Selenocysteine, identified as the penultimate C-terminal residue in human T-cell thioredoxin reductase, corresponds to TGA in the human placental gene

The possible relationship of selenium to immunological function which has been suggested for decades was investigated in studies on selenuim 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-terminaldeduced 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 Cterminal 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

Selenoproteins—Tracing the Role of a Trace Element in Protein Function

Selenium is an important component of several enzymes, replacing sulfur in cysteine residues. Its discovery and significance are described in this primer

CLOSTRIDIUM STICKLANDII NOV. SPEC

A clostridium, strain HF, originally associated with Methanococcus vannielii in formate enrich-ment cultures (Stadtman and Barker, 1951) was subsequently isolated in pure culture and found to be an amino acid fermenting organism (Stadt-man, 1954). Biochemical studies revealed that a characteristic property of the organism is its ability to obtain its energy for growth from coupled oxidation-reduction reactions between certain amino acid pairs, i.e., a "Stickland reac-tion " (Stadtman and White, 1954). It is dis-tinctly different, both morphologically and in various of its biochemical characteristics, from the other clostridia known to catalyze these re-actions; viz., Clostridium sporogenes, the organis