We describe the existence of at least two thyroxine 5′-deiodinases in rat liver. They co-fractionate with NADPH-cytochrome c reductase, the marker enzyme for membranes of the endoplasmic reticulum. Subcellular-localization studies of the most active microsomal thyroxine 5′-deiodinase were performed under substrate saturation and at optimal pH 6.8. This enzyme was a Km(app.) of about 3 microM-thyroxine and a Vmax. of about 8 ng of tri-iodothyronine/min per mg of protein. Our study confirms in part the earlier reports of microsomal localization of thyroxine 5′-deiodination. However, this process is not mediated by only a single enzyme.

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.