Engormix/Dairy Cattle/Technical articles

Selenium metabolism in animals

Selenium metabolism in animals: the relationship between dietary selenium form and physiological response

Published on: 5/30/2007
Author/s : KATE A. JACQUES - North American Biosciences Center, Alltech Inc.

The mineral complement of the animal and, to a large extent, the human diet is the fraction that historically we have made little attempt to supply in the form in which it occurs in natural mammalian foods. For the most part the strategy of using inorganic minerals, ie. oxides, sulfates and carbonates, has successfully provided the nutrients needed for growth and production of domestic livestock. Limitations in bioavailability or metabolism of inorganic sources due to either chemical or physical form have been well compensated by low costs and widespread market availability.

However it is an increasing challenge to formulate diets that allow highly productive, intensively-reared modern livestock and poultry to reach genetic potential; and in recent years there has been a growing understanding that marginal trace element status is a factor limiting health and productivity. This is partly because important physiological roles of most of the trace elements are in the body’s disease resistance mechanisms. Along with this health-related perspective on trace mineral nutrition has come the realization that mineral form is critical to mineral function. The metabolic pathways along which the highly oxidized inorganic forms move can differ markedly from the routes followed by the more reduced, ‘organic’ mineral compounds naturally present in plants.

The difference between metabolism of plant-derived and inorganic selenium sources by animals is a pointed example of the importance of nutrient form in physiological function. While the predominant form of selenium supplement in animal feeds is currently the inorganic sodium selenite, the major natural form of selenium that occurs in food is L-selenomethionine, a selenium analogue of the amino acid methionine (Schrauzer, 2000).

Most common species of plants, marine algae, bacteria and yeast can synthesize both methionine and selenomethionine, however animals can form neither. For this reason methionine is listed among the dietary essential amino acids for higher animals. In reviewing the role dietary selenium form plays in its metabolism, it becomes increasingly clear that selenomethionine may be considered equally essential. This is because selenomethionine derived primarily from plants is the main source of easily metabolized and easily retained form of this critical trace element for animals, including humans.

The following reviews the chemistry and metabolism of dietary selenium of food animals and the role of selenium in physiology.


Selenium chemistry and occurrence in forages and cereals

SELENIUM CHEMISTRY: SIMILAR TO SULFUR

The chemistry of selenium has much in common with sulfur. Like sulfur, selenium can exist in selenide (-2), elemental (0), selenite (+4) and selenate (+6) states (Table 1). Inorganic forms of sulfur and selenium include the selenides hydrogen sulfide and hydrogen selenide while common corresponding organic sulfides and selenides are cysteine and selenocysteine, methionine and selenomethionine. The similarity in size and chemistry of the two minerals brings them into interaction in biological systems; however there are important differences as well. The protonated forms of selenium, H2Se and H2SeO3, are more acidic than corresponding sulfur compounds and in solution at neutral pH are dissociated and ionized. Also, sulfurcontaining compounds tend to be oxidized in the body with sulfate produced as the end product. In contrast, selenium-containing compounds are usually reduced to produce selenides or methylated in preparation for excretion via lungs or kidney (Brody, 1994).


Table 1. Similarities in chemistry of sulfur and selenium.

Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 1

Adapted from Brody (1994).



SELENIUM METABOLISM IN SOILS, PLANTS (INCLUDING S. CEREVISIAE)

Where soil conditions allow, plants take up soil selenite, selenate and selenomethionine, though no requirement by higher plants for selenium has yet been found. Selenate is absorbed by roots in strong preference to selenite; and for this reason the selenate form is preferred for addition to fertilizers (Oldfield, 1999). Soils also contain some selenomethionine, which is readily taken up by wheat seedlings (Marschner, 1995).

Soil conditions such as pH and aeration largely determine the availability of selenium for plant uptake. In acidic, poorly aerated soils, selenium is in an insoluble selenide or elemental selenium form and is unavailable to plants.

Selenium is available in selenite form in acidic, well-aerated and neutral pH soils; and when soils are alkaline and are dry/well-aerated, the selenium is in a soluble selenate form, readily available to plants (Mayland, 1986). In contrast, soils with a very acid pH or those that are very moist (i.e., poorlyaerated) contain reduced selenium (elemental selenium and selenides), which forms insoluble adsorption complexes with soil iron hydroxide (Fe+3) and becomes unavailable (Mayland, 1986). As a result, total soil selenium is not a good indicator of plant selenium content.

The similarities between sulfur and selenium cause a number of biological interactions. Sulfate and selenate compete for common uptake sites in plant roots; and therefore selenate uptake can be inhibited by high sulfate supplies (Marschner, 1995). Likewise, the toxic effect of selenium to some plant species is due to interference with sulfur metabolism. The similarity of selenoamino acids to their sulfur analogues of cysteine and methionine can disrupt biochemical reactions and cellular enzyme function. Those cellular components most sensitive to selenium toxicity are those that require sulfur for an essential reaction, eg. the –SH group of some enzymes loses its reactivity when selenium substitutes for sulfur (Mikkelsen et al., 1989).

Plants are considered in three categories with respect to selenium content. The first two groups, accumulator plants and selenium indicator plants, absorb high quantities of selenium when grown on high selenium soils. While animals normally avoid such species, selenosis or selenium toxicity occurs when they are grazed. The third group, non-accumulator plants, includes grains and grasses of nutritional and agronomic importance and many forbs that do not accumulate selenium in levels toxic to animals when grown on seleniferous soils.

