[Show abstract][Hide abstract] ABSTRACT: The interaction between an essential micronutrient, Zn and a toxic non-essential element, Cd has been comprehensively reviewed based on our experiments conducted with Ceratophyllum demersum L. in a hydroponic system. Since Cd and Zn belong to the group IIB transition elements and show similarities in chemistry, geochemistry and environmental properties, it would be one of the elemental pairs of choice to investigate metal-metal interactions. Evidence in support of the protective role of Zn against Cd toxicity in Ceratophyllum demersum L. is presented in this review. Based on our experimental results, we conclude that the antioxidant properties of Zn play an important role in counteracting Cd toxicity.
Brazilian Journal of Plant Physiology 03/2005; 17(1). DOI:10.1590/S1677-04202005000100002
[Show abstract][Hide abstract] ABSTRACT: To understand the interaction between Zn, an essential micronutrient and Cd, a non-essential element, Cd-10 microM and Zn supplemented (10, 50, 100, and 200 microM) Cd 10 microM treated Ceratophyllum demersum L. (Coontail), a free floating freshwater macrophyte was chosen for the study. Cadmium at 10 microM concentration decreased thiol content, enhanced oxidation of ascorbate (AsA) and glutathione (GSH) to dehydroascorbate (DHA) and glutathione disulfide (GSSG), respectively, a clear indication of oxidative stress. Zinc supplementation to Cd (10 microM) treated plants effectively restored thiols, inhibited oxidation of AsA and GSH maintaining the redox molecules in reduced form. Cd-10 microM slightly induced ascorbate peroxidase (APX, E.C. 220.127.116.11) but inhibited monodehydroascorbate reductase (MDHAR, E.C. 18.104.22.168), dehydroascorbate reductase (DHAR, E.C. 22.214.171.124) and glutathione reductase (GR, E.C. 126.96.36.199), enzymes of ascorbate-glutathione cycle (AGC). Zn supplementation restored and enhanced the functional activity of all the AGC enzymes (APX, MDHAR, DHAR and GR). Gamma-glutamylcysteine synthetase (gamma-GCS, E.C. 188.8.131.52) was not affected by Cd as well as Zn, but Zn supplements increased glutathione-S-transferase (GST, E.C. 184.108.40.206) activity to a greater extent than Cd and simultaneously restored glutathione peroxidase (GSH-PX, E.C. 220.127.116.11) activity impaired by Cd toxicity. Zn-alone treatments did not change above investigated parameters. These results clearly indicate the protective role of Zn in modulating the redox status of the plant system through the antioxidant pathway AGC and GSH metabolic enzymes for combating Cd induced oxidative stress.
[Show abstract][Hide abstract] ABSTRACT: The mechanism by which Zn protects the conformation of carbonic anhydrase (CA; carbonate hydrolyase, E.C. 18.104.22.168) in Cd-exposed Ceratophyllum demersum L. was analyzed. Cd 10 μM, Zn (200 μM) supplemented Cd (10 μM) and Zn 200 μM alone treated plants of C. demersum were compared with the control plants. The circular dichroism spectra of native CA from control plants indicated α-helix secondary conformation, whereas CA from Cd 10 μM treatments indicated unfolding of the enzyme to a random coil, denaturation, protein-cross linking and aggregation as depicted by the decrease in ellipticities and diminishing of distinct α-helical peaks. Cd-induced protein unfolding lead to the quenching of tryptophan and tyrosine intrinsic fluorescence subsequently reflected in ultraviolet (UV) spectra also, decreased thiol (–SH) groups, increased the hydrophobicity measured as 8-anilino-1-naphthalene sulfonic acid (ANS)-protein fluorescence and enhanced the formation of oxidised components, dityrosine and carbonyls. Zn (200 μM) supplementation to Cd 10 μM treatments restored α-helical conformation with increased helical ellipticities, –SH groups, tryptophan and tyrosine intrinsic fluorescence, as well as UV spectra, decreased ANS-protein fluorescence and formation of oxidised dityrosine and carbonyl components. Zn alone treatments did not change the conformation or properties of the native CA. The clear inhibition of distortion in secondary structure, protection of hydrophobic core in the protein, inhibition of residual side chain oxidation or formation of insoluble aggregates by supplemented Zn indicates the potential role of Zn in combating Cd-induced oxidative damage to CA.
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