Antioxidants selenium

Even though selenium itself is not an antioxidant, selenium is an important cofactor for the enzyme, glutathione peroxidase, which has been shown to break down hydrogen peroxide (l) as well as organic hydroperoxides (2). The reactions may well be important in removing peroxides which might interact with and damage DNA. 

Keywords Free radicals, antioxidants, selenium, tellurium, hemolytic substitution, chiral stannanes, enantioselective... 

Selenium is an essential element and is beneficial at low concentrations, serving as an antioxidant. Lack of selenium affects thyroid function, and selenium deficiencies have been linked to Keshan Disease (34). Selenium at high levels, however, is toxic. Hydrogen selenide (which is used in semiconductor manufacturing) is extremely toxic, affecting the mucous membranes and respiratory system. However, the toxicity of most organ oselenium compounds used as donor compounds for organic semiconductors is not weU studied. 

Vitamin E acetate [58-95-7] serves primarily as an antioxidant and is closely associated with selenium. Vitamin E usually is present in... 

A variety of other metals and their complexes have been studied for radioprotective activity. Among these are copper glycinate, strontium chloride, ZnNa -diethylenetriaminepentaacetate (ZnDTPA), and selenium, which has been studied because of its relationship to endogenous antioxidant mechanisms, especiaHy GSH peroxidase and vitamin E. 

Rubber. The mbber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural mbber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfudess and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useflil as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

There are numerous synthetic and natural compounds called antioxidants which regulate or block oxidative reactions by quenching free radicals or by preventing free-radical formation. Vitamins A, C, and E and the mineral selenium are common antioxidants occurring naturally in foods (104,105). A broad range of flavonoid or phenoHc compounds have been found to be functional antioxidants in numerous test systems (106—108). The antioxidant properties of tea flavonoids have been characterized using models of chemical and biological oxidation reactions. 

Figure 45-6. Interaction and synergism between antioxidant systems operating in the lipid phase (membranes) of the cell and the aqueous phase (cytosol). (R-,free radical PUFA-00-, peroxyl free radical of polyunsaturated fatty acid in membrane phospholipid PUFA-OOH, hydroperoxy polyunsaturated fatty acid in membrane phospholipid released as hydroperoxy free fatty acid into cytosol by the action of phospholipase Aj PUFA-OH, hydroxy polyunsaturated fatty acid TocOH, vitamin E (a-tocopherol) TocO, free radical of a-tocopherol Se, selenium GSH, reduced glutathione GS-SG, oxidized glutathione, which is returned to the reduced state after reaction with NADPH catalyzed by glutathione reductase PUFA-H, polyunsaturated fatty acid.)...

Situnayake et al., 1991). No correlation between disease activity and serum vitamin E concentrations was found, but it was su ested that such patients might suffer a reduced antioxidant capacity. However, it is conceivable that a decreased serum antioxidant status is a primary event in the evolution of RA. Recent studies (Heliovaara etal., 1994) have demonstrated that lowered levels of vitamin E, /3-carotene and selenium (required for glutathione peroxidase) together may be a risk fector for subsequent development of RA. 

Guyan et al. 1990) have used several markers of lipid peroxidation (9-cis-, 11-tmns-isomer of linoleic acid, conjugated dienes and ultraviolet fluorescent products) to demonstrate significant increases in the duodenal aspirate after secretin stimulation in patients with acute and clinic pancreatitis. They interpreted this as indicating induction of hepatic and pancreatic drug-metabolizing enzymes in the face of a shortfidl of antioxidant defences, more marked in chronic pancreatitis. Subsequent studies in patients with chronic pancreatitis have confirmed decreased serum concentrations of selenium, -carotene and vitamin E compared with healthy controls (Uden et al., 1992). Basso aol. (1990) have measured increases in lipid peroxides in the sera of patients with chronic... 

Only one study to date has been conducted on the treatment of acute pancreatitis with antioxidants. Clemens et al. (1991) were unable to show any difference in the incidence or severity of post-endoscopic retrograde cholangiopancreatography (ERCP) pancreatitis in a prospective, randomized, double-blind, placebo-controlled trial of allopurinol. However, Salim (1991) performed a similar trial of the effect of allopurinol and DMSO in patients with pain from recurrent pancreatitis, and found significant benefit. On the basis that depletion of antioxidants is important in the pathogenesis of chronic pancreatitis, the administration of a cocktail of antioxidants was assessed for its effect on pain in this disease. Treatment with a combination of organic selenium, d-carotene, vitamins C and E, and methionine was of benefit in the initial pilot study, and in a placebo-controlled trial (San-dilands etal., 1990 Uden et al., 1990). 

