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J APPL POULT RES 2009. 18:193-202. doi:10.3382/japr.2008-00019
© 2009 Poultry Science Association
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Research Reports

The effects of dietary oxidized fat and selenium source on performance, glutathione peroxidase, and glutathione reductase activity in broiler chickens1

J. R. Upton, F. W. Edens2 and P. R. Ferket

Department of Poultry Science, North Carolina State University, Raleigh 27695

2 Corresponding author: fwedens{at}mindspring.com


   SUMMARY
TOP
 SUMMARY
DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
Normal or elevated selenium status of broilers, which is influenced by dietary selenium sources, improves the bird’s ability to overcome the adverse effects of reactive oxygen metabolites. The objective of this study was to evaluate the effects of feeding graded levels of peroxidized poultry fat on blood and hepatic glutathione peroxidase (GSH-Px), and hepatic glutathione reductase activity in broiler chickens fed either inorganic sodium selenite (SEL) or organic selenium enriched in the organic selenium yeast product Sel-Plex (SP). Nine starter diets, varying in levels of oxidized fat (0, 3, and 6 mEq/kg) and dietary selenium sources, were fed to 360 male chicks from hatch to 21 d of age. Sel-Plex or SEL was added to the basal diet to provide either 0 or 0.2 ppm of supplemental selenium in the diets. Blood and hepatic samples were obtained for each treatment group at 21 d of age. Neither peroxidized fat nor selenium source significantly altered the activity of hepatic glutathione reductase (P ≤ 0.05). Blood GSH-Px was influenced significantly by both fat and selenium source (P ≤ 0.05), but the fat x selenium source interaction was not significant (P ≥ 0.3). A selenium source effect on the hepatic GSH-Px activity (P ≤ 0.05) was evidenced by higher GSH-Px activity, even in the basal diet with no added peroxidized fat. An increase in GSH-Px activity was seen in the erythrocyte and hepatic samples in both the SEL and SP treatments when peroxidized fat was given at 3 mEq/kg, but in the erythrocytes and in the hepatic tissues from SEL-supplemented birds, there was an apparent inhibition of GSH-Px activity. This inhibition was not seen in the hepatic tissue samples from SP-fed birds. Because elevated GSH-Px activity is indicative of oxidative stress, it was concluded that dietary SP supplementation resulted in better selenium and redox status in broilers than did SEL. These results indicate that the dietary selenium supplied in an organic form (selenium yeast as SP) improved the selenium and redox status in broilers, leading to greater resistance to oxidative stress than when the inorganic form of selenium (SEL) was fed.

Key Words: selenium • selenium yeast • Sel-Plex glutathione peroxidase • glutathione reductase • oxidized fat • broiler


   DESCRIPTION OF PROBLEM
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
Efficient broiler production is facilitated by the feeding of well-balanced diets to highly productive lines of birds. Natural antioxidants play an important role in maintaining bird health, productivity, and reproductive characteristics. Oxidation of fats in feeds is known to produce undesirable results, such as loss of fat-soluble vitamins, degradation of xanthophyll and β-carotene, generation of malodors, palatability problems, and even certain deficiency syndromes [1]. Products of lipid oxidation or destruction of the vital nutrients have been implicated as causes of steatitis in swine and cats, encephalomalacia in poultry, and, symptomatically, may become manifest as exudative diathesis, muscular dystrophy, necrotic tissue in various organisms, and poor fertility and hatchability [2–7]. These are readily detectable consequences of feeding severely oxidized feed, but there are other, more subtle consequences, such as reduced BW and feed efficiency [1]. Although these effects can be difficult to detect, and their economic consequences are even more difficult to quantify; antioxidants are typically added to feeds to help protect against these problems [8] and others.

Many of the oxidative processes that occur in animal diets have been characterized, and the mechanisms by which oxidative processes occur have been defined [1]. Free radicals formed by unsaturated fatty acids react with oxygen to form peroxides, which serve as the entry point to a multitude of reactions, producing numerous by-products and the decomposition of vital nutrients contained in the feed [1].

