J. Anim Sci. 2008. 86:3100-3109. doi:10.2527/jas.2007-0595
© 2008 American Society of Animal Science
Effect of dietary supplementation with selenium-enriched yeast or sodium selenite on selenium tissue distribution and meat quality in beef cattle1
D. T. Juniper*,2,
R. H. Phipps*,
E. Ramos-Morales* and
G. Bertin
* Animal Science Research Group, School of Agriculture, Policy and Development University of Reading, Earley Gate, Reading RG6 6AR, UK; and
Alltech France, EU Regulatory Affairs Department, 14 Place Marie-Jeanne Bassot, 92300 Levallois-Perret, France
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Abstract
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The objective was to determine the concentration of total Se and the proportion of total Se comprised as selenomethionine (SeMet) and selenocysteine (SeCys) in postmortem tissues of beef cattle offered diets containing graded additions of selenized enriched yeast (SY; Saccharomyces cerevisiae CNCM I-3060) or sodium selenite (SS). Oxidative stability and tissue glutathione peroxidase (GSH-Px) activity of edible muscle tissue were assessed 10 d postmortem. Thirty-two beef cattle were offered, for a period of 112 d, a total mixed ration that had been supplemented with SY (0, 0.15, or 0.35 mg of Se/kg of DM) or SS (0.15 mg of Se/kg of DM). At enrollment (0 d) and at 28, 56, 84, and 112 d following enrollment, blood samples were taken for Se and Se species determination, as well as whole blood GSH-Px activity. At the end of the study beef cattle were killed and samples of heart, liver, kidney, and skeletal muscle (LM and psoas major) were retained for Se and Se species determination. Tissue GSH-Px activity and thiobarbituric acid reactive substances were determined in skeletal muscle tissue (LM only). The incorporation into the diet of ascending concentrations of Se as SY increased whole blood total Se and the proportion of total Se comprised as SeMet, as well as GSH-Px activity. There was also a dose-dependent response to the graded addition of SY on total Se and proportion of total Se as SeMet in all tissues and GSH-Px activity in skeletal muscle tissue. Furthermore, total Se concentration of whole blood and tissues was greater in those animals offered SY when compared with those receiving a comparable dose of SS, indicating an improvement in Se availability and tissue Se retention. Likewise, GSH-Px activity in whole blood and LM was greater in those animals offered SY when compared with those receiving a comparable dose of SS. However, these increases in tissue total Se and GSH-Px activity appeared to have little or no effect in meat oxidative stability.
Key Words: beef • selenium • selenocysteine • selenomethionine • thiobarbituric acid reactive substance
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INTRODUCTION
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In the early 1970s a specific biological role for Se became apparent with the discovery of the first selenoprotein, glutathione peroxidase (GSH-Px; Rotruck et al., 1973
). Glutathione peroxidases catalyze the reduction of lipid and hydrogen peroxides to less harmful hydroxides via the oxidation and subsequent reduction of selenocysteine (SeCys), which is the active center of this enzyme (Arteel and Sies, 2001).
The antioxidant functions of Se, via GSH-Px activity, have been shown to persist postmortem in poultry muscle tissue (DeVore et al., 1983), delaying the onset of oxidation reactions, which affects adversely both the nutritive value and flavor of meat products (Morrissey et al., 1998).
Selenium content of plants is variable and to a large extent is dependent upon the area of growth. Within the European Union the Se content of soil is low and necessitates the need for Se supplementation of livestock diets. Selenium supplements are in 2 forms, inorganic mineral salts, such as sodium selenite (SS; Na2SeO3) or selenate (Na2SeO4), or organic forms such as Se enriched yeast (SY), in which selenomethionine (SeMet) is the predominant form of Se (Korhola et al., 1986).
Surai (2006) reported that SeMet is retained in tissue proteins to a greater extent than SeCys and the inorganic Se forms. Furthermore, Seko et al. (1989) showed that SS may act as a pro-oxidant and is toxic at increased dietary concentrations, whereas SeMet is not. However, there is little information available on the effect of organic or inorganic sources of Se on the distribution and speciation of tissue Se and subsequent meat quality of beef cattle.
