The method of competitive labelling with [3H]acetic anhydride as the labelling reagent was used to determine the properties of the active-centre lysine residue of rabbit muscle aldolase. This residue is much less reactive than a normal exposed lysine residue towards this reagent, and its reactive properties did not parallel the pH–activity profile for aldolase. At higher pH values it became reactive, but this was shown to be due to disruption of the enzyme structure. The binding of the competitive inhibitor phosphate did not alter the reactive properties. It is concluded that the active-centre lysine has an apparent pKa greater than 11.5 and probably is made nucleophilic during the catalytic process, perhaps by proton abstraction.