An excessive production of oxygen-based free radicals, especially the highly toxic hydroxyl radical (·OH), leads to indiscriminate damage to macromolecules, including DNA, proteins, and membrane lipids [9]. To resist damage caused by the presence of reactive oxygen metabolites, organisms have evolved nonenzymatic and enzymatic antioxidant defense mechanisms. The nonenzymatic antioxidant defense mechanisms include direct free radical scavengers, such as vitamin E, vitamin C, and glutathione [10], and several iron chelators, such as ferritin [9]. The enzymatic defenses are provided by superoxide dismutase, glutathione peroxidase (GSH-Px), glutathione reductase (GR), and glucose-6-phosphate dehydrogenase, the limiting factor in nicotinamide adenine dinucleotide phosphate (NADPH) production [11]. Both GSH-Px and GR are involved in the redox cycling of glutathione [12]. Glutathione peroxidase oxidizes glutathione (GSSG) while reducing toxic endogenous and exogenous peroxides, whereas GR restores the reduced glutathione (GSH) status [12].

Selenium is an essential ultratrace element supplemented in poultry diets for maintenance of the GSH-Px cellular antioxidant system. In the chicken, selenium deficiency, especially in combination with low vitamin E supply, is responsible for the development of a range of diseases, including exudative diathesis [13, 14], nutritional encephalomalacia [15, 16], and nutritional pancreatic atrophy [17–19]. It has been postulated that lipid peroxidation is important in the etiology of these diseases [20]. In this regard, progress in understanding the importance of different selenoproteins as elements of antioxidant systems could aid in explaining some of the clinical signs of the diseases. Furthermore, the importance of the functions of selenoproteins in the antioxidant system needs to be investigated further. The experiment reported herein was conducted to examine the positive responses of organic selenium supplementation previously seen in broiler chickens [21–25] and to determine whether the functions of the selenoproteins GR and GSH-Px are partially responsible for the favorable results that have been documented. In addition, the study model incorporates the use of oxidized fat to induce a practical nutritional stress factor.


   MATERIALS AND METHODS
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
Animal Welfare
This project was approved and conducted under the supervision of the North Carolina State University Animal Care and Use Committee, which has adopted Animal Care and Use Guidelines governing all animal use in experimental procedures.

Birds and Experimental Treatments
One-day-old male broilers of a commercial strain [26] were obtained from the North Carolina Agricultural Research Service Poultry Field Research Laboratory. On the day of hatching, 360 chicks were individually neck-banded [27] and placed in 36 groups of 10, and BW were recorded. The chicks were randomly placed in battery brooder cages [28], and feed was added to all-stainless steel pans. Water in stainless steel pans and feed were available ad libitum to all chicks in the experiment. The 9 starter diets consisted of varying levels of oxidized fat (approximately 0, 3, and 6 mEq of oxidized poultry fat/kg of finished feed) and dietary selenium sources, provided as either 1) no supplemental selenium, 2) organic selenium as selenium yeast (Sel-Plex; SP) [29], or 3) sodium selenite (SEL) [30]. Unstabilized poultry fat was used, and fats were oxidized by heating them to 80°C with aeration and stirring until the desired level of peroxide production was reached (Figure 1Go). Peroxide levels were determined by performing peroxide value tests [31]. The composition of the basal diet used in this experiment is presented in Table 1Go. The feed was stored at –15°C until feed was given to the birds to reduce the potential of further autoxidation. Body weights and FCR were determined on a weekly basis for 3 wk.


Figure 1
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  		Figure 1. Peroxidation of poultry fat incorporated into experimental diets.

 

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  		Table 1. Diet composition
 
At 3 wk of age, 5 birds from each treatment were randomly chosen for blood and tissue sampling. Blood samples were obtained via cardiac puncture and collected in heparinized vials for storage for both whole-blood and erythrocyte analysis. Chickens were then killed by carbon dioxide asphyxiation, and hepatic samples (1 g) were collected for enzyme analyses.

