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      Structural, chemical and biological aspects of antioxidants for strategies against metal and metalloid exposure

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          Abstract

          Oxidative stress contributes to the pathophysiology of exposure to heavy metals/metalloid. Beneficial renal effects of some medications, such as chelation therapy depend at least partially on the ability to alleviate oxidative stress. The administration of various natural or synthetic antioxidants has been shown to be of benefit in the prevention and attenuation of metal induced biochemical alterations. These include vitamins, N-acetylcysteine, α-lipoic acid, melatonin, dietary flavonoids and many others. Human studies are limited in this regard. Under certain conditions, surprisingly, the antioxidant supplements may exhibit pro-oxidant properties and even worsen metal induced toxic damage. To date, the evidence is insufficient to recommend antioxidant supplements in subject with exposure to metals. Prospective, controlled clinical trials on safety and effectiveness of different therapeutic antioxidant strategies either individually or in combination with chelating agent are indispensable. The present review focuses on structural, chemical and biological aspects of antioxidants particularly related to their chelating properties.

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          Most cited references233

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          Toxic metals and oxidative stress part I: mechanisms involved in metal-induced oxidative damage.

          Toxic metals (lead, cadmium, mercury and arsenic) are widely found in our environment. Humans are exposed to these metals from numerous sources, including contaminated air, water, soil and food. Recent studies indicate that transition metals act as catalysts in the oxidative reactions of biological macromolecules therefore the toxicities associated with these metals might be due to oxidative tissue damage. Redox-active metals, such as iron, copper and chromium, undergo redox cycling whereas redox-inactive metals, such as lead, cadmium, mercury and others deplete cells' major antioxidants, particularly thiol-containing antioxidants and enzymes. Either redox-active or redox-inactive metals may cause an increase in production of reactive oxygen species (ROS) such as hydroxyl radical (HO.), superoxide radical (O2.-) or hydrogen peroxide (H2O2). Enhanced generation of ROS can overwhelm cells' intrinsic antioxidant defenses, and result in a condition known as "oxidative stress". Cells under oxidative stress display various dysfunctions due to lesions caused by ROS to lipids, proteins and DNA. Consequently, it is suggested that metal-induced oxidative stress in cells can be partially responsible for the toxic effects of heavy metals. Several studies are underway to determine the effect of antioxidant supplementation following heavy metal exposure. Data suggest that antioxidants may play an important role in abating some hazards of heavy metals. In order to prove the importance of using antioxidants in heavy metal poisoning, pertinent biochemical mechanisms for metal-induced oxidative stress should be reviewed.
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            Structure-antioxidant activity relationships of flavonoids and phenolic acids.

            The recent explosion of interest in the bioactivity of the flavonoids of higher plants is due, at least in part, to the potential health benefits of these polyphenolic components of major dietary constituents. This review article discusses the biological properties of the flavonoids and focuses on the relationship between their antioxidant activity, as hydrogen donating free radical scavengers, and their chemical structures. This culminates in a proposed hierarchy of antioxidant activity in the aqueous phase. The cumulative findings concerning structure-antioxidant activity relationships in the lipophilic phase derive from studies on fatty acids, liposomes, and low-density lipoproteins; the factors underlying the influence of the different classes of polyphenols in enhancing their resistance to oxidation are discussed and support the contention that the partition coefficients of the flavonoids as well as their rates of reaction with the relevant radicals define the antioxidant activities in the lipophilic phase.
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              Oxidized redox state of glutathione in the endoplasmic reticulum.

              The redox state of the endoplasmic reticulum (ER) was measured with the peptide N-Acetyl-Asn-Tyr-Thr-Cys-NH2. The peptide diffused across cellular membranes; some became glycosylated and thus trapped within the secretory pathway, and its cysteine residue underwent reversible thiol-disulfide exchanges with the surrounding redox buffer. Glycosylated peptides from cells were disulfide-linked to glutathione, indicating that glutathione is the major redox buffer in the secretory pathway. The redox state of the secretory pathway was more oxidative than that of the cytosol; the ratio of reduced glutathione to the disulfide form (GSH/GSSG) within the secretory pathway ranged from 1:1 to 3:1, whereas the overall cellular GSH/GSSG ratio ranged from 30:1 to 100:1. Cytosolic glutathione was also transported into the lumen of microsomes in a cell-free system. Although how the ER maintains an oxidative environment is not known, these results suggest that the demonstrated preferential transport of GSSG compared to GSH into the ER lumen may contribute to this redox compartmentation.
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                Author and article information

                Journal
                Oxid Med Cell Longev
                OMCL
                Oxidative Medicine and Cellular Longevity
                Landes Bioscience
                1942-0900
                1942-0994
                Sep-Oct 2009
                : 2
                : 4
                : 191-206
                Affiliations
                Defense Research and Development Establishment; Division of Pharmacology and Toxicology; Gwalior, India
                Author notes
                Correspondence to: Swaran J.S. Flora; Division of Pharmacology and Toxicology; Defence Research and Development Establishment; Jhansi Road, Gwalior 474 002 India; Email: sjsflora@ 123456hotmail.com
                Article
                1942-0900-2-4-3
                10.4161/oxim.2.4.9112
                2763257
                20716905
                0a33cde9-0d63-456f-8877-d77f871639ee
                Copyright © 2009 Landes Bioscience
                History
                : 23 May 2009
                : 28 May 2009
                : 28 May 2009
                Categories
                Review

                Molecular medicine
                combination therapy,oxidative stress,metal toxicity,antioxidant,chelation therapy

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