14C-Labeled 2-carboxyarabinitol-1,5-bisphosphate was bound to both nonactivated and CO2and Mg2+activated forms of ribulose bisphosphate carboxylase/oxygenase. The complex could be precipitated with 20% polyethylene glycol and 20 mm MgCl2for quantitation of the moles of the affinity label bound per mole of enzyme. The [14C]carboxyarabinitol-P2bound to the nonactivated enzyme could be exchanged with a 100-fold excess of the unlabeled compound. With the activated enzyme the binding of [14C]carboxyarabinitol-P2was so tight that it did not exchange with the unlabeled compound and a binding stoichiometry of one molecule per active site was assumed. This tight binding was dependent upon pretreatment of the enzyme with both CO2and MgCl2in the same manner that enzyme activation depended on CO2and Mg2+concentrations. Various enzyme preparations from spinach leaves tightly bound [14C]carboxyarabinitol-P2in proportion to their specific activities. By extrapolating to a maximum binding of 8 mol of [14C]carboxyarabinitol-P2per mole of this A8B8enzyme a theoretical specific activity of 2.8 μmol · min-1· mg protein-1was indicated. Enzyme preparations purified from spinach leaves generally have a specific activity in the range of 1.0 to 2.3. © 1981.
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