Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: KEGG:D02011 (FAD)
 5,530 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The possible relationship of selenium to immunological function which has been suggested for decades was investigated in studies on selenium metabolism in human T cells. One of the major 75Se-labeled selenoproteins detected was purified to homogeneity and shown to be a homodimer of 55-kDa subunits. Each subunit contained about 1 FAD and at least 0.74 Se. This protein proved to be thioredoxin reductase (TR) on the basis of its catalytic activities, cross-reactivity with anti-rat liver TR antibodies, and sequence identities of several tryptic peptides with the published deduced sequence of human placental TR. Physicochemical characteristics of T-cell TR were similar to those of a selenocysteine (Secys)-containing TR recently isolated from human lung adenocarcinoma cells. The sequence of a 12-residue 75Se-labeled tryptic peptide from T-cell TR was identical with a C-terminal-deduced sequence of human placental TR except that Secys was present in the position corresponding to TGA, previously thought to be the termination codon, and this was followed by Gly-499, the actual C-terminal amino acid. The presence of the unusual conserved Cys-Secys-Gly sequence at the C terminus of TR in addition to the redox active cysteines of the Cys-Val-Asn-Val-Gly-Cys motif in the FAD-binding region may account for the peroxidase activity and the relatively low substrate specificity of mammalian TRs. The finding that T-cell TR is a selenoenzyme that contains Se in a conserved C-terminal region provides another example of the role of selenium in a major antioxidant enzyme system (i.e., thioredoxin-thioredoxin reductase), in addition to the well-known glutathione peroxidase enzyme system.
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PMID:Selenocysteine, identified as the penultimate C-terminal residue in human T-cell thioredoxin reductase, corresponds to TGA in the human placental gene. 865 Feb 34

Mammalian selenocysteine-containing thioredoxin reductase (TR) isolated from HeLa cells and from human lung adenocarcinoma cells was separated into two major enzyme species by heparin-agarose affinity chromatography. The low-affinity enzyme forms that were not retained on heparin agarose showed strong crossreactivity in immunoblot assays with anti-rat liver TR polyclonal antibodies, whereas the high-affinity enzyme forms that were retained by the heparin column were not detected. Both low and high heparin-affinity enzyme forms contained FAD, were indistinguishable on SDS/PAGE analysis, and exhibited similar catalytic activities in the NADPH-dependent DTNB [5,5'-dithiobis(2-nitrobenzoate)] assay. The C-terminal amino acid sequences of 75Se-labeled tryptic peptides from lung adenocarcinoma low- and high heparin-affinity enzyme forms were identical to the predicted C-terminal sequence of human placental TR. These two determined peptide sequences were -Ser-Gly-Ala-Ser-Ile-Leu-Gln-Ala-Gly-Cys-Secys-(Gly). Occurrence of the Se-carboxymethyl derivative of radioactive selenocysteine in the position corresponding to TGA in the gene confirmed that UGA is translated as selenocysteine. The presence of cysteine followed by a reactive selenocysteine residue in this C-terminal region of the protein may explain some of the unusual properties of the mammalian TRs.
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PMID:Heparin-binding properties of selenium-containing thioredoxin reductase from HeLa cells and human lung adenocarcinoma cells. 917 83

Mammalian thioredoxin reductases (TrxR) are dimers homologous to glutathione reductase with a selenocysteine (SeCys) residue in the conserved C-terminal sequence -Gly-Cys-SeCys-Gly. We removed the selenocysteine insertion sequence in the rat gene, and we changed the SeCys(498) encoded by TGA to Cys or Ser by mutagenesis. The truncated protein having the C-terminal SeCys-Gly dipeptide deleted, expected in selenium deficiency, was also engineered. All three mutant enzymes were overexpressed in Escherichia coli and purified to homogeneity with 1 mol of FAD per monomeric subunit. Anaerobic titrations with NADPH rapidly generated the A(540 nm) absorbance resulting from the thiolate-flavin charge transfer complex characteristic of mammalian TrxR. However, only the SeCys(498) --> Cys enzyme showed catalytic activity in reduction of thioredoxin, with a 100-fold lower k(cat) and a 10-fold lower K(m) compared with the wild type rat enzyme. The pH optimum of the SeCys(498) --> Cys mutant enzyme was 9 as opposed to 7 for the wild type TrxR, strongly suggesting involvement of the low pK(a) SeCys selenol in the enzyme mechanism. Whereas H(2)O(2) was a substrate for the wild type enzyme, all mutant enzymes lacked hydroperoxidase activity. Thus selenium is required for the catalytic activities of TrxR explaining the essential role of this trace element in cell growth.
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PMID:Essential role of selenium in the catalytic activities of mammalian thioredoxin reductase revealed by characterization of recombinant enzymes with selenocysteine mutations. 1084 37