Effect of dietary selenium level on growth performance, body composition and hepatic glutathione peroxidase activities of largemouth bass Micropterus salmoide
Abstract
An 8-week growth trial was conducted to determine the effect of dietary selenium (Se) level on growth performance, body composition and hepatic glutathione peroxidase (GPx) activities of largemouth bass. Sodium selenite was added to the fish meal basal diet at 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mg kg−1 Se providing 0.97, 1.17, 1.42, 1.60, 1.85 and 2.06 mg Se kg−1 diet respectively. Each diet was fed to triplicate groups of fish (initial mean body weight: 4.95 ± 0.03 g) in a closed indoor recirculating system. The Se concentration in the rearing water was not detectable during the whole experimental period. The highest weight gain was obtained in fish fed diets with 1.60 mg Se kg−1, which was significant higher (P < 0.05) than the basal diet with 0.97 mg Se kg−1 and did not differ significantly (P > 0.05) with the other treatments. Feed conversion ratio, protein efficiency ratio, protein productive value, apparent digestibility coefficients of dry matter and muscle composition were not significantly impacted (P > 0.05) by dietary treatments. Fish fed diets with ≥1.42 mg Se kg−1 obtained higher liver lipid contents than treatments with lower dietary Se levels. Hepatic malondialdehyde (MDA) was unchanged (P > 0.05) in relation to dietary Se concentration. Hepatic GPx and glutathione reductase (GR) activities markedly increased and decreased (P < 0.05) with increasing dietary Se concentration, respectively, and both reached a plateau with ≥1.85 mg Se kg−1. Based on growth performance, hepatic MDA and enzymatic responses of GPx and GR, the highest Se concentration (2.06 mg kg−1) employed in our study was not harmful for largemouth bass, and the optimal dietary level should be 1.60–1.85 mg Se kg−1 from sodium selenite, at a dietary vitamin E level of 400 IU kg−1.
Introduction
As an essential trace element for animals including fish (Kohrle, Brigelius-Flohe & Bock 2000), selenium (Se) is an integral part of the active centre of glutathione peroxidase (GPx) in the form of selenocysteine (Levander & Burk 1994). This enzyme catalyses reactions necessary for the conversion of hydrogen peroxide and fatty acid hydroperoxides into water and fatty acid alcohol by using reduced glutathione (GSH), thereby protecting cell membranes against oxidative damage (Watanabe, Kiron & Satoh 1997). However, the partition line between Se deficiency and toxicity is very narrow. In fact, in high cellular concentration Se can interact with cellular sulphydryls leading to a depletion of GSH and an increase of lipid peroxidation (Combs & Combs 1986). In fish, either Se deficiency or excess dietary Se was associated with higher mortality, depressed growth, decreased GPx activity in plasma and tissues, increased lipid peroxidation and tissue degeneration (Poston, Combs & Leibovitz 1976; Hilton, Hodson & Slinger 1980; Hilton & Hodson 1983; Gatlin & Wilson 1984; Lin & Shiau 2005; Deng, Hung & Teh 2007; Wang, Han, Li & Xu 2007). In addition, Se is also included in other functionally active selenoproteins as the type 1 iodothyronine 5′-deiodinase which interacts with iodine to prevent abnormal hormone metabolism (Foster & Sumar 1997). Furthermore, Se is also required for the efficient functioning of many components of the immune system (Kiremidjian-Schumacher & Stotzky 1987; Arthur, McKenzie & Beckett 2003), which is of particular importance in intensive fish culture.
As an important freshwater sport fish species native of lakes and small rivers in North America (Coyle, Tidwell & Webster 2000), largemouth bass was first introduced into Guangdong Province in 1983 from where it quickly disseminated through sub-tropical areas of China such as Zhejiang and Jiangsu Province, and is today becoming one of the most commercially important fish in China. In the latest decades, a few nutrition studies on M. salmoides have focused on macronutrient requirements (Portz, Cyrino & Martino 2001; Bright, Coyle & Tidwell 2005; Amoah, Coyle, Webster, Durborow, Bright & Tidwell 2008) and alternative protein and lipid sources other than fish meal and oil (Tidwell, Coyle, Bright & Yasharian 2005; Subhadra, Lochmann, Rawles & Chen 2006a,b; Tidwell, Coyle & Bright 2007). However, the nutrition status of trace element such as Se in M. salmoides is still unknown. Therefore, this study was conducted to determine the effect of dietary selenium level on growth, feed utilization, body composition and hepatic GPx activities of M. salmoides.