The objective of this study was to determine the distribution of total Se and the proportion of total Se comprised as the selenized AA SeMet and SeCys, as well as the oxidative stability, within the postmortem tissues of beef cattle that had either received diets containing graded additions of SY or comparable doses of SY and SS.
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MATERIALS AND METHODS
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The work was conducted under the authority of the UK Animals (Scientific procedures) Act 1986 (Home Office, 1986), and procedures were undertaken by staff holding appropriate license authorities under the act. All cattle were housed on straw bedding with sufficient space as prescribed by Home Office guidelines. Fresh potable water was available at all times.
Thirty-two castrated 20-mo-old male Limousin x Holstein-Friesian cattle, with a mean initial BW of 489 ± 42.9 kg, were enrolled onto the study. Animals were blocked by BW and then randomly allocated to 1 of 4 experimental treatments: an unsupplemented control (T1), supplementation with SS to achieve a total mixed ration (TMR) total Se concentration of 0.30 mg of Se/ kg of DM (T2), or supplementation with ascending concentrations of SY at rates sufficient to achieve TMR total Se concentrations of 0.30 (T3) or 0.50 mg of Se/ kg of DM (T4). For 112 d, cattle were offered TMR that contained (DM basis) 790 g/kg of corn silage, 90 g/kg of soybean meal, 50 g/kg of rapeseed meal, 50 g/kg of ground wheat, and 20 g/kg of a vitamin and mineral supplement that differed in the quantity of SY [Sel-Plex (Saccharomyces cerevisiae CNCM; Collection Nationale de Culture de Microorganism, I-3060), Alltech, Nicholasville, KY] or SS, depending on treatment designation. Diets were prepared fresh daily and were offered for ad libitum access (refusal maintained at 100 g/kg of DMI) to group-housed animals that were individually fed through Calan-Broadbent electronic feed gates (American Calan Inc., Northwood, NH).
Sampling Procedures and Measurements
Feed Analyses.
Representative samples of each TMR were taken weekly and stored at –20°C until analysis. At the end of the study feed samples were bulked, subsampled, and then analyzed for nutritional composition and total Se concentration (Table 1).
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Table 1. Laboratory determined nutrient composition of total mixed ration diets un-supplemented (T1), supplemented with sodium selenite (T2) or selenized yeast (T3 and T4) for a period of 112 d (g/kg of DM unless otherwise stated)
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Oven DM concentration was determined by drying samples in a forced-air oven at 100°C for 24 h. Ash was obtained by incineration in a muffle furnace at 550°C for 16 h. Concentration of NDF was determined by Ankom Fiber analyzer 200 (Ankom Technology, Macedon, NY) by procedures described by Ministry of Agriculture, Fisheries and Food (MAFF, 1993) using SDS and 
amylase solutions and corrected for residual ash (MAFF, 1982). Starch was determined by polarimetry (MAFF, 1982) with random samples analyzed using the enzyme technique (MacRae and Armstrong, 1968) to verify the calibration of the polarimeter. Nitrogen concentration was measured by the Kjeldahl technique, and water-soluble carbohydrate (WSC) concentration was measured spectrophotometrically (MAFF, 1986). Neutral detergent cellulose + gammanase digestibility was determined and ME estimated (MAFF, 1993).
Feed Preparation.
All TMR were prepared fresh daily using a self-propelled feeder wagon (Calan Data Ranger, American Calan Inc.) fitted with a weighing device (Weightronix Model 1015, Fairmount, WV), which recorded the individual weights of the different feed ingredients added to the hopper. Three samples for each constituent ingredient were analyzed for total Se before the start of the study so that required doses could be calculated to meet target concentrations. Required quantities of SY or SS were incorporated into 4 vitamin and mineral supplements, with respect to treatment, at the point of manufacture (Dairy Direct, Bury St Edmunds, UK). These mineral mixes were subsequently mixed with the soy bean meal, rapeseed meal, and ground wheat fraction of the TMR to form 4 identified concentrate blends. These concentrate blends were incorporated into the SY and SS designated TMR at the time of mixing at forage to concentrate ratio of 3.8:1. Selenized yeast was included in the mineral supplements of T3 and T4 at a rate sufficient to achieve TMR total dietary Se concentrations of 0.30 or 0.50 mg of Se/kg of DM, respectively, or SS was included in the mineral mix of T2 to achieve a TMR total Se concentration of 0.30 mg Se/kg of DM.
