Ultracentrifugal characterization of cytosol-binding proteins for thyroxine and triiodothyronine in human liver.

Sucrose density gradient ultracentrifugation was performed on human liver cytosol containing double tracers of 131I-thyroxine(T4) and 125I-triiodothyronine(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 microgram/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 thtat the human liver cytosol contains limited-capacity binding proteins: the one specific for T4, and the other relatively specific for T3.