Catalytic Activities of Human Alpha Class Glutathione Transferases toward Carcinogenic Dibenzo[a,l]pyrene Diol Epoxides

In this study, human glutathione transferases (GSTs) of alpha class have been assayed with the ultimate carcinogenic (−)-anti- and (+)-syn-diol epoxides (DEs) derived from the nonplanar dibenzo[a,l]pyrene (DBPDE) and the (+)-anti-diol epoxide of the planar benzo[a]pyrene [(+)-anti-BPDE] in the presence of glutathione (GSH). In all DEs, the benzylic oxirane carbon reacting with GSH, possess R-absolute configuration. GSTA1-1 demonstrated activity with all DEs tested whereas A2-2 and A3-3 only were active with the DBPDE enantiomers. With GSTA4-4, no detectable activity was observed. GSTA1-1 was found to be the most efficient enzyme and demonstrated a catalytic efficiency (k<INF>cat</INF>/K<INF>m</INF>) of 464 mM^-1 s^-1 with (+)-syn-DBPDE. This activity was about 7-fold higher than that observed with (−)-anti-DBPDE and more than 65-fold higher than previously observed with less complex fjord-region DEs. GSTA3-3 also demonstrated high k<INF>cat</INF>/K<INF>m</INF> with the DEs of DBP and a high preference for the (+)-syn-DBPDE enantiomer [190 vs 16.2 mM^-1 s^-1 for (−)-anti-DBPDE]. Lowest k<INF>cat</INF>/K<INF>m</INF> value of the active enzymes was observed with GSTA2-2. In this case, 30.4 mM^-1 s^-1 was estimated for (+)-syn-DBPDE and 3.4 mM^-1 s^-1 with (−)-anti-DBPDE. Comparing the activity of the alpha class GSTs with (−)-anti-DBPDE and (+)-anti-BPDE revealed that GSTA1-1 was considerable more active with the former substrate (about 25-fold). Molecular modeling studies showed that the H-site of GSTA1-1 is deeper and wider than that of GSTA4-4. This is mainly due to the changes of Ser212→Tyr212 and Ala216→Val216, which cause a shallower active site, which cannot accommodate large substrates such as DBPDE. The higher activity of GSTA1-1 with (+)-syn-DBPDE relative to (−)-anti-DBPDE is explained by the formation of more favorable interactions between the substrate and the enzyme−GSH complex. The presence of GSTA1-1 in significant amounts in human lung, a primary target tissue for PAH carcinogenesis, may be an important factor for the protection against the harmful action of this type of potent carcinogenic intermediates.