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

Abstract

Male Sprague-Dawley rat liver cytosol mediated regioselective conjugation of styrene 7,8-oxide (STO) enantiomers with glutathione in completely trans-ring-openmg manner to afford (1 S)- S-(1-phenyl-2-hydroxyethyl) glutathione and (2 R)- S-(2-phenyl-2-hydroxyethyl)glutathione m the ratio 22:1 for ( R)-STO and also to afford (1 R)- S-(1-phenyl-2-hydroxyethyl)glutathione and (2 S)- 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 K cat K m 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 K cat K m 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.