Formation of a selenium-substituted rhodanese by reaction with selenite and glutathione: possible role of a protein perselenide in a selenium delivery system.

Selenophosphate is the active selenium-donor compound required by bacteria and mammals for the specific synthesis of Secys-tRNA, the precursor of selenocysteine in selenoenzymes. Although free selenide can be used in vitro for the synthesis of selenophosphate, the actual physiological selenium substrate has not been identified. Rhodanese (EC ) normally occurs as a persulfide of a critical cysteine residue and is believed to function as a sulfur-delivery protein. Also, it has been demonstrated that a selenium-substituted rhodanese (E-Se form) can exist in vitro. In this study, we have prepared and characterized an E-Se rhodanese. Persulfide-free bovine-liver rhodanese (E form) did not react with SeO(3)(2-) directly, but in the presence of reduced glutathione (GSH) and SeO(3)(2-) E-Se rhodanese was generated. These results indicate that the intermediates produced from the reaction of GSH with SeO(3)(2-) are required for the formation of a selenium-substituted rhodanese. E-Se rhodanese was stable in the presence of excess GSH at neutral pH at 37 degrees C. E-Se rhodanese could effectively replace the high concentrations of selenide normally used in the selenophosphate synthetase in vitro assay in which the selenium-dependent hydrolysis of ATP is measured. These results show that a selenium-bound rhodanese could be used as the selenium donor in the in vitro selenophosphate synthetase assay.