The metabolism of selenocompounds and distribution of selenium on a worldwide basis have been presented.3-7 As much as 80% of the total selenium in some accumulator plants is present as SeMCYS, which until recently was thought to be absent in nonaccumulator plants. Selenocompounds present in plants may have a profound effect on the health of animals and humans. The total selenium content cannot be used as an indication of its efficacy but the knowledge of individual selenocompounds is critical for proper interpretation of the results. Thus, speci-ation of the selenocompounds has moved to the forefront. Because animals and humans are dependent on plants for their nutritional requirements, this makes the types of selenocompounds in plants even more critical.
When rats are injected with selenite, the majority of the selenium is present in tissues in the form of selenocysteine.89 As expected, no Semet was found under the conditions of these studies. In contrast to plants, there is no known pathway in animals for synthesis of Semet from inorganic selenium, and thus they must depend on plant or microbial sources for this selenoamino acid. However, animals can convert Semet to selenocysteine. One day after injection of Semet there is about three times as much Semet as selenocysteine in tissues, but 5 or more days afterward the majority (46 to 57%) of the selenium is present as selenocysteine.910
Selenium exists in the form of selenoproteins. A total of 25 selenoproteins have been identified in eukaryotes.1112 These selenoproteins have been subdivided into groups based on the location of the selenocysteine. The first group (including glutathione peroxidase, GPX) is the most abundant and includes proteins in which selenocysteine is located in the N-terminal portion of a relatively short functional domain. This group includes the four GPXs, selenoproteins P, Pb, W, W2, T, T2, and BthD (from Drosophila). The second group of eukaryotic selenoproteins is characterized by the presence of selenocysteine in C-terminal sequences. These include the three thioredoxin reductases and the G-rich protein from Drosophila. Other eukaryotic selenoproteins are currently placed in the third group that consists of the three deiodinase isozymes, selenoproteins R and N, the 15-kDa selenoprotein and selenophosphate synthetase.
The four GPXs are located in different parts of tissues and all detoxify hydrogen peroxide and fatty acid-derived hydroperoxides and thus are considered antioxidant selenoenzymes. The three deiodinases convert thyroxine to triiodot-hyronine, thus regulating thyroid hormone metabolism. The thioredoxin reduc-tases reduce intramolecular disulfide bonds and, among other reactions, regenerate vitamin C from its oxidized state. These reductases can also affect the redox regulation of a variety of factors, including ribonucleotide reductase, the gluco-corticoid receptor, and the transcription factors.13 Selenophosphate synthetase synthesizes selenophosphate, which is a precursor for the synthesis of selenocys-teine.14 The functions of the other selenoproteins have not been definitely identified.
Selenium is present in all eukaryotic selenoproteins as selenocysteine.11 Semet is incorporated randomly in animal proteins in place of methionine. By contrast, the incorporation of selenocysteine into proteins known as selenopro-teins is not random. Thus, in contrast to Semet, selenocysteine does not randomly substitute for cysteine. In fact, selenocysteine has it own triplet code (UGA) and is considered to be the 21st genetically coded amino acid. Interestingly, UGA has a dual role in the genetic code, serving as a signal for termination and also a codon for selenocysteine insertion. Whether it serves as a stop codon or encodes selenocysteine depends on the location of what is called the selenocysteine insertion sequence.14 The selenocysteine insertion sequences (seven so far) for the various selenoproteins have been presented.12
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