Blood and Tissue Sampling.
Individual blood samples were taken from each animal via jugular venipuncture at 0, 28, 56, 84, and 112 d of the study. Samples were taken aseptically using the Vacutainer system (BD Diagnostics, Oxford, UK) using a 21-gauge needle into two 5-mL lithium heparin pretreated tubes, for subsequent determination of total Se, Se species, and whole blood GSH-Px activity.
At the end of the treatment period (112 d), all cattle were killed and samples of heart, liver, kidney, and skeletal muscle (LM and psoas major) were retained for determination of total Se and Se species. Glutathione peroxidase and thiobarbituric acid reactive substances (TBARS) were determined in the LM.
Se Analyses.
Total Se in TMR, whole blood samples and tissues (heart, liver, kidney, and skeletal muscle) was determined according to the method of Mester et al. (2006). Briefly, 1 g of each sample was mineralized in 4 mL of 16 M HNO3 and 2 mL of 9.8 M H2O2 within a closed-vessel heating block system. The solution was further diluted with water and Se subsequently determined using inductively coupled plasma mass spectrometry (ICPMS; 6100 ICPMS, Perkin Elmer Elan, Waltham, MA).
Selenium speciation in blood was determined using the method described by Palacios et al. (2005). Samples were initially incubated for 5 h with DL-dithiothreitol and iodoacetamide to reduce and alkylate SeCys. Samples were then spiked with SeMet77 and subsequently incubated for 24 h at 37°C with a mixture of protease and lipase maintained at a pH 7.5. Following incubation the mixture was centrifuged (9,500 x g for 10 min at 20 to 25°C) and the supernatant separated and purified by cell exclusion liquid chromatography. Aliquots of the supernatant were analyzed by reversed-phase HPLC-ICPMS using an ICPMS equipped with a collision cell (6100 ICPMS, Perkin Elmer Elan).
Selenium speciation in tissues was determined according to the method of Bierla et al. (2008). Samples were initially pooled, with respect to tissue type and treatment, before analysis, as fresh samples. Samples were subsequently lyophilized, mixed, and sieved after which a representative subsample was taken. Urea was added to the subsample and the subsample sonicated after which it was reduced, alkylated, and submitted to proteolysis. The extract was then purified by cell exclusion chromatography and the AA quantified by reverse phase HPLC-ICPMS.
Whole Blood Glutathione Peroxidase Analysis.
Glutathione peroxidase activity in whole blood was determined using the Olympus AU400 Chemistry Analyzer (Olympus UK, Watford, UK) based on the method of Anderson et al. (1978).
Meat Quality Assessment.
Samples of LM were analyzed for TBARS and GSH-Px. Samples were packed into modified atmosphere packs (MAP; O2:CO2, 75:25) and subjected to simulated retail display (3°C, 700 lx for 16 h out of 24 h).
Thiobarbituric acid reactive substances were analyzed on d 10 of display, by the method of Tarladgis et al. (1960) modified by the use of a Büchi 321 distillation unit (BÜCHI Labortechnik AG, Postfach, Switzerland). Glutathione peroxidase activity was determined in samples of LM taken immediately following euthanasia and in samples that had undergone 10 d aging in MAP, using a modified method of the coupled assay procedure of Paglia and Valentine (1967), modified by DeVore and Greene (1982), Daun et al. (2000), and K. Raes of Ghent University, Ghent, Belgium (unpublished). Tissue GSH-Px activity is expressed as nanomoles of NAD phosphate-oxidase/(min x mg of protein).
Statistical Analysis
Statistically significant differences among the 4 treatment groups for whole blood Se concentration and erythrocyte GSH-Px activity were determined by ANOVA using PROC MIXED (SAS Inst. Inc., Cary, NC). Sources of variation within the model included treatment (3 df), block (3 df), and time (3 df) where individual animal formed the repeated subject and time the repeated measure. Statistical tests were undertaken for main effects and treatment x time interaction.
Statistical differences among the 4 treatment groups and linear and quadratic contrasts between SY doses for tissue total Se concentration and meat quality assessments were determined by ANOVA using a GLM. Sources of variation within the model included treatment. Results are presented as least square means with the SEM and P-value. Tukey simultaneous test was used to establish statistical differences (P < 0.05) between individual treatment means.
