Kinetics of enzymic reductive deiodination of iodothyronines. Effect of pH

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Abstract
5''-Deiodination of thyroxine [T4] (yielding 3,3'',5-triiodothyronine [T3]; reaction I) and of 3,3'',5''-triiodothyronine approximately (yielding 3,3''-diiodothyronine [T2]; reaction II) and 5-deiodination of T4 (yielding 1-T3; reaction III) and of T3 (yielding T2; reaction IV) as catalyzed by rat liver microsomal fraction were studied at pH 6.5, 7.2 and 8.0. It was found that: the Km of reaction I was relatively independent of pH (approximately 3 .mu.M), whereas V [velocity] was highest at pH 6.5 (63 pmol of T3/min per mg of protein); the Km of reaction II was lowest at pH 6.5 (0.035 .mu.M), but V was highest at pH 8.0 (829 pmol of T2/min per mg of protein); T4 inhibited reaction II competitively; Ki values were identical at pH 6.5 and 8.0 (1 .mu.M); for both reactions III and IV Km was lowest and V was highest at pH 8.0. Reactions I and II are apparently mediated by a single enzyme (iodothyronine 5''-deiodinase) and reactions III and IV are catalyzed by a 2nd enzyme (iodothyronine 5-deiodinase).