Adenosine-5'-phosphosulfate (APS) as sulfate donor for assimilatory sulfate reduction in Rhodospirillum rubrum

Crude extracts of Rhodospirillum rubrum catalyzed the formation of acid-volatile radioactivity from (35S) sulfate, (35S) adenosine-5'-phosphosulfate, and (35S) 3'-phosphoadenosine-5'-phosphosulfate. An enzyme fraction similar to APS-sulfotransferases from plant sources was purified 228-fold from Rhodospirillum rubrum. It is suggested here that this enzyme is specific for adenosine-5'-phosphosulfate, because the purified enzyme fraction metabolized adenosine-5'-phosphosulfate; 3'-phosphoadenosine-5'-phosphosulfate, however, only at a rate of 1/10 of that with adenosine-5'-phosphosulfate. Further, the reaction with 3'-phosphoadenosine-5'-phosphosulfate was inhibited with 3'-phosphoadenosine-5'-phosphate whereas this nucleotide had no effect on the reaction with adenosine-5'-phosphosulfate. For this activity with adenosine-5'-phosphosulfate the name APS-sulfotransferase is suggested. This APS-sulfotransferase needs thiols for activity; good rates were obtained with either dithioerythritol or reduced glutathione; other thiols like cysteine, 2'-3'-dimercaptopropanol or mercaptoethanol are less effective. The electron donor methylviologen did not catalyze this reaction. The pH-optimum was about 9.0; the apparent Km for adenosine-5'phosphosulfate was determined to be 0.05 mM with this so far purified enzyme fraction. Enzyme activity was increased with K2SO4 and Na2SO4 and was inhibited by 5'-AMP. These properties are similar to assimilatory APS-sulfotransferases from spinach and Chlorella.