Zinc and cadmium in 5-aminolevulinic acid dehydratase. Equilibrium,kinetic, and 113Cd-nmr-studies

5-Aminolevulinic acid dehydratase (ALAD) from bovine liver contains zinc that is partially lost during the isolation of the enzyme. ALAD has its maximal activity at 10^?5 M ZnCl₂. It binds 7.4 Zn per octameric protein with an association constant of 5.3 × 10⁶ M^?1. ALAD is inactivated by 1,10-phenanthroline or ethylenediaminetetraacetic acid (EDTA) but not by monodentate anions like cyanide or sulfide. After removal of zinc by chelating agents, the enzyme activity may be restored by Zn²⁺ or Cd²⁺. Removal or zinc by EDTA increases K_M 60-fold and decreases V_max to about $\text{1}\text{2}$ of its original value. The ¹¹³Cd nuclear magnetic resonance spectrum of the enzyme reconstituted with ¹¹³Cd-acetate exhibits a single sharp resonance signal at 79 ppm. It does not change by the addition of substrate but disappears when the inhibitor lead acetate is added. Therefore, an immediate interaction between the metal ion of the enzyme and the substrate is excluded, whereas lead changes the environment of cadmium and probably of zinc too.