Glutathione – an important intracellular chelator of uranium

Plants are able to accumulate uranium (U) in roots and shoots /1/. Hence, we could detect an accumulation of this metal inside suspension cells of canola (Bassica napus) via fluorescence microscopy. Such uptake of highly toxic heavy metals requires skilled complex cellular defense mechanisms.
One example is the formation of metal complexes in the cytosol. A prominent candidate might be the tripeptide glutathione (GSH) bearing metal chelating functionalities like carboxylato- and thiol residues. Whereas at pH 5.7 the formed complex was stable over 6 hours, the initial complexes at pH > 6.0 degraded within 30 minutes. A reduction of U(VI) by GSH seems to be possible, because of strong spectral changes contributed by thiolate functionality. However, the spectra of the later formed chelates indicated an involvement of SH-groups in U complexation.
This is necessary for the formation of phytochelatins based on GSH. These metal binding peptides could be detected by HPLC in the cytosol 24 hours after U contact. U was bound to these peptides revealed by gelelectrophoresis of isolated cytoplasmic peptides and subsequent Laser-induced Fluorescence Spectroscopy. Despite these GSH consuming processes during U contact, the cytoplasmic GSH content decreased only slightly but not significantly compared to the control. Artificial depletion of GSH induced an up to twofold increased metabolic activity of cells challenging with U concentrations ≤ 10 µM whereas higher U contents dramatically reduced the metabolism. These data underlines the tight cellular regulation and the importance of GSH in maintaining defense reactions against U.

/1/ K. Viehweger, G. Geipel, Environmental and Experimental Botany 69 (2010) 39–46