Glutathione conjugation of trans-3,4-dihydroxy 1,2-epoxy 1,2,3,4-tetrahydrobenzo[c]phenanthrene isomers by human glutathione transferases.
Each of the four stereoisomers of trans-3,4-dihydroxy 1,2-epoxy 1,2,3,4-tetrahydrobenzo[c]phenanthrene [(+)- and (-)-anti-BPhDE and (+)- and (-)-syn-BPhDE] has been incubated with the human glutathione transferase (GST) isoenzymes GST A1-1, GST M1-1 and GST P1-1, representing class alpha, mu and pi respectively, and glutathione (GSH). The conjugates formed were analyzed by HPLC and the results demonstrate that all GST isoenzymes catalyze the formation of GSH conjugates of all BPhDE isomers. However, a marked variation in catalytic efficiencies was observed (0.122-1.28/mM/s). These values are considerably lower than those previously estimated for the bay-region diol epoxides of benzo[a]pyrene (B[a]P) and human GSTs. The (+)-syn and (-)-anti-BPhDE (1R,2S-epoxide absolute configuration) were in general better substrates than the corresponding 1S,2R-epoxides. In accordance with previous observations with the diolepoxides of B[a]P, GST P1-1 was highly selective towards the BPhDE isomer with 4R,3S-diol 2S,1R-epoxide absolute configuration, i.e. (-)-anti-BPhDE, whereas GST A1-1 and M1-1 preferentially catalyzed the conjugation of (+)-syn-BPhDE (4R,3S-diol 2R,1S-epoxide absolute configuration). Overall, the most active isoenzyme was GST A1-1. Analysis by NMR spectroscopy of the GSH conjugates of BPhDE demonstrate that the reaction with GSH generally takes place by trans-addition of the thiol group at the benzylic C-1 carbon. The low catalytic efficiencies of human GSTs with BPhDE as compared to diolepoxides of B[a]P may be explained in part by the more crowded bay-region and substantially lower chemical reactivity (e.g. delta Edeloc/beta) of the former compounds.
Jernström
B
B
Department of Toxicology, Karolinska Institutet, Stockholm, Sweden.
Seidel
A
A
Funk
M
M
Oesch
F
F
Mannervik
B
B
D002273:Carcinogens / Q000737:chemistry* / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D005978:Glutathione / Q000737:chemistry* / Q000378:metabolism; D005982:Glutathione Transferase / Q000378:metabolism*; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism*; D007536:Isomerism; D008099:Liver / Q000201:enzymology; D009682:Magnetic Resonance Spectroscopy; D010616:Phenanthrenes / Q000737:chemistry* / Q000378:metabolism; D010920:Placenta / Q000201:enzymology; D013056:Spectrophotometry, Ultraviolet; D013237:Stereoisomerism
D002273:Carcinogens; D007527:Isoenzymes; D010616:Phenanthrenes; C090203:1,2-epoxy-3,4-dihydroxy-1,2,3,4-tetrahydrobenzo(c)phenanthrene; D005982:Glutathione Transferase; D005978:Glutathione