ID 1.21.99.3 DE thyroxine 5-deiodinase. AN diiodothyronine 5'-deiodinase. AN iodothyronine 5-deiodinase. AN iodothyronine inner ring monodeiodinase. AN type III iodothyronine deiodinase. CA 3,3',5'-triiodo-L-thyronine + A + H(+) + iodide = AH2 + L-thyroxine. CC -!- The enzyme activity has only been demonstrated in the direction of CC 5-deiodination. CC -!- This removal of the 5-iodine, i.e. from the inner ring, largely CC inactivates the hormone thyroxine. CC -!- Formerly EC 1.97.1.11. DR P49898, IOD3_AQUCT ; Q5I3B1, IOD3_BOVIN ; O42412, IOD3_CHICK ; DR P55073, IOD3_HUMAN ; Q91ZI8, IOD3_MOUSE ; Q6QN11, IOD3_PIG ; DR P49897, IOD3_RAT ; Q6DN07, IOD3_SHEEP ; A7YD35, IOD3_SPAAU ; DR P49899, IOD3_XENLA ; //