Perturbation of the tyrosine degradation pathway by dichloroacetic acid (DCA): A mechanism for DCA-induced multiorgan toxicity and hepatocarcinogenesis.

摘要

Dichloroacetic acid (DCA) is a disinfection by-product (DBP) present in many municipal water supplies. DCA, as a noncompetitive inhibitor of pyruvate dehydrogenase kinase, is an investigational therapeutic compound for clinical management of lactic acidosis. Environmental and therapeutic exposure to DCA is a concern; multiorgan toxicities in rodents, dogs, and humans and liver tumors in rodents are observed after prolonged exposure to DCA. The mechanisms of toxicity and hepatocarcinogenesis of DCA are poorly understood.; DCA is a substrate and mechanism-based inactivator of glutathione transferase zeta (GSTZ1-1), which catalyzes the isomerization of maleylacetoacetic acid (MAA) to fumarylacetoacetate (FAA), the penultimate step of the tyrosine degradation pathway. We hypothesized that toxic and carcinogenic effects of DCA may be associated with inactivation of GSTZ1-1 and perturbation of MAA-metabolism. The general objective of this thesis was to determine the effects of DCA-induced inactivation of GSTZ1-1 on MAA-metabolism.; Steady-state kinetic analyses with recombinant hGSTZ1-1 showed that the DCA-inactivated enzyme lacks isomerase activity. The loss of isomerase activity was confirmed in rats treated with DCA for 5 days, wherein, immunohistochemical, immunoblotting and activity analyses showed loss of GSTZ1-1 expression and activities in most tissues tested. DCA-treated rats excreted MAA-derived maleylacetone (MA). Though MA was toxic to hepatocytes in vitro (EC₅₀ ∼350 μM after 6 hr), little toxicity was observed in liver of rats given DCA for 5 days. During the kinetic analysis, we observed that MA covalently modified GSTZ1-1 by Michael-addition reactions with solvent-accessible thiol moieties. These data support the concept that MA-dependent alkylation of proteins underlies toxic effects of DCA. MA-formation has significant implications for assessing the safety of other GSTZ1-1-inactivating α-haloacids—the National Toxicology Program has classified some α-haloacids as ‘high priority agents of public health concern.’; Given the therapeutic potential of α-haloacids, the toxicity of chlorofluoroacetic acid (CFA) and difluoroacetic acid (DFA), which inhibit pyruvate dehydrogenase kinase but do not inactivate GSTZ1-1, was determined. CFA and DFA were acutely nephrotoxic in rats given ≥0.6 mmol/kg/day for 5 days. These compounds are thus unsuitable alternatives for treatment of lactic acidotic disorders.