Identification of a thioselenurane intermediate in the reaction between phenylaminoalkyl selenoxides and glutathione

Selenium has a long history of association with human health and disease, and a low concentration of selenium in plasma has been identified in epidemiological studies as a risk factor for several disorders associated with oxidative stress. This association suggests that organoselenium compounds capable of propagating a selenium redox cycle might supplement natural cellular defenses against oxidants, such as peroxynitrite and hydrogen peroxide. While several such organoselenium compounds are under active investigation as potential therapeutic agents, chemical characterization of reaction intermediates involved in their redox cycling has been problematical. We now report evidence that the reaction between phenylaminoalkyl selenoxides and glutathione (GSH) proceeds through the intermediacy of a thioselenurane species. The results of stopped-flow kinetic experiments were consistent with a rapid and stoichiometric initial reaction of GSH with selenoxide to generate a kinetically-detectable intermediate, followed by a slower reaction of this intermediate with a second molecule of GSH to produce the final selenide and GSSG products. Flow injection ESI-MS and ESI-MS/MS experiments confirmed that the reaction intermediate is indeed a thioselenurane. Final structural characterization of the thioselenurane intermediate was obtained from analysis of the daughter ions produced in flow injection ESI-MS/MS experiments. These results help to elucidate the chemical nature of the redox cycling of phenylaminoalkyl selenides, and represent, to our knowledge, the first evidence for the intermediacy of a thioselenurane species in the reaction of thiols with selenoxides.