A PAPS-Dependent Sulfotransferase in Cyanophora paradoxa

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 P A P S ; the K m for this nucleotide was determined to be 8.3 juu. 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 /uu for 5'-AM P; 290 /um for 5'-ADP; and 0.3 f iu for APS. These inhibitions seem to be competetive to PAPS. The role of these inhibitors for a possible regulation of assimi­ latory sulfate reduction are discussed.