Cytosolic Protein Extraction
Dissected liver tissues were placed directly into ice-cold protein sample buffer (50 mM potassium phosphate buffer, pH 7.4, 1 mM phenylmethyl sulfonyl fluoride, and 5 mM EDTA). A sample (0.5 to 1.0 g) from each liver was washed 3 times to rinse blood from the surface of the samples, and samples were then minced in 4 mL of cold protein sample buffer for protein extraction. Subsequently, the samples were homogenized [32] and the resulting homogenates were centrifuged at 6,500 x g for 30 min. Supernatants were removed and further centrifuged at 50,000 x g for 30 min. Total protein content was measured [33, 34] by using a linear range (r2 ≥ 0.98) from 0 to 80 µg/mL of BSA.

Whole-Blood GSH-Px and Erythrocyte GR Analysis
Whole-blood GSH-Px activity was determined in triplicate samples by a spectrophotometric microplate method based on the decrease in NADPH absorbance at 340 nm (A340) modified for a plate reader [35]. The rate of decrease at A340 is directly proportional to the GSH-Px activity [36].

Erythrocyte GR
The erythrocyte GR was determined by a spectrophotometric method based on the measurement of the rate of NADPH oxidation at A340 with a synchronized reduction of GSSG by using a commercial kit system [36]. Blood samples were prepared following the manufacturer’s directions [36].

Hepatic GSH-Px and GR Analysis
The GSH-Px activity in liver cytosolic protein extract was determined by a spectrophotometric method [35] based on the decrease in NADPH absorbance at A340. The rate of absorbance decrease at A340 is directly proportional to the GSH-Px activity. Hepatic GR activity in liver cytosolic protein extract was determined spectrophotometrically based on the measurement of the rate of NADPH oxidation at A340 with synchronized reduction of GSSG by using a commercial kit system [36].

Statistical Analysis
Data from the experiment were subjected to statistical analysis by using the GLM procedures of SAS [37]. A 3 x 3 factorial arrangement of treatments was implemented, with the main effects being selenium supplementation and level of oxidized fat inclusion, and interactions of main effects were determined. Differences among main effect means and interactions (P ≤ 0.05) were assessed via least squares comparisons [37].


   RESULTS AND DISCUSSION
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
Performance
Mean BW at 7, 14, and 21 d and FCR from 1 to 21 d were not affected significantly by selenium source or fat oxidation (Table 2Go). A difference in BW gain among treatments was expected, but in retrospect, the lack of response to selenium treatment could be a result of background selenium levels in the feed. The background selenium level (0.115 ppm) in our experimental basal diets should have been barely adequate to prevent development of any extreme, classical signs of selenium deficiency during the experimental period if the birds did not experience severe oxidative stressors [21]. It has been reported that the minimum dietary requirement of selenium for chickens is 0.1 ppm [38], and as little as 0.05 ppm of selenium in feed can sustain GSH-Px activity in many animal species [39–41]. Thus, it is possible that, in both selenium-supplemented and non-selenium-supplemented birds, there was sufficient selenium just from the background, which would be in the organic form, to stimulate synthesis of basal GSH-Px activity to prevent differences in BW gains among peroxidized fat treatments within and among selenium sources. The BW of SP-fed birds was 48 g less than the BW of SEL-fed birds, and this might have been due to more rapid feathering of the SP-fed birds, which is a characteristic of organic selenium-fed broilers [22, 42]. Although feathering rate was not among the objectives of this study, it was noted that birds fed no supplemental selenium and SP-fed birds had subjectively greater feather growth than did SEL-fed broilers. Increased feather growth results from redistribution of energy from protein accretion in muscle to protein deposition in the feathers, and therefore results in slightly smaller BW gain rates until a full feather coat is realized between 4 and 5 wk of age. When maturation of the feather coat is realized, an increased rate of gain and improved FCR in SP-fed vs. SELfed birds can be demonstrated [21–23, 42]. The effect of feeding high levels of peroxidized fat, although controversial, has been found to affect performance adversely [1, 39, 43–45], but conflicting results have also been reported [46–53]. In this study, we did not find any significant adverse effect of peroxidized fat on BW or FCR. Nevertheless, it has been reported that 4 mEq or less of peroxidized fat caused no BW gain suppression in young chickens, but dietary levels of 7 mEq of peroxidized fat or greater caused inhibition of BW gain [45]. In this study, it is possible that sufficient peroxidized fat (6 mEq) was not given to suppress BW gain.


