Metabolic Fate of Glutathione Conjugate of Benzo[a]pyrene-(7R,8S)-diol (9S,10R)-epoxide in Human Liver

Benzo[a]pyrene-(7R,8S)-diol (9S,10R)-epoxide [(+)-anti-BPDE] is believed to be the activated form of the widely spread environmental pollutant benzo[a]pyrene. Glutathione (GSH) S-transferase (GST)-catalyzed conjugation of (+)-anti-BPDE with GSH is an important mechanism in its cellular detoxification. Here, we report that the GSH conjugate of (+)-anti-BPDE [(−)-anti-BPD-SG] is a potent inhibitor (Ki 15 μM) of class Mu human GST isoenzyme, which, among human liver GSTs, is a highly efficient detoxifier of (+)-anti-BPDE. Thus, the inhibition of GST activity by (−)-anti-BPD-SG may hinder GSH conjugation of (+)-anti-BPDE, unless the conjugate is metabolized and/or eliminated. The results of the present study show that γ-glutamyltranspeptidase (γ-GT) can metabolize (−)-anti-BPD-SG at a rate of about 0.29 nmol/min/mg protein. Our studies also show that (−)-anti-BPD-SG is transported across the human canalicular liver plasma membrane (cLPM) in an ATP-dependent manner at a rate of about 0.33 nmol/min/mg protein. The ATP-dependent transport of (−)-anti-[3H]BPD-SG across human cLPM follows Michaelis–Menten kinetics (Km 84 μM; Vmax 0.33 nmol/min/mg). In conclusion, the results of the present study suggest that both γ-GT-mediated metabolism and ATP-dependent canalicular transport may be important steps in overall detoxification of (+)-anti-BPDE in the human liver.