Zinc Detoxification Research Articles

Subcellular distribution of zinc and cadmium in the hepatopancreas and gills of the decapod crustacean Penaeus indicus

The decapod crustacean Penaeus indicus accumulated Cd and Zn in different subcellular compartments of hepatopancreas and gill cells. Most of the Cd and part of the Zn accumulates within the soluble fraction of the cells, while the remainder of the Zn is found in insoluble inclusions, associated with P, Ca, Mg and Si in B-, F- and R-cells in the hepatopancreas, and haemocytes, nephrocytes and epithelial cells in the gills. No presence of Cd was observed in metal-rich inclusions in any cell analysed. Metallothionein-like proteins (MTLP), analysed by differential pulse polarography, were present in the hepatopancreas (12–18 mg g−1) and gills (7–8 mg g−1) of metal-exposed prawns. Binding to MTLP is the detoxification mechanism for cadmium, while the detoxification of zinc involves both binding to MTLP and incorporation into insoluble metal-rich inclusions.

Two genes encoding Arabidopsis halleri MTP1 metal transport proteins co-segregate with zinc tolerance and account for high MTP1 transcript levels.

The zinc hyperaccumulator plant Arabidopsis halleri is able to naturally accumulate 100-fold higher leaf zinc concentrations when compared with non-accumulator species such as the closely related A. lyrata and A. thaliana, without showing toxicity symptoms. A novel member of the cation diffusion facilitator (CDF) protein family, an A. halleri metal tolerance protein 1 (MTP1), and the homologous A. thaliana Zn transporter (ZAT)/AtMTP1 metal-specifically complement the zinc hypersensitivity of a Saccharomyces cerevisiae zrc1 cot1 mutant strain. A fusion of the AhMTP1 protein to green fluorescent protein (GFP) localizes to the vacuolar membrane of A. thaliana protoplasts. When compared with A. lyrata and A. thaliana, the total MTP1 transcript levels are substantially higher in the leaves and upregulated upon exposure to high zinc concentrations in the roots of A. halleri. The high MTP1 transcript levels in A. halleri can be primarily attributed to two genetically unlinked genomic AhMTP1 gene copies. The two corresponding loci co-segregate with zinc tolerance in the back-cross 1 generation of a cross between the zinc-tolerant species A. halleri and the zinc-sensitive species A. lyrata. In contrast, a third MTP1 gene in the genome of A. halleri generates only minor amounts of MTP1 transcripts and does not co-segregate with zinc tolerance. Our data suggests that zinc tolerance in A. halleri involves an expanded copy number of an ancestral MTP1 gene, encoding functional proteins that mediate the detoxification of zinc in the cell vacuole. At the transcript level, MTP1 gene copies of A. halleri are regulated differentially and in response to changes in zinc supply.

Zinc detoxification by a cyanobacterium from a metal contaminated bay in Brazil

We describe here the trapping of zinc in polyphosphate granules of the cyanobacterium Synechocystis aquatilis NPBS-3. Cells were cultured in 25 µM of zinc chloride and prepared for electron microscopy and energy-dispersive X-ray analysis. Some ultrastructural features were changed by zinc exposure, the increase of glycogen granules number being the main change. The polyphosphate granules contained phosphorus, sulphur, calcium, iron and zinc. The trapping of zinc in polyphosphate granules seemed to be an effective way of detoxifying the metal and surviving in the bay. As a non-specific mechanism, these polyphosphate granules could also be effective in trapping other metals in excess.

Induction of the ZRC1 Metal Tolerance Gene in Zinc-limited Yeast Confers Resistance to Zinc Shock

