LAUSR

Abstract Human carboxylesterase 1 (hCES1) is an important carboxylesterase responsible for the hydrolysis of various esters and amides, and plays very important roles in drug metabolism and pharmacokinetics. In this study, we investigated hCES1-catalyzed hydrolysis mechanism of a common drug, methylphenidate (Ritalin), which bears an amino group at β- position of carboxylester structure. The results show that methylphenidate undergoes an orthoester intermediate, and the free energy barrier of the reaction is thus significantly elevated. These results indicate that orthoester intermediates might exist in hCES1-catalyzed hydrolysis process of other β-aminocarboxylesters. Moreover, our study shows that the enzymatic hydrolysis product of methylphenidate may form a thermodynamically stable internal salt in the catalytic site of hCES1. These findings would be helpful in understanding the related drug metabolism and pharmacokinetics, drug designing, as well as the catalytic mechanism of hCES1 and other carboxylesterases.