Substrate specificity and enantioselectivity of arylcarboxylic acid glucuronidation

A general and sensitive HPLC method using a precolumn switching system was developed for the separation and quantification of the individual diastereoisomeric glucuronides of the 2-phenylpropionic acid optical isomers. Kinetic properties of rat liver glucuronidation of several arylcarboxylic acids (1- and 2-naphthylacetic acids, clofibric acid, (R)-(-)- and (S)-(+)-2-phenylpropionic acids) are characterized. The results show that rat liver microsomes glucuronidate 1-naphthylacetic acid more efficiently than its regioisomer (higher Vmax/Km ratio because of a 6-fold lower Km value). Furthermore, 2-phenylpropionic acid glucuronidation occurs stereoselectively and is characterized by an enantiomeric ratio R/S = 1.60. Specific inducers of different UDP-glucuronosyltransferase isoforms and the Gunn rat strain are used to define the substrate specificity of the conjugation reaction towards arylcarboxylic aglycones. Acyl glucuronide formation is induced by phenobarbital. Gunn rats are not deficient in conjugation of these arylcarboxylic acids. These results indicate that these compounds behave similarly to classically defined group 2 substrates. In addition, the stereospecificity of 2-phenylpropionic acid conjugation is unchanged by pretreatment of animals with inducers, in vitro detergent activation, and enantiomeric inhibition. This suggests that the optical isomers of 2-phenylpropionic acid can be either conjugated by the same form or very closely regulated forms of UDPGT.