GLUTATHIONE TRANSFERASE-ZETA CATALYZES THE OXYGENATION OF THE CARCINOGEN DICHLOROACETIC-ACID TO GLYOXYLIC-ACID

Dichloroacetic acid (DCA), a common drinking-water contaminant, is hep atocarcinogenic in rats and mice, and is a therapeutic agent used clin ically in the management of lactic acidosis. DCA is biotransformed to glyoxylic acid by glutathione-dependent cytosolic enzymes in vitro and is metabolized to glyoxylic acid in vivo. The enzymes that catalyse t he oxygenation of DCA to glyoxylic acid have not, however, been identi fied or characterized. In the present investigation, an enzyme that ca talyses the glutathione-dependent oxygenation of DCA was purified to h omogeneity (587-fold) from rat liver cytosol, SDS/ PAGE and HPLC gel-f iltration chromatography showed that the purified enzyme had a molecul ar mass of 27-28 kDa. Sequence analysis showed that the N-terminus of the purified protein was blocked. An internal sequence of 30 amino aci d residues was obtained that matched the recently discovered human glu tathione transferase Zeta well [Board, Baker, Chelvanayagam and Jermii n (1997) Biochem. J. 328, 929-935]. Western-blot analysis showed that the purified rat-liver enzyme cross-reacted with rabbit antiserum rais ed against recombinant human glutathione transferase Zeta. The apparen t K-m and V-max values of the purified enzyme with DCA as the variable substrate were 71.4 mu M and 1334 nmol/min per mg of protein, respect ively; the K-m for glutathione was 59 mu M. Both the purified rat-live r enzyme and the recombinant human enzyme showed high activity with DC A as the substrate. These results demonstrate that the glutathione-dep endent oxygenation of DCA to glyoxylic acid is catalysed by a Zeta-cla ss glutathione transferase.