Differential Catalytic Efficiency and Enantioselectivity of Murine GlutathioneS-Transferase Isoenzymes in the Glutathione Conjugation of CarcinogenicAnti-Diol Epoxides of Chrysene and Benzo(g)chrysene

Abstract

The kinetics of the conjugation of carcinogenicanti-diol epoxides of chrysene (anti-CDE) and benzo(g)chrysene [anti-B(g)CDE] with glutathione (GSH) catalyzed by GSHS-transferase (GST) isoenzymes mGSTP1-1, mGSTM1-1, mGSTA3-3, mGSTA4-4, and GST 9.5 of female A/J mouse tissues has been investigated. When GST activity was measured as a function of varyinganti-CDE oranti-B(g)-CDE concentrations at a fixed concentration of GSH, each isoenzyme obeyed Michaelis–Menten kinetics. The catalytic efficiencies(kcat/Km)of murine GSTs in the GSH conjugation ofanti-CDE were in the order of GST 9.5 > mGSTP1-1 > mGSTM1-1 > mGSTA3-3 > mGSTA4-4. While each GST isoenzyme examined in the present study exhibited preference for the GSH conjugation of (+)-anti-CDE with the(R,S)-diol(S,R)-epoxide absolute configuration, which is a far more potent carcinogen than the (−)-anti-CDE [(S,R)-diol(R,S)-epoxide absolute configuration], the enantioselectivity was relatively more pronounced for mGSTP1-1 compared with other murine GSTs.Anti-B(g)CDE was a relatively poor substrate for each GST isoenzyme examined compared withanti-CDE. The catalytic efficiencies of murine GSTs in the GSH conjugation ofanti-B(g)CDE were in the order of GST 9.5 > mGSTP1-1 > mGSTM1-1 > mGSTA3-3. With the exception of mGSTM1-1, all other murine GSTs exhibited preference for the GSH conjugation ofanti-B(g)CDE enantiomer with the(R,S)-diol(S,R)-epoxide absolute configuration. In summary, the results of the present study indicate that the murine GSTs significantly differ in their catalytic efficiency and enantioselectivity in the GSH conjugation of bothanti-CDE andanti-B(g)CDE, and thatanti-B(g)CDE is a relatively poor substrate for murine GSTs compared withanti-CDE, which may partially account for the observed relatively higher carcinogenic potency of the former compound.