Activation of ribulose 1,5-bisphosphate carboxylase oxygenase by inorganicphosphate under nocturnal conditions

In vivo activation states of ribulose 1,5-bisphosphate carboxylase/oxygenas e (RuBisCO; EC 4.1.1.39) in the dark and light phases were measured in inta ct leaves of Phaseolus and radish. The activation slate was high in the dar k and comparable to the activation state under illumination at saturating l ight intensity. Then, we examined, using RuBisCO purified from spinach leav es, a mechanism for the activation of RuBisCO in the dark when the stroma i s neutralized and lossess Mg2+ partly. Activation was not obserevd when the enzyme was incubated at air-level CO2 and 10 mM Mg2+ 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 activ ation show that the promotion of activation was through stabilization of th e active form of the enzyme by inorganic phosphate, not by altering the pKa of the activator epsilon-amino group of Lys-201, The physiological signifi cance of the activation of RuBisCO by inorganic phosphate In the dark is di scussed.