Zinc has been reported to activate δ-aminolevulinic acid dehydratase (ALAD) and to reverse lead-induced inhibition of human blood ALAD activity in vitro. Inasmuch as cadmium lies just below zinc in Group IIB of the periodic table of elements, similar studies were initiated with cadmium. Using normal adult human whole blood hemolysates, cadmium was found to activate normal erythrocytic ALAD and to reverse the in vitro lead-induced inhibition of ALAD at concentrations 40- to 100-fold lower than those of zinc. At higher concentrations cadmium was observed to inhibit erythrocytic ALAD activity, with a 50% inhibition of ALAD activity occurring at a concentration approximately 20 times higher than that of lead. The inhibition caused by cadmium, unlike that caused by lead, could not be reversed by an in vitro addition of zinc. The present results indicate that, with respect to concentration, cadmium is a more potent activator of normal adult human erythrocytic ALAD activity than is zinc. It is suggested that the role of cadmium and zinc in increasing ALAD activity and reversing lead induced inhibition of ALAD activity is a structural one related to binding of the metal to peripheral sulfhydryl groups.
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