A hydrogen peroxide electrode assay to measure thiol peroxidase activity for organoselenium and organotellurium drugs

Molecular mimics of the enzyme glutathione peroxidase (GPx) are increasingly being evaluated as redox active drugs. Their molecular mechanism of action parallels that of the native enzyme; however, a major distinction is that GPx mimics can use alternative thiol substrates to glutathione. This generic thiol peroxidase activity implies that it is necessary to assess a GPx mimic's recognition of a range of cellular thiols in order to determine its potential therapeutic effects. We report an electrochemical assay that, by measuring the rate of decrease of the peroxide substrate, allows the activity of GPx mimics to be directly compared against an array of thiols. The derived pseudo zero-order rate constants, k(obs), for representative GPx mimics range between 0 and 6.6min(-1) and can vary by more than an order of magnitude depending on the thiol electron donor. An additional advantage of the assay is that it enables synergistic interactions between GPx mimics and cellular proteins to be evaluated. Here we report that glutathione disulfide reductase, which is commonly used to evaluate GPx mimic activity, recognizes the GPx mimic ebselen as a substrate, increasing its apparent k(obs). Therefore, reports relying on glutathione disulfide reductase to evaluate GPx mimic activity may exaggerate drug antioxidant action.