Selenium, an essential trace element, is incorporated into the polypeptide chain of proteins in the form of a selenocysteine residue and demonstrates various physiological activities as selenoproteins. Both organic selenium (selenomethionine, selenocysteine, and selenium-containing proteins) and inorganic selenite serve as an effective selenium source for the biosynthesis of selenoproteins. However, molecular details of metabolism of such selenium sources in the cell are not clear. Because levels of free selenide in cells are extremely low, it is thought that there is a system that efficiently relays a selenium substrate to enzymes involved in selenoprotein biosynthesis. Selenocysteine lyase (SCL) decomposes L-selenocysteine to form an enzyme-bound perselenide (-S-SeH) on the active site cysteine residue and is proposed to function as a selenium delivery protein for selenophosphate synthetase. We revealed the mechanism for selenium-specific substrate recognition of SCL.

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Selenium is a naturally occurring trace element that belongs to the group 16 of the Periodic Table. The mineral is required as an essential micronutrient for mammals, birds, reptiles, amphibians, fishes, algae, and some microorganisms including archaea. Higher plants can accumulate selenium under the circumstances where selenium is abundant in soil, but their essentiality in plants is yet to be established. Inorganic forms of selenium take one of the four oxidation states including selenate SeO₄ ^2-, selenite SeO₃ ^2-, elemental selenium Se⁰ and divalent selenide Se^2-. An activated form of divalent selenide, monoselenophosphate HSe-PO₃H₂ serves as a reactive substrate for the biogenesis of SeCys-tRNA^UGA, which is cotranslationally inserted in the growing polypeptide chain at the in-frame UGA codon. This review summarizes present understanding regarding the reductive assimilation pathway of inorganic oxyanions of selenium, and introduces confronting two hypotheses on how the selenium could be specifically reduced and efficiently utilized for monoselenophosphate synthetase.

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Phospholipid hydroperoxide glutathione peroxidase(PHGPx) is an intracellular antioxidant enzyme that directly reduces peroxidized phospholipids. PHGPx is strongly expressed in the mitochondria of testis and spermatozoa. We previously found a significant decrease in the expression of PHGPx in spermatozoa from 30% of infertile human males diagnosed with oligoasthenozoospermia. To clarify whether defective PHGPx in testis causes male infertility, we established testis-specific PHGPx knockout mice using a Cre-loxP system. All the testis-specific PHGPx knockout male mice were found to be infertile in spite of normal plug formation after mating and displayed a significant decrease in the number of spermatozoa. These spermatozoa showed significant reductions of forward motility and the mitochondrial membrane potential. These impairments were accompanied by the structural abnormality, such as a hairpin-like flagella bend at the midpiece and swelling of mitochondria in the spermatozoa. These results demonstrate that depletion of PHGPx in spermatozoa causes severe abnormalities in the spermatozoa, which may be one of the causes of male infertility in mice and humans.

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Although selenium (Se) and tellurium (Te) are non-essential elements in plants, they are utilized in metabolic pathways to form the metalloid-containing compounds having carbon-metalloid covalent bond(s) (organic seleno/tellurometabolites). Therefore, it is necessary to identify seleno/tellurocompounds to determine the metabolic pathway of Se and Te, and understand the beneficial or toxicological effects of these compounds. Many literatures showed that the complementary use of HPLC-ICP-MS and HPLC-ESI (APCI)-MS/MS is a powerful tool for the speciation and identification of these metabolites. In this review, our recent works on the complementary use of ICP-MS, ESI-MS/MS and NMR from the viewpoint of identification of unknown seleno/tellurometabolites in plant samples were summarized. In particular, (1) speciation of sulfur (S) and Se-containing amino acids in Allium plants, (2) identification of a novel selenocompound in selenized Japanese pungent radish, (3) evaluation of function and translation efficacy of selenomethionine (SeMet)-containing proteins, (4) identification of SeMet-metabolites in wheat germ extracts, and (5) evaluation of Te metabolism in the Se-accumulating plant were mentioned.

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