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Metabolism and Enzymology

Specific Selenium-Containing Macromolecules in the Marine Diatom Thalassiosira pseudonana¹

Department of Botany, University of British Columbia, Vancouver, B.C., V6T 1W5, Department of Oceanography, University of British Columbia, Vancouver, B.C., V6T 1W5

Thalassiosira pseudonana Husedt (Hasle and Heimdal) clone 3H was grown in axenic culture in artificial seawater medium containing 10^–8 molar Na₂⁷⁵SeO₃. Biochemical distribution of radiolabeled Se was determined by solvent extraction techniques, gel filtration, and polyacrylamide gel electrophoresis. Of the total cellular Se, 51% was protein bound. Two soluble macromolecules of 21 and 29 kilodaltons contained ⁷⁵Se. These results are the first to provide evidence of specific Se-containing compounds in a photosynthetic organism. Glutathione peroxidase (GSH-Px) activity was measured in cell-free extracts and on nondenaturing polyacrylamide gels by a glutathione-reductase coupled assay. Two enzymes showing GSH-Px activity were present. One enzyme was active with H₂O₂ and tert-butyl hydroperoxide (tBOOH); consistent with known Sedependent GSH-Pxs, but the other enzyme was only active with tBOOH. Co-migration of the H₂O₂-active GSH-Px and ⁷⁵Se on nondenaturing polyacrylamide gels provides evidence that T. pseudonana contains a Sedependent GSH-Px. The molecular weight of one of the ⁷⁵Se-labeled macromolecules is identical with the weight of previously characterized GSH-PX subunits. We conclude that the obligate requirement for Se in Thalassiosira pseudonana is due in part to the presence of the selenoenzyme glutathione peroxidase.

¹ Funding provided by the Natural Sciences and Engineering Research Council of Canada.

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