Vol. 26, Issue 1, 52-55, January 1998

Comparison of Human Liver and Small Intestinal Glutathione S-Transferase-Catalyzed Busulfan Conjugation in Vitro

John P. Gibbs, Ji-Sun Yang and John T. Slattery

Department of Pharmaceutics (J.P.G., J.-S.Y, J.T.S.) and University of Washington and Fred Hutchinson Cancer Research Center (J.T.S.)

The apparent oral clearance of busulfan has been observed to vary as much as 10-fold in the population of children and adults receiving high-dose busulfan. The only identified elimination pathway for busulfan involves glutathione conjugation. The reaction is predominantly catalyzed by glutathione S-transferase (GST) A1-1, which is present in both liver and intestine. The purpose of this study was to compare busulfan V_max/K_m in cytosol prepared from adult human liver and small intestine. Tetrahydrothiophenium ion formation rate per milligram of cytosolic protein was constant along the length (assessed in 30-cm segments) of three individual small intestines. A 30-cm-long intestinal segment 90-180 cm from the pylorus was chosen to be representative of intestinal cytosolic busulfan conjugating activity. Busulfan V_max/K_m (mean ± SD) in cytosol prepared from 23 livers and 12 small intestines was 0.166 ± 0.066 and 0.176 ± 0.085 µl/min/mg cytosolic protein, respectively, in incubations with 5 µM busulfan, 1 mM glutathione, and 2 mg of cytosolic protein. The relative content of GST (A1-1, A1-2, and A2-2) was compared for human liver and intestinal cytosol using Western blot. The levels of GST in liver and intestinal cytosol were 1.12 ± 0.56 and 1.36 ± 0.32 integrated optimal density units/5 µg cytosolic protein, respectively. Busulfan conjugation in vitro was comparable per milligram of cytosolic protein in liver and intestinal cytosol.