Thioredoxin reductase (E.C.1.8.1.9.) together with glutathione are the most important intracellular antioxidants. Regeneration of thioredoxin by thioredoxin reductase belongs to very important antioxidant pathways. Together with other antioxidant enzymes it helps to remove free radicals and reactive oxygen species from the cells. Thioredoxin reductase is a protein, which in reduced form is ubiquitous in all cells. This selenoenzyme is the major disulfide reductant in the cells. It reduces the ascorbyl radical back to vitamin C, lipid hydroperoxides, hydrogen peroxide, lipoic acid, selenites and others. Thioredoxin reductase takes part in the regulation of the nitroxide synthase activity. It applies in DNA synthesis, fertilization, skin protection against UV irradiation, in bone remodulation, it has protective ability against oxidative stress in central nervous system, it prevents carcinogenesis. On the other hand, if the cancer is already present, it may paradoxically protect the tumor cells against the therapy based on free radicals (T-cells, X-irradiation, some cytostatic drugs) with respect to its antioxidant ability and to the protection of protein kinase C. Inhibition of thioredoxin reductase may be used in treatment of chronic inflammatory diseases such as rheumatoid arthritis.

        Key words: thioredoxin reductase, antioxidants, free radicals, selenium, tumors.