Liver carboxylesterase 1 also known as carboxylesterase 1 (CES1, hCE-1 or CES1A1) is an enzyme that in humans is encoded by the CES1gene. The protein is also historically known as serineesterase 1 (SES1), monocyte esterase and cholesterol esterhydrolase (CEH). Three transcript variants encoding three different isoforms have been found for this gene. The various protein products from isoform a, b and c range in size from 568, 567 and 566 amino acids long, respectively.
Carboxylesterase 1 is a serine esterase and member of a large multigene carboxylesterase family. It is also part of the alpha/beta fold hydrolase family. These enzymes are responsible for the hydrolysis of ester- and amide-bond-containing xenobiotics and drugs such as cocaine and heroin. They also hydrolyze long-chain fatty acid esters and thioesters. As part of phase II metabolism, the resulting carboxylates are then often conjugated by other enzymes to increase solubility and eventually excreted.
This enzyme is known to hydrolyze aromatic and aliphatic esters and can manage cellular cholesterol esterification levels. It may also play a role in detoxification in the lung and/or protection of the central nervous system from ester or amide compounds.
CES1 can activate or deactivate various drugs. CES1 is responsible for the activation of many prodrugs such as angiotensin-converting enzyme (ACE) inhibitors, oseltamivir, and dabigatran.  Genetic variants of CES1 can significantly affect both pharmaocokinetics and pharmacodynamics of drugs metabolized by CES1, such as methylphenidate and clopidogrel. The ability of CES1 to metabolize heroin and cocaine among other drugs has suggested a therapeutic role for the enzyme.
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