Interaction of Gold(I) with the Active Site of Selenium-Glutathione Peroxidase

Gold(I) thioglucose in the presence of excess glutathione (GSH) leads to strong and reversible inhibition of selenium-glutathione peroxidase (EC 1.11.19) around neutral pH. Binding at equilibrium and competition studies demonstrated that the most reduced form of the active site selenocysteine is the only binding site for gold(I) Steady-state kinetics indicate that gold(I) forms a dead-end complex with glutathione peroxidase in competition with the reduction of hydroperoxide. The apparent K₁ is 2 3 μM at pH 7.6,37°C and 1 mM GSH. Theoretical models of inhibition were assessed by the use of linear least-squares fitting to a generalized integrated rate equation. The results are consistent with trapping of gold(I) at the active site in the form of a mixed bidentate selenolato-thiolate complex involving GSH and the active site selenocysteine. The kinetics of inhibition imply that the resting form of glutathione peroxidase in the presence of excess GSH is also within the enzyme cycle. This rules out the existence of selenium(+lV) species in the redox cycle of the active site when t-butylhydroperoxide is used as a substrate. Electronic properties of selenium and gold as well as a large relief of inhibition by selenocysteine suggest that a very stable interaction should be obtained between Se(-II) and gold(I) through covalent bonding. These results suggest that glutathione peroxidase could be a target of gold drugs used in the treatment of rheumatoid arthritis.