Is there another player in the game of Rubisco regulation?

Leaves of French bean plants, whether collected in the dark or in the light, contained a tight-binding inhibitor of ribulose 1,5-bisphosphate carboxylase/ oxygenase (Rubisco). Wheat leaves contained such an inhibitor only in the light. The inhibitor in wheat leaves was not 2-carboxy-D-arabinitol 1-phosphate (CA1P) and not xylulose 1,5-bisphosphate. It was stable in acid conditions, but was destroyed during low pressure anion exchange chromatography and in alkaline conditions, especially during concentration of solutions. Although the kinetics of inhibition suggested tight-binding, the inhibitor did not survive attempts at purification by binding it to purified Rubisco followed by denaturation of the protein. The retention time of the inhibitory component on HPLC anion-exchange columns suggested that the substance was a sugar bisphosphate. Reduction with tritiated sodium borohydride and dephosphorylation produced radioactive sugar alcohols identified by TLC as ribitol and arabinitol. The small amounts of this inhibitor detected do not suggest a major physiological role, but instability during extraction may be obscuring its true importance.