Stereochemical aspects of conjugation reactions catalyzed by rat liver glutathione S-transferase isozymes.

Substrate enantioselectivity in the conjugation of phenethyl halides catalyzed by the glutathione S-transferases was studied with partially purified isozymes from rat liver. All of the isozymes tested possessed measurable activity with phenethyl chloride. Tranferase A was the most active isozyme tested. Each of the isozymes demonstrated a high degree of substrate enantioselectivity, with transferase A being the most enantioselective isozyme. The enantioselectivity was determined by high-pressure liquid chromatographic analysis of the enzymatically formed diastereomeric products. The effect of limiting glutathione concentrations on the stereochemical outcome of the transferase A catalyzed conjugation of the chiral substrate, (S)-phenethyl chloride (4 mM), was investigated. The stereochemical course of the enzymatic reaction was not significantly altered at glutathione concentrations as low as 25 microM. The major product of conjugation had the opposite stereochemistry at the benzylic carbon, indicating that the reaction proceeded primarily with inversion of configuration. The glutathione conjugates, S-[(R)-1-phenylethyl]glutathione, S-[(S)-1-phenylethyl]glutathione, S-benzylglutathione, and S-methylglutathione were studied as inhibitors of the transferase A catalyzed conjugation of 1-chloro-2,4-dinitrobenzene. The order of the inhibitory potency was S-[(S)-1-phenylethyl]glutathione = S-benzylglutathione greater than S-[(R)-1-phenylethyl]glutathione greater than S-methylglutathione. This represented the first demonstration of the stereoselective product inhibition of the glutathione S-transferases.