Busulfan conjugation by glutathione S-transferases alpha, mu, and pi

Busulfan is eliminated by glutathione S-transferase (GST)-catalyzed conjugation with glutathione (GSH). We have characterized the busulfan-conjugating activity of purified human liver GSTA1-1, GSTA1-2, GSTA2-2, GSTM1-1, and placental GSTP1-1. Isoforms were purified from cytosol by GSH-affinity chromatography and chromatofocusing. In addition, the busulfan-conjugating activity of cDNA-expressed GTH1 and GTH2, corresponding to GSTA1-1 and GSTA2-2, were characterized. The major product of busulfan conjugation, a thiophenium ion (THT+), was assayed by GC/MS after conversion to tetrahydrothiophene (THT). THT+ formation rate increased linearly with busulfan concentration up to its solubility limit for all GST isoforms. Because Vmax and KM could not be determined separately, the slope of the velocity vs. substrate concentration plot, Vmax/KM was used to compare isoform activities. Vmax/KM for GSTA1-1 was 7.95 microliters/min/mg protein, the highest busulfan-conjugating activity of all human liver and placenta isoforms evaluated. GSTM1-1 and GSTP1-1, respectively, had 46% and 18% of the activity of GSTA1-1. Since the polymorphic mu-class GST catalyzed busulfan conjugation, we examined busulfan clearance in 50 patients undergoing high-dose busulfan before bone marrow transplantation. Busulfan clearance was normally distributed, suggesting that GSTM1-1 does not contribute significantly to the elimination of busulfan from the body. We conclude that GSTA1-1 is the major isoform catalyzing busulfan conjugation, whereas GSTM1-1 and GSTP1-1 may be important in the protection of specific cells.