Materials and methods
Experimental design and diets
Diets were based on commercial available ingredients and the proximate composition of the test diets were shown in Table 1. Except fish meal which was imported from Peru, the other feed ingredients were originated from mainland of China. Fish meal and soybean meal were used as protein source. Wheat flour was used as carbohydrate source, and corn oil and soy lecithin were used as lipid source. Six isoproteic (48%) and isolipidic (9%) diets were formulated and differed only by the supplementation level of Se at 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mg kg−1 as sodium selenite. As basal fishmeal-based diets contained 0.97 mg Se kg−1, measured concentrations of Se in the tested diets were 0.97, 1.17, 1.42, 1.60, 1.85 and 2.06 mg Se kg−1 diet respectively. Yttrium oxide (Y2O3) was added to the diets at an inclusion rate of 0.01% as the digestibility marker.
| Ingredients | % |
|---|---|
| Fish meal | 45 |
| Soybean meal | 30 |
| Wheat flour | 16.8 |
| Corn oil | 2 |
| Soy lecithin | 4 |
| Vitamin mixa | 0.2 |
| Mineral mixb | 0.5 |
| Ascorbicacid phosphate ester | 0.1 |
| Choline chloride | 0.4 |
| Monocalcium phosphate monohydrate | 0.99 |
| Yttrium oxide | 0.01 |
| Proximate analysis (analysed,% in dry matter) | |
| Moisture | 10.28 |
| Protein | 47.88 |
| Lipid | 9.21 |
| Ash | 11.59 |
- a Vitamin mix (mg kg−1 of diet): thiamine, 50; riboflavin, 200; vitamin A, 10 000 IU; vitamin E, 400 IU; vitamin D3, 2000 IU; menadione, 40; pyridoxine HCl, 50; cyanocobalamin, 1; biotin, 5; calcium pantothenate, 500; folic acid, 15; niacin, 750; inositol, 2000; and cellulose was used as a carrier.
- b Mineral mix (g kg−1 diet): Ca(H2PO4).H2O, 0.536; Calcium Lactate,1.332; FeC6H5O7.5H2O, 0.132; MgSO4.7H2O, 0.532; KH2PO4, 0.932; NaH2PO4.2H2O, 0.332; AlCl3.6H2O, 0.028; ZnCl2, 0.08; CuSO4, 0.04; MnSO4.H2O, 0.028; KI, 0.028; and cellulose was used as a carrier.
Diet ingredients were ground through a 60 mesh size. Oil and distilled water (350 mL kg−1 dry ingredients mixture) were added to the premixed dry ingredients, and were thoroughly mixed until homogenous in a Hobart-type mixer. The 1.5 mm diameter pellets were wet extruded, air dried outdoor, sealed in plastic bags and stored at −20°C until used.
Supply and maintenance of fish
Eight hundred feed trained largemouth bass were purchased from a commercial hatchery (Shunde, China) and kept into 18 circular fibreglass tanks (48 cm × 48 cm × 42 cm) connected with a closed indoor recirculating system with continuous filtered freshwater. After a 2-week acclimatization period, during which fish were fed the basal diet without Se supplementation, largemouth bass were graded to a similar size (4.95 ± 0.03 g) and randomly stocked into 18 circular fibreglass tanks (48 cm × 48 cm × 42 cm) at 30 fish per tank. Ten fish were taken prior to stocking for baseline whole-body proximate analysis. For the next 8 weeks, fish in triplicate were fed their perspective diets twice daily at 09:00 and 16:00. The feeding rate was 3% of whole body weight. Natural light-dark cycle was used in the whole experimental period. Fish were weighed every 2 weeks, and the daily rations were adjusted accordingly. Throughout the experimental period, water temperature, dissolved O2, pH and ammonia were maintained at about 27.28 ± 1.5°C, 8.23 ± 0.42 mg L−1, 7.45 ± 0.05 and 0.15 ± 0.02 mg L−1 respectively.
