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 pH 6.5, 7.2 and 8.0 It was found that: (1) the Km of reaction I was relatively independent of pH (approx. 3 microM), whereas V was highest at pH 6.5 (63 pmol of 3,3′,5-tri-iodothyronine/min per mg of protein); (2) the Km of reaction II was lowest at pH 6.5 (0.035 microM), 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; Ki values were identical at pH 6.5 and 8.0 (1 microM); (4) for both reactions III and IV Km was lowest and V was 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).

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