Accumulator and non-accumulator plants metabolize selenium differently. In non-accumulator plants soil selenite or selenate is converted primarily into selenomethionine after reduction to selenide form and conversion to selenocysteine (Figure 1). Selenomethionine is then incorporated into plant protein in place of methionine. This occurs because the tRNA for methionine does not distinguish between methionine and its selenium analog (Schrauzer, 2000).

Other selenocompounds indentified in plants include smaller amounts of selenocysteine, methylselenocysteine and selenomethylmethionine (Brody, 1994). Selenomethionine represented around 50% of the selenium in cereal grains in a study by Olson and Palmer in the mid-1970s (1976); while a more recent analysis of corn, wheat and soybeans revealed 80-81% of the selenium in selenomethionine form (cited by Schrauzer, 2000). In contrast, in accumulator plants such as many Astragalus spp., the formation of selenomethionine seems to be impaired. The selenocysteine is transformed into small, water-soluble non-protein compounds such as selenomethylcysteine (Marschner, 1995).

As part of the plant kingdom, S. cerevisiae yeast take up selenium salts from media and form selenoamino acids. Certain strains are capable of assimilating as much as 2-3000 ppm Se with >90 % in the form of seleno methionine. Kelly and Power (1995) found that 67% of selenium in Sel-Plex was located in the cell cytosol and another 11% in the mitochondria. Fractionation of the cell wall-free extract determined that a significant portion of the Se was associated with soluble proteins.


Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 2


Figure 1. Selenium assimilation in accumulator plants versus assimilation in nonaccumulator species, marine algae, many bacteria and yeast (adapted from Marschner, 1995 and Schrauzer, 2000).



Absorption of selenium


ABSORPTION OF INORGANIC AND ORGANIC SELENIUM FORMS FROM THE SMALL INTESTINE

Animal diets contain variable amounts of organic selenium (predominantly selenomethionine) and whatever amounts of inorganic selenium we add at formulation. Not surprisingly, inorganic and organic forms of selenium in animal diets are absorbed by different mechanisms. Selenite is absorbed from the intestine by a simple diffusion process, whereas selenate is actively absorbed in the ileum by co-transport with sodium ions. Unlike selenate, the absorption of which is competitively inhibited by sulfate owing to a shared absorption route, selenite absorption is not influenced by sulfate (Wolffram et al., 1986).

Selenomethionine contained in forages and grains is absorbed in the small intestine via the Na+-dependent neutral amino acid transport system, a mechanism shared with, and competitively inhibited by, methionine. Selenocysteine is thought to be transported in a similar way by the carrier mechanism for basic amino acids, as cysteine transport was similarly inhibited by selenocysteine, lysine and arginine (Wolffram et al., 1989a,b).

Soluble forms of selenite, selenate, selenomethionine and selenocysteine have been shown under research conditions to be efficiently absorbed from the intestine (Levander, 1986). However, numerous vagaries of the digestive process and practical diet formulation act to reduce the quantity of selenium actually absorbed. The amount of absorbable inorganic selenium presented at absorption sites depends on interactions with a range of interfering substances in the diet or drinking water including iron, sulfur, phytate and antioxidants, among others.

The amount of selenomethionine in forage or grain available for absorption will depend on the digestibility of the source, which is a function of both the species of animal and the nature of the ingredient, as well as the nutritional adequacy of the diet. In general, the selenium in high selenium yeast, wheat and alfalfa is considered very available, other plant sources of moderate availability and the selenium in meat and fishmeal is poorly available (Combs and Combs, 1986; Table 2). It should be noted that while the selenium in yeast, wheat and alfalfa is selenomethionine, the chemical form in meat or fish by-products is not well defined in the literature.


Table 2.Summary of factors affecting the availability of selenium in feedstuffs.

Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 3

Adapted from Combs and Combs (1986).



THE RUMEN AND SELENIUM METABOLISM

The ability of ruminants to absorb selenium from dietary selenite or selenate is notably poor owing to the highly reducing nature of the rumen environment.

Rumen microbes reduce much of dietary inorganic selenium to unabsorbable elemental or inorganic selenide forms (Wright and Bell, 1966). As a result, estimates of absorption of selenite-derived selenium vary widely depending on the method and response criteria used, but tend to be low. Harrison and Conrad (1984), using the balance method, found selenium availability to range from 17 to 50% of selenium intake in non-lactating dairy cows given a variety of diets. Typical figures quoted for selenite selenium availability to ruminants are 25-30%.

The role of rumen microbes in selenium metabolism is more complicated than the well-known problem with inorganic selenium implies, and is analogous in many respects to the way in which microbes mediate the quality of dietary protein. Bacteria are able to synthesize methionine in addition to cysteine and the selenium analogues, selenomethionine and selenocysteine; and these selenoamino acids are incorporated into microbial protein.

The selenium content of rumen microbes is enriched relative to the diet; and ruminal microbes have been shown to alter its chemical form in ways other than reduction. Analysis of the selenium content of rumen microbes from sheep fed a range of natural diets revealed that microbial selenium content was higher than that of the diet, whether considered relative to diet dry matter (average of 46-fold), nitrogen (average of 11.3-fold) or sulfur content (26-fold) (Whanger et al., 1978). The enrichment of selenium in microbial cells (2-78-fold) was greater than the enrichment of either nitrogen (~4- fold) or sulfur (~1.6-fold). The dietary adequacy of sulfur affected microbial selenium concentration as would be expected given its necessity in formation of microbial protein.

The form of selenium presented to microbes and the relative amount of concentrate in the diet also affect rumen microbial selenium metabolism. Van Ryssen et al. (1989) found that selenomethionine was the predominant selenoamino acid in bacteria when rumen microbes were incubated with selenomethionine; however the predominant selenoamino acid was selenocysteine when microbes were incubated with selenite. The latter may represent a route by which a portion of inorganic selenium supplements in ruminant diets finds its way into a form metabolizable by animals.

