An enzyme that releases P(i) from 2-carboxy-D-arabinitol 1-phosphate, a naturally occurring tightly binding inhibitor of ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39), was purified from leaves of French bean seedlings. It was a monomeric protein of M(r) about 56,000. Catalytic activity was stimulated by increased concentrations of inorganic salts to a maximum at an ionic strength above 0.2. NADPH and D-fructose 1,6-bisphosphate increased the activity of the enzyme in both the presence and absence of 0.2 M-KCl. The pure enzyme did not require dithiothreitol for activity. The pH optimum was 7, the Km for 2-carboxy-D-arabinitol 1-phosphate was 0.43 mM and the specific activity 6.8 mumol/min per mg of protein. The enzyme had little or no activity against phosphate ester intermediates of photosynthetic metabolism and glycolysis but hydrolysed the 1,5-bisphosphates of 2′-carboxy-D-ribitol and 2′-carboxy-D-arabinitol more rapidly than 2′-carboxy-D-arabinitol 1-phosphate.

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