The adenosine-5'-phosphosulfate sulfotransferase from spinach (Spinacea oleracea L.). Stabilization, partial...

Adenosine-5'-phosphosulfate (APS) sulfotransferase was purified 25-fold from spinach (Spinacea oleracea L.) leaves by Sephadex-G-200 gel filtration and chromatography on DEAE-cellulose. Enzyme activity was stabilized with 0.05 M Tris-HCl pH 8.0 containing 10 mM mercaptoethanol (ME), 10 mM MgCl2, and 30% glycerol. The molecular weight of the APS-sulfotransferase was estimated by gel filtration to be about 110,000 daltons. The enzyme is specific for the sulfonucleotide APS; PAPS is not a sulfur donor for this reaction. The apparent Km for APS was found to be 13 μM. The enzyme activity was determined with dithioerythritol (DTE) as acceptor, which has an apparent Km of 0.6 mM. Glutathione can substitute for DTE; other thiols such as mercaptoethanol and cysteine are less effective. The APS-sulfotransferase activity is inhibited by 5'-AMP, which increases the Km for APS but does not change Vmax, suggesting a competetive inhibition. Reduced methylviologen cannot substitute for a thiol in the spinach enzyme system. Thus it seems that assimilatory APS-sulfotransferase from spinach is different from the dissimilatory APS-reductase from Desulfovibrio or Thiobacillus, where methylviologen can be used as the electron donor.