Inhibition of 5-aminolevulinic acid dehydratase by mercury in excised greening maize leaf segments

Mercury (Hg), a potent metallic toxicant, is known for having inhibitory effect on chlorophyll biosynthesis. In vivo supply of HgCl2 inhibited 5-aminolevulinic acid dehydratase (ALAD, EC 4.2.1.24) activity in excised greening maize (Zea mays) leaf segments. The inhibition caused by Hg was alleviated by addition of KNO3. Amongst the nutrients and metabolites tested, NH4Cl and sucrose increased the inhibitory effect of Hg on enzyme activity, while glutamine and glutathione decreased it. The inhibitors, levulinic acid and 5,5′ dithio bis 2-nitrobenzoic acid, also reduced the % inhibition of enzyme activity caused by Hg supply. In vitro inclusion of Hg during assay of the enzyme preparations obtained from the tissue treated without Hg (−Hg enzyme) and with Hg (+Hg enzyme) caused the inhibition of −Hg enzyme but activation of +Hg enzyme. Almost similar trend was observed for the in vitro inclusion of Hg in the presence of levulinic acid. It is suggested that two forms of enzyme exist in Hg-treated tissue, i.e. the usual Mg dependent form and an unusual Hg modified form. Kinetic studies for the two enzymes, −Hg enzyme and +Hg enzyme, involving the effect of varying concentrations of δ-aminolevulinic acid yielded distinct apparent Km and apparent Vmax values being 532 μM and 118 units g−1 fr. wt., respectively, for −Hg enzyme and 347 μM and 52 units g−1 fr. wt., respectively, for +Hg enzyme indicating that +Hg enzyme has higher affinity for δ-aminolevulinic acid but lower activity as compared to the −Hg enzyme.