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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