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

Abstract

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, 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 K _m 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.