Glutathione conjugation of styrene 7,8-oxide enantiomers by major glutathione transferase isoenzymes isolated from rat livers

Male Sprague-Dawley rat liver cytosol mediated regioselective conjugation of styrene 7,8-oxide (STO) enantiomers with glutathione in completely trans-ring-opening manner to afford (1S)-S-(1-phenyl-2-hydroxyethyl)glutathione and (2R)-S-(2-phenyl-2-hydroxyethyl)glutathione in the ratio 22:1 for (R)-STO and also to afford (1R)-S-(1-phenyl-2-hydroxyethyl)glutathione and (2S)-S-(2-phenyl-2-hydroxyethyl)glutathione in the ratio 12:1 for (S)-STO. In the above cytosolic reactions, (R)-STO was conjugated 1.8 times faster than (S)-STO, while the (R)- to (S)-ratio in rate of the conjugation was 2.7 when racemic STO was used as a substrate. A kinetic study, carried out by using six major glutathione transferase (GST) isoenzymes isolated from the cytosol, indicated that GSTs 3-3, 3-4 and 4-4 (class mu enzymes) had much higher Kcat/Km values towards both STO enantiomers than the other three major isoenzymes, GSTs 1-1, 1-2 and 2-2 (class alpha enzymes). All the class mu enzymes mediated preferential glutathione conjugation of (R)-STO to (S)-STO. On the contrary, the class alpha enzymes catalysed the conjugation of (S)-STO preferentially to (R)-STO. The kinetic study strongly suggested that GSTs determining the higher enantioselectivity towards (R)-STO in the rat liver cytosol were the class mu enzymes, especially GST 3-3, which had the highest Kcat/Km value towards (R)-STO as well as the highest (R) to (S) ratio in the enantioselectivity among the six isoenzymes examined. GST 7-7, isolated as a major enzyme from the liver cytosol of the animals bearing hepatic hyperplastic nodules which were induced by chemical carcinogens, catalysed preferential GSH conjugation of (S)-STO to (R)-STO.