Approximately 2 weeks prior to trial end, faeces samples were collected at 11:00 and 18:00 using syphoning method according to Lin, Liu, Tian, Wang, Zheng, Huang and Chen (2004). Collected faeces were gently rinsed with freshwater, oven-dried at 50°C overnight, and stored at −20°C until analysed for Y2O3 concentration.
Sample collection and chemical analysis
At the termination of the 8-week feeding trial, approximately 24 h after the last feeding, all fish were counted and weighed to determine survival rate, weight gain (WG), feed conversion ratio (FCR) and protein efficiency ratio (PER). After obtaining the final total weight of fish in each aquarium, 12 fish per tank were randomly selected and anaesthetized (MS-222; Sigma, St Louis, MO, USA at 10 mg L−1). Three fish were used for analysis of whole-body composition, and six fish were measured for individual body weight and length, and then dissected to obtain viscera, liver, intraperitoneal fat and dorsal white muscle samples, and protein productive value (PPV), condition factor (CF), viscerosomatic index (VSI), hepatosomatic index (HSI) and intraperitoneal fat ratio (IPF) were calculated. The dissected livers of the remaining three fish per tank were immediately frozen in liquid nitrogen and stored at −80°C until used for enzymatic analysis of hepatic GPx, glutathione reductase (GR) and measurement of hepatic malondialdehyde (MDA) content.
Proximate analysis consisted of determining moisture, protein, lipid and ash contents of feeds and various tissues using standard methods (AOAC 1995). Crude protein (N × 6.25) was determined by following the Kjeldahl method after an acid digestion by an auto-Kjeldahl System (1030-Auto-analyzer; Tecator, Sweden). Crude lipid was determined by following the ether-extraction method using Soxtec System HT (Soxtec System HT6; Tecator, Sweden). Moisture was determined by oven drying at 105°C for 24 h and ash was determined using a muffle furnance at 550°C for 24 h.
The Se concentrations of the experimental diets were determined by hydride generation atomic absorption spectrophotometer (Z-5000; Hitachi Ltd., Tokyo, Japan) according to Tinggi (1999). The Y2O3 contents of the feed and faeces were determined by inductively coupled plasma atomic emission spectrophotometer (ICP; model: IRIS Advantage (HR), Thermo JarrelAsh Corporation, Boston, MA, USA) after wet digestion with nitric acid and perchloric acid.
Apparent digestibility coefficients (ADC) of dry matter were derived from the equation ADC (%) = (1−YD/YF) × 100, where YD was concentration of Y2O3 in diets, and YF was concentration of Y2O3 in faeces (NRC 1993).
Liver MDA measurement
The measurement of MDA was carried out using the method of Rueda-Jasso, Conceicao, Dias, De Coen, Gomes, Rees, Soares, Dinis and Sorgeloos (2004) with the following modifications. A volume of 500-μL homogeneous tissue sample was mixed with 400-μL 15% thichloroacetic acid and 800-μL thiobarbituric acid (TBA, 0.67%, diluted in 0.3 M NaOH). After heating at 100°C for 30 min and cooling to 25°C, the protein precipitate was removed by centrifugation (860 g at 4°C, for 10 min). The supernatant was recovered and measured in a spectrophotometer at 532 nm. The sample concentrations were calculated from a standard curve established with TBA–MDA. The results were expressed as μmol of MDA g−1 liver tissue.
Hepatic enzymatic analysis
Samples of liver were homogenized in nine volumes of 20 mM phosphate buffer pH 7.4, 1 mM EDTA and 0.1% Triton X-100, the homogenates were centrifuged (860 g at 4°C, for 10 min) to remove debris, and the resultant supernatants were used directly for enzyme assays. GPx activity was analysed with the procedure of Bell, Cowey, Adron and Shanks (1985). Glutathione reductase activity was assayed as described by Racker (1955). The protein concentration of the supernatant solutions was determined by following the biuret method using bovine serum albumin as the standard.
