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

Abstract

In the presence of ATP and Mg2+, ATP sulphurylase from Saccharomyces cerevisiae catalysed the conversion of selenate into a compound with the electrophoretic and acid-lability properties of adenosine 5'-sulphatophosphate. Structural characterization, involving extensive purification of adenosine 5'-selenophosphate, proved impossible. However, we showed ATP-, Mg2+-and ATP sulphurylase-dependent, and inorganic pyrophosphatase-stimulated, production of elemental selenium from selenate in the presence of GSH (reduced glutathione). Since selenate was not reduced by GSH, this reaction proved that ATP sulphurylase had formed an active selenate. The enzymecatalysed formation of elemental selenium had the same kinetics and GSH-dependency as the non-enzymic reduction ofselenite to elemental selenium by GSH. In the presence of inorganic pyrophosphatase, 2mol of Pi was released for each mol of 'active selenate' formed. This was shown by a spectrophotometric assay for elemental selenium. The observed reactivity with thiols and the instability of the enzymic product were those predicted for selenium anhydrides. By analogy with the chemistry of sulphur, 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 selenium. We conclude that ATP sulphurylase can catalyse the formation of adenosine 5'-selenophosphate. The anhydride can be reduced by thiols in a manner similar to the reduction of selenite. These results probably explain the ability of mammals, lacking a sulphate reductase system, to incorporate selenium from selenate into seleno-amino acids.

Selenium uptake, translocation, assimilation and metabolic fate in plants

“…There is evidence that APSe can be nonenzymatically reduced by glutathione in vitro (Dilworth and Bandurski 1977), but in E. coli this reductive step occurs enzymatically (Muller et al 1997). Moreover, considering that the reaction equilibrium of ATP sulfurylase favors the reverse direction, APS reductase is essential in plants because APS and pyrophosphate, the products of the ATP sulfurylase reaction, have to be rapidly metabolized in order for sulfate activation to proceed towards APS (Saito 2004).…”

Effects of glutathione depletion on selenite- and selenate-induced embryotoxicity in cultured rat embryos

“…Capacities of selenite to induce chromosome aberrations and DNA repair in human fibroblasts were enhanced by an S-9 activation mixture, but those of selenate were not [26]. Although little is known about the metabolism of selenate, it has been proposed that selenate requires GSH and ATP on reduction to selenite at slow rates in non-enzymatic pathways [27]. It is possible that under GSH-depleted conditions this requirement causes oxidative stress, a known cause of teratogenicity [28].…”

Metabolism of Metals and Metalloids by the Sulfate-Reducing Bacteria

“…Thus, it can replace sulfate in its reaction with ATP sulfurylase, the first enzyme in the sulfate reduction pathway [79]. The resulting adduct, adenosine 5′-selenophosphate (APSe), is then reduced to selenite [27]. This is the same reaction catalyzed in the activation of sulfate to APS and subsequent reduction to sulfide.…”