Thioredoxin reductase is a redox enzyme that plays an important role in cell proliferation. Through thioredoxin it provides reducing equivalents for ribonucleotide reductase, the first unique step in DNA synthesis, for methionine sulfoxide reductase, and for vitamin K epoxide reductase. The protein exhibiting catalytic activities typical of mammalian thioredoxin reductases, i.e. reduction of disulfide bonds in thioredoxins and DTNB, was found in human lung adenocarcinoma, HeLa, and T-cells. The 12-residue tryptic peptides from the human lung adenocarcinoma and T-cell thioredoxin reductases were identical in sequence to the predicted C- terminal peptide of the placental enzyme except that TGA stop-codon was decoded as selenocysteine followed by glycine. Subsequent studies revealed that there are two selenium-containing forms of thioredoxin reductase in the human lung adenocarcinoma cells and also in HeLa cells. One form of the enzyme that is adsorbed to heparin affinity column matrices fails to react with the polyclonal rat liver enzyme antibodies, whereas the other form that is not retained on the heparin affinity matrix is recognized by the same antibody preparation. Experiments are planned to find out the chemical and structural basis of the different antigenic properties and affinities to heparin of these two forms of thioredoxin reductase. For this purpose two sets of antibodies will be elicited to the two enzyme forms. The specific chemical modification of selenocysteine residues will be carried out to clarify its role in catalytic activity and structural stability of enzyme. We are also planning to clone the thioredoxin reductase gene from human lung adenocarcinoma cells cDNA library to get selenocysteine/ cysteine mutant enzyme to compare its properties with that of wild type enzyme.