Abstract

The carboxylesterases (CESs) are a family of serine hydrolases that hydrolyze compounds containing an ester, amide, or thioester. In humans, two dominant forms, CES1 and CES2, are highly expressed in organs of first-pass metabolism and play an important role in xenobiotic metabolism. The current study was conducted to better understand species-related differences in substrate selectivity and tissue expression of these enzymes. To elucidate potential similarities and differences between these enzymes, a series of 4-nitrophenyl esters and a series of gemcitabine prodrugs were evaluated using enzyme kinetics as substrates of expressed and purified CESs from beagle dog, cynomolgus monkey, and human genes. For the substrates examined, human and monkey CES2 more efficiently catalyzed hydrolysis compared to CES1, whereas CES1 was the more efficient enzyme in dog. Quantitative real-time PCR and western blot analyses indicate the pattern of CES tissue expression in monkey is similar to that of human, but the CES expression in dog is unique, with no detectable expression of CES in the intestine. Loperamide, a selective human CES2 inhibitor, was also found to be a CES2-selective inhibitor in both dog and monkey. This is the first study to examine substrate specificity between dog, human, and monkey CESs.