ULTRACENTRIFUGAL CHARACTERIZATION OF CYTOSOL-BINDING PROTEINS FOR THYROXINE AND TRIIODOTHYRONINE IN HUMAN LIVER

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Abstract
Sucrose density gradient ultracentrifugation was performed on human liver cytosol containing double tracers of 131I-thyroxine(T4) and 125I-triiodo-thyronine(T3) together with or without large amount of stable hormones. Tracer T4 bound to the cytosol indicated 2 peaks of 8.3S and 4.7S, both of which were displaced with stable T4, but not with T3. Maximal binding capacity of the 8.3S component averaged 43.2 μg/100ml of 33% cytosol. Tracer T3 in the cytosol showed a single peak of 4.2S which was displaced with stable T3 and also with T4. The results indicate that the human liver cytosol contains limited-capacity binding proteins: the one specific for T4, and the other relatively specific for T3.

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