Structural and functional consequences of catechol estrogen mediated inactivation of glutathione S-transferases

Estrogen replacement therapy (ERT) has been correlated with an increased cancer risk, although the numerous benefits of ERT may negate this harmful side effect. 4-Hydroxyequilenin (4-OHEN) is the Phase I catechol metabolite of equine estrogens present in Premarin® (Wyeth-Ayerst), the most widely prescribed estrogen replacement formulation. The ability of 4-OHEN to modulate the activities of drug metabolizing enzymes was investigated. 4-OHEN was found to be a potent inhibitor of glutathione S-transferase (GST) M1-1, glutathione reductase, P450 reductase, and GST P1-1, which are drug metabolizing enzymes and play a role in maintaining cellular redox state. With GST M1-1, oxidation of 4-OHEN to o-quinone and/or formation of GSH conjugates of 4-OHEN are associated with the mechanism(s) of 4-OHEN-mediated inactivation of the enzyme. Further experiments with GST P1-1 showed that electrophilic/redox active quinoids of 4-OHEN were involved in the inhibition mechanism(s) probably through covalent modification and/or oxidative damage of critical amino acid residues in the enzyme. These data may contribute to the potential toxic mechanisms(s) of 4-OHEN through protein inactivation.