Rumen microbes also appear to take up more of the organic than inorganic form of selenium. Koenig et al. (1997) found that in sheep fed either forage or concentrate-based diets, bacterial protein contained more labeled organic selenium than labeled inorganic selenium. In a comparison of selenite and Sel-Plex (contains predominantly selenomethionine) in 55% and 75% concentrate diets, van Ryssen (1998) found that in either diet microbial selenium content was higher in diets that included Sel-Plex (Figure 2).


Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 4


Figure 2.Effect of selenium level and form on selenium content of rumen bacteria from sheep fed 75% or 55% concentrate diets (Control contained 0.17 ppm background Se; Selenite, 0.3 ppm Se added; Sel-Plex, 0.3 ppm Se added; Selenite + Sel-Plex, 0.3 ppm added from each source). The difference between Sel-Plex (0.3 ppm Se) and Sel-Plex + selenite (0.6 ppm Se) was not significant for any tissue or rumen microbes) (From van Ryssen, 1998).



The forage:concentrate ratio, its effects on total microbial activity, pH and microflora populations also play a role in ruminant selenium metabolism. Koenig et al. (1997) and van Ryssen (1998) noted that at higher levels of dietary concentrate, bacteria isolated from the rumen of sheep incorporated more selenium. Both reports attributed this response to the greater rumen microbial activity in the more nutrient-dense concentrate diet. Koenig et al. (1997) suggested that the diet effect on rumen pH (6.44 vs 5.41) and the population of rumen microbes might be reflected in the bacterial selenium profile. Among the key species present in the rumen of animals fed foragebased diets (Butyrivibrio spp., Prevotella ruminicola), Prevotella is known to reduce selenium to elemental form (Se0). In contrast, Selenomonas ruminantium, an organism predominating in the concentrate-fed rumen, incorporates selenium into selenoamino acids (as does Butyrivibrio).

Total selenium retention in other tissues may also be affected by diet concentrate content. Van Ryssen (1998) compared Sel-Plex organic selenium and selenite in sheep fed either 55% or 75% concentrate diets and found that selenium content of liver, kidney, heart and muscle were higher in animals fed the concentrate diets, particularly when the organic source was fed (Figure 3). In the previously mentioned study by Koenig et al. (1997), both absorption and retention of selenium in a standard balance study was higher in sheep fed the concentrate diet with absorption differing more due to diet than retention (Figure 4).


Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 5


Figure 3.Effect of dietary concentrate level on selenium content of liver, kidney cortex, heart and muscle (from van Ryssen, 1998). (abMeans differ, P<0.01).



Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 6


Figure 4.Effect of diet on absorption and retention of selenium (absorption: forage vs concentrate differ, P<0.05) (adapted from Koenig et al., 1997).



Metabolism, storage and excretion of selenium

CONVERSION OF DIETARY SELENIUM TO BIOLOGICALLY USEFUL FORM


Once absorbed, dietary selenium of either inorganic or organic origin is either used in synthesis of selenoproteins, stored or excreted. The biologically active selenoproteins, of which around 20 have been identified, are primarily redox enzymes that contain a selenocysteine residue at the active site (Low and Berry, 1996). Though metabolic fates of dietary inorganic and organic selenium forms differ, in order for either to be used in selenoprotein synthesis (including dietary selenocysteine) it must first be converted to the inorganic selenide, hydrogen selenide (H2Se) before the process of forming and incorporating specific selenocysteine into the active site of one of the selenoproteins (Daniels, 1996) (Figure 5).

Selenate is reduced to selenite and selenite is reduced via formation of selenodiglutathione to selenide (Foster and Sumar, 1997). Selenomethionine is activated initially by adenosylation, demethylated and converted to selenocysteine via selenohomocysteine and selenocystathione in analogy to methionine (Schrauzer, 2000). Pyridoxine-dependent enzymes are involved in the activation of selenomethionine and in this way vitamin B6 status and selenomethionine metabolism are related. The selenocysteine formed is then degraded in the liver to serine and selenide (Schrauzer, 2000).


Excretion

The inorganic selenide formed is either used to synthesize selenoproteins or methylated and excreted. Some dimethylselenium is excreted via the lungs while most of the excess selenide is excreted via urine in the form of the trimethylselenonium ion (Brody, 1994).


Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 7


Figure 5. Routes of dietary selenium metabolism in animals (adapted from Schrauzer, 2000; Low and Berry, 1996; Daniels, 1996).



Biosynthesis of selenoproteins


The biosynthetic pathways of selenoproteins in bacteria are well known (Low and Berry, 1996). For higher animals the process is less comprehensively understood, but key similarities and differences between the prokaryotic and eukaryotic pathways have been established. A tRNA designated tRNAsec is aminoacylated with serine and converted to sectRNA( ser)sec using selenophosphate by selenocysteine synthase (Figure 5) (The latter step is characterized in prokaryotes but not yet confirmed in eukaryotes.) Selenocysteine is inserted into the growing polypeptide chain at a UGA codon. A stem loop structure, termed the selenocysteine insertion sequence, that interacts with the elongation factor is nearby the UGA codon in the selenoprotein mRNA (Burk and Hill, 1999).


SELENOMETHIONINE VS INORGANIC FORMS: STORAGE IN BODY PROTEINS

The key difference in the physiological value of dietary inorganic and organic selenium sources lies in the ability of selenomethionine to be incorporated non-specifically into body proteins. As in plants and bacteria, tRNAMet in animals does not discriminate between methionine and selenomethionine (Daniels, 1996).