Statistical analysis
Results were presented as mean ± SD of three replicates. All data were subjected to one-way anova using spss 11.5 for Windows (SPSS Inc., Chicago, IL, USA). When significant difference (P < 0.05) was found, a Duncan's new multiple range test was used to rank the groups.
Results
Growth, feed utilization and biometric parameters
The survival rates in all treatments were quite high (above 95%) at the conclusion of the 8-week study. The highest WG was obtained in fish-fed diets with 1.60 mg Se kg−1, which was significant higher (P < 0.05) than the basal diet with 0.97 mg Se kg−1 and did not differ significantly (P > 0.05) with the other treatments (Table 2). FCR, PER, PPV, condition factor (CF), viscerosomatic index (VSI) and intraperitoneal fat ratio (IPF) were not significantly impacted (P > 0.05) by dietary treatments. There was an increasing trend in ADCs of dry matter with Se supplementation diets than the basal diet; however, the differences were not significant (P > 0.05) between them. Fish-fed diets with 1.42 mg Se kg−1 had significantly lower (P < 0.05) hepatosomatic index (HSI) than the other treatments, with no marked differences (P > 0.05) between them.
| Se supplementation levels (mg Se kg−1) | 0 (0.97) | 0.2 (1.17) | 0.4 (1.42) | 0.6 (1.60) | 0.8 (1.85) | 1 (2.06) |
|---|---|---|---|---|---|---|
| IBWa | 4.98 ± 0.04 | 4.96 ± 0.06 | 4.89 ± 0.02 | 4.97 ± 0.07 | 4.93 ± 0.06 | 4.96 ± 0.04 |
| WGb | 618.06 ± 20.12b | 639.82 ± 14.05ab | 627.61 ± 19.29ab | 669.29 ± 24.62a | 644.85 ± 11.24ab | 653.59 ± 17.20ab |
| FCRc | 1.02 ± 0.03 | 1.08 ± 0.10 | 1.03 ± 0.04 | 1.04 ± 0.08 | 1.06 ± 0.06 | 0.97 ± 0.06 |
| PERd | 1.95 ± 0.20 | 2.01 ± 0.16 | 1.86 ± 0.12 | 1.94 ± 0.13 | 1.86 ± 0.10 | 2.01 ± 0.04 |
| PPVe | 0.39 ± 0.03 | 0.39 ± 0.03 | 0.35 ± 0.03 | 0.37 ± 0.06 | 0.35 ± 0.02 | 0.39 ± 0.01 |
| ADCf | 68.14 ± 2.95 | 68.57 ± 0.68 | 69.65 ± 3.21 | 69.44 ± 0.51 | 68.89 ± 1.55 | 71.32 ± 1.37 |
| CFg | 1.93 ± 0.06 | 1.98 ± 0.07 | 2.05 ± 0.05 | 2.02 ± 0.08 | 1.93 ± 0.06 | 2.04 ± 0.06 |
| VSIh | 5.80 ± 0.21 | 6.01 ± 0.14 | 5.92 ± 0.18 | 5.99 ± 0.23 | 5.86 ± 0.28 | 5.97 ± 0.04 |
| HSIi | 1.00 ± 0.10a | 1.09 ± 0.08a | 0.83 ± 0.15b | 1.02 ± 0.06a | 0.91 ± 0.07a | 0.94 ± 0.18a |
| IPFj | 1.11 ± 0.04 | 1.11 ± 0.25 | 1.12 ± 0.10 | 1.16 ± 0.13 | 1.08 ± 0.06 | 1.23 ± 0.07 |
- Values represents means ± SD of triplicates, and values with the same row with different letters were significantly different (P < 0.05, one-way anova).
- a IBW, initial mean body weight (g).
- b Weight gain = 100 × (final mean body weight − initial mean body weight)/initial body weight.
- c Feed conversion ratio = g dry feed consumed/g wet weight gain.
- d Protein efficiency ratio = fish wet weight gain/protein intake.
