Activation of Ribulose 1, 5-Bisphosphate Carboxylase/Oxygenase by Inorganic Phosphate under Nocturnal Conditions

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In vivo activation states of ribulose 1, 5-bisphosphate carboxylase/oxygenase(RuBisCO; EC 4.1.1.39) in the dark and light phases were measured in intact leaves of Phaseolus and radish. The activation state was high in the dark and comparable to the activation state under illumination at saturating light intensity. Then, we examined, using RuBisCO purified from spinach leaves, a mechanism for the activation of RuBisCO in the dark when the stroma is neutralized and lossess Mg^<2+> partly. Activation was not obserevd when the enzyme was incubated at air-level CO_2 and 10 mM Mg^<2+> at pH ranging from 6.2 to 7.5. However, the activation was highly promoted in this pH range when the activation mixture contained 10 mM inorganic phosphate. The activation state was 50 to 60% between pH 7.0 and 7.8 and maximum over pH 8.2 in the presence of 10 mM inorganic phosphate. Studies of the initial rate of activation show that the promotion of activation was through stabilization of the active form of the enzyme by inorganic phosphate, not by altering the pKa of the activator ε-amino group of Lys-201. The physiological significance of the activation of RuBisCO by inorganic phosphate in the dark is discussed.