Abstract
Various mechanisms have been proposed to explain the biological dissimilatory reduction of selenite (SeO3(2-)) to elemental selenium (Se(o)), although none is without controversy. Glutathione, the most abundant thiol in the eukaryotic cells, the cyanobacteria, and the alpha, beta, and gamma groups of the proteobacteria, has long been suspected to be involved in selenium metabolism. Experiments with the phototrophic alpha proteobacterium Rhodospirillum rubrum showed that the rate of selenite reduction was decreased when bacteria synthesized lower than normal levels of glutathione, and in Rhodobacter sphaeroides and Escherichia coli the reaction was reported to induce glutathione reductase. In the latter organism superoxide dismutase was also induced in cells grown in the presence of selenite, indicating that superoxide anions (O2-) were produced. These observations led us to investigate the abiotic (chemical) reduction of selenite by glutathione and to compare the features of this reaction with those of the reaction mediated by R. rubrum and E. coli. Our findings imply that selenite was first reduced to selenodiglutathione, which reached its maximum concentration within the 1st min of the reaction. Formation of selenodiglutathione was paralleled by a rapid reduction of cytochrome c, a known oxidant for superoxide anions. Cytochrome c reduction was inhibited by superoxide dismutase, indicating that O2- was the source of electrons for the reduction. These results demonstrated that superoxide was produced in the abiotic reduction of selenite with glutathione, thus lending support to the hypothesis that glutathione may be involved in the reaction mediated by R. rubrum and E. coli. The second phase of the reaction, which led to the formation of elemental selenium (Se(o)), developed more slowly. Se(o) precipitation reached a maximum within 2 h after the beginning of the reaction. Secondary reactions leading to the degradation of the superoxide significantly decreased the yield of Se(o) in the abiotic reaction compared with that of the bacterially mediated selenite reduction. Abiotically formed selenium particles showed the same characteristic orange-red color, spherical structure, and size as particles produced by R. rubrum, again providing support for the hypothesis that glutathione is involved in the reduction of selenite to elemental selenium in this organism.
Highlights
Selenium is an essential trace element in the nutrition of many organisms, but it can be highly toxic depending on its concentration and speciation
We investigate the kinetics of formation of selenodiglutathione, superoxide anions, and elemental selenium during abiotic reduction of selenite by glutathione, and we compare the features of this reaction with those of the reaction mediated by R. rubrum and E. coli
According to Bebien et al [5], who observed a large induction of two types of superoxide dismutase (SOD) in E. coli grown in the presence of selenite, we propose that Reaction 4 takes place in this organism and that it may constitute the first step of the dissimilatory reduction of selenite in all cells containing high levels of glutathione and performing intracellular selenite reduction
Summary
Selenium is an essential trace element in the nutrition of many organisms, but it can be highly toxic depending on its concentration and speciation. In their investigations of selenite toxicity in prokaryotes, Kramer and Ames [4] did not observe any nonspecific incorporation of selenium into proteins They demonstrated that a mutant strain of Salmonella typhimurium, which is able to overexpress oxidative stress proteins such as catalase and superoxide dismutase (SOD), is significantly more resistant to selenite toxicity than the wild type. They considered the high reactivity of selenite with sulfhydryl groups and the formation of oxygen radicals when selenium reacted with cysteine or glutathione and concluded that selenite toxicity in bacteria might be the result of oxidative damage Consistent with these results Bebien et al [5] observed that two types of SOD are induced in cultures of Escherichia coli exposed to selenite, confirming the involvement of free radicals in selenium toxicity. Ganther [9] proposed that the unstable selenopersulfide of glutathione (GS-SeϪ) dismutates into elemental selenium (Se°) and reduced glutathione according to the following stoichiometry
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