Selenium-dependent regulation of type I 5'-deiodinase expression.

Selenite concentration regulates activity and expression of the p27 substrate-binding subunit of type I 5'deiodinase (5'-D) and of a protein labeled with bromoacetylthyroxin (BrAcT4), or p30, with yet unknown function in a porcine-kidney epithelial cell line (LLC-PK1) cultured in serum-free medium. p27 is metabolically labeled by 75-selenite and affinity labeled by BrAc[125I]T4. Compared with glutathione peroxidase, expression of the p27 5'D subunit (5'-DI) is observed at 10-fold-lower concentrations of selenium in the growth medium, suggesting an intracellular hierarchy of selenite utilization. Selenium deficiency retards cell growth and prevents 5'-DI expression and may thus impair thyroid hormone action in vivo.