alpha-Bromoisovalerylurea as model substrate for studies on pharmacokinetics of glutathione conjugation in the rat. I. (Bio-) synthesis, analysis and identification of diastereomeric glutathione conjugates and mercapturates.

In order to find a model substrate for kinetic characterization of glutathione conjugation in vivo alpha-bromoisovalerylurea (BIU) was studied. After administration of racemic [14C]urea BIU to rats, two radioactive metabolites were found in bile by high-performance liquid chromatography. The identity of these metabolites was established by various methods. Based on the hydrolytic activity of gamma-glutamyltranspeptidase (presence of the gamma-glutamyl moiety), high resolution nuclear magnetic resonance (isovaleryl and glutathionyl moieties) and fast atom bombardment mass spectrometry (molecular weight and fragmentation pattern), they were identified as glutathione conjugates of BIU. Because both conjugates in bile had these characteristics in common they must be diastereomers. Incubation of BIU with glutathione in the presence of rat liver cytosol resulted in formation of the same diastereomeric glutathione conjugates. Chemical synthesis of the diastereomers confirmed their identity. The major urinary excretion products of [14C]urea BIU in the rat were identified as diastereomeric mercapturates. A convenient chromatographic separation of the diastereomeric glutathione conjugates and the mercapturic acids is described. Electrochemical detection was used to determine the presence of the thioethers in both urine and bile. Pharmacokinetic results on BIU conjugation are described in the accompanying paper.