Inhibition of δ-aminolevulinic acid dehydratase activity by cadmium in excised etiolated maize leaf segments during greening

Abstract

Supply of 0.1–0.5 mmol CdCl₂ inhibited δ-aminolevulinic acid dehydratase (EC 4.2.1.24, ALAD) activity and total chlorophylls in excised etiolated segments of maize leaves during greening. Due to cadmium supply δ-aminolevulinic acid (ALA) content was reduced significantly at 0.5 mmol Cd only. Also the Cd treatment decreased the protein content and accumulated significantly the Cd in the tissue. Significant correlation between Cd accumulation in the leaves and various parameters measured is observed, with the R-squared values being 0.727 with ALAD activity, 0.885 with ALA content, 0.902 with total chlorophylls and 1.00 with proteins. The % inhibition of ALAD activity by Cd was decreased in the presence of nitrogenous compounds, glutamine and NH₄NO₃ and the observed inhibition was 25% and 16%, respectively. More substantial reduction in % inhibition of enzyme activity by Cd was observed during treatment with glutathione, a ubiquitous thiol and levulinic acid, a competitive inhibitor of ALAD, with the inhibition being only 2% and 4%, respectively. Supply of some essential metal ions, such as Mg, Zn, and Mn, also reduced the % inhibition of enzyme activity by Cd. Inclusion of varying concentrations of ALA during assay also affected the % inhibition of enzyme activity by Cd showing an increased inhibition from 17% to 53% with increasing ALA concentration. It is suggested that Cd inhibits ALAD activity by affecting the ALA binding to the enzyme and/or disrupting thiol interaction.