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.
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.
Arnér, E.S.J. and A. Holmgren. 2000. Physiological functions of thioredoxin reductase. Eur. J. Biochem. 267:6102-6109.
Arthur, J.R. 1993. The biochemical functions of selenium: relationships to thyroid metabolism and antioxidant systems. In: Rowett Research Institute Annual Report. Bucksburn, Aberdeen, UK.
Awadeh, F.T., M.M. Abdelrahman, R.L. Kinkaid and J.W. Finley. 1998a. Effect of selenium supplements on the distribution of selenium among serum proteins in cattle. J. Dairy Sci. 81:1089-1094.
Awadeh, F. T., R.L. Kincaid and K.A. Johnson. 1998b. Effect of level and source of dietary selenium on concentrations of thyroid hormones and immunoglobulins in beef cows and calves. J. Anim Sci. 76:1204.
Beck, M.A. 1998. The influence of antioxidant nutrients on viral infection. Nutrition Reviews 56(1):S140-S146.
Beilstein, M.A. and P.D. Whanger. 1986. Chemical forms of selenium in rat tissues after administration of selenite or selenomethionine. J. Nutr. 116(9):1711-1719.
Brody, T. 1994. Nutritional Biochemistry. Academic Press, Inc. New York, NY.
Burk, R.F. and K.E. Hill. 1993. Regulation of selenoproteins. Annu. Rev. Nutr. 13:65-81.
Burk, R.F. and K.E. Hill. 1999. Orphan selenoproteins. BioEssays 21(3):231- 237.
Burk, R.F., K.E. Hill, M.E. Boeglin, F.F. Ebner and H.S. Chittum. 1997. Selenoprotein P associates with endothelial cells in rat tissues. Histochem. Cell. Biol. 108:11-15.
Cantor, A.H. 1997. The role of selenium in poultry nutrition. In: Biotechnology in the Feed Industry, Proceedings of the 13th Annual Symposium. (T.P. Lyons and K.A. Jacques, eds). Nottingham University Press, Nottingham, UK.
Cantor, A.H. and M.L. Scott. 1974. The effect of selenium in the hen’s diet on egg production, hatchabihty, performance of progeny and selenium concentrations in eggs. Poultry Sci. 53:1870.
Carstens, G. 1994. Cold thermoregulation in the newborn calf. In: Perinatal Mortality in Beef Calves. Veterinary Clinics of North America: Food Animal Practice. Vol. 10(1):69-106.
Combs, G.F. and S. B. Combs. 1986. The Role of Selenium in Nutrition. Academic Press, New York.
Conrad. H.R. 1985. The role of selenium and vitamin E in bovin reproduction. In: Selenium Responsive Diseases in Food Animals. Proc. Symposium, Western States Veterinary Conference, Las Vegas, NV.
Daniels, L.A. 1996. Selenium metabolism and bioavailability. Biol. Trace Elem. Res. 54(3):185-199.
Edens, F.W., T.A. Carter, C.R. Parkhurst and A. E. Sefton. 2000. Effect of selenium source and litter type on broiler feathering. J. Appl. Poultry Res. 9:407-413.
Foster, L.H. and S. Sumar. 1997. Selenium in health and disease: a review. Critical Reviews in Food Science and Nutrition 37(3):211-228.
Harrison, J.H. and H.R. Conrad. 1984. Effect of selenium intake on selenium utilization by the non-lactating cow. J. Dairy Sci. 67:219.
Kelly, M.P. and R.F. Power. 1995. Fractionation and identification of the major selenium-containing compounds in selenized yeast. J. Dairy Sci. (Suppl. 1):237.
Kincaid, R.L., M. Rock and F. Awadeh. 1999. Selenium for ruminants: comparing organic and inorganic selenium for cattle and sheep. In: Biotechnolofgy in the Feed Industry, Proceedings of the 15th Annual Symposium. (T.P. Lyons and K.A. Jacques, eds). Nottingham University Press, UK.
Klasing, K.C. 1998. Comparative Avian Nutrition. CAB International, New York, NY.
Knowles, S.O., N.D. Grace, K. Wurms and J. Lee. 1999. Significance of amount and form of dietary selenium on blood, milk and casein selenium concentrations in grazing cows. J. Dairy Sci. 82:429-437.
