Selenium Concentration Controls Cellular Glutathione Peroxidase (CGPx) Synthesis by Translational Mechanisms Without Changing Polysome Patterns
Selenium (Se)-dependent CGPx contains selenocysteine (SeCys) in the primary structure. SeCys is incorporated during translation from SeCys t-RNA. We investigated the effects of Se concentration on mRNA, polysome distribution of mRNA, protein synthesis, and enzymatic activity of CGPx. Se-deficient cultured human (Caki-2, HL60 and HK-2) and mouse (EG-7) cells incorporate Se into CGPx more efficiently from selenite than from selenomethionine (SeMet): selenite raised CGPK activity to maximal levels (20-fold over Sedeficient) at 3x10-s M, whereas SeMet raised GPx activity to maximal levels at lxl0-s M. This level of Se caused only a 2-3 fold change in the steady-state level of CGPx mRNA. The effects of Se concentration on the rate of synthesis of CGPx was determined by immunoprecipitation of metabolically-labeled 3sS-CGPx from cells grown under equilibrium conditions of low, moderate and high concentratons of selenite (lx10-9, 6x10-9, and 1x10-7 M, respectively). The rate of protein synthesis was linear for 8 hrs at all concentrations of Se. Furthermore, the effects of Se on the rate of synthesis of CGPx protein account for more than 80% of the effects of Se on enzyme activity. On sucrose gradients, transcripts for CGPx are found only on polysomes of very low ribosome number (1-4 ribosomes, suggesting translational control) regardless of the concentration of Se. Therefore We propose that Se controls CGPK synthesis and activity by translational mechanisms that are not reflected by changes in polysome distribution patterns.
Whitin, JC; Tham, DM; Bhamre, S; Cohen, HJ
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