Comparative enzymology of the adenosine triphosphate sulphurylases from leaf tissue of selenium-accumulator and non-accumulator plants

[摘要]

1. ATP sulphurylases were partially purified (20–40-fold) from leaf tissue of Astragalus bisulcatus, Astragalus racemosus (selenium-accumulator species) and Astragalus hamosus and Astragalus sinicus (non-accumulator species). Activity was measured by sulphate-dependent PP_i–ATP exchange. The enzymes were separated from pyrophosphatase and adenosine triphosphatase activities. The properties of the Astragalus ATP sulphurylases were similar to the spinach enzyme. 2. The ATP sulphurylases from both selenium-accumulator and non-accumulator species catalysed selenate-dependent PP_i–ATP exchange; selenate competed with sulphate. The ratio of V(selenate)/V(sulphate) and K_m(selenate)/K_m(sulphate) was approximately the same for the enzyme from each species. 3. Sulphate-dependent PP_i–ATP exchange was inhibited by ADP, chlorate and nitrate. The kinetics of the inhibition for each enzyme were consistent with an ordered reaction mechanism, in which ATP is the first substrate to react with the enzyme and PP_i is the first product released. 4. Synthesis of adenosine 5′-[³⁵S]sulphatophosphate from [³⁵S]sulphate was demonstrated by coupling the Astragalus ATP sulphurylases with Mg²⁺-dependent pyrophosphatase; the reaction was inhibited by selenate. An analogous reaction using [⁷⁵Se]selenate as substrate could not be demonstrated.