GLUTATHIONE CONJUGATION OF BAY-REGION AND FJORD-REGION DIOL EPOXIDES OF POLYCYCLIC AROMATIC-HYDROCARBONS BY GLUTATHIONE TRANSFERASES M1-1 AND P1-1

Metabolism of polycyclic aromatic hydrocarbons in mammalian cells resu lts in the formation of vicinal diol epoxides considered as ultimate c arcinogens if the oxirane ring is located in a bay-or fjord-region of the parent compound. In the present study, individual stereoisomers of the bay-region diol epoxides of chrysene, dibenz[a, h] anthracene, an d benzo[a]pyrene as well as of the fjord-region diol epoxides of benzo [c]phenanthrene, benzo[c]chrysene, and benzo[g]chrysene have been incu bated with GSH in the presence of human glutathione transferases GSTM1 -1 (a mu-class enzyme) and GSTP1-1 (a pi-class enzyme). As previously shown with GSTA1-1 (an alpha-class enzyme) both M1-1 and P1-1 demonstr ate considerable activity toward a number of the diol epoxides studied , although a great variation in catalytic efficiency and enantioselect ivity was observed. With GSTM1-1, the bay-region diol epoxides, in par ticular the syn-diastereomers were in most cases more efficiently conj ugated with GSH than the fjord-region analogues. GSTM1-1 demonstrated an enantioselectivity ranging from no preference (50%) to high prefere nce (greater than or equal to 90%) for conjugation of the enantiomers with R-configuration at the benzylic position of the oxirane ring. Wit h GSTP1-1, the enzyme demonstrated appreciable activity toward both ba y-and fjord-region diol epoxides and, in most cases, a preference for the anti-diastereomers. In contrast to GSTM1-1 and as previously shown for GSTA1-1, GSTP1-1 showed an exclusive preference for conjugation o f the enantiomers with R-configuration at the benzylic oxirane carbon. With both GSTM1-1 and GSTP1-1, the chemically most reactive diol epox ide, the (+)-syn-enantiomer of droxy-9,10-epoxy-7,8,9,10-tetrahydroben zo[a]pyrene (BPDE), was the best substrate. As for GSTA1-1, no obvious correlation between chemical reactivity or lipophilicity of the compo unds and catalytic efficiencies was observed. Molecular modeling of di ol epoxides in the active sites of GSTP1-1 and -A1-1 is in agreement w ith the assumption, based on functional studies, that the H-site of GS TA1-1 [Jernstrom et al. (1996) Carcinogenesis 17, 1491-1498] can accom modate stereoisomers of different sizes. Further, modeling of the enan tiomers of anti-and syn-BPDE in the active site of GSTP1-1 provides an explanation for the exclusive preference for the enantiomers with R-c onfiguration at the benzylic oxirane carbon. These isomers could be sn uggly fitted in the H-site close to the GSH sulfur, whereas those with opposite stereochemistry could not.