Prevention of vascular disease is one of the goals of a study in progress in Sweden, in which newly diagnosed diabetic children have been randomized in a doubleblind study where one group receives placebo and the other a preparation containing ascorbic acid, )3-carotene, nicotinamide, selenium and vitamin E (Ludvigsson, 1992). Future research with antioxidants may attempt to prevent the onset of pancreatic beta-cell destruction in the prediabetic phase of susceptible individuals. 

Supplementation with antioxidant micronutrients, for example, vitamin E and selenium, in the elderly and in dementia subjects (Tolonen et al., 1985) has indicated that such treatments may be of some limited benefit. The value of ascorbate as a cerebroprotective antioxidant against excitotoxic neuronal injury has been proposed (Griinewald, 1993). 

The synergistic effect observed in the presence of all three antioxidants implies that there is an interaction between the individual antioxidant components. The direct interaction of the a-tocopherol radical and ascorbic acid is already well established (Bisby and Parker 1995) and a study by Mayne and Parker (1989) on chicks deficient in vitamin E and selenium showed that the... 

Imam, S.Z., Newport, G.D., Islam, F., Slikker, W.J., Ali, S.F. Selenium, an antioxidant, protects against methamphetamine-induced dopaminergic neurotoxicity. Brain Res. 18 575, 1999. 

All selenium-containing proteins and enzymes in animals, microorganisms and plants incorporate selenocysteine non-specifically105 or as part of Se-dependent antioxidant enzymes such as glutathione peroxidase, (EC 1.11.1.9) which has a Se-cysteine residue in its active site.116 120 An active form of Se, selenophosphate, is produced by selenophosphate synthetase in several bacteria. This active form is required for the production of Secys-tRNA, a precursor for Se-cysteine.121 In a similar vein, a Se-containing modified-tRNA nucleoside, 5-methylaminomethyl-2-selenouridine, encodes a selenouridine synthase which replaces sulfur in tRNA with selenium.122... 

The effects of antioxidants on protein oxidation were also studied in animal experiments. Barja et al. [73] demonstrated that feeding guinea pigs with vitamin C decreased endogenous protein oxidative damage in the liver. Administration of the mixture of antioxidants containing Trolox C, ascorbic palmitate, acetylcysteine, (3-carotene, ubiquinones 9 and 10, and (+)-catechin in addition to vitamin E and selenium to rats inhibited heme protein oxidation of kidney homogenates more efficiently than vitamin E + selenium [74]. 

Mammalian thioredoxin reductases are a family of selenium-containing pyridine nucleotide-disulfide oxidoreductases. These enzymes catalyze NADPH-dependent reduction of the redox protein thioredoxin (Trx), which contains a redox-active disulfide and dithiol group and by itself may function as an efficient cytosolic antioxidant [77]. One of the functions of Trx/ thioredoxin reductase system is the NADPH-catalyzed reduction of protein disulfide [78] ... 

The possible involvement of free radicals in the development of hypertension has been suspected for a long time. In 1988, Salonen et al. [73] demonstrated the marked elevation of blood pressure for persons with the lowest levels of plasma ascorbic acid and serum selenium concentrations. In subsequent studies these authors confirmed their first observations and showed that the supplementation with antioxidant combination of ascorbic acid, selenium, vitamin E, and carotene resulted in a significant decrease in diastonic blood pressure [74] and enhanced the resistance of atherogenic lipoproteins in human plasma to oxidative stress [75]. Kristal et al. [76] demonstrated that hypertention is accompanied by priming of PMNs although the enhancement of superoxide release was not correlated with the levels of blood pressure. Russo et al. [77] showed that essential hypertension patients are characterized by higher MDA levels and decreased SOD activities. 

Goede, A.A. and H.T. Wolterbeek. 1994. The possible role of selenium in antioxidation in marine waders a preliminary study. Sci. Total Environ. 144 241-246. 

Selenium is essential for many species including man, on account of its presence in a number of enzymes, notably glutathione peroxidase, an important antioxidant enzyme. It is incorporated into selenoenzymes in the form of selenocysteine. 

---