AN ALPHA-CLASS MOUSE GLUTATHIONE-S-TRANSFERASE WITH EXCEPTIONAL CATALYTIC EFFICIENCY IN THE CONJUGATION OF GLUTATHIONE WITH PHA,10-ALPHA-OXY-7,8,9,10-TETRAHYDROBENZO(A)PYRENE

The kinetics of the conjugation of glutathione (GSH) with anti-7 beta, 8 alpha-dihydroxy-9 alpha,10 alpha-oxy-7,8,9,10- tetrahydrobenzo(a)pyr ene (anti-BPDE) catalyzed by GSH S-transferase (GST) isoenzymes purifi ed from the liver and forestomach of female A/J mouse has been investi gated. The GST isoenzymes studied included an alpha class isoenzyme of forestomach (GST 9.5), alpha class hepatic isoenzymes mGSTA3-3 and mG STA4-4, pi class hepatic isoenzyme mGSTP1-1, and mu class hepatic isoe nzyme mGSTM1-1. When the concentration of (+)-anti-BPDE was varied (5- 120 mu m) at a fixed GSH concentration (2 mM), linear Lineweaver-Burk plots were observed for each isoenzyme. The k(cat) values for GST 9.5, mGSTA3-3, mGSTP1-1, and mGSTA4-4 were 2.0, 0.02, 0.40, 0.05, and 0.01 s(-1), respectively, with corresponding K-m values of 16, 12, 29, 27, and 49 mu m. The catalytic efficiency (k(cat)/K-m) of GST 9.5 in the conjugation of GSH with (+)-anti-BPDE, which is believed to be the ult imate carcinogenic metabolite of benzo(a)pyrene, was about 9-625-fold higher as compared with other mouse GST isoenzymes. These results indi cate that GST 9.5 of forestomach is different among mammalian alpha cl ass GSTs because (+)-anti-BPDE has been shown to be a poor substrate f or alpha class rate or human GST isoenzymes. The catalytic efficiency of GST 9.5 was approximately 4.5-fold higher than that of pi class hum an isoenzyme (hGSTP1-1), which among human GSTs is reported to be most efficient in the detoxification of (+)-anti-BPDE. Unlike rat GST isoe nzymes, linear Lineweaver-Burk plots were observed for mouse GSTs when GSH was used as a variable substrate. The catalytic efficiencies of t he mouse GSTs toward (+)-anti-BPDE were about 2-20-fold higher as comp ared with the (-)-enantiomer of anti-BPDE. The results of the present study suggest that GST 9.5 may play an important role in the detoxific ation of (+)-anti-BPDE.