Studies of sulfate utilization by algae 17. Reactions of the adenosine 5′-phosphosulfate (APS) sulfotransferase from Chlorella and studies of model reactions which explain the diversity of side products with thiols<xref ref-type="fn" rid="fn1"><sup>1</sup></xref>

Abstract

Adenosine 5′-phosphosulfate sulfotransferase has been partially purified from Chlorella and is shown to catalyze the transfer of the sulfate group of adenosine 5′-phosphosulfate to a variety of thiol acceptors to form the corresponding organic thiosulfate. While the normal acceptor in the sulfate reducing pathway is thought to be a peptide carrier containing a thiol group the enzyme is very non-specific with respect to the thiols to which it will transfer leading to a large number of side reactions which are possible when thiols are added to the system. Using adenosine 5′-phosphosulfate and the enzyme, monothiols form sulfite and the organic thiosulfate of the thiol, with dithiols which readily form intramolecular disulfides, sulfite is the only product, while with vicinal dithiols, sulfite and finally thiosulfate is formed. The -SO3− sulfur of the thiosulfate originates from adenosine 5′-phosphosulfate while the -S−- sulfur is supplied by the vicinal dithiol. The same products can be obtained using glutathione-S-sulfonate in place of adenosine 5′-phosphosulfate and the enzyme, in a non-enzymatic reaction with the same thiols. Thus it appears that the enzymatic reaction catalyzes the transfer of the sulfate group of adenosine-5′-phosphosulfate to a thiol carrier or to any other thiol. When these other thiols are present, however, sulfite, thiosulfate or organic thiosulfates of the thiols are formed in non-enzymatic side reactions. The transferase from Chlorella is specific for adenosine 5′-phosphosulfate and will not catalyze the reaction with adenosine-3′-phosphate-5′-phosphosulfate.