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  		Table 2. Influence of selenium source and peroxidized fat on broiler performance from hatch to 3 wk of age1
 
GSH-Px
The effect of dietary supplementation with different selenium sources and peroxidized fat on GSH-Px activity in erythrocytes is shown in Table 3Go. Erythrocyte GSH-Px was influenced significantly by both peroxidized fat (P ≤ 0.01) and selenium source (P ≤ 0.05), but the fat x selenium source interaction was not significant (P ≥ 0.3).


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  		Table 3. Influence of selenium source and peroxidized fat on erythrocyte glutathione peroxidase (GSH-Px) activity1
 
The effect of dietary supplementation with selenium source and peroxidized fat on hepatic GSH-Px is shown in Table 4Go. There was a selenium source effect on the hepatic GSH-Px activity (P ≤ 0.05), with selenium supplementation increasing GSH-Px activity even in the basal diets with no peroxidized fat.


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  		Table 4. Influence of selenium source and peroxidized fat on hepatic glutathione peroxidase (GSH-Px) activity1
 
The GSH-Px activity in erythrocytes and hepatic tissue yielded some unexpected results. In the non-selenium-supplemented birds given 3 mEq of peroxidized fat, GSH-Px activities in both erythrocytes and hepatic tissue were not statistically different from those of selenium-supplemented diets, although there was a numeric increase in activity. These results were in agreement with observations reported previously [40, 54]. The reason for this effect resides in the reactivity of this enzyme to the presence of the background dietary selenium (0.115 ppm), which should facilitate induction of selenium-dependent GSH-Px activity at a nominal level. The feeding of peroxidized fat at 6 mEq/kg caused an inhibition of GSH-Px activity in the erythrocytes and in the hepatic samples from SEL-fed birds. When selenium in the diet is insufficient or when animals are confronted with oxidative stressors that overwhelm the selenium-dependent antioxidant system, it is possible that selenium-dependent GSH-Px can be inhibited [55]. If selenium is sufficient in the diet, the GSH-Px enzyme activity increases, and when substrate increases, enzyme levels increase to bind the substrate, reaching maximum steady-state rates. Therefore, in the non-selenium-supplemented treatments, the decrease in GSH-Px activity found in blood and hepatic samples of those birds given 6 mEq of peroxidized fat, as compared with those given 3 mEq of peroxidized fat, might be an indication of an antioxidant system that was overwhelmed [55]. Inhibition of erythrocyte and hepatic GSH-Px in SEL-supplemented chickens and erythrocytes of SP-supplemented chickens given 6 mEq of peroxidized fat was evident (Table 3Go). However, in the liver of SP-supplemented chickens given 6 mEq of peroxidized fat, GSH-Px activity was maximized, as compared with the 0- and 3-mEq peroxidized fat treatments (Table 4Go). In contrast to these observations, Cabel et al. [45] reported that feeding a high level of oxidized vegetable fat in broiler diets did not cause depression in either selenium-dependent or selenium-independent GSH-Px.

The higher activities of hepatic GSH-Px in selenium-supplemented animals than in non-selenium-supplemented animals is in accordance with various literature reports [13, 56–58] that have demonstrated significant differences among different selenium formulations in their capacity to meet selenium requirements of the animals. In addition, differences are reported for the same compounds in their effectiveness to protect against many of the manifestations of selenium deficiency. Conflicting conclusions in these reports are likely due to the confounding effect of the background selenium levels in the experimental diets. If the background levels of selenium are high, the effects of selenium supplementation can be masked. Comparative efficiency of dietary selenium compounds, especially SEL and feed-grade selenomethionine, in raising GSH-Px activity is the subject of much controversy. A roughly equal availability of selenium from selenomethionine and SEL in chicks [57] and rats [41] has been documented. However, a review of the literature [59] indicated that selenomethionine in yeast (SP) has greater availability than SEL.

