Purification and Properties of 2-Carboxy-d-Arabinitol 1-Phosphatase

Carboxyarabinitol 1-phosphatase (2-carboxy-d-arabinitol 1-phosphate phosphohydrolase), a chloroplast enzyme that metabolizes the naturally occurring inhibitor of ribulose-1,5-bisphosphate carboxylase/oxygenase, was isolated from tobacco (Nicotiana tabacum) leaves. The enzyme was purified more than 3500-fold using a protocol that included ammonium sulfate fractionation and gel filtration, ion-exchange, and hydrophobic interaction chromatography. Analysis of the final preparation by sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed the presence of a single polypeptide with a molecular mass of 53 kilodaltons. The enzyme exhibited an apparent K(m) (carboxyarabinitol 1-phosphate) of 33 micromolar and a pH optimum of 7.5. Enzyme activity did not require divalent cations and was unaffected by the metal chelators EDTA and cysteine. Carboxyarabinitol 1-phosphatase activity was inhibited by zinc, copper and molybdate and stimulated by sulfate. Chloroplast metabolites that affected activity included inorganic phosphate and ATP, which were inhibitory, and ribulose-1,5-bisphosphate, fructose-1,6-bisphosphate and NADPH which stimulated activity 2.5-fold. Activation of carboxyarabinitol 1-phosphatase activity by these positive effectors, together with the previously reported requirement for dithiothreitol, explain the light/dark modulation of carboxyarabinitol 1-phosphatase activity in vivo.