Next Page Selenium (Se) is not an essential element for plants. However, selenium is an essential micronutrient for animals because of the role of selenocysteine as the 21st essential amino acid (Stadtman, 1996) and is obtained via a food web that ultimately rests on the inadvertent uptake of Se by higher plants, for which no essential nor beneficial role has been firmly established except for Se-hyperaccumulator plants (Lauchli, 1993; Stadtman, 1996). The chemistry and biochemistry of selenium is more easily expressed in terms of the better-known sulfur chemistry because of the great similarities in chemical properties that they share by virtue of being adjacent Group VIA elements, leading to significant interactions from both a chemical and biological standpoint. The sulfur equivalents of selenide (Se2-), elemental Se, thioselenate (Se2O32-), selenite (SeO32-) and selenate (SeO42- are sulfide, elemental S, thiosulfate, sulfite, and sulfate, respectively. When Se is assimilated into amino acids by bacteria, plants, and animals, the Se analogs of S amino acids, Se-cysteine and Se-methionine, are synthesized. The function of the Se analogs is not equivalent to the S amino acids and excessive Se assimilation by plants results in toxicity, growth limitation, and death. The selenium chemistry of plants has been reviewed several times (Brown and Shrift, 1982; Anderson and Scarf, 1983), most recently by Lauchli (1993).
Interaction between S and Se uptake and assimilation by plants is expected, particularly when S and Se are present as sulfate (SO42-) and selenate (SeO42-), their most chemically stable forms in aerated, neutral and near-neutral natural waters. Antagonistic sulfate/selenate interactions in uptake have been reported in single cells, excised roots, and whole plants (Lauchli, 1993).
Dietary intake of crops grown in Se-poor soils has been associated with increased rates of certain cancers in humans (Jackson, 1988) and Se is thought to play a role in reducing the growth of cancerous tumors in animal systems (Axley et al., 1991) and lung, colorectal, and prostate cancers in humans (Clark et al., 1996). In studies using Se-enriched garlic, Ip and Ganther (1992) demonstrated organoselenium compounds are more active than S analogs in chemoprevention. Further investigation revealed Se-enriched garlic was superior to unenriched garlic in suppression of mammary tumors in cancer-treated mice (Ip et al., 1992; Ip and Lisk, 1995). Feeding Se-enriched garlic and onion to cancer-induced rats reduced total tumor yield but did not cause excessive Se accumulation in animal tissues (Ip and Lisk, 1994a and 1994b), suggesting that Se-enriched vegetables may be a better delivery source for organoselenium analogs than the commonly used selenite or selenomethionine. At present, the active organoselenium chemopreventative agents in preparations from Se-enriched medium plant tissues have not yet been fully identified. Cai et al. (1995b) proposed that enhanced levels of selenocysteine were responsible for a reduction in mammary tumor growth in carcinogen-treated mice fed a Se-enriched garlic diet, although clinical evidence has yet to be produced. Furthermore, there are potentially scores of Se-analogs of specific organosulfur compounds in alliaceous species that may require investigation for phytopharmaceutical activity.
References:
Anderson, J. W.; Scarf, A. R. Selenium and Plant Metabolism. In Metals
and Micronutrients: Uptake and Utilization by Plants; Robb, D. A., Pierpoint,
W. S.; Eds.; Academic Press: London, 1983.
Axley, M. J.; Bock, A.; Stadtman, T. C. Catalytic Properties of an E. coli
formate dehydrogenase Mutant in which Sulfur Replaces Selenium. Proc. Natl.
Acad. Sci. U. S. A. 1991, 88:8450-8454.
Brown, T.A.; Shrift, A. Selenium: Toxicity and Tolerance in Higher Plants.
Biol. Rev. 1982, 57:59-84.
Cai, X-J.; Block, E.; Uden, P. C.; Zhang, X.; Quimby, B. D.; Sullivan, J. J.
Allium Chemistry: Identification of Selenoamino Acid in Ordinary and
Selenium-enriched Garlic, Onion, and Broccoli Using Gas Chromatography with
Atomic Emission Detection. J. Agric. Food Chem. 1995b, 43:1754-1757.
Clark, L. C.; Combs, G. F.; Turnbull, B. W.; Slate, E. H.; Chalker, D. K.;
Chow, J.; Davis, L. S.; Glover, R. A.; Graham, G. F.; Gross, E. G.; Krongrad,
A.; Lesher, J. L.; Park, H. K.; Sanders, B. B.; Smith, C. L.; Taylor, J. R.
Effects of Selenium Supplementation for Cancer Prevention in Patients With
Carcinoma of the Skin: A Randomized Controlled Trial. J. Am. Med. Assoc. 1996,
276:1957-1963.
Ip, C.; Ganther, H. Comparison of Selenium and Sulfur Analogs in Cancer
Prevention. Carcinogenesis. 1992, 1167-1170.
Ip, C.; Lisk, D. J. Characterization of Tissue Selenium Profiles and
Anticarcinogenic Responses in Rats Fed Natural Sources of Selenium-rich
Products. Carcinogenesis. 1994a, 15:573-576.
Ip, C.; Lisk, D. J. Enrichment of Selenium in Allium Vegetables for Cancer
Prevention. Carcinogenesis. 1994b, 15:1881-1885.
Ip, C.; Lisk, D.J. Efficacy of Cancer Prevention by High-selenium Garlic Is
Primarily Dependent on the Action of Selenium. Carcinogenesis. 1995,
16:2649-2652.
Jackson, M.L. Selenium: Geochemical Distribution and Associations with Human
Heart and Cancer Death Rates and Longevity in China and the Us. Biol. Trace.
Elem. Res. 1988, 15:13-21.
Lauchli, A. Selenium in Plants: Uptake, Functions, and Environmental Toxicity.
Bot. Acta. 1993, 455-68.
Stadtman, T. C. Selenocysteine. Annu. Rev. Biochem. 1996, 65:83-100.
For a comprehensive look at selenium, see "Environmental chemistry of selenium" by WTFrankenberger, Jr., and RA Engberg (eds.) New York : Marcel Dekker, c1998. 736 pp.
This page was last modified by Phillip Barak, Univ. of Wisconsin, on 20 Mar 2000. All rights reserved.