Koenig, K.M., L.M. Rode, R.D.H. Cohen and W.T. Buckley. 1997. Effects of diet and chemical form of selenium on selenium metabolism in sheep. J. Anim. Sci. 75:817-827.
Köhrle, J., R. Brigelius-Flohe, A. Bock, R. Gartner, O. Meyer and L. Flohe. 2000. Selenium in biology: facts and medical perspectives. Biol. Chem. 381(9-10):849-864.
Levander, O.A. 1986. Selenium. In: Trace Elements in Human and Animal Nutrition. (W. Mertz, ed.) Academic Press, Harcourt Brace Jovanovich, New York.
Low, S.C. and M.J. Berry. 1996. Knowing when not to stop: Selenocysteine incorporation in eukaryotes. TIBS 21:203-208.
MacPherson, A. 1994. Selenium, vitamin E and biological oxidation. In: Recent Advances in Animal Nutrition (P.C. Garnsworthy and D.J.A. Cole, eds). Nottingham University Press, Nottingham, UK.
Mahan, D.C. 2000. Effect of organic and inorganic selenium sources and levels on sow colostrum and milk selenium concentration. J. Anim. Sci. 78:100.
Mahan, D.C. and Y.Y. Kim. 1996. Effect of inorganic or organic selenium at two dietary levels on reproductive performance and tissue selenium concentrations in first parity gilts and their progeny. J. Anim. Sci. 74:2711.
Marschner, H. 1995. Mineral Nutrition of Higher Plants. Academic Press, Harcourt Brace & Company, London, New York.
Maas, J. 1998. Studies on selenium metabolism in cattle: deficiency, supplementation and environmental fate of supplemented selenium. In: Selenium-Tellurium Development Association, 6th International Symposium, May 10-12, Scottsdale, AZ.
Mayland, H.F. 1986. Selenium in soils and plants. In: Selenium Responsive Diseases in Food Animals. Proceedings, Western States Veterinary Conference, February 18. Schering Corporation, Kenilworth, NJ.
Mikkelsen, R.L., A.L. Page and F.T. Bingham. 1989. Factors affecting selenium accumulation by agricultural crops. In: Selenium in Agriculture and the Environment. SSSA Special Publication Number 23. American Society of Agronomy, Inc., Soil Science Society of America, Inc.
Mitch, M.E. and A.L. Goldberg. 1996. Mechanisms of muscle wasting. New England Journal of Medicine 335(25):1897-1905.
Mustacich, D. and G. Powis. 2000. Thioredoxin reductase. Biochem. J. 346:1-8.
Naylor, A.J., M. Choct and K.A. Jacques. 2000. Effects of selenium source and level on performance and meat quality in male broilers. Poultry Sci. 79 (Suppl. 1):117.
Oldfield, J.E. 1999. The case for selenium fertilization: an update! Bulletin of the Selenium-Tellurium Development Association, 301 Borgtstraat, Brussels, Belgium. August.
Olson, O. E., and I. S. Palmer. 1976. Selenoamino acids in tissues of rats administered inorganic selenium. Metabolism 25:299.
Pagan, J.D., P. Karnezos, M.A.P. Kennedy, T. Currier and K.E. Hoekstra. 1999. Effect of selenium source on selenium digestibility in exercised thoroughbreds. Equine Nutrition and Physiology, pp. 135-140.
Roch, G., M. Boulianne, and L. De Roth, 2000. Effect of dietary antioxidants on the incidence of pulmonary hypertension syndrome in broilers. In: Proceedings of Alltech’s 16th Annual Symposium, Biotechnology in the Feed Industry, T. P. Lyons and K. A. Jacques, eds. Nottingham University Press, Nottingham, NG11 0AX, United Kingdom. 16: 261-276.
Rotruck, J. T., A.L. Pope, H.E. Ganther, A.B. Swanson, D.G. Hafeman and W.G. Hoekstra. 1973. Selenium: biochemical role as a component of GSHPx. Science 170:588.
Schrauzer, G.N. 2000. Selenomethionine: A review of its nutritional significance, metabolism, and toxicity. J. Nutri. 130:1653-1656.
Sordillo, L.M., K. Shafer-Weaver and D. DeRosa. 1997. Immunobiology of the mammary gland. J. Dairy Sci. 80:1851-1865.