Selenomethionine not used in selenoprotein synthesis is taken up by organs and tissues with high rates of protein synthesis such as skeletal muscle, erythrocytes, pancreas, liver, kidney, stomach and the gastrointestinal mucosa (Schrauzer, 2000). In contrast, little selenite selenium finds its way into body proteins. Most of the inorganic selenium not used immediately in the liver for selenoprotein synthesis is quickly excreted via the urine. Although selenocysteine is synthesized both from selenite and via conversion from selenomethionine in animals, substitution for cysteine in protein synthesis is probably not a major metabolic fate of selenocysteine due to differences in the chemical properties of these two amino acids (Beilstein and Whanger, 1986).

Selenomethionine incorporation in body proteins provides two major physiological advantages to organic selenium as a means of supplying dietary selenium. First, it provides a means of establishing tissue reserves against periods of increased demand such as disease challenge or reproduction/ parturition. Second, higher selenium content of embryonic and fetal tissues, along with selenoamino acid inclusion in milk or egg protein ensures an easily metabolized supply of selenium to the neonate.

Recycling of selenium stored in proteins: the role of proteasomes The significance of tissue storage of any nutrient is availability during periods when either demand is higher than dietary supply or dietary supplies are precluded due to low or no feed intake. Protein turnover is the continual process of synthesis and degradation of structural and functional proteins.

Following synthesis, most proteins are degraded within a few hours to days (Mitch and Goldberg, 1996). Functional proteins such as enzymes have very high rates of turnover while structural proteins turn over more slowly.

When the diet includes organic selenium, selenomethionine and to some extent selenocysteine, are incorporated in body proteins such as muscle.

Protein turnover releases stored selenomethionine into the free amino acid pool where it is made available for synthesis of other proteins, including selenoproteins (Figure 6).


Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 8


Figure 6.Protein turnover releases stored selenomethionine to the free amino acid pool.



The mechanisms responsible for protein degradation include lysosomes such as cathepsins, which are active in pathological states such as starvation or remodeling following injury. Some cells do not have lysosomes, and structural proteins are too large to be internalized intact. Cytosolic proteases such as the calpains and those in organelles called proteasomes are important in degrading structural proteins (Klasing, 1998). Proteasomes are very large organelles. The average protein might be 40,000-80,000 Da while proteasomes are ~2 million Da. These specialized structures degrade proteins tagged with ubiquitin markers thereby releasing amino acids for synthesis of new proteins (Mitch and Goldberg, 1996).

In times of stress, activity in proteasomes speeds up as the pool of amino acids generated is needed for formation of immune defense cells and enzymes, including GSH-Px. During such times feed intake and thus gut absorption of minerals are in short supply, therefore reserves in tissues are of primary importance. When the diet includes organic selenium in the form of selenoamino acids, selenomethionine and selenocysteine are randomly distributed in proteinaceous tissues. Protein turnover frees reserves of selenomethionine and selenocysteine to the free amino acid pool for conversion to needed selenoproteins.

Though energetically expensive, the continual process of protein turnover is vital for homeostatic and homeorhetic processes as it allows 1) rapid change in protein amounts, 2) release of amino acids during periods of nutritional deprivation, 3) removal of faulty or damaged proteins, 4) removal of signal sequences of the protein necessary to establish tertiary structure, and 5) repair of proteins. As such, it is easy to see why rates of protein degradation are very high in fast-growing young animals and much slower in older or slower-growing species (Klasing, 1998).


Functions of the selenoproteins

Discovery of the physiological roles of selenium began in the 1970s when Rotruck et al. (1973) first identified selenium as a stoichiometric component of glutathione peroxidase. Starting in the mid-1980s, more selenoproteins were discovered and the scope of selenium biochemistry began to broaden from antioxidant defense to several aspects of mammalian metabolism including hormone activation, participation in the thioredoxin system and male fertility. At present less than 20 selenoproteins have been identified, though estimates of the existence of 30-50 such selenoproteins have been made based on electrophoretic separation (Körhle et al., 2000). Of the identified selenoproteins (Table 3), only about half have been functionally described; however, their roles explain many of the observed seleniumdeficiency syndromes we see in animals.


Table 3. Mammalian selenoproteins.

Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 9

From Köhrle et al. (2000)



THIOREDOXIN REDUCTASE

Cell surface and extracellular proteins are rich in stabilizing disulfides owing to the oxidizing nature of that environment, however the inside of the cell is maintained in a very reduced state. Intracellular proteins contain many free sulfydryl groups (-SH) and disulfides are rare. The major disulfide reductase responsible for maintaining proteins in a reduced state is thioredoxin. Together with glutathione, the other system involved in thiol status in cells, intracellular redox potential is kept low and free –SH groups are maintained (Arnér and Holmgren, 2000).

The ability of thioredoxin to transfer reducing power to cellular proteins is catalyzed by thioredoxin reductase, an enzyme that uses electrons from NADPH to reduce oxidized thioredoxin. All mammalian thioredoxin reductases are Se-dependent flavoproteins, though their identification by Tamura and Stadtman (1996) as selenoproteins is very recent. Zhong et al. (2000) determined that the active redox site consisted of selenocysteine and neighboring cysteine residue. While the link to cellular selenium status is now well-established, the relationship to selenium nutrition has not been investigated to any degree.

