Adenosine 5'-Phosphosulfate Sulfotransferase and Adenosine 5'-Phosphosulfate Reductase Are Identical Enzymes

Adenosine 5'-phosphosulfate (APS) sulfotransferase and APS reductase have been described as key enzymes of assimilatory sulfate reduction of plants catalyzing the reduction of APS to bound and free sulfite, respectively. APS sulfotransferase was purified to homogeneity from <i>Lemna minor</i> and compared with APS reductase previously obtained by functional complementation of a mutant strain of <i>Escherichia coli</i> with an <i>Arabidopsis thaliana</i> cDNA library. APS sulfotransferase was a homodimer with a monomer <i>M</i>_r of 43,000. Its amino acid sequence was 73% identical with APS reductase. APS sulfotransferase purified from <i>Lemna</i> as well as the recombinant enzyme were yellow proteins, indicating the presence of a cofactor. Like recombinant APS reductase, recombinant APS sulfotransferase used APS (<i>K</i>_m = 6.5 µM) and not adenosine 3'-phosphate 5'-phosphosulfate as sulfonyl donor. The <i>V</i>_max of recombinant <i>Lemna</i> APS sulfotransferase (40 µmol min^-1 mg protein ^-1) was about 10 times higher than the previously published <i>V</i>_max of APS reductase. The product of APS sulfotransferase from APS and GSH was almost exclusively SO₃^-2. Bound sulfite in the form of <i>S</i>-sulfoglutathione was only appreciably formed when oxidized glutathione was added to the incubation mixture. Because SO₃^-2 was the first reaction product of APS sulfotransferase, this enzyme should be renamed APS reductase.