Zinc is an essential nutrient but toxic to cells with overaccumulation. For this reason, intracellular zinc levels are tightly controlled. In the yeast Saccharomyces cerevisiae, the Zrc1 and Cot1 proteins have been implicated in the storage and detoxification of excess zinc in the vacuole. Surprisingly, transcription of ZRC1 is induced in zinc-limited cells by the zinc-responsive transcription factor Zap1. We show here that this increase in ZRC1 expression is a novel mechanism of zinc homeostasis and stress tolerance. Zinc-limited cells also express high levels of the plasma membrane zinc uptake transporters. As a consequence, when zinc-limited cells are resupplied with small amounts of zinc, large quantities quickly accumulate in the cell, a condition we refer to as "zinc shock." We show here that ZRC1 and its induction in zinc-limited cells are required for resistance to this zinc shock. Experiments using the zinc-responsive fluorophore FuraZin-1 as an indicator of vacuolar zinc levels indicated that Zrc1 is required for the rapid transport of zinc into the vacuole during zinc shock. We also present evidence that cytosolic zinc rises to higher levels in cells unable to sequester this excess zinc. Thus, the increase in ZRC1 expression occurs prior to the zinc shock stress for which this induction is important. We propose that this "proactive" strategy of homeostatic regulation, such as we document here for ZRC1, may represent a common but largely unrecognized phenomenon.

Detoxification of Aqueous Zinc Using Fluorapatite - Bearing Lean Grade Rock Phosphate

Detoxification of aqueous zinc was studied using the adsorption technique by application of fluorapatite bearing low grade phosphate as a sorbent. The effects of particle size of adsorbent, contact time, initial concentration of zinc and temperature on the adsorption process were studied. Initial rate of adsorption of zinc increases with time up to 30 min and thereafter it reaches equilibrium. Percent adsorption is adversely affected by increase in initial concentration of zinc and increase in particle size of sorbent. Adsorption processes follow first order kinetics and are found to be exothermic.

Characteristic induction of 70,000 da-heat shock protein and metallothionein by zinc in HeLa cells.

The synthesis of a 70,000 dalton-heat shock protein (hsp70) is one of several heat shock proteins induced in HeLa cells during the incubation in medium containing zinc sulphate. The synthesis of hsp70 was increased in the presence of 200 microM zinc sulphate and above, but not at 100 microM zinc sulphate. On the other hand, the synthesis of metallothionein was activated in the presence of 100 microM zinc sulphate and above. Uptake of zinc into the cells depended on the concentration of zinc sulphate in the medium. The separation of intracellular zinc into three fractions by gel filtration chromatography; high molecular, metallothionein, and low molecular fractions, showed that zinc in the low molecular weight and metallothionein fractions was elevated in the presence of 100 microM zinc sulphate in the medium, whereas increase in the zinc content of the high molecular weight fraction occurred at 200 microM zinc sulphate and above. Inhibition of cell growth and cellular protein synthesis was also observed at 200 microM zinc sulphate and above, but not at 100 microM. From these findings, since the induction of hsp70 synthesis and inhibition of cell growth occurred concomitantly with the increase of zinc in the high and low molecular weight fractions, hsp70 seemed not to function in the detoxification of zinc, but it may participate in the repair of zinc-induced damage.

THE UPTAKE OF ZINC AND CALCIUM FROM SOLUTION CULTURE BY ZINC‐TOLERANT AND NON‐TOLERANT SILENE MARITIMA WITH. IN RELATION TO CALCIUM SUPPLY

SUMMARYInter‐population variation in the uptake of calcium from solution culture was studied in a range of zinc‐tolerant and non‐tolerant populations of Silene maritima under conditions of high zinc and calcium. The populations exhibited great variability in their patterns of calcium uptake similar to that previously reported for zinc. The effects of variation in calcium supply on yield and calcium and zinc uptake by one zinc‐tolerant and one non‐tolerant population are described. Little difference in yield‐response to calcium was detected between the two populations but they showed major differences in the uptake and accumulation of zinc and calcium and their transport from root to shoot. Total zinc uptake was little affected by increases in calcium level in the tolerant plants but was decreased significantly in the non‐tolerant plants. At the higher calcium levels, total zinc uptake by tolerant plants was also significantly greater than by the non‐tolerant but the populations did not differ in total calcium uptake. Evidence for a reduction in zinc transport in both populations resulting from increased calcium uptake is presented. Marked stimulation of zinc accumulation by the roots of the tolerant population resulting from increases in the external calcium level is interpreted as a possible corollary of zinc tolerance in this tolerant population. The role of calcium in the internal detoxification of zinc is discussed.