Stereoselectivity of human liver and intestinal cytosolic fractions as well as purified human glutathione S-transferase isoenzymes towards 2-bromoisovalerylurea enantiomers.

Glutathione (GSH) conjugation of 2-bromoisovalerylurea (BIU) enantiomers is stereoselective in humans in vivo. Administration of racemic BIU results in a higher plasma elimination and urinary excretion of R-BIU and its mercapturate, respectively, than of S-BIU and its mercapturate. In order to relate the in vivo BIU pharmacokinetics to the activity of glutathione S-transferase (GST) isoenzymes, the GSH conjugation of BIU enantiomers was studied with human liver and intestinal cytosolic fractions as well as purified human class alpha (GSTA1-1, GSTA2-2), mu (GSTM1a-1a) and pi (GSTP1-1) GST isoenzymes. Stereoselective GSH conjugation of BIU enantiomers was observed for most human liver and intestinal cytosolic fraction. In general, the cytosolic fractions preferentially conjugated S-BIU. Stereoselective preference for GSH conjugation of S-BIU was also observed for GSTA2-2 and GSTM1a-1a, whereas GSTA1-1 was not selective for either of the BIU enantiomers. GSTP1-1 did not catalyse conjugation of R- and S-BIU. Quantification of the GST isoenzymes in the liver cytosolic fractions showed that the stereoselectivity towards S-BIU was related to the profile and amount of GST subunits in the cytosolic fractions. The discrepancy in stereoselectivity between the BIU pharmacokinetics in vivo and the GSH conjugation of BIU enantiomers in vitro is discussed. In addition, since in contrast to human GSTM1a-1a, rat class Mu isoenzymes prefer R-BIU, the present results indicate that related isoenzymes in different species may have a different stereoselectivity.