Dietary Selenium Absorption and Mechanisms of Incorporation of Selenium into Selenoproteins

Rich food sources of selenium in human diets include Brazil nuts, offal, shellfish, and some other types of fish, although there is uncertainty about the extent of selenium bioavailability in some foods, which may in turn be linked to problems of mercury contamination. In the United Kingdom, cereal foods account for approximately 20% of total selenium intake, whereas meat, poultry, and fish account for 30-40%.

Selenium is readily absorbed, especially in the duodenum but also in the caecum and colon.

Seleno-amino acids are almost completely absorbed: selenomethionine via the gut methionine transporter and selenocysteine probably via the cysteine transporter. Both selenite and selenate are >50% absorbed, selenite more readily so than selenate, and for these forms there is competition with sulphate transport. Selenite is more efficiently retained then selenate because part of the latter is rapidly excreted into the urine. Vitamins A, E, and C can modulate selenium absorption, and there is a complex relationship between selenium and vitamin E that has not been entirely elucidated for man. A combined deficiency of both nutrients can produce increases in oxidative damage markers (malondialdehyde, F2 isoprostanes, and breath hydrocarbons) and in pathological changes that are not seen with either deficiency alone. Inorganic Se is reduced to selenide by glutathione plus glutathione reductase and is then carried in the blood plasma, bound mainly to protein in the very low-density lipoprotein fraction. Selenomethionine is partly carried in the albumin fraction.

Figure 1 summarizes the main pathways of interconversion of selenium in mammalian tissues. Selenium appears not to be an essential element for plants, but it is normally taken up readily into their tissues and is substituted in place of sulfur, forming the seleno-amino acids selenomethionine and selenocysteine, which are then incorporated at random in place of the corresponding sulfur amino acids into plant proteins. All branches of the animal kingdom handle selenium in essentially similar ways. When ingested, plant selenium-containing proteins liberate free seleno-methionine and selenocysteine, either for incorporation at random into animal proteins or for metabolic turnover, to liberate inorganic selenide, which is the precursor of active selenium to be inserted at the active site(s) of the selenoproteins. Selenide is also supplied by the reduction of selenite and selenate that enters the diet from nonorganic sources (i.e., from the environment) or from dietary supplements of inorganic selenium. The inorganic forms of the element are absorbed with approximately 50-90% efficiency (i.e., only slightly less than the >90% efficiency of absorption of selenomethionine).

Selenide represents the 'crossroads' of selenium metabolism, from which it may either be committed to specific selenoprotein synthesis or be removed from the body by urinary excretion pathways that involve its detoxification by methylation to methyl selenides, of which the largest fraction is usually trimethyl selenonium. If used for selenoprotein synthesis, selenide combines with a chaperone protein, and the first metabolic step is its conversion to selenophosphate by the ATP-requiring enzyme

SELENOPROTEIN SYNTHESIS

Selenophosphate (SePO| )

DIET

Selenide (Se2-)

Peptidyl selenomethionine tJ

Diet: Selenomethionine

EXCRETION

Methyl selenol

Peptidylserine Selenocysteine in transfer-RNA (tRNAsec)

Dimethyl selenol

Trimethyl selenonium

Functional selenoproteins Figure 1 Interconversion of different selenium species in animal and human tissues.

selenophosphate synthetase, which is a selenoprotein. This then becomes the precursor for selenocysteinyl-soluble (or transfer) RNA, which is synthesized from a serine moiety attached to a specific soluble (transfer) RNA identified as tRNAsec. This serine-tRNA complex is first dehydrated to aminoacrylyl-tRNA in a reaction that requires a vitamin B6 cofactor, pyridoxal phosphate. This product then reacts with selenophosphate in a reaction that requires magnesium and the enzyme selenosynthase. The resulting selenocysteinyl-tRNA then recognizes a UGA codon in the messenger RNA sequence. This codon is also used as a stop sequence; therefore, the adjacent mRNA structure has to provide the correct 'context' (e.g., a stem-loop structure) to direct the incorporation of selenocysteine into the growing polypeptide chain of the selenoprotein. Other gene products are involved, and although the sequence of reactions and the participatory proteins have been studied in detail and largely elucidated for prokaryotes such as Escherichia coli, the analogous pathways are only partly understood for eukaryotes such as mammals. Specific selenoprotein synthesis is often tissue specific, with different versions of structurally similar selenoproteins being made at different tissue sites. In liver, for instance, provided that the selenium supply is generous, there is a considerable accumulation of cytosolic glutathione peroxidase type I, which can act as a storage repository of selenium for later liberation and redistribution.

Degradation of selenocysteine is catalyzed by selenocysteine lyase, which releases elemental Se, and this is then reduced to selenide by glutathione or other thiols. The urinary excretion pathway is very important for selenium homeostasis of the tissues. Urinary selenium tends to reflect recent intake rather than tissue status, but it can be a useful source of information about possible selenium overload.

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