Studies of sulfate utilization by algae 13. Adenosine 5′-phosphosulfate (APS) as an intermediate in the conversion of adenosine 3′-phosphate 5′-phosphosulfate (PAPS) to acid-volatile radioactivity

An intermediate, identified as adenosine 5′-phosphosulfate (APS), has been found in the enzymatic conversion of PAP³⁵S to acid-volatile radioactivity by enzyme fractions "A" and "S" from Chlorella pyrenoidosa. "A" was found to be a Mg²⁺-dependent 3′-nucleotidase specific for 3′5′-diphosphonucleosides which converts adenosine 3′-phosphate 5′-phosphosulfate (PAPS) to APS. Authentic AP³⁵S is a substrate for "S" and yields acid-volatile radioactivity in the presence of Mg²⁺ and a thiol. This is of some interest since Chlorella is an assimilatory sulfate reducer and APS has been reported as a substrate only for dissimilatory sulfate reduction. PAPS production is undoubtedly concerned with providing a source of active sulfate for esterification reactions, but since fraction "A" can convert PAPS to APS, this may be yet another mechanism for forming APS despite the unfavorable equilibrium of the ATP sulfurylase reaction.