Adenosine 5‘-Phosphosulfate Sulfotransferase from Norway Spruce: Biochemical and Physiological Properties*

Biochemical and physiological properties of adenosine 5′-phosphosulfate sulfotransferase, a key enzyme of assimilatory sulfate reduction, from spruce trees growing under field conditions were studied. The apparent K_m for adenosine 5′-phosphosulfate (APS) was 29 ± 5.5μM, its apparent M_r was 115,000. 5′-AMP inhibited the enzyme competitively with a K_i of 1 mM, but also stabilized it. MgS0₄ at 800 mM increased adenosine 5′-phosphosulfate sulfotransferase activity by a factor of 3, concentrations higher than lOOOmM were inhibitory. Treatment of isolated shoots with nutrient solution containing 1 or 2 mM sulfate, and 3 or 10 mM glutathione, respectively, induced a significant decrease in extractable adenosine 5′-phosphosulfate sulfotransferase activity over 24h, whereas GSH as well as S^2- up to 5mM cysteine and up to 200 mM SO₃^2- had no effect on the in vitro activity of the enzyme. As with other enzymes involved in assimilatory sulfate reduction, namely ATP sulfurylase (EC 2.7.7.4), sulfite reductase (EC 1.8.7.1) and O-acetyl-L.-serine sulfhydrylase (EC 4.2.99.8), adenosine 5′-phosphosulfate sulfotransferase was still detected at appreciable activities in 2- and 3-year-old needles. Adenosine 5′-phosphosulfate sulfotransferase activity was low in buds and increased during shoot development, parallel to the chlorophyll content. The enzyme activity was characterized by an annual cycle of seasonal changes with an increase during February and March.