Chloride and other anions inhibit dichloroacetate-induced inactivation of human liver GSTZ1 in a haplotype-dependent manner

• Inactivation of GSTZ1 by dichloroacetate (DCA) controls DCA pharmacokinetics.
• This work reports factors affecting mechanism-based inactivation of GSTZ1-1 by DCA.
• The physiologically important nucleophile, Cl− protected GSTZ1 from inactivation by DCA.
• A lower concentration of Cl− was required to protect GSTZ1C than GSTZ1A.
• Br−, I− and SO32− protected GSTZ1, but F−, SO42−, CO32−, CN− and CH3COO− did not.

The in vivo elimination rate of dichloroacetate (DCA), an investigational drug; is determined by the rate of its biotransformation to glyoxylate, catalyzed by glutathione transferase ζ1 (GSTZ1). DCA is a mechanism-based inactivator of GSTZ1, thus elimination of DCA is slowed with repeated dosing. We observed that chloride, a physiologically important anion, attenuated DCA-induced GSTZ1 inactivation in human liver cytosol in a concentration and GSTZ1 haplotype-dependent way. In the absence of chloride, incubation with 0.5 mM DCA resulted in inactivation of GSTZ1 with a half-life of 0.4 h (samples with the KRT haplotype) to 0.5 h (EGT haplotype). At the hepatic physiological chloride concentration, 38 mM, samples with the EGT haplotype retained more activity (80%) following a 2-h incubation with 0.5 mM DCA than those possessing the KRT haplotype (55%). The chloride concentration that protected 50% of the GSTZ1 activity following 2-h incubation with 0.5 mM DCA (EC50) was 15.0 ± 3.1 mM (mean ± S.D., n = 3) for EGT samples and 36.2 ± 2.2 mM for KRT samples. Bromide, iodide and sulfite also protected GSTZ1 from inactivation by DCA, however fluoride, sulfate, carbonate, acetate, cyanide did not. Protection by bromide varied by GSTZ1 haplotype: EC50 was 1.3 ± 0.3 mM for the EGT haplotype and 5.0 ± 0.60 mM for the KRT haplotype. The EC50 values for iodide and sulfite in liver cytosol samples with EGT haplotype were respectively 0.14 ± 0.06 mM and 9.6 ± 1.1 mM (mean ± S.D., n = 3). Because the in vivo half-life of DCA is determined by the fraction of active GSTZ1 in the liver, identifying factors that regulate GSTZ1 activity is important in determining appropriate DCA dosing in humans.

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