- e Protein productive value = 100 × (final body protein − initial body protein)/total protein fed.
- f Apparent digestibility coefficient of dry matter (%).
- g Condition factor = fish weight (g) × 100/body length3 (cm).
- h Viscerosomatic index = 100 × (viscera weight/whole body weight).
- i Hepatosomatic index = 100 × (liver weight/whole body weight).
- j Intraperitoneal fat ratio = 100 × (IPF weight/whole body weight).
Whole body, liver and muscle composition
The results of whole body, liver and muscle composition were presented in Table 3. Whole-body lipid, protein contents and muscle composition (moisture, lipid and protein) were not markedly affected (P > 0.05) by dietary treatments. Fish-fed diets with 1.17 and 1.60 mg Se kg−1 obtained significantly lower (P < 0.05) whole-body moisture contents than the other treatments, with no marked differences (P > 0.05) between both of them. The liver moisture content of fish-fed diets with 1.17 mg Se kg−1 was significantly lower (P < 0.05) than that of fish-fed diets with 1.42 mg Se kg−1, but it was significantly higher (P < 0.05) than the other treatments. Fish-fed diets with ≥1.42 mg Se kg−1 obtained higher liver lipid contents than treatments with lower dietary Se levels. The liver protein content in fish-fed diets with 1.42 mg Se kg−1 was significantly lower (P < 0.05) than the other treatments, with on marked differences (P > 0.05) between them.
| Se supplementation levels (mg Se kg−1) | 0 (0.97) | 0.2 (1.17) | 0.4 (1.42) | 0.6 (1.60) | 0.8 (1.85) | 1 (2.06) |
|---|---|---|---|---|---|---|
| Whole body | ||||||
| Moisture | 71.87 ± 0.22a | 71.34 ± 0.28b | 71.85 ± 0.08a | 71.39 ± 0.19b | 72.29 ± 0.16a | 72.03 ± 0.42a |
| Lipid | 5.61 ± 0.24 | 5.85 ± 0.50 | 5.85 ± 0.70 | 5.64 ± 0.33 | 5.31 ± 0.31 | 5.74 ± 0.37 |
| Protein | 19.05 ± 0.91 | 19.37 ± 0.44 | 18.70 ± 0.72 | 18.96 ± 0.67 | 18.84 ± 0.07 | 19.35 ± 0.85 |
| Liver | ||||||
| Moisture | 77.30 ± 1.65a | 77.00 ± 0.86b | 77.49 ± 2.27c | 76.95 ± 1.31a | 77.10 ± 0.28a | 77.00 ± 0.65a |
| Lipid | 1.61 ± 0.23b | 1.53 ± 0.18b | 2.16 ± 0.11a | 1.87 ± 0.28ab | 2.29 ± 0.32a | 2.11 ± 0.26a |
| Protein | 13.38 ± 0.78a | 12.33 ± 0.75a | 9.93 ± 0.46b | 12.86 ± 0.22a | 13.40 ± 0.50a | 13.24 ± 1.07a |
| Muscle | ||||||
| Moisture | 73.57 ± 0.17 | 72.76 ± 0.09 | 74.60 ± 0.09 | 73.78 ± 0.09 | 73.64 ± 0.01 | 73.69 ± 0.08 |
| Lipid | 1.06 ± 0.08 | 1.13 ± 0.31 | 0.70 ± 0.27 | 0.81 ± 0.17 | 1.19 ± 0.20 | 1.04 ± 0.29 |
| Protein | 24.46 ± 0.29 | 25.26 ± 0.19 | 23.17 ± 0.10 | 23.82 ± 0.20 | 24.72 ± 0.08 | 24.57 ± 0.07 |
- Values represents means ± SD of triplicates, and values with the same row with different letters were significantly different (P < 0.05, one-way anova).
