Investigation of the kinetic and stereochemical recognition of arene and azaarene oxides by isozymes A2 and C2 of glutathione S-transferase.

The stereoselectivity of the closely related isozymes A2 and C2 of rat liver glutathione S-transferase toward several arene and azaarene oxides is examined. Isozyme C2 is stereospecific, catalyzing attack of glutathione at the oxirane carbon of R absolute configuration for a series of K-region arene oxides including phenanthrene 9,10-oxide, 1. Substitution of nitrogen in the biphenyl system of 1 causes a loss in stereospecificity. Isozyme A2 exhibits a low degree of stereoselectivity toward both arene and azaarene oxides. Kinetic studies of the two isozymes show that although isozyme C2 turns over 1 faster than does isozyme A2 the opposite is true when 4,5- diazaphenanthrene 9,10-oxide is the substrate. The kinetic and stereochemical behavior of the homodimeric isozymes A2 and C2 can be used to predict the stereoselectivity of the heterodimeric isozyme AC perhaps suggesting that catalysis is insensitive to different subunit-subunit interactions in the three isozymes.