Mechanism of differential catalytic efficiency of two polymorphic forms of human glutathione S-transferase P1-1 in the glutathione conjugation of carcinogenic diol epoxide of chrysene.

The kinetics of the conjugation of glutathione (GSH) with anti-1, 2-dihydroxy-3,4-oxy-1,2,3,4-tetrahydrochrysene (anti-CDE), the activated form of the widespread environmental pollutant chrysene, catalyzed by two naturally occurring polymorphic forms of the pi class human GSH S-transferase (hGSTP1-1), has been investigated. The polymorphic forms of hGSTP1-1, which differ in their primary structure by a single amino acid in position 104, exhibited preference for the GSH conjugation of (+)-anti-CDE, which is a far more potent carcinogen than (-)-anti-CDE. When concentration of anti-CDE was varied (5-200 microM and the GSH concentration was kept constant at 2 mM, both hGSTP1-1(I104) and hGSTP1-1(V104) obeyed Michaelis-Menten kinetics. However, the Vmax of GSH conjugation of anti-CDE was approximately 5.3-fold higher for the V104 variant than for the I104 form. Calculation of catalytic efficiency (kcat/Km) thus resulted in a value for hGSTP1-1(V104), 28 mM-1 s-1, that was 7.0-fold higher than that for hGSTP1-1(I104), 4 mM-1 s-1. The mechanism of the differences in the kinetic properties of hGSTP1-1 isoforms toward anti-CDE was investigated by molecular modeling of the two proteins with GSH conjugation products in their active sites. These studies revealed that the enantioselectivity of hGSTP1-1 for (+)-anti-CDE and the differential catalytic efficiencies of the V104 and I104 forms of hGSTP1-1 in the GSH conjugation of (+)-anti-CDE were due to the differences in the active-site architecture of the two proteins. The results of the present study, for the first time, provide evidence for the toxicological relevance of GSTP1-1 polymorphism in humans and suggest that the population polymorphism of hGSTP1-1 variants with disparate enzyme activities may, at least in part, account for the differential susceptibility of individuals to environmental carcinogens such as anti-CDE and possibly other similar carcinogens.