Kinetics of Enzymic Reductive Deiodination of lodothyronines

5'-Deiodination of thyroxine (yielding 3,3',5-tri-iodothyronine; reaction 1) and of 3,3',5'tri-iodothyronine (yielding 3,3'-di-iodothyronine; reaction ll) and 5-deiodination of thyroxine (yielding 3,3',5'-tri-iodothyronine; reaction III) and of 3,3',5-tri-iodothyronine (yielding 3,3'-di-iodothyronine; reaction 1V) as catalysed by rat liver microsomal fraction were studied at pH 6.5, 7.2 and 8.0. It was found that: (1) the Kn, of reaction 1 was relatively independent of pH (approx. 3 /(M), whereas Vwas highest at pH 6.5 (63 pmol of 3,3',5-triiodothyronine/min per mg of protein); (2) the Km of reaction 1I was lowest at pH 6.5 (0.035 pM), but Vwas highest at pH 8.0 (829pmol of 3,3'-di-iodothyronine/min per mg of protein); (3) thyroxine inhibited reaction II competitively; Ki values were identical at pH 6.5 and 8.0 (1 pM); (4) for both reactions III and IV Km was lowest and Vwas highest at pH 8.0. The results are compatible with the view that reactions I and II are mediated by a single enzyme (iodothyronine 5'-deiodinase) and that reactions III and IV are catalysed by a second enzyme (iodothyronine 5-deiodinase).