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RESULTS AND DISCUSSION
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Feed Analyses
Total Se content of each concentrate blend was 0.32, 0.98, 0.98, and 1.98 mg of Se/kg of DM for T1, T2, T3, and T4, respectively. These blends, following incorporation into each experimental TMR, resulted in laboratory determined TMR total Se concentrations of 0.16, 0.30, 0.30, and 0.51 mg of Se/kg of DM for T1, T2, T3, and T4, respectively.
Se Concentration of Blood and Tissues
Whole blood total Se concentration and the total Se concentration of heart, liver, kidney, and skeletal muscle (LM and psoas major) are shown in Tables 2 and 3, respectively.
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Table 2. Whole blood total selenium concentration and glutathione peroxidase activity of beef cattle offered diets that were unsupplemented [T1 (0.16 mg of total Se/kg of DM)] or supplemented with sodium selenite [T2 (0.3 mg/kg of DM total Se)] or selenized yeast [T3 (0.3 mg of total Se/kg of DM) and T4 (0.5 mg of total Se/kg of DM)] for a period of 112 d
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Table 3. Total Se concentration (mg/kg of DM) in heart, liver, kidney, and skeletal muscle of beef cattle offered diets that were unsupplemented [T1 (0.16 mg of total Se/kg of DM)] or supplemented with sodium selenite [T2 (0.3 mg of total Se/kg of DM)] or selenized yeast [T3 (0.3 mg of total Se/kg of DM) and T4 (0.5 mg of total Se/kg of DM)] for a period of 112 d
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Se Concentration in Whole Blood.
There were significant (P < 0.001) differences between treatments in whole blood total Se concentration with T1 values being predictably least and relatively constant throughout the study. There were significant differences (P < 0.05) between the 2 Se sources in whole blood Se concentration, values being greater in SY supplemented animals [T3; 158 ng of Se/g of fresh weight (FW)] when compared with those that had received a comparable dose of SS (T2; 142 ng of Se/g of FW), indicating better uptake and incorporation of Se derived from SY. These observations are in line with those of Van Ryssen et al. (1989) in sheep and Gunter et al. (2003) in cows, who reported greater whole blood Se concentrations in animals supplemented with organic Se when compared with those offered diets of comparable Se content where SS was the source of supplementary Se.
A trend line analysis of whole blood total Se concentrations over time is shown in Figure 1. There was a linear (P < 0.001) dose-dependent response on whole blood Se concentration to the graded addition of SY to the diet. Similar linear effects of dietary Se increases, either for organic or inorganic Se supplements, were reported in dairy cows (Juniper et al., 2006) and wether sheep (Cristaldi et al., 2005). Extrapolation of the resultant curve indicated that whole blood Se values, irrespective of treatment, increased over time without achieving a Se steady state following 112-d exposure to the diets. Van Ryssen et al. (1989) also reported that whole blood Se did not reach a plateau during the administration of a diet supplemented with organic or inorganic Se (1 mg/kg of DM) over a similar period in sheep. The estimations of time required to reach a Se steady state indicated that treatment T2, T3, and T4 would achieve asymptotic values of 177, 207, and 274 ng/g of FW, respectively, after approximately 150 d of exposure to the diets, reflecting the average lifespan of erythrocytes (Mahan et al., 1999), which incorporate the seleno-enzyme GSH-Px (Figure 2).
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Figure 1. Trend line analysis of the relationship between whole blood total Se concentration and days of sampling of beef cattle offered diets that were unsupplemented [T1 (0.16 mg of total Se/kg of DM)] or supplemented with sodium selenite [T2 (0.3 mg of total Se/kg of DM)] or selenized yeast [T3 (0.3 mg of total Se/kg of DM) and T4 (0.5 mg of total Se/kg of DM)] for a period of 112 d.
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Figure 2. Whole blood total Se and the proportion of total Se comprising selenomethionine (SeMet), selenocysteine (SeCys), and other Se species of beef cattle offered diets that were unsupplemented [T1 (0.16 mg of total Se/kg of DM)] or supplemented with sodium selenite [T2 (0.3 mg of total Se/kg of DM)] or selenized yeast [T3 (0.3 mg of total Se/kg of DM) and T4 (0.5 mg of total Se/kg of DM)] for 0, 56, or 112 d.