Hepatic MDA content and enzymatic activities
The data of hepatic MDA concentration and enzymatic activities were shown in Table 4. Hepatic MDA content was not markedly impacted (P > 0.05) by dietary treatments. Hepatic GPx activities were highest in fish-fed diets with ≥1.85 mg Se kg−1, followed by 1.42 and 1.60 mg Se kg−1, then 1.17 mg Se kg−1 and lowest in fish-fed the basal diet. Hepatic GR activities of fish-fed diet with ≥1.60 mg Se kg−1 were significantly lower (P < 0.05) than those of fish-fed diet with 0.97 (basal diet) and 1.17 mg Se kg−1.
| Se supplementation levels (mg Se kg−1) | MDA (μmol g−1) | GPx (nmol NADPH min−1 mg protein−1) | GR (nmol NADPH min−1 mg protein−1) |
|---|---|---|---|
| 0 (0.97) | 0.16 ± 0.02 | 27.8 ± 1.2d | 5.52 ± 0.45a |
| 0.2 (1.17) | 0.14 ± 0.01 | 32.4 ± 2.2c | 5.25 ± 0.26a |
| 0.4 (1.42) | 0.17 ± 0.02 | 34.1 ± 1.8b | 5.11 ± 0.41ab |
| 0.6 (1.60) | 0.15 ± 0.02 | 33.2 ± 1.5b | 4.7 ± 0.33b |
| 0.8 (1.85) | 0.15 ± 0.01 | 37.5 ± 2.0a | 3.7 ± 0.42c |
| 1 (2.06) | 0.15 ± 0.04 | 40.8 ± 2.6a | 4.2 ± 0.32bc |
- Values represents means ± SD of triplicates and values with the same row with different letters were significantly different (P < 0.05, one-way anova).
Discussion
Hamilton (2003) reviewed residue-based Se toxicity threshold for freshwater fish and suggested that dietary Se threshold should be as low as 3 mg Se kg−1 diet. For most fish species, the requirement range is 0.25–1.42 mg Se kg−1 diet from sodium selenite (Hilton et al. 1980; Gatlin & Wilson 1984; Cotter, Craig & McLean 2008). The minimum Se requirement of grouper was 0.7 mg kg−1 from selenomethionine, and growth depression was evidenced with ≥2.02 mg Se kg−1 (Lin & Shiau 2005). Therefore, a relative narrow range of Se levels (0.97–2.06 mg kg−1) was chosen in the present study. In this study, the highest WG was obtained in fish-fed diets with 1.60 mg Se kg−1, which was significant higher than the basal diet with 0.97 mg Se kg−1 and did not differ significantly with the other treatments (Table 2), which indicated that dietary Se supplementation was necessary and the optimal dietary Se level may be 1.60 mg kg−1 based on WG. The discrepancies of optimal Se level in various studies may be attributed to fish species-dependent physiological difference, the Se concentrations in rearing water, Se source (organic or inorganic) and the amount of dietary vitamin E (Gatlin & Wilson 1984; Wang & Lovell 1997; Lin & Shiau 2005; Abdel-Tawwab, Mousa & Abbass 2007). Hilton, Hodson and Slinger (1982) reported that rainbow trout could readily take up water-borne Se at such low concentrations. Organic Se (selenomethionine or selenoyeast) has been reported to have higher bioavailability than the inorganic Se (sodium selenite) for Atlantic salmon (Bell & Cowey 1989; Lorentzen, Maage & Julshamn 1994), channel catfish (Wang & Lovell 1997) and crucial carp (Wang et al. 2007). However, independent of Se source (organic or inorganic), the optimal Se level was 1.42 mg kg−1 for hybrid striped bass (Cotter et al. 2008) and an almost equal effect in related to immunity and disease resistance was evident in prawn (Chiu, Hsieh, Yeh, Jian, Cheng & Liu 2010). The digestibility of residual Se from diets containing fishmeal is 46–54%, which is significantly lower than from semi-purified diets supplemented with organic Se compounds (Rider, Davies, Jha, Clough & Sweetman 2010). In this study, inorganic sodium selenite was used as Se source and the Se concentration of the rearing freshwater was not detectable, the relative higher optimum Se level (1.60 mg kg−1) was conceivable. In accordance with the results reported in crucial carp (Wang et al. 2007; Zhou, Wang, Gu & Li 2009) and in hybrid striped bass (Cotter et al. 2008), the feed utilization (FCR, PER and PPV) was not affected by Se supplementation in the present experiment. There was an increasing trend in ADCs of dry matter with Se supplementation diets than the basal diet, but the differences were not significant. Se is included in the type 1 iodothyronine 5′-deiodinase, which interacts with iodine to prevent abnormal hormone metabolism (Foster & Sumar 1997). The effect of selenium on growth performances of largemouth bass might be associated with these forenamed functions of Se.
