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A 3′-phosphoadenosine-5′-phosphosulfate (PAPS) dependent sulfotransferase was partly purified from Cyanophora paradoxa. This enzyme has a pH-optimum of 8.5 and was found to be specific for PAPS ; the K m  for this nucleotide was determined to be 8.3 μᴍ. In crude extracts monothiols like glutathione, mercaptoethanol, or cysteine catalyzed a rate of about 80% of the rate obtained with dithioerythritol; however after purification of the PAPS-sulfotransferase the monothiol activity decreased to about 20% of the rate obtained with dithiols. Addition of heated, crude extract partly restored the monothiol activity. This PAPS-sulfotransferase was inhibited by 5′-ADP and 5′-AMP, and especially by adenosine-5′-phosphosulfate (APS). Practically no inhibition was found with 2′-AMP, 3′-AMP, c-AMP, 3′-5′-ADP and 2′-5′-ADP. The inhibitor constants have been determined to be 430 μᴍ for 5′-AMP; 290 μᴍ for 5′-ADP; and 0.3 μᴍ for APS. These inhibitions seem to be competetive to PAPS. The role of these inhibitors for a possible regulation of assimilatory sulfate reduction are discussed.

A PAPS-Dependent Sulfotransferase in Cyanophora paradoxa Inhibited by 5ʹ-AMP, 5ʹ-ADP, and APS