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Se Concentration in Tissues.
The Se content of tissues differed among tissue types (Figures 3 and 4). Total Se concentration was greatest in kidney, with values ranging between 4.5 and 6.4 mg/kg of DM, followed by liver, heart, and skeletal muscle. This hierarchy of tissue Se concentration is consistent with those reported by Taylor (2005) in wether lambs that had been offered diets containing up to 2.9 mg/kg of DM of organic Se.
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Figure 3. Total Se and the proportion of total Se comprising selenomethionine (SeMet), selenocysteine (SeCys), and other Se species in liver and kidney of beef cattle offered diets that were either unsupplemented [T1 (0.16 mg of total Se/kg of DM)] or supplemented with sodium selenite [T2 (0.3 mg of total Se/kg of DM)] or selenized yeast [T3 (0.3 mg of total Se/kg of DM) and T4 (0.5 mg of total Se/kg of DM)] for a period of 112 d.
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Figure 4. Total Se and the proportion of total Se comprising selenomethionine (SeMet), selenocysteine (SeCys), and other Se species in heart and skeletal muscle of beef cattle offered diets that were either unsupplemented [T1 (0.16 mg of total Se/kg of DM)] or supplemented with sodium selenite [T2 (0.3 mg of total Se/kg of DM)] or selenized yeast [T3 (0.3 mg of total Se/kg of DM) and T4 (0.5 mg of total Se/kg of DM)] for a period of 112 d.
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There were treatment (P < 0.001) and linear dose responses (P < 0.001) on kidney, liver, heart, and skeletal muscle tissues to the graded addition of SY to the diet. Furthermore, there were notable differences (P < 0.05) between Se sources, with values being greater in those animals that had received SY compared with those that had received similar dose of SS for kidney (5.0 vs. 6.0 mg of Se/kg of DM), liver (1.56 vs. 1.95 mg of Se/kg of DM), heart (0.72 vs. 0.86 mg of Se/kg of DM), and LM (0.31 vs. 0.46 mg of Se/kg of DM), and numerically greater in psoas major (0.29 vs. 0.36 mg of Se/kg of DM). These results are in agreement with those reported by Qin et al. (2007) who showed significantly greater total Se concentration in the kidney, liver, and muscle tissues of lambs that had been offered SY supplemented diets (0.10 mg of Se/kg of diet) when compared with those offered comparable doses of SS.
Selenized AA Content of Whole Blood.
Selenocysteine was the predominant selenized AA within pooled whole blood samples, irrespective of treatment and time point. Furthermore, increases seen in whole blood total Se concentration over time, irrespective of treatment, seemed to be more attributable to increases in the proportion of total Se comprised as SeCys than SeMet or other unidentified Se fractions. The proportion of total Se comprised as SeMet increased with ascending SY inclusion, the greatest increase being between treatments T3 and T4.
When changes in the proportions of each of the selenized AA are expressed as a function of d-0 values, Se-Cys increased by 90 to 100%, irrespective of the inclusion rate of supplementary Se or Se source. The content of SeMet in whole blood increased by 29 and 63% of d-0 values for T2 and T3, respectively, the greater uptake in T3 most likely attributable to the high concentration of SeMet in the SY product (SeMet comprised 54 to 74% of total Se; Rayman, 2004). This is even more apparent in T4 as SeMet values are seen to increase appreciably (167%) between d 0 and the completion of the study at 112 d.
Selenized AA Content of Tissues.
