COMPARISON OF HUMAN LIVER AND SMALL-INTESTINAL GLUTATHIONE S-TRANSFERASE-CATALYZED BUSULFAN CONJUGATION IN-VITRO

The apparent oral clearance of busulfan has been observed to vary as m uch as 10-fold in the population of children and adults receiving high -dose busulfan. The only identified elimination pathway for busulfan i nvolves glutathione conjugation, The reaction is predominantly catalyz ed by glutathione S-transferase (GST) A1-1, which is present in both l iver and intestine, The purpose of this study was to compare busulfan V-max/K-m in cytosol prepared from adult human liver and small intesti ne, Tetrahydrothiophenium ion formation rate per milligram of cytosoli c protein was constant along the length (assessed in 30-cm segments) o f three individual small intestines, A 30-cm-long intestinal segment 9 0-180 cm from the pylorus was chosen to be representative of intestina l 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 mu l/min/mg cytosolic protein, resp ectively, in incubations with 5 mu M busulfan, 1 mM glutathione, and 2 mg of cytosolic protein, The relative content of GST alpha (A1-1, A1- 2, and A2-2) was compared for human liver and intestinal cytosol using Western blot, The levels of GST alpha in liver and intestinal cytosol were 1.12 +/- 0.56 and 1.36 +/- 0.32 integrated optimal density units /5 mu g cytosolic protein, respectively, Busulfan conjugation in vitro was comparable per milligram of cytosolic protein in liver and intest inal cytosol.