Like many of the selenoenzymes, knowledge of the physiological functions of the thioredoxin system is steadily increasing. Thioredoxins are high capacity electron donors for reductive enzymes including ribonucleotide reductase, thioredoxin peroxidase, and through thiol/disulfide exchange reduce key cysteine residues in transcription factors that increases binding to DNA and alters gene transcription (Mustacich and Powis, 2000).

Other functions identified for thioredoxins include cell growth factors and the ability to inhibit apoptosis (cell death). In addition, another characteristic of the mammalian thioredoxin reductases is broad substrate specificity. In addition to acting on thioredoxin, this selenoenzyme reduces hydroperoxides, ascorbate and selenite (Arnér and Holmgren, 2000). Burk and Hill (1999) suggested that ascorbate, because it has been shown in vitro to recycle tocopheroxyl to tocopherol, may be the biochemical link between vitamin E and selenium. Some of the known biological functions of thioredoxin and thioredoxin reductase are illustrated in Figure 7 and Table 4.


Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 10


Figure 7.Reactions and functions of thioredoxin reductase in the cell (from Mustacich and Powis, 2000).



Table 4.Roles of thioredoxin in various organisms.

Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 11

Adapted from Arnér and Holmgren (2000).



SELENOPROTEIN P


Along with plasma/extracellular GSH-Px, selenoprotein P is the only other known extracellular selenoprotein. It is also unique among the selenoproteins in that it contains multiple selenocysteine residues. Selenoprotein P has been isolated from various tissues of the rat, mouse, human and bovine; and cDNA cloning and sequencing revealed 62-87% implied amino acid sequence identity among the species (Burk and Hill, 1999).

Selenoprotein P represents a large portion of the selenium in plasma. More than 60% of plasma selenium was found in selenoprotein P in rats, while the corresponding percentage in humans is ~50% (Burk and Hill, 1999; Xia et al., 1992). Though most of the selenium in ruminant blood is in the red blood cell, selenoprotein P contains most of the plasma selenium. Selenoprotein P was found to represent ~20% of total serum protein in beef cows given either 2.4, 4.7 or 8.7 mg Se from selenite or 4.8 mg from Sel- Plex organic selenium daily (Awadeh et al., 1998) (Table 5). This selenoprotein contained nearly 70% of total serum selenium when averaged across treatments.

While a specific biochemical function has not been identified for selenoprotein P, both extracellular antioxidant and selenium transport roles have been suggested. A transport role, originally suggested due to its high selenium content and extracellular location, is now thought unlikely both because of the apparent energy waste it would imply and the fact that selenium in selenoprotein P is incorporated during protein synthesis rather than just bound to the protein (Daniels, 1996; Burk and Hill, 1999). Burk and colleagues (1997) found selenoprotein P to be associated with endothielial cells in liver, kidney and brain of rats. Secreted into plasma by the liver and into interstitial spaces by cells in virtually all tissues, selenoprotein P binds heparin proteoglycans on cells and in the interstitial matrix (Burk and Hill, 1999). These authors suggest that selenoprotein P functions to protect endothelial cells against oxidants; and its ability to bind metals supports this point (Burk and Hill, 1993).


Table 5.Effects of selenium level and source on the distribution of serum proteins and the percentage of selenium in specific fractions.

Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 12

Adapted from Awadeh et al. (1998a)
abMeans in a row differ, P<0.05.
1 Micromoles of NADPH oxidized per minute per ml of Hb.



Selenoprotein P is responsive to dietary selenium and is reduced to 5- 10% of control levels in rats fed deficient diets. This selenoprotein has a higher priority for synthesis and declines less rapidly than GSH-Px when dietary selenium is limiting (Yang et al., 1989; Burk and Hill, 1993). While the percentages of selenium present in serum protein fractions were little affected by dietary form or level, the concentrations of selenium in whole blood and serum increased in beef cows and calves as selenium intake went up or the organic form was provided (Awadeh et al., 1998a).


SELENOPROTEIN W

Another selenoprotein without a confirmed function, selenoprotein W has been isolated from the rat, mouse, human, monkey and sheep (Burk and Hill, 1999). This selenoprotein is located in the cell cytosol and tissue occurrence is somewhat species dependent (Yeh et al., 1997a). Because highest concentrations of selenoprotein W occur in muscle and heart of sheep, interest arose as to its possible involvement in etiology of white muscle disease.

Yeh et al. (1997a) fed sheep diets containing either a basal seleniumdeficient diet or the same diet with 3 mg/kg Se (selenite). Selenoprotein Wwas present in greatest quantities in muscle, heart, brain, tongue and spleen, but little was found in liver tissue. All tissue levels of selenoprotein W were sensitive to dietary selenium content except brain. The brains of deficient sheep contained the same amount of selenoprotein W as those given the very high level, despite the fact that there was a 53% reduction in total brain selenium content and a 30% reduction in GSH-Px activity. This indicated that regulation of selenoprotein W synthesis differed from other tissues. Studies with rats by the same research group demonstrated that tissues differed in rates of increase in selenoprotein W content in response to graded increases in dietary selenium. Further, as in the sheep there was little correlation with tissue GSH-Px activity or tissue selenium content (Yeh et al., 1997b).

Like selenoprotein P, an antioxidant role is suspected. Both selenoproteins possess the selenol/thiol pairs analogous to the active site in thioredoxin reductase (Burk and Hill, 1999).


