Activation of selenate by adenosine 5′-triphosphate sulphurylase from Saccharomyces cerevisiae

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Abstract
In the presence of ATP and Mg2+, ATP sulfurylase from S. cerevisiae catalyzed the conversion of selenate into a compound with the electrophoretic and acid-lability properties of adenosine 5''-sulfatophosphate. Structural characterization, involving extensive purification of adenosine 5''-selenophosphate, proved impossible. ATP-, Mg2+- and ATP sulfurylase-dependent, and inorganic pyrophosphatase-stimulated, production of elemental Se from selenate selenate in the presence of GSH (reduced glutathione) was shown. Since selenate was not reduced by GSH, this reaction proved that ATP sulfurylase had formed an active selenate. The enzyme-catalyzed formation of elemental Se had the same kinetics and GSH-dependency as the non-enzymic reduction of selenite to elemental Se by GSH. In the presence of inorganic pyrophosphatase, 2 mol of Pi was released for each mol of active selenate formed. This was shown by a spectrophotometric assay for elemental Se. The observed reactivity with thiols and the instability of the enzymic product were those predicted for selenium anhydrides. By analogy with the chemistry of sulfur, the product of the thiolytic cleavage of a selenium anhydride would be converted into selenite. The selenite would then be reduced by the thiol to elemental Se. Apparently, ATP sulfurylase catalyzed the formation of adenosine 5''-selenophosphate. The anhydride was reduced by thiols in a manner similar to the reduction of selenite. These results probably explain the ability of mammals, lacking a sulfate reductase system, to incorporate Se from selenate into seleno-amino acids.