Thioredoxin reductase (TrxR) contains the cofactor FAD and catalyzes the reduction of the disulfide form of thioredoxin (TRX -S2) to the dithiol form (TRX -(SH)2):
particular, the enzyme reduces selenite directly and is a lipid hydroperoxide reductase (5). Recently, it was shown that human adrenocarcinoma thioredoxin reductase contains a selenocysteine (SEC) residue (6). The enzyme is homologous to glutathione reductase, and has an elongation that contains the conserved C-terminal sequence -Gly-Cys-Sec-Gly in human, rat, and bovine enzymes (Fig. 1). Insertion of Sec requires a selenocysteine-insertion sequence (SECIS), present in the 3′-untranslated region, to decode the UGA normal stop codon as selenocysteine (7). The -Sec-Gly sequence is required for catalytic activity, because its removal by carboxypeptidase digestion inactivates the enzyme (7). The selenocysteine in mammalian thioredoxin reductase explains its wide substrate specificity and inhibition by 1-chloro-dinitrobenzene (DCNB) (8). The latter compound selectively modifies both the Sec and Cys residues in the enzyme and induces a 30-fold increase in NADPH-oxidase activity in the alkylated enzyme (9). The requirement for selenium in the enzyme may explain why selenite is required to grow some cells in synthetic tissue-culture media. Tumor cells often have a strong up-regulation of TrxR (10), which may protect the cells. The selenocysteine nature of the enzyme also explains why it is inhibited by some drugs used clinically, such as gold thioglucose or nitrosourea compounds (11).
Figure 1. The primary structure of one mammalian thioredoxin reductase subunit from the rat that has 498 residues (7). The conserved N-terminal active -site disulfide region, which is identical to that of glutathione reductase (Cys-Val-Asn-Val-Gly-Cys, or CVNVGC ), is shown, plus the conserved C-terminal (Gly-Cys-Sec-Gly, or GCUG) selenocysteine- containing sequence in the human, rat, and bovine enzymes. The positions of the FAD-binding, NADPH-binding, and interface domains are indicated.
Trx-S2 from many species
Vitamin K Alloxan
Selenite and selenodiglutathione 5,5′-Dithiobis-(2-nitrobenzoic acid) (Ellman’s reagent)
Protein disulfide isomerase (PDI)
Lipid hydroperoxides and H2O2 S-Nitrosoglutathione(GSNO)