Selenomethionine metabolism

Hoffman, D.J., G.H. Heinz, and A.J. Krynitsky. 1989. Hepatic glutathione metabolism and lipid peroxidation in response to excess dietary selenomethionine and selenite in mallard ducklings. Jour. Toxicol. Environ. Health 27 263-271. 

Selenium is present in meat, seafood and cereals. The former two contain the highest levels. It is present in soil as inorganic selenium that enters the food chain via plants. In plant protein, it is present as selenomethionine and in animals as selenocysteine this difference is due to the metabolism of selenomethionine in the liver as part of the normal catabolic pathway for methionine (Chapter 8). Somewhat surprisingly, selenocysteine is incorporated into protein via a specific tRNA which possesses a UCA anticodon for this amino acid. 

Schrauzer, G.N. 2000. Selenomethionine a review of its nutritional significance, metabolism and toxicity. J. Nutr. 130, 1653-1656. 

Tuve, T. and Williams, H.H. 1961. Metabolism of selenium by Escherichia coli biosynthesis of selenomethionine. J. Biol. Chem. 236, 597-601. 

G. N. Schrauzer, The nutritional significance, metabolism and toxicology of selenomethionine, Adv. Food Nutr. Res., 47 (2003), 73D112. 

Plants absorb Se from soil primarily as selenate and translocate it to the chloroplast, where it follows the sulfur assimilation pathway. Se is reduced (enzymatically and non-enzymatically) to selenide, which reacts with serine to form selenocysteine (76). It can be further metabolized to selenomethionine (79) and methylated to form products such as. Se-methyl selenomethionine (89). Alternatively, selenocysteine-specific methyl transferase may form -methyl selenocysteine (83), allowing the plant to accumulate extraordinarily large amounts of Se. 

Proposed Pathway for Formation of Dimethyl Selenide from Selenite in Animals 3-6. Activation and Reduction of Selenate to Selenite in Yeast Saccharomyces cerevisiae 3-7. Conceptual Representation of a Physiologically Based Pharmacokinetic (PBPK) Model for a Hypothetical Chemical Substance 3-8. Selenite Model, a Kinetic Model for Selenite Metabolism 3-9. Selenomethionine Model, a Kinetic Model for Selenomethionine Metabolism 3-10.Existing Information on Health Effects of Selenium 6-1. Frequency of NPL Sites with Selenium Contamination... 

Selenomethionine metabolism to selenide and the incorporation into selenium-specific proteins may occur by two pathways metabolism to methane selenol and selenide or via selenocysteine. Evidence that the incorporation of selenium from selenomethionine into protein is by the transsulfuration pathway (methionine to cysteine) comes from studies of selenomethionine metabolism in lymphoblast cell lines deficient in cystathionine lyase and cystathionine synthetase, enzymes of the transsulfuration pathway (Beilstein and Whanger 1992). Deficiency in these enzymes greatly reduces the incorporation of selenomethionine into glutathione peroxidase. 

Figure 3-9. Selenomethionine Model, a Kinetic Model for Selenomethionine Metabolism...

Little is known about the specific biochemical mechanism(s) by which selenium and selenium compounds exert their acute toxic effects. Long-term effects on the hair, skin, nails, liver, and nervous system are also well documented, and a general theory has been developed to explain the toxicity of exposure to excess selenium, as discussed below. Generally, water-soluble forms are more easily absorbed and are generally of greater acute toxicity. Mechanisms of absorption and distribution for dermal and pulmonary uptake are unknown and subject to speculation, but an active transport mechanism for selenomethionine absorption in the intestine has been described (Spencer and Blau 1962). The mechanisms by which selenium exerts positive effects as a component of glutathione peroxidase, thioredoxin reductase, and the iodothyronine 5 -deiodinases are better understood, but the roles of other selenium-containing proteins in mammalian metabolism have not been clarified. 

Griffiths NM, Stewart RDH, Robinson MF. 1976. The metabolism of [75Se]selenomethionine in four women. Br J Nutr 35 373-382. 

---