Subcellular localization of thyroxine-5-deiodinase in rat liver.

L-Thyroxine (T4) is monodeiodinated to 3,3',5'-triiodothyronine (rT3) and 3,3',5'-triiodothyronine (T3). A recent work has demonstrated that T4-5'-deiodinase is localized solely in the microsomes of rat liver, as was originally suggested. In this study identical subcellular localization was also found for T4-5-deiodinase. This is in contrast with a previous report, indicating that T4-5-deiodinase is present in the soluble cytosol fraction of rat liver. Our results strongly indicate that the thyromimetically active T3 as well as the regulatory active rT3 are both formed from T4 in a microsomal enzyme complex.