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 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).
D000818:Animals; D004104:Diiodothyronines / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007453:Iodide Peroxidase / Q000378:metabolism; D007455:Iodine / Q000378:metabolism; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology*; D051381:Rats; D013970:Thyronines / Q000378:metabolism*; D013974:Thyroxine / Q000378:metabolism*; D014284:Triiodothyronine / Q000378:metabolism
D004104:Diiodothyronines; D013970:Thyronines; D014284:Triiodothyronine; D007455:Iodine; D007453:Iodide Peroxidase; D013974:Thyroxine