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Metabolism and Enzymology

Bicarbonate Inhibits Ribulose-1,5-Bisphosphate Carboxylase

Institut für Pflanzenwissenschaften, Eidgenössische Technische Hochschule, Universitätstrasse 2, CH-8092 Zürich, Switzerland

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) rapidly extracted from leaves of wheat (Triticum aestivum) and purified activated RuBPCO were incubated in the presence and absence of 20 millimolar HCO₃^– and changes in activation state were followed. Rapid inactivation occurred in the presence, but not in the absence, of HCO₃^–. Effects of CO₂ concentration and pH during preincubation before assay on activation state of RuBPCO were investigated in equilibrium studies. Twenty percent inactivation occurred at high CO₂ concentration if pH was high, but not if it was low, suggesting that RuBPCO was inactivated by HCO₃^–. The inactivation by HCO₃^– was more rapid than the dissociation of activating CO₂ in CO₂-free buffer (both in the presence of 20 millimolar MgCl₂), suggesting that HCO₃^– was bound to the active enzyme complex. The dissociation of inactivating HCO₃^– from the enzyme was slow enough that inhibition could be demonstrated in experiments with HCO₃^– treatments during preincubation and constant conditions during assay. Inorganic phosphate did not seem to interfere with the binding of HCO₃^–.