Specific Selenium-Containing Macromolecules in the Marine Diatom Thalassiosira pseudonana

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Abstract
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 was present. One enzyme was active with H2O2 and tert-butyl hydroperoxide (tBOOH); consistent with known Se-dependent 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 Se-dependent GSH-Px. The molecular weight of one of the 75Se-labeled marcromolecules 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.

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