A comparison of the effects of dietary selenium on selenoprotein expression in rat brain and liver

In studies with rodents, when dietary supplies of the essential nutrient Se are restricted, in most tissues there are parallel substantial losses of the element and the important antioxidant selenoenzyme glutathione peroxidase (GPx) for which it is a cofactor. In brain, however, there appears to be both a sequestration of Se and a conservation of GPx activity when dietary Se is limited. To further explore the relation between these phenomena, we have undertaken a comparison of the effects of diets low, normal and high in Se on GPx activity, and labeling of selenoproteins following short-term (72 h) in vivo exposure to 75Se, in subcellular fractions from rat brain and liver, the latter serving as a representative tissue which does not retain Se and is depleted of most GPx activity following dietary restriction. Brains and livers from animals on the three diets showed different patterns of response with respect to both GPx activity and retention of the 75Se dose. The low-Se diet (0.006 ppm) substantially reduced GPx activity in liver but not brain, while high levels (1 ppm) did not increase GPx in either tissue relative to a normal (0.1 ppm) intake. The 75Se was retained in brain homogenates and subcellular fractions to the greatest extent by rats on the restricted diet, while in liver, retention was greater in rats fed the normal supplement than in animals on either the low- or high-Se diets. Levels of non-protein-bound 75Se were higher in brain than liver and increased with dietary Se in both tissues. When proteins in brain and liver homogenates and subcellular fractions where separated by one-dimensional SDS-PAGE and exposed to X-ray film, the resulting autoradiograms revealed the existence of seven distinct selenoprotein bands in brain and eight in liver. Different patterns of selenoprotein expression were observed in subcellular fractions isolated from both tissues. Dependence of levels of individual selenoproteins on diet paralleled the effects on 75Se retention. Dietary influences on expression of protein bands tentatively identified as GPx were more pronounced in liver than brain. All of these observations provide further evidence of the unique nature of Se metabolism in brain.