KINETICS OF ENZYMIC REDUCTIVE DEIODINATION OF IODOTHYRONINES - EFFECT OF PH

5'-Deiodination of thyroxine (yielding 3,3',5-tri-iodothyronine; reaction I) and of 3,3',5'-tri-iodothyronine (yielding 3,3'-di-iodothyronine; reaction II) 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 IV) as catalysed by rat liver microsomal fraction were studied at pH6.5, 7.2 and 8.0. It was found that : (1) the K(m) of reaction I was relatively independent of pH (approx. 3 μM), whereas V was highest at pH 6.5 (63 pmol of 3,3',5-tri-iodothyronine/min per mg of protein); (2) the K(m) of reaction II was lowest at pH 6.5 (0.035 μM), but V was highest at pH 8.0 (829 pmol of 3,3'-di-iodothyronine/min per mg of protein);(3) thyroxine inhibited reaction II competitively; K(i) values were identical at pH 6.5 and 8.0 (1μM);(4) for both reactions III and IV K(m) was lowest and V was highest at pH8.0. 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).