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Biochemical pharmacology

Glutathione conjugation of the alpha-bromoisovaleric acid enantiomers in the rat in vivo and its stereoselectivity. Pharmacokinetics of biliary and urinary excretion of the glutathione conjugate and the mercapturate.


PMID 2597178

Abstract

The glutathione (GSH) conjugation of (R)-and (S)-alpha-bromoisovaleric acid (BI) in the rat in vivo, and its stereoselectivity, have been characterized. After administration of racemic [1-14C]BI two radioactive metabolites were found in bile: only one of the possible diastereomeric BI-GSH conjugates, (R)-I-S-G (35 +/- 2% of the dose), and an unidentified metabolite "X" (6 +/- 1%). In urine, only one of the possible BI-mercapturates, (R)-I-S-MA (14 +/- 1%), minor unidentified polar metabolites (5 +/- 1%) and unchanged BI (13 +/- 2%) were excreted. When (R) or (S)-BI were administered separately, the same metabolites were found. However, a ten-fold difference in excretion half lives of the biliary metabolites was observed following (S)-and (R)-BI administration, (S)-BI being more rapidly excreted. The excretion of the mercapturate in urine shows the same difference in excretion rate: its half life after administration of (R)-BI was more than 10 times longer than after a dose of (S)-BI. More of the dose of (S)-BI was excreted after 5 hr in bile and urine: 58% and 23% respectively for (S)- and (R)-BI. Therefore, a pronounced stereoselectivity in GSH conjugation exists for the (R) and (S) enantiomers of BI in the rat in vivo, which is a major determinant of their pharmacokinetics. The results suggest that (slow) inversion of the chiral centre of BI occurred in the rat in vivo.