Selenomethionine was the predominant selenized AA within kidney tissue (54 to 60% of total Se), irrespective of treatment. In cardiac tissue, SeCys comprised 72% of total Se in treatment T1, with the proportion of SeMet increasing (40 to 50% of total Se) as additional Se in the form of SY was added to the diet. In liver, SeCys was the predominant form of Se for T1, T2, and T3 (from 55 to 65% of total Se), whereas SeMet was the predominant selenized AA for T4 (55% of total Se). These differences may reflect changes in Se partitioning from incorporation into functional seleno-proteins in the form of Se-Cys to the nonspecific incorporation of SeMet into liver tissue protein. Similarly, in skeletal muscle (LM and psoas major), SeCys was the predominant form of Se in T1, T2, and T3, whereas SeMet was the main form for T4. Furthermore, when the proportions of total Se comprised as SeMet and SeCys in T2, T3, and T4 are expressed as their relative increase with respect to T1, the most notable increase is seen in the proportion of SeMet, the greatest change being apparent within hepatic tissue. Therefore, increases in the proportion of total Se shown in all tissues with ascending inclusion of SY were attributable to increases of proportion of total Se comprised as SeMet. It could be concluded that the supplementation of animal diets with SY, when compared with comparable doses of SS, led to greater bioaccumulation of Se as SeMet, which could be used advantageously in animals that require a sustained Se source. Selenomethionine incorporated into actively regenerating body proteins can be removed during the degradation and resynthesis of proteins and transformed to selenide for utilization or excretion (Schrauzer, 2003). In addition SY supplementation may also enhance the nutritional quality of livestock products with regard to Se content, thus increasing the current intake of Se, an essential nutrient for human health, which has been estimated to be deficient in several countries (Rayman, 2000; Combs, 2001). Selenium has been shown to be protective against cancers in some animal models (McIntosh et al., 2006) and human studies (Combs, 2005; Finley, 2007). However, Se intakes in both studies were greater than those considered essential for normal nutritional requirements, and Se-enriched animal products would only improve human Se status with regard to recognized recommended daily intakes.
Whole Blood Glutathione Peroxidase Activity.
There were notable differences (P < 0.001) in GSH-Px activity between treatment groups (Table 2
), values being greater (P < 0.05) in those animals offered SY [135.9 GSH-Px u/mL of red blood cells (RBC)] when compared with those that received a comparable dose of SS (97.3 GSH-Px u/mL of RBC). These results were not unexpected because whole blood Se concentration is closely correlated with GSH-Px activity (Koller et al., 1984), although other authors (Beilstein and Whanger, 1986; Van Ryssen et al., 1989) reported that the percentage of Se in erythrocytes associated with GSH-Px may vary depending on Se source. Qin et al. (2007) also showed significantly greater whole blood Se concentrations and GSH-Px activities in lambs fed SY-enriched diets (0.10 mg/kg of diet) when compared with lambs fed similar concentrations of SS. However, other authors did not report differences in blood GSH-Px activity in sheep (Van Ryssen et al., 1989; Gunter et al., 2003) that had been offered diets containing comparable doses of SY and SS.
The plot of whole blood Se concentration against GSH-Px activity (Figure 5) indicated a positive curvilinear response (R2 = 0.795) to ascending whole blood Se values. Furthermore, increases in GSH-Px activity were better correlated with the proportion of total Se comprised as SeCys (R2 = 0.984; P < 0.001) than SeMet (R2 = 0.902; P < 0.001) in whole blood. These results would tend to indicate that a larger proportion of whole blood total Se was incorporated into functional seleno-enzymes.
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Figure 5. Trend line analysis of the relationship between total Se concentration and glutathione peroxidase (GSH-Px) activity in whole blood of beef cattle offered diets that were either unsupplemented [T1 (0.16 mg of total Se/kg of DM)] or supplemented with sodium selenite [T2 (0.3 mg of total Se/kg of DM)] or selenized yeast [T3 (0.3 mg of total Se/kg of DM) and T4 (0.5 mg of total Se/kg of DM)] for a period of 112 d.
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Glutathione Peroxidase Activity and TBARS in LM.
There were no statistical differences in TBARS values for LM among treatments (Table 4). However, there was a quadratic dose effect (Figure 6; P = 0.01) to the graded addition of SY to the diet, possibly a consequence of greater TBARS values in T3 than treatments T1 and T4. Despite this effect, the difference between individual treatments has little biological significance. Other authors (Sk
ivanová et al., 2007) have also reported a lack of effect of SY supplementation on the oxidative stability (TBARS) of the LM in calves, when compared with inorganic sources of Se, and Taylor et al. (2008) reported that beef steaks from cattle supplemented with Se had similar shelf life attributes to those from unsupplemented animals despite having greater Se contents.
