The mutual sparing effects of selenium and vitamin E in animal nutrition may be further explained by the discovery that mammalian thioredoxin reductase is a selenoenzyme.

The recent discovery that mammalian thioredoxin reductase is a selenoprotein furnishes an additional explanation of the mutual sparing roles of selenium and vitamin E in cellular antioxidant systems. Thioredoxin reductases isolated from human lung adenocarcinoma cells, human Jurkat T-cells and HeLa cells contain selenocysteine which is located in a C-terminal tripeptide, -Cys-SeCys-Gly.