Comparative Enzymology of the Adenosine Triphosphate Sulfurylases from Leaf and Swollen Hypocotyl Tissue of Beta vulgaris

ATP sulfurylase activity was partially purified from the swollen hypocotyl of beetroot (Beta vulgaris); activity was measured by sulfate-dependent PPi-ATP exchange. The ATP sulfurylase activity was separated from pyrophosphatase and ATPase activities which interfere with the assay of ATP sulfurylase activity. The ATP sulfurylase activity from hypocotyl tissue was invariably resolved into two approximately equal activities (hypocotyls I and II) by ion exchange chromatography and polyacrylamide gradient gel electrophoresis. Both enzymes catalyzed selenate- and sulfate-dependent PPi-ATP exchange; the affinity of hypocotyl II for these substrates was greater than for hypocotyl I. It is unlikely that the two activities arise by allelic variation or as an artifact of purification; they are most probably isoenzymes. Studies of the subcellular localization of the two hypocotyl enzymes were inconclusive.ATP sulfurylase was also purified from leaf tissue. Ion exchange chromatography resolved the ATP sulfurylase from leaf tissue into a major activity (which accounted for 98% of the total leaf activity) and a minor activity. The major leaf and hypocotyl II ATP sulfurylases were indistinguishable as judged by the properties investigated.