acyl-glucuronide Among these esterases, CES enzymes are well known for tion of a number of clinically used drugs and prodrugs. CES1 their involvement in drug metabolism. Some human esterases is predominantly expressed in the human liver and margin- other than CES enzymes have also been demonstrated ally expressed in the gastrointestinal tract.8¥ The monomeric to hydrolyze several drugs. For example, we found that molecular weight of CES1 is 60 kDa, and CES1 is present as arylacetamide deacetylase ¤AADAC¥, which was previously a 180-kDa trimer.9¥ CES2 is expressed in the liver as well as recognized as a lipase, and Æ/Ç hydrolase containing ¤ABHD¥ in extrahepatic tissues, such as the gastrointestinal tract and 10, whose function was previously unknown, are involved kidney.10¥ In contrast to CES1, CES2 is a 60-kDa mono- in the hydrolysis of clinical drugs.2®5¥ Therefore, recent mer.11¥ These CES enzymes contain a hydrophobic signal evidence demonstrates that various esterases are involved in peptide at the N-terminus for trafficking through the drug metabolism ¤Table 1¥. A comprehensive understanding endoplasmic reticulum and a retention sequence at the of esterases would be useful in clinical pharmacotherapy C-terminus for interacting with the Lys-Asp-Glu-Leu and drug development. This review describes the novel ¤KDEL¥ receptor. Therefore, it has been hypothesized that aspects of the roles of the enzymes that are responsible for CES enzymes are specifically localized in the lumen side drug hydrolysis. of the endoplasmic reticulum.12¥ However, CES1 and CES2 enzymes are also present in human liver cytosol at com- Carboxylesterase parable levels; although, it is unclear whether cytosolic CESs CES enzymes are members of the serine esterase super- initially possess no signal peptide, the signal peptide had been family that possesses the serine residue of the conserved removed before CESs move to the endoplasmic reticulum, -Gly-X-Ser-X-Gly- motif at the active site and are respon- or CESs in endoplasmic reticulum move to the cytosol.10,13¥ sible for the hydrolysis of a wide variety of xenobiotic Although the amino acid homology of CES1 and CES2 and endogenous compounds. Human CES enzymes are around a serine residue in an active site is high, they exhibit classified into 5 subfamilies: CES1, CES2, CES3, CES4A, significant differences in their substrate specificity. Clinical and CES5A.6¥ The CES1 subfamily is composed of 2 genes, drugs such as imidapril, methylphenidate, osertamivir, and CES1 ¤a functional gene, previously termed CES1A1¥ and clopidogrel are specifically hydrolyzed by human CES1.14®17¥ CES1P1 ¤a pseudogene, previously termed CES1A3¥. Pre- Capecitabine and oxybutinin are hydrolyzed by CES1 but viously, another functional CES1 gene ¤previously termed are also hydrolyzed by CES2 to a minor extent.18,19¥ CPT-11 CES1A2¥ was reported to be located at the locus that corre- and prasugrel are preferentially hydrolyzed by CES2.20,21¥ sponds to the CES1P1 gene, but our later study determined A small alcohol group and large acyl group are common it to be a CES1P1 variant.7¥ The other human CES subfamilies features of CES1 substrates, except for oxybutinin, whereas are composed of the corresponding single gene. CES1 and CES2 substrates have a large alcohol group and small acyl CES2 are reported to be responsible for the biotransforma- group ¤Table