Differences in the way the body handles organic and inorganic selenium contribute to the ultimate activity of selenium-dependent GSH-Px. Inorganic selenium as SEL forms loose associations with thiol groups and is easily lost from the body under times of stress [59], but organic selenium as selenomethionine is chiefly incorporated into body proteins [59]. As muscle protein is turned over, the stored selenomethionine is made available as a free amino acid and is recycled into selenoproteins and newly synthesized structural proteins. In times of oxidative stress, the proteasome, a cytosolic organelle, increases the rate of protein degradation and increases amino acid availability for synthesis of other proteins and selenoproteins. In fast-growing, high-yielding broiler chickens, the need for additional antioxidant enzyme is apparent because of the high rate of muscle accretion and its metabolism [59]. Thus, if the inorganic form of selenium can be lost rapidly from the body under periods of extreme stress, it is not surprising that when faced with an extreme stressor such as high dietary levels of peroxidized fat, insufficient activity of GSH-Px might become manifest. Because elevated GSH-Px is also indicative of responsiveness to oxidative stress [58, 60], the dietary SP supplementation reported here resulted in a better selenium status than in birds fed SEL.

GR
The effect of dietary selenium supplementation and peroxidized fat inclusion on GR activity in hepatic tissues is shown in Table 5Go. In general, the GR activity in hepatic tissues of selenium-supplemented animals was not statistically different (P ≥ 0.05) from that in tissues of non-selenium-supplemented animals, indicating that selenium supplementation alone did not alter the activity of this enzyme, similar to other reported observations [60–64]. Linear regression analysis of the data failed to indicate any significant correlation between GSH-Px and GR activities.


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  		Table 5. Influence of selenium source and peroxidized fat on hepatic glutathione reductase (GR) activity1
 
However, GR, which functions in the bioreductive activation of glutathione (GSH) from glutathion disulfide (GSSG), is regulated by the redox status of the cell itself [65]. Glutathione can be deactivated by its own reducing agent, NADPH, and it can be reactivated by elevation of oxidized glutathione (GSSG). Therefore, the activity of GR appears to reflect the cell redox status during oxidative stress. Thus, if GSH-Px activity is sufficient, GSSG is produced at levels to stimulate GR and prevent its deactivation by NADPH. Under the conditions of this investigation, the GR activity in the liver was adequate to provide GSH for reduction of additional peroxides, even in birds given no supplemental selenium. Unfortunately, the background selenium (0.115 ppm) in the experimental basal feed was great enough to sustain GSH-Px activity at a level to overshadow the effects of supplemental selenium sources. Nevertheless, when one examines the GSH-Px activities induced by organic selenium in SP, it suggests that the improved availability of organic selenium in SP was able to continue to provide stimulus for elevated GSH-Px activity in the liver, even in the face of peroxide levels that tended to inhibit GSH-Px in young SEL-fed chickens given high levels of peroxidized poultry fat in the feed.


   CONCLUSIONS AND APPLICATIONS
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
REFERENCES AND NOTES
 

  1. Neither selenium source nor peroxidized fat level significantly altered the performance parameters or hepatic GR activity.
  2. Elevated GSH-Px, an indicator of the presence of oxidative stress, was observed in response to dietary inclusion of peroxidized poultry fat.
  3. Erythrocyte GSH-Px activity was elevated by addition of 3 mEq of peroxidized poultry fat by both inorganic and organic selenium supplements. In birds fed 6 mEq of peroxidized poultry fat, GSH-Px activity was lower than in those birds fed 3 mEq of peroxidized fat, suggesting that the selenium-dependent antioxidant system was overwhelmed by the lipid peroxides.
  4. When SP-fed birds were given 6 mEq of peroxidized fat, the hepatic GSH-Px activity did not show as large a decrease as that seen in SEL-fed birds, suggesting that the selenium from organic selenium in SP was more available for incorporation into selenium-dependent GSH-Px enzyme than was SEL.
  5. Organic selenium, as compared with inorganic selenium, improves the selenium status in broilers because it is stored in protein until liberated by natural turnover of the protein.


   FOOTNOTES
 
1 Mention of the trade name of a product in this publication does not indicate endorsement of that product nor does it indicate criticism of a similar product(s) by the North Carolina Agricultural Research Service. Back


   REFERENCES AND NOTES
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 

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