Swecker Jr., W.S., D.E. Eversole, C.D. Thatcher, D.J. Blodgett and G.G. Shurig. 1989. Influence of supplemental selenium on humoral immune responses in weaned beef calves. Am. J. Vet. Res. 50:1760.
Swecker, W.S., C.D. Thatcher, D.E. Eversole, D.J. Blodgett and G.G. Schurig. 1995. Effect of selenium supplementation on colostrum IgG concentration in cows grazing selenium-deficient pastures and on postsuckle serum IgG concentration in their calves. Am. J. Vet. Res. 56:450.
Tamura, T. and T.C. Stadtman. 1996. A new selenoprotein from human lung adenocarcinoma cells: purification, properties and thioredoxin reductase activity. Proc. Natl. Acad. Sci. USA 93:1006-1011.
Ursini, F., M. Maiorino, R. Brigeliusflohe, K.D. Aumann, A. Roveri, D. Schomburg and L. Flohe. 1995. Diversity of glutathione peroxidases. Meth. Enzymol. 252:38-53.
Ursini, F., S. Heim, M. Kiess, M. Maiorino, A. Roveri, J. Wissing and L. Flohe. 1999. Dual function of the selenoprotein PHGPx during sperm maturation. Science 285:1393-1396.
van Ryssen, J.B., J.T. Deagen, M.A. Beilstein and P.D. Whanger. 1989. Comparative metabolism of organic and inorganic selenium by sheep. J. Agric. Food Chem. 37:1358-1363.
van Ryssen, J.B. 1998. Effect of different conditions in the rumen of sheep on the metabolism of selenium in the organic and inorganic form. Department of Animal Science, University of Natal, Pietermaritzburg.
Whanger, P.D., P.H. Weswig and J.E. Oldfield. 1978. Selenium, sulfur and nitrogen levels in ovine rumen microorganisms. J. Anim. Sci. 46:515-519.
Wolffram, S., E. Anliker and E. Scharrer. 1986. Uptake of selenate and selenite by isolated brush border membrane vesicles from pig, sheep and rat intestine. Biol. Trace Elem. Res. 10:293-306.
Wolffram, S., B. Berger, B. Grenacher and E. Scharrer. 1989a. Transport of selenoamino acids and their sulfur analogues across the intestinal brush border membrane. J. Nutr. 119:706-712.
Wolffram, S., B. Berger and E. Scharrer. 1989b. Transport of selenomethionine and methionine across the intestinal brush border membrane. In: Selenium in Biology & Medicine, Proceedings of the 4th International Symposium on Selenium in Biology & Medicine. A. Wendel, ed., pp. 109-113, Springer Verlag, Berlin, Heidelberg, New York.
Wolffram, S. 1999. Absorption and metabolism of selenium: differences between organic and inorganic sources. In: Biotechnology in the Feed Industry (T.P. Lyons and K.A. Jacques, ed.). Nottingham University Press.
Wright, P.L. and M.C. Bell. 1966. Comparativc metabolism of selenium and tellurium in sheep and swine. Am. J. Physiol. 211:6-10.
Xia, Y., L. Zhao, L. Zhu and P.D. Whanger. 1992. Metabolism of selenate and selenomethionine by a selenium-deficient population of men in China. J. Nutr. Biochem. 3:202-210.
Yang, J.G., K.E. Hill and R.F. Burk. 1989. Dietary selenium intake controls rat plasma selenoprotein P concentration. J. Nutr. 119:1010-1012.
Yeh, J., Q. Gu, M.A. Beistein, N.E. Forsberg and P.D. Whanger. 1997a. Selenium influence tissue levels of selenoprotein W in sheep. J. Nutr. 127:394-402.
Yeh, J., S.C. Vendeland, Q. Gu, J.A. Butler, B-R. Ou and P.D. Whanger. 1997b. Dietary selenium increases selenoprotein W levels in rat tissues. J. Nutr. 127:2165-2172.
Zhong, L., E.S.J. Arnér and A. Holmgren. 2000. Structure and mechanism of mammalian thioredoxin reductase: oxidation of the C-terminal cysteine/ selenocysteine active site forms a thioselenide and replacement of selenium with sulfur markedly reduces catalytic activity. Proc. Natl. Acad. Sci. USA 97:2521-2526.