GLUTATHIONE TRANSFERASE ZETA-CATALYZED BIOTRANSFORMATION OF DICHLOROACETIC ACID AND OTHER ALPHA-HALOACIDS

Dichloroacetic acid (DCA) is a common drinking-water contaminant, is h epatocarcinogenic in rats and mice, and is a therapeutic agent used cl inically in the management of lactic acidosis. Recent studies show tha t glutathione transferase Zeta (GSTZ) catalyzes the oxygenation of DCA to glyoxylic acid [Tong et al. (1998) Biochem. J. 331, 371-374]. In t he present studies, the substrate selectivity of GSTZ, the kinetics of DCA metabolism, and the fate of DCA and glutathione were investigated . The results showed that GSTZ catalyzed the oxygenation of bromochlor o-, bromofluoro-, chlorofluoro-, dibromo-, and dichloroacetic acid, bu t not difluoroacetic acid, to glyoxylic acid. GSTZ also catalyzed the biotransformation of fluoroacetic acid to S-(carboxymethyl)glutathione , and of(R, S)-2-bromopropionic acid, (R)-, (S)-, and (R,S)-2-chloropr opionic acid, and (R,S)-2-iodopropionic acid, but not (R,S)-2-fluoropr opionic acid, to S-(alpha-methylcarboxymethyl)glutathione; and of 2,2- dichloropropionic acid to pyruvate. No biotransformation of 3,3-dichlo ropropionic acid was detected, and no GSTZ-catalyzed fluoride release from ethyl fluoroacetate and fluoroacetamide was observed. The relativ e rates of DCA biotransformation by hepatic cytosol were mouse > rat > human. Immunoblotting showed the presence of GSTZ in mouse, rat, and human liver cytosol. C-13 NMR spectroscopic studies showed that [2-C-1 3]glyoxylic acid was the only observable, stable metabolite of [2-13C] DCA. Also, glutathione was required, but was neither consumed nor oxid ized to glutathione disulfide, during the oxygenation of DCA to glyoxy lic acid. These results are consistent, with a reaction mechanism that involves displacement of chloride from DCA by glutathione to afford S -(alpha-chlorocarboxymethyl)glutathione, which may undergo hydrolysis to give the hemithioacetal S-(alpha-hydroxycarboxymethyl)glutathione. Elimination of glutathione from the hemithioacetal would give glyoxyli c acid.