Bicarbonate inhibits ribulose-1,5-bisphosphate carboxylase.

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 HCO(3) (-) and changes in activation state were followed. Rapid inactivation occurred in the presence, but not in the absence, of HCO(3) (-). Effects of CO(2) concentration and pH during preincubation before assay on activation state of RuBPCO were investigated in equilibrium studies. Twenty percent inactivation occurred at high CO(2) concentration if pH was high, but not if it was low, suggesting that RuBPCO was inactivated by HCO(3) (-). The inactivation by HCO(3) (-) was more rapid than the dissociation of activating CO(2) in CO(2)-free buffer (both in the presence of 20 millimolar MgCl(2)), suggesting that HCO(3) (-) was bound to the active enzyme complex. The dissociation of inactivating HCO(3) (-) from the enzyme was slow enough that inhibition could be demonstrated in experiments with HCO(3) (-) treatments during preincubation and constant conditions during assay. Inorganic phosphate did not seem to interfere with the binding of HCO(3) (-).