IODOTHYRONINE DEIODINASES


The second largest group of selenoproteins are the iodothyronine deiodinases (ID), all three of which have been identified as selenoproteins. The three deiodinases control the local availability and concentration of the active thyroid hormone, 3,3N,5-triiodothyronine (T3). These enzymes catalyse the conversion of the prohormone thyroxin (T4), which is secreted by the thyroid gland to T3 (Type I and II ID) or the deiodination of T4 and T3 to non-active metabolites (Type III ID) (Figure 8). The three isoenzymes are encoded by different genes and have tissue and development-specific patterns of expression and regulation (Köhrle et al., 2000). Type I ID (ID-I) activity in the liver and kidney as well as the thyroid is responsible for producing most of the circulating T3 (Brody, 1994). It is found in liver, kidney, brain, pituitary and ruminant brown adipose tissue. Type II ID is found in brain and pituitary of all species examined and brown adipose tissue of humans (Arthur, 1997).

An increase in thyroid hormone levels causes an increase in basal metabolic rate (BMR). This is associated with various reactions that use ATP and increase activity of the respiratory chain and O2 reduction. Most closely associated with higher thyroid hormone levels are activities of the Na-K-ATPase (the sodium pump) and fatty acid synthesis. Fatty acid synthesis increases when the rise in thyroid activity diverts fatty acids in the liver away toward oxidation. The net effect is a futile cycle of fatty acid oxidation and synthesis that produces heat (Brody, 1994).

While thyroid hormone regulation is tied to a wide range of metabolic activities that include growth and nutrient utilization, the relationship between selenium status and thyroid hormone circulation is of particular importance to calves and lambs born in cold weather. The sources of cold-induced thermogenesis in neonatal ruminants are shivering thermogenesis in skeletal muscle and non-shivering thermogenesis in brown adipose tissue. Brown adipose tissue is a specialized organ found in cold-adapted animals, hibernators and most newborn mammals. It has been identified in calves, lambs and kids, but not piglets. Brown adipose tissue, under influence of locally-produced T3, increases production of an uncoupling protein which allows respiration to be uncoupled from ATP synthesis (oxidative phosphorylation) with the energy used to produce heat instead (Carstens, 1994).


Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 13


Figure 8.Conversion of T4 to T3 and to inactive catabolites (Brody, 1994).



Any decline in ID-I activity due to selenium deficiency would reduce T3, therefore limit heat production and render the newborn calf or lamb susceptible to cold stress (Arthur, 1993). ID-I is the most sensitive of the three deiodinases to dietary deficiencies of selenium. One of the characteristics of selenium deficiency is an acute increase (up to 2-fold) in T4 concentration and a smaller decrease in T3 concentration (Arthur, 1997).

Calves and lambs born to selenium-deficient dams have lower circulating T3 levels and relatively higher T4 levels. Kincaid and co-workers found that beef calves born to cows supplemented with Sel-Plex had higher circulating T3 concentrations at birth than those given an equal amount of selenite (Table 6, Awadeh et al., 1998b). In similar studies with sheep, T3 concentrations were 39% lower in control ewes at lambing and tended to be higher in lambs born to selenium-supplemented ewes though neither were affected by selenium source. Concentrations of selenium in whole blood of newborn lambs was increased 2-fold and 4-fold by selenite and Sel-Plex supplements, respectively, relative to the unsupplemented controls (Kincaid et al., 1999).


Table 6.Effect of maternal intakes of selenium on selenium status and thyroid hormones in day-old calves1.

Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 14

1Adapted from Awadeh et al., 1998b.
EU = nmol NADPH oxidized/min/mg hemoglobin.
abMeans with different superscripts differ, P<0.05.




The relationship between selenium status and thyroid hormone activity has a number of other implications in cattle as well as other species. Reduced thyroid hormone activity explains why selenium-deficient animals grow more slowly as effects of this hormone are chiefly anabolic (Arthur, 1993). This could explain in part the ‘ill-thrift’ syndrome associated with low selenium status. Feathering in poultry is related to thyroid hormone activity; and both faster feathering rate and improved feed efficiency have been associated with supplementation of organic selenium in broilers (Edens et al., 2000).

Reduced ID-I activity in the pituitary is associated with lower levels of growth hormone during selenium deficiency in some species (MacPherson, 1994).


GLUTATHIONE PEROXIDASE


Glutathione peroxidase (GSH-Px) was the first selenoprotein to be described and is still the most comprehensively understood. To date four structurally and genetically different forms of selenium-containing GSH-Px have been functionally described (Ursini et al., 1995) and exist in different tissues or parts of the cell. Classical or cytosolic GSH-Px (cGSH-Px) is the most abundant selenoprotein in most mammalian tissues.

In conjunction with glutathione (reducing agent), the pentose phosphate cycle (generates NADPH + H+ for reduction of diglutathione) and glutathione reductase (regeneration of glutathione), GSH-Px efficiently metabolizes hydrogen peroxide (H2O2) as well as unesterified fatty acid hydroperoxides (Wolffram, 1999). A second cytosolic GSH-Px, functionally indistiguishable from classic GSH-Px, has been identified in gastrointestinal tract cells and is termed GSH-Px-GI. The third GSH-Px is an extracellular form present in plasma (pGSH-Px). Extracellular GSH-Px has been found in bronchial lavage, is excreted by the placenta into maternal circulation and is the only form found in human milk (Daniels, 1996).

A membrane-bound form of GSH-Px, phospholipid hydroperoxide GSHPx (GSH-Px-PH) offers part of the explanation for the interaction of vitamin E and selenium. Together with vitamin E, GSH-Px-PH may act as a chainbreaking antioxidant to protect phospholipid membranes. Phospholipid GSHPx may also be involved in regulation of leukotriene biosynthesis. This enzyme is preferentially expressed in reproductive and endocrine tissues (the testes is the tissue containing highest amounts of GSH-Px-PH) and has been determined to explain the role of selenium in male reproduction (Köhrle et al., 2000).


