Affiliations: Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, CA 94720–3200, USA | Department of Physiological Chemistry, University of Braunschweig, Mascheroder Weg 1, D‐38124 Braunschweig, Germany | Department of Molecular and Cell Biology, University of California Berkeley, 251 Life Sciences Addition, Berkeley, CA 94720–3200, USA Tel.: +1 510 642 1872; Fax: +1 510 642 8313
Note: [] On leave from: Department of Biochemical Sciences “A. Rossi Fanelli” and CNR Center of Molecular Biology, University of Rome “La Sapienza”, Piazzale Aldo Moro 5, 00185 Rome, Italy.
Note: [] Corresponding author.
Abstract: Human thioredoxin reductase was recently shown to contain a TGA encoded selenocysteine residue at the penultimate position of its amino acid chain. Depending on the availability of selenium during biosynthesis, an authentic selenocysteine‐containing or a selenium‐free enzyme truncated at the penultimate position is expected to be formed. Correspondingly, the enzymatic activity should be altered by selenium restriction, if the selenocysteine residue is functionally important. In order to check the catalytic role of the selenocysteine residue, four different human cell lines were grown in selenium deficient media or with adequate selenium supplementation (40 nM sodium selenite) and thioredoxin reductase activity was measured as NADPH‐dependent DTNB reduction or thioredoxin‐mediated insulin reduction. Thioredoxin reductase activities, like glutathione peroxidase activities, were consistently higher in selenium supplemented cells, whereas glutathione reductase activity was not affected by the selenium. The dose‐response was similar for thioredoxin reductase and glutathione peroxidase, but the recovery of glutathione peroxidase activity upon selenium supplementation was faster than with thioredoxin reductase. Also the increase of glutathione peroxidase activities was substantially higher than that of thioredoxin reductase (400–1200% versus a maximum of 250%). These observations clearly indicate a catalytic role of the selenocysteine residue in the thioredoxin reductase, but suggest either the existence of a selenium‐unresponsive isoenzyme or a residual disulfide reductase activity in the selenium‐free truncated protein made under conditions of selenium deficiency.
