## 4 Discussion
Therefore, after fermentation condition optimization (containing 30.0 \(\mu\)g/mL Na\({}_{2}\)SeO\({}_{3}\) and an adding time of 9 h after inoculation), the Se content of the Se-enriched DB-yeast reached 14.95 mg/L (approximately 2.0 mg/g), which indicated a considerably high Se accumulation capacity. The maximum biomass of the Se-enriched DB-yeast was 7.3 g/L, which was relatively low. This may be attributed to variations in the degree of growth performance of the DB-yeast in the beer industry fermentation medium. Through bioconversion, inorganic Se is transformed into organic Se such as selenomethionine or selenocysteine, possibly via replacement of the sulfur in methionine and cysteine, finally forming newly synthesized protein [5; 30]. Therefore, compared with DB-yeast, the sulfur-containing amino-acid content of Se-enriched DB-yeast decreased. Interestingly, on the one hand, the total amino-acid content of the Se-enriched DB-yeast increased, which may be attributed to Se reducing the protein degradation resulting from oxidative damage; on the other hand, alkaline amino-acid contents increased, whereas acidic amino-acid content decreased, revealing the stress response of DB-yeast for Se\({}_{2}\)O\({}_{3}\)[2]-. Se is also a key component of numerous functional selenoproteins including five glutathione peroxididases (GSH-Px), two deiodinases, thioredoxin reductases, and selenophosphate synthetases [5]. These five GSH-Px can detoxify H\({}_{2}\)O\({}_{2}\) and fatty-acid-derived hydroperoxides, thus contributing to the antioxidant defense against reactive molecules and free radicals, complementing the effects of vitamin E [10]. Therefore, compared with the DB-yeast extracted protein, the reducing capacity, hydroxyl radical removal capacity, and sulfhydryl content after treatment with H\({}_{2}\)O\({}_{2}\) of the Se-enriched DB-yeast extracted protein were obviously increased. These increased antioxidant defense abilities may due to functional selenoprotein synthesis through inorganic Se bioconversion, indicating that Se-enriched DB-yeast can be used as the bioavailable organic Se resource for fodder Se-supplemented additive. For the GIFT bioassay, the Se content of the fodder additives was set as 0.6 mg/kg Se, which was three times more than the mouse bioassay (0.2 mg/kg Se). The Se content increase was because the fish fodder was diluted by water in the GIFT bioassay. [PERSON] et al. [31] demonstrated that Se supplementation in broiler diets significantly improved weight gain, final body weight, and meat quality without increasing the feeding cost. Similarly, both Se sources, Na\({}_{2}\)SeO\({}_{3}\) and Se-enriched DB-yeast, could improve mouse and GIFT growth performance. However, the growth performance of mice and GIFT treated with Se-enriched DB-yeast was not significantly increased compared with those treated with Na\({}_{2}\)SeO\({}_{3}\), which was slightly different from the results reported by [PERSON] et al. [32],where organic Se supplementation improved growth compared with inorganic Se supplementation in broiler chickens; however, this may because of variations in the growth performance of different species in the presence of Se. The GSH-Px activity level in the liver and serum is indicative of Se the status and antioxidant level of the organism [11; 33]. GIFT treated with Se-enriched DB-yeast had significantly higher serum GSH-Px activities than those treated with Na\({}_{2}\)SeO\({}_{3}\) or no Se-additive, which was attributed to the Se sedimentation content in the serum, indicating that Se-enriched DB-yeast can be used as an effective Se-enriched fodder additive for GIFT. Both mouse and GIFT had a higher Se sedimentation content of the liver, intestine, and spleen than that of muscle, indicating that the viscera, particularly the liver and spleen, were the major locations of Se sedimentation, similar to the results from [PERSON] et al. [34] for _Oncorhynchus mykiss_ treated with selenomethionine.