Selenium-responsive conditions

In production of food animals we are interested in both the classicallydescribed deficiency syndromes, which occur in many regions of the world despite supplementation, and problems brought about by marginal deficiencies. The physiological and histological manifestations of primary selenium deficiency have been described in many species (Table 7), and are broadly similar. Many have been associated with failure of antioxidant protection of specific tissues. On a practical basis, Maas (1998) divided the selenium-responsive diseases or syndromes that appear in ruminants into four categories: musculoskeletal, reproductive, gastrointestinal/efficiency and immunologic, which can be generalized to other species with the possible addition of circulatory/edema disorders, particularly where poultry are concerned. An additional category of responses to selenium that has emerged recently centers on the oxidative stability of food animal products.


MUSCULOSKELETAL DYSFUNCTIONS: NUTRITIONAL MUSCULAR DYSTROPHY

Nutritional muscular dystrophy (NMD) occurs throughout the world wherever there are grazing ruminants; but has been observed in a wide range of animal species including foals, rabbits, marsupials (Levander, 1986) fish (Combs and Combs, 1986). While NMD has received widest attention in calves and lambs (‘white muscle disease’), it is also noted in chicks in association with exudative diathesis and in poults and in ducks. Exudative diathesis is a noninflammatory degeneration or necrosis of varying severity of the skeletal or cardiac muscle or both (Levander, 1986). The cardiac form of NMD is most severe, with necrosis and calcification of the heart and intercostal muscles. It occurs within 2-3 days of birth and is often fatal.

Calves or lambs affected at 1-4 weeks of age appear lame or stiff and reluctant to move. Neonatal weakness is a less severe manifestation, while adult myodegeneration is usually associated with exercise or parturition (Maas, 1998).


Table 7.Selenium deficiency syndromes of various animal species.

Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 15

To enlarge the image, click here
From Combs and Combs (1986)
aPartial protection
bOnly at high intake level
cPartial protection by cyst(e)ine
dOther factors may also protect
eMay relate to immune function
fExacerbated by high levels of PUFA
gNot well understood




Selenium status of the dam determines health and livability of the progeny

Progeny born to selenium-deficient dams are at highest risk of deficiency syndromes. Mobilization and transfer of body stores to the fetus during gestation and the shift of nutrients from blood to form colostrum reduces nutrient reserves of the dam; and feedstuffs low in organic selenium force reliance on inorganic supplements.

Comparisons of selenite and Sel-Plex organic selenium supplement forms in beef and dairy cows, sheep, hens and sows have consistently demonstrated that when the dam is given Sel- Plex during gestation, selenium status is better at parturition or hatch. Gilts given 0.3 ppm Sel-Plex selenium were better able to maintain serum selenium content as farrowing approached and had higher loin, liver and pancreatic selenium content than gilts given 0.3 ppm Se from selenite (Mahan and Kim, 1996). Kincaid et al. (1999) observed that whole blood selenium of ewes given Sel-Plex organic selenium was higher at lambing than those given selenite. Newborn lambs from Sel-Plex-fed ewes also had higher blood selenium content (Figure 9). These data provide further evidence that organic selenium forms are more readily transported across the placenta.

Dietary organic selenium is also used in formation of colostrum and milk proteins and provides an increased supply of easily metabolized selenium to nursing calves (Knowles et al., 1999), lambs (Kincaid et al., 1999) and piglets (Mahan, 2000).


REPRODUCTIVE DISORDERS

Selenium deficiency adversely affects reproduction in both sexes and all species including humans (MacPherson, 1994). The problem is most extensively described in cattle and sheep; and while there have been reports of selenium deficiency as a direct cause of abortion, the related problems of increased disease susceptibility and retained placenta make it difficult to view infertility in the dam in terms of a single factor (Maas, 1998). Selenium supply to particular tissues may also be important. Conrad (1985) reported that uterine involution was completed eight days sooner in cows supplemented with selenium and vitamin E. He also found that increasing whole blood selenium was associated with higher selenium in the ovary and that GSHPx activity in follicular fluid correlated with plasma GSH-Px. Significant GSH-Px activity was present in luteal tissue as well.


Selenium metabolism in animals: the relationship between dietary selenium form and physiological response - Image 16


Figure 9. Effect of selenium supplement form on whole blood selenium content of ewes at lambing and newborn lambs (from Kincaid et al., 1999; abP#0.001).



Egg production is depressed by selenium deficiency in both chickens and turkeys; however the impact on hatchability is more severe in turkeys (Cantor and Scott, 1974; Cantor, 1997).

While the role of selenium status in female fertility is more recognized than understood, recent work has provided much clearer insight into the well known association between selenium and male fertility through identification of the membrane-bound phospholipid hydroperoxide GSH-Px in the testes.

In sperm, selenium is largely associated with the keratin-like material in the mitochondrial helix in the midpiece of spermatozoa and was previously referred to as the ‘mitochondrial capsule selenoprotein’. Recently it has been determined that GSH-Px-PH is abundantly expressed in spermatogenic cells, but exists as an enzymatically-inactive structural protein in mature sperm where it contributes to formation of the mitochondrial capsule (Ursini et al., 1999). In spermatozoa, GSH-Px-PH therefore replaces ‘sperm capsule selenoprotein’ as the link between selenium and fertility. Morphological defects of sperm in selenium-deficient males can be attributed to inadequate GSH-Px synthesis (Köhrle et al., 2000).


