Glutathione S-transferase A1-1-catalysed conjugation of bay and fjord region diol epoxides of polycyclic aromatic hydrocarbons with glutathione

Mammalian metabolism of polycyclic aromatic hydrocarbons results in the formation of vicinal diol epoxides (existing as enantiomeric pairs of two diastereomers) considered as important ultimate carcinogens if the oxirane ring is located in a bay or fjord region of the parent hydrocarbon, In the present study, individual stereoisomers of the bay region diol epoxides of chrysene, dibenz[a,h]-anthracene and 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 incubated with glutathione (GSH) in the presence or absence of human glutathione S-transferase isoenzyme GST A1-1, a class Alpha enzyme, The formation of GSH conjugates was determined and quantified by HPLC, The results demonstrate that the GST A1-1 isoenzyme catalyzes the formation of GSH conjugates of all diol epoxides tested, although a marked variation in catalytic efficiency (>20-fold) was observed, With both bay and fjord region anti-diol epoxides a significant preference for conjugation of the enantiomer with the R configuration at the benzylic position of the oxirane ring was noted, Among the syn diastereomers of the fjord region diol epoxides a similar substrate enantioselectivity was noted, i.e. the enantiomer with the corresponding R configuration was again preferentially conjugated, In contrast, for the bay region syn-diol epoxides this substrate selectivity was reversed, resulting in a preference for the enantiomer with the S configuration. The chemically more reactive syn diastereomers were ire general better substrates for GST A1-1 than the corresponding anti diastereomers. However, a comparison between different diol epoxide diastereomers revealed no obvious correlation between chemical reactivity of the compounds and catalytic efficiencies, Furthermore, no significant correlation between diol epoxide lipophilicity and catalytic efficiency was observed, It is suggested that stereochemical factors, including the size and the geometry of the aromatic ring system and the preferred conformation of the diol epoxide, are involved as the major determinant for the rate of catalysis by GST A1-1.