Rat liver homogenate was incubated at 37 degrees C with thyroxine, 3,3′,5-tri-iodothyronine, 3,3′,5′-tri-iodothyronine or 3,3′-di-iodothyronine. The degradation or accumulation of these compounds was measured by specific radioimmunoassays. (1) Production of 3,3′,5-tri-iodothyronine from thyroxine was highest at pH 6.0–6.5 and was markedly stimulated by the addition of dithiothreitol and effectively inhibited in the presence of 6-propyl-2-thiouracil. (2) Accumulation of 3,3′,5′-tri-iodothyronine on incubation of thyroxine with homogenate was only observed above pH 8.5. Otherwise the product was converted into 3,3′-di-iodothyronine too rapidly to allow its measurement. By measuring 3,3′-di-iodothyronine it was deduced that 5-deiodination of thyroxine was most effective at approx. pH 8.0. Dithiothreitol powerfully stimulated this reaction and 6-propyl-2-thiouracil strongly inhibited. (3) Monodeiodination of the tyrosine ring of 3,3′,5-tri-iodothyronine was the slowest reaction, was optimal at pH 8.0 and was less affected by dithiothreitol and 6-propyl-2-thiouracil than the above reactions. (4) 5′-Deiodination of 3,3′,5′-tri-iodothyronine was extremely rapid, with a pH optimum probably at about 6.5. Owing to the high reaction rate under the conditions used it was not possible to assess the effects of dithiothreitol and 6-propyl-2-thiouracil.

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