A phosphatase from chloroplast stroma of Nicotiana tabacum hydrolyses 2′‐carboxyarabinitol 1 ‐phosphate, the natural inhibitor of Rubisco to 2′‐carboxyarabinitol

An enzyme has been identified in the stroma of tobacco chloroplasts that degrades the natural inhibitor of Rubisco, 2′‐carboxyarabinitol 1‐phosphate (2CA1P). The products of the catalysis, determined by HPLC and phosphate analysis are 2′‐carboxyarabinitol and inorganic phosphate indicating that the enzyme is a phosphatase. The only other phosphate esters found to be degraded by the enzyme were the branched‐chain carboxylic sugar bisphosphates, 2′‐carboxyarabinitol and ribitol 1,5‐bisphosphates that were converted to the corresponding 5‐monophosphates. Other intermediary metabolites of the photosynthetic cycle were essentially unaffected by the phosphatase. The kinetic parameters of the enzyme are consistent with those required to relieve the inhibition of rubisco in vivo.