Differential Catalytic Efficiency of Allelic Variants of Human GlutathioneS-Transferase Pi in Catalyzing the Glutathione Conjugation of Thiotepa

Alkylating agents are extensively used in the treatment of cancer. The clinical usefulness of this class of anticancer drugs, however, is often limited by the emergence of drug-resistant tumor cells. Increased glutathione (GSH) conjugation through catalysis by GSHS-transferases (GSTs) is believed to be an important mechanism in tumor cell resistance to alkylating agents. In the present study, we report that the allelic variants of human Pi class GST (hGSTP1-1), which differ in their primary structures at amino acids in positions 104 and/or 113, exhibit significant differences in their activity in the GSH conjugation of alkylating anticancer drug thiotepa. Mass spectrometry revealed that the major product of the reaction between thiotepa and GSH was the monoglutathionyl–thiotepa conjugate. While nonenzymatic formation of monoglutathionyl–thiotepa was negligible, the formation of this conjugate was increased significantly in the presence of hGSTP1-1 protein. The hGSTP1-1-catalyzed GSH conjugation of thiotepa was time and protein dependent and followed Michaelis–Menten kinetics. The catalytic efficiency of hGSTP1-1(I104,A113) variant was approximately 1.9- and 2.6-fold higher compared with hGSTP1-1(V104,A113) and hGSTP1-1(V104,V113) isoforms, respectively. The results of the present study indicate that the hGSTP1-1 polymorphism may be an important factor in GST-mediated tumor cell resistance to thiotepa, and that subjects homozygous for thehGSTP1-1(I104,A113)allele, which is most frequent in human populations, are likely to be at a greater risk for developing GST-mediated resistance to thiotepa than heterozygotes or homozygotes with valine 104 background.