‘ILL-THRIFT’, POOR EFFICIENCY

Selenium deficient cattle often present signs of ill-thrift, diarrhea or abnormal weight gains (Maas, 1998). Arthur (1993) points out that the role of selenium in thyroid hormone function may well explain the slower growth rates of deficient animals. In addition, studies in the past few years comparing inorganic and organic selenium supplements have reported improved growth rate and/or efficiency in broilers (Edens, this volume; Naylor et al., 2000; Roch et al., 2000) and in feedlot steers during the grower phase (see Clyburn et al., this volume) in association with higher selenium status when diets included Sel-Plex. In all of these studies diets were supplemented with either 0.2 or 0.3 ppm Se in addition to that contained in ingredients. These data suggest that the requirement for selenium in rapidly growing modern genetic stock may be higher than previously thought.


IMMUNOLOGIC RESPONSES TO SELENIUM

Selenium deficiency is well known to decrease both cellular and humoral immune function in man and laboratory animals (Combs and Combs, 1986).

While knowledge of specific mechanisms in domestic livestock is less detailed than in laboratory animals, the increase in susceptibility to disease in deficient/ marginally deficient livestock is well recognized (Maas, 1998). Selenium deficiency is an established risk factor in mastitis incidence; and has been correlated with decreased bactericidal activity of neutrophils and the severity and duration of mastitis infection (Sordillo et al., 1997).

Humoral immune responses to selenium have been reported including increased colostral IgG in beef cows (Swecker et al., 1995) and increased antibody response to antigen in weaned calves (Swecker et al., 1989).

Awadeh et al. (1998) noted that concentrations of IgG and IgM in serum of cows and their calves were significantly lower when given salt with 20 ppm Se. Serum IgM concentrations were higher (P<.05) in cows given salt with 60 ppm Se as Sel-Plex (~4.7 mg Se/day) compared to cows supplemented with 60 ppm Se as selenite.

While we recognize that nutritional deficiencies in general tend to increase susceptibility to bacterial and viral infection, nutritional status is now known to affect viral virulence. This work began when researchers recognized that incidence of Keshan disease in China peaked at different times in different parts of the country in association with periods of greatest infectious disease. Among the viruses isolated from Keshan disease victims was a coxsackie B4 virus. Subsequent testing revealed that a normally avirulent strain of the virus expressed virulence and caused heart damage in selenium deficient, but not selenium sufficient, mice due to point mutations in the viral genome (Beck, 1998). This information is of keen interest in human medicine and has engendered new studies of relationships between regional nutrition and emerging viruses; however it also holds implications for our intensivelyreared food animals.


CIRCULATORY

Exudative diathesis is described in chickens, turkeys and ducks and Japanese quail (Combs and Combs, 1986). It has the appearance of massive subutaneous hemorrhages and arises from abnormal permeability of the capillary walls. This failure of antioxidant membrane protection can be prevented with either vitamin E or selenium, however they clearly interact.

Chicks fed selenium-deficient diets required 100-110 IU vitamin E per kg of diet to prevent the disorder and when fed diets containing no vitamin E they required 0.10-0.15 ppm Se for similar protection (Combs and Combs, 1976).

Selenium status and its impact on capillary membrane stability has also been found to be related to ascites incidence. The main mechanism for pulmonary hypertension syndrome (ascites) is an increase in intravascular pressure occurring secondary to right ventricular failure. The increased pressure pushes fluid out of blood vessels and into the abdominal cavity, hence the ascites (Roch et al., 2000). During hypoxia, various mechanisms increase free radical production; and various researchers have suggested that antioxidant status is lower in broilers subject to ascites.

Roch and coworkers hypothesized that dietary vitamin E level and selenium form would interact to affect ascites incidence by increasing stability of the red blood cell membrane. They found that ascites mortality rate decreased with addition of 250 IU/kg vitamin E, 0.3 ppm Se from organic selenium alone, or the combination of vitamin E and 0.6 ppm Se from selenite. Interestingly, the biggest difference in ascites mortality rate was observed with organic selenium alone or with the combination of vitamin E and organic selenium (P<0.5).

Selenium status may also have an impact on oxygen metabolism during athletic work. Exercise is known to increase oxidative stress and therefore may lead to free radical generation and lipid peroxidation/damage in both respiratory tissues and working muscle. Pagan et al. (1999) found that plasma and whole blood selenium (and selenium excretion) increased when Thoroughbred horses began an exercise bout on the treadmill and declined slowly with time following exercise. At 24 hrs post-exercise, plasma selenium in horses given selenite had returned to pre-exercise levels but remained elevated in horses given Sel-Plex.


Conclusions

The physiological roles of selenium are expressed through a range of functional selenoproteins, most of which appear to have redox functions throughout the body in intracellular and extracellular locations, as well as in cell membranes. This series of antioxidant protective functions translates into wide-ranging effects on metabolism of nutrients and growth, disease resistance and reproduction, many of which are not yet fully understood.

Knowledge of the importance of selenium status in human health and animal health and performance is increasing rapidly; and ongoing work on the human medical and animal sciences will be mutually beneficial.

Selenium status during critical points of growth, reproduction, stress or disease challenge depends on the presence of readily mobilized tissue reserves. When the diet includes organic selenium in the form of selenomethionine from forages, grains or selenium yeast, this methionine analogue is incorporated into general body proteins. This prevents loss via urinary excretion and releases needed selenomethionine for selenoprotein synthesis through normal protein turnover mechanisms. Incorporation into fetal, milk and egg proteins also provides needed selenium for embryonic and post-natal development; a function which cannot be fulfilled by inorganic sources at a rate to meet needs of modern livestock and poultry breeds.


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Author: KATE A. JACQUES
North American Biosciences Center, Alltech Inc., Nicholasville, KY, USA


 
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