Impact of uranium (U) on the cellular glutathione pool and resultant consequences for the redox status of U |
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Authors: | Email author" target="_blank">Katrin?ViehwegerEmail author Gerhard?Geipel Gert?Bernhard |
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Institution: | (1) Biogeochemistry Division, Institute of Radiochemistry, Helmholtz-Zentrum Dresden-Rossendorf eV, P.O. Box 510119, 01314 Dresden, Germany |
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Abstract: | Uranium (U) as a redox-active heavy metal can cause various redox imbalances in plant cells. Measurements of the cellular
glutathione/glutathione disulfide (GSH/GSSG) by HPLC after cellular U contact revealed an interference with this essential
redox couple. The GSH content remained unaffected by 10 μM U whereas the GSSG level immediately increased. In contrast, higher
U concentrations (50 μM) drastically raised both forms. Using the Nernst equation, it was possible to calculate the half-cell
reduction potential of 2GSH/GSSG. In case of lower U contents the cellular redox environment shifted towards more oxidizing
conditions whereas the opposite effect was obtained by higher U contents. This indicates that U contact causes a consumption
of reduced redox equivalents. Artificial depletion of GSH by chlorodinitrobenzene and measuring the cellular reducing capacity
by tetrazolium salt reduction underlined the strong requirement of reduced redox equivalents. An additional element of cellular
U detoxification mechanisms is the complex formation between the heavy metal and carboxylic functionalities of GSH. Because
two GSH molecules catalyze electron transfers each with one electron forming a dimer (GSSG) two UO2
2+ are reduced to each UO2
+ by unbound redox sensitive sulfhydryl moieties. UO2
+ subsequently disproportionates to UO2
2+ and U4+. This explains that in vitro experiments revealed a reduction to U(IV) of only around 33% of initial U(VI). Cellular U(IV)
was transiently detected with the highest level after 2 h of U contact. Hence, it can be proposed that these reducing processes
are an important element of defense reactions induced by this heavy metal. |
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