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Table 4. Thiobarbituric acid reactive substances (TBARS) measured at 10 d postmortem and glutathione peroxidase (GSH-Px) at 0 and 10 d postmortem in LM of beef cattle offered diets that were unsupplemented [T1 (0.16 mg of total Se/kg of DM)] or supplemented with sodium selenite [T2 (0.3 mg of total Se/kg of DM)] or selenized yeast [T3 (0.3 mg of total Se/kg of DM) and T4 (0.5 mg of total Se/kg of DM)] for a period of 112 d
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Figure 6. Trend line analysis of the relationship between total Se concentration and thiobarbituric reactive substances (TBARS) of the LM of beef cattle offered diets that were either unsupplemented [T1 (0.16 mg of total Se/kg of DM)] or supplemented with sodium selenite [T2 (0.3 mg of total Se/kg of DM)] or selenized yeast [T3 (0.3 mg of total Se/kg of DM) and T4 (0.5 mg of total Se/kg of DM)] for a period of 112 d.
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There was a treatment effect on tissue GSH-Px activity at 0 (P = 0.02) and 10 d postmortem (P = 0.02). Furthermore, GSH-Px activity, at both 0 and 10 d postmortem, was significantly greater (P < 0.05) in those animals that had received SY when compared with those that had received a comparable dose of SS. There was a linear dose effect (P = 0.007) to the graded addition of SY to the diet on tissue GSH-Px activity at 0 d postmortem. This effect, although not as marked, was still apparent at 10 d postmortem and showed a quadratic response (P = 0.009) to the graded addition of SY to the diet. Subsequent regression analysis (Figure 7) indicated a relationship between the total Se concentration of LM and GSH-Px activity at 0 d postmortem (R2 = 0.651) and at 10 d postmortem (R2 = 0.446), although the latter appeared to plateau at tissue total Se values greater than 0.5 mg/kg of DM. A significant correlation between GSH-Px activity and tissue Se has previously been reported in beef muscle tissue (De Vore and Greene, 1982; Gatellier et al., 2004). However, Van Ryssen et al. (1989), in sheep, and Zhan et al. (2007), in pigs, reported similar tissue GSH-Px activities in animals that were offered organic or inorganic Se, although animals offered organic sources had greater total tissue Se concentrations. These authors attributed this finding to the different metabolic pathways that differing sources of Se follow for incorporation into selenoproteins.
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Figure 7. Relationship between total Se concentration of LM and tissue glutathione peroxidase (GSH-Px) activity at 0 (shaded symbols) and 10 d (open symbols) postmortem from beef cattle offered diets unsupplemented [T1 (0.16 mg of total Se/kg of DM)] or supplemented with sodium selenite [T2 (0.3 mg of total Se/kg of DM)] or selenized yeast [T3 (0.3 mg of total Se/kg of DM) and T4 (0.5 mg of total Se/kg of DM)] for a period of 112 d.
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In conclusion, the incorporation of graded additions of SY into the diets of finishing cattle resulted in increases in total Se in a dose-dependent manner, and based on limited speciation data much of the increase in total Se appeared to be attributable to greater increases in the proportion of total Se as SeMet rather than as SeCys. Furthermore, when comparing comparable doses from different Se sources, responses in blood and tissue were markedly greater in animals offered SY as opposed to SS, again the result of the greater proportion of total Se comprised as SeMet. These increases in both blood and tissue Se concentrations were positively related to increases in both blood and tissue GSH-Px activity, although it is more likely that SeCys content of blood and tissues would influence GSH-Px activity rather than total Se. Despite the greater GSH-Px activity in the muscle tissue of animals offered diets containing SY, it did not appear to result in any appreciable effect on oxidative stability. It is therefore likely that increases in the Se content of meat products from SY-supplemented animals, rather than resulting in any benefits regarding product shelf-life, may improve the Se status of humans consuming such products in Se-deficient areas.
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Footnotes
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1 This research was funded by Alltech EU Regulatory Affairs Department (trial ref. SEL/BEF/EFF/03/0505/UK). The authors also give special thanks to Ryszard Lobinski (UT2A laboratories) for performing the selenium analyses. 
2 Corresponding author: d.t.juniper{at}rdg.ac.uk
Received for publication September 19, 2007.
Accepted for publication May 28, 2008.
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LITERATURE CITED
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Abstract
INTRODUCTION
