Polymorphism- and Species-Dependent Inactivation of Glutathione Transferase Zeta by Dichloroacetate

Glutathione transferase zeta catalyzes the glutathione-dependent oxidation or conjugation of a range of α-haloacids. Repeated administration of dichloroacetate to human subjects increases its plasma elimination half-life, and the activity of glutathione transferase zeta is decreased in rats given dichloroacetate. The objective of the studies presented here was to investigate the kinetics and mechanism of the dichloroacetate-induced decrease in glutathione transferase zeta activity. The rate constants (k inact) for the dichloroacetate-dependent inactivation of glutathione transferase zeta in liver cytosol are in the following order: rat > mouse > human; the half-maximal inhibitory concentration (K inact) of DCA did not differ among the species that were studied. In contrast to dichloroacetate, chlorofluoroacetate produced much less inactivation of mouse liver glutathione transferase zeta activity. Moreover, the addition of N-acetyl-l-cysteine or potassium cyanide did not fully block the dichloroacetate-induced inactivation of glutathione transferase zeta. The k inact values for the dichloroacetate-induced inactivation of four polymorphic variants of recombinant human glutathione transferase zeta (hGSTZ1-1) were in the following order: variant 1a-1a < 1b-1b ≈ 1c-1c ≈ 1d-1d. The dichloroacetate-induced inactivation of hGSTZ1-1 was irreversible. The binding of radioactivity from [1-14C]dichloroacetate and from [35S]glutathione to recombinant hGSTZ1c-1c was demonstrated, indicating covalent modification of the protein. These results show that dichloroacetate is a mechanism-based inactivator of glutathione transferase zeta and is biotransformed to electrophilic metabolites that covalently modify and, thereby, inactivate the enzyme.