mRNA stability and selenocysteine insertion sequence efficiency rank gastrointestinal glutathione peroxidase high in the hierarchy of selenoproteins

The recently described gastrointestinal glutathione peroxidase (GI-GPx) is the fourth member of the family of the selenoenzymes glutathione peroxidase s (GPx). In contrast to the more uniform distribution of, for example, the classical glutathione peroxidase (cGPx), it is expressed exclusively in the gastrointestinal tract and has, therefore, been suggested to function as a primary barrier against alimentary hydroperoxides. In order to get an idea of its relative importance we investigated its position in the hierarchy o f selenoprotein expression. The selenium-dependent expression of GI-GPx was analyzed in comparison with that of other GPx types at the level of mRNA a nd protein in HepG2 and CaCo-2 cells. Furthermore, the selenocysteine inser tion sequence (SECIS) efficiencies of GI-GPx, phospholipid hydroperoxide gl utathione peroxidase (PHGPx) and cGPx in response to selenium were determin ed by a reporter-gene assay in human hepatoma cells and baby hamster kidney cells. GI-GPx mRNA levels increased during selenium deficiency, whereas cG Px mRNA levels decreased and PHGPx mRNA levels remained almost unaffected. In cells grown in selenium-poor media, all GPx-types were low in both activ ity and immunochemical reactivity. Upon selenium repletion immunoreactive G I-GPx protein reached a plateau after 10 h, whereas cGPx started to be expr essed at 24 h and did not reach its maximum level before 3 days. SECIS effi ciencies decreased in the order PHGPx > cGPx > GI-GPx. The augmentation of SECIS efficiencies by selenium was highest for cGPx and intermediate for PH GPx, whereas it was marginal for GI-GPx. The high mRNA stability under sele nium restriction, the speed of biosynthesis upon selenium repletion and the marginal effect of selenium on the SECIS efficiency indicate that of the G Px isotypes, GI-GPx ranks highest in the hierarchy of selenoproteins and po int to a vital role of GI-GPx in the gastrointestinal tract.