Measurement of the Enzyme-CO2-Mg2+ Form of Spinach Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase

When the amount of activation of ribulose 1,5-bisphosphate carboxylase has been measured, two forms of the enzyme, not one, are actually determined experimentally. Only the enzyme-activator CO(2)-Mg(2+) form can bind ribulose bisphosphate for reaction with substrate CO(2) or O(2). A method is presented which measures only this catalytically active form by stabilizing it with ribulose bisphosphate just before dilution and assay in Mg(2+)-free reaction medium.