Thyroxine 5′-Deiodinase Activity in Brown Adipose Tissue

Homogenates of rat brown adipose tissue (BAT) and epididymal fat pad (EPI) catalyze the 5'-deiodination of 3,3',5'-triiodothyronine (rT3) in the presence of dithiothreitol. BAT, but not EPI, catalyzes the 5'-deiodination of thyroxinE (T4) generating equal amounts of 3,5,3'-triiodothyronine (T3) and iodide. BAT 5'-deiodinase activity was unaffected by propylthiouracil (PTU) and was elevated in hypothyroid rats, whereas EPI 5'-deiodinase was totally inhibited by PTU and was depressed in hypothyroidism. These results indicate that type II (PTU-insensitive) 5'-iodothyronine deiodinase activity is present in BAT.