In line with the study of Elia, Prearo, Pacini, Dorr and Abete (2011), CF evidenced no significant differences in relation to dietary Se concentrations. Hilton et al. (1980) reported that no significant deviation in HSI was detected in Rainbow trout raised on diets containing 0–13 mg Se kg−1. The reason for significant smaller HSI value obtained with 1.42 mg Se kg−1 than the other treatments was unclear in this study. In the present experiment, muscle composition were not affected by dietary Se level, and similar results were obtained in crucial carp employed organic or inorganic Se sources (Wang et al. 2007; Zhou et al. 2009). Proximate analysis of the whole body of African catfish indicated a decline in lipid content of fish fed on 0.5 g organic Se kg−1 and an inverse relationship between total protein and lipid contents (Abdel-Tawwab et al. 2007). In this study, only whole-body moisture content changed with dietary Se levels. Interestingly, fish fed diets with ≥1.42 mg Se kg−1 obtained higher liver lipid contents than treatments with lower dietary Se levels. It seemed that higher Se concentration may exert an impact on hepatic lipid metabolism, and the specific mechanism of Se on hepatic fatty acid synthesis, lipoprotein absorption and secretion deserves further investigations.
MDA, a product of lipid peroxidation, is frequently measured as a marker of oxidative stress (Esterbauer, Schaur & Zollner 1991; Dotan, Lichtenberg & Pinchuk 2004). In this study, the unchanged hepatic MDA with dietary Se concentration may be attributed to the ample dietary antioxidant (such as vitamin E: 400 IU kg−1, vitamin C phosphate ester: 1000 mg kg−1), which reacted with the possibly endogenous reactive oxygen species (ROS) (Fontagné, Bazin, Brèque, Vachot, Bernarde, Rouault & Bergot 2006) and may mask the antioxidant effect of Se on oxidative stress. To date, the vitamin E requirement of largemouth bass was unclear, and the vitamin premix in this study was prepared according to Wanakowat, Boonyaratpalin, Pimoljinda and Assavaaree (1989). It is well established that vitamin E and Se function synergistically in animal tissues to form an important antioxidant defence system (Wise, Tomasso, Gatlin, Bai & Blazer 1993; Lin & Shiau 2009). Determination of the GPx activity can be an effective way to estimate the bioavailability of Se (Lin & Shiau 2005; Wang & Xu 2008). Hepatic GR activities would also reflect the Se supplementation level up to the optimum level (Lin & Shiau 2005). In the present experiment, hepatic GPx and GR activities increased and decreased with increasing dietary Se concentration, respectively, and both reached a plateau with ≥1.85 mg Se kg−1, which further confirmed the necessity of Se supplementation. Similar responses were reported in many fish species for activities of GPx (Hilton et al. 1980; Gatlin & Wilson 1984; Wang & Lovell 1997; Lin & Shiau 2005) and GR (Lin & Shiau 2005; Elia, Dorr, Taticchi, Prearo & Abete 2007), with increasing Se concentration unless at an excess level. Adequate dietary Se (1.32 mg kg−1) was reported to increase the mRNA expression of GPx in abalone (Wu, Mai, Zhang, Ai, Xu, Wang, Ma & Liufu 2010).
Conclusion
The overall results suggested that the highest Se concentration (2.06 mg kg−1) employed in our study was not harmful for largemouth bass, and the optimal dietary level should be 1.60–1.85 mg Se kg−1 from sodium selenite, at a dietary vitamin E level of 400 IU kg−1.
Acknowledgment
The authors are grateful for the financial support by National Natural Science Foundation of China (NSFC, no. 30972264) and the coworkers for their help in sampling.
