Demonstration and some properties of cytosol-binding proteins for thyroxine and triiodothyronine in human liver.

Cytosol-binding proteins for L-thyroxine (T4) and triiodo-L-thyronine (T3) were studied in human liver specimens obtained at autopsy from 5 male and 2 female subjects. The liver cytosol containing 131I-T4 or T3, together with or without added stable hormones, was fractionated by Pevikon thin-layer electrophoresis at pH 8.6, 8.0, and7.4. It was demonstrated in all the specimens that besides a small amount of serum T4-binding globulin, there existed three T4-binding proteins, termed hT4-1, hT4-2 and hT4-3, with the electrophoretic mobilities of alpha2- and beta-globulins, and two T3-binding proteins, termed hT3-1 and hT3-2, with the mobilities of gamma-globulin. Binding of hormones by the cytosol proteins was pH-dependent, and a preliminary dialysis had no effect on the hormone binding. The major band of T4, hT4-2, bound more than half the tracer T4, and possessed the maximal binding capacity of 110 mug/100 ml of 33% cytosol at pH 7.4. However, it showed no apparent affinity for T3, because the bound T4 could not be displaced with a T3 load of 600 mug/100 ml. The major band of T3, hT3-2, bound more than 70% of the tracer T3, and appeared to have a large capacity for the hormone although secondary binding sites on the same molecule might be responsible for the large capacity. The binding sites appeared almost specific for T3, because only a small, insignificant displacement was noted with a T4 load of 600 mug/100 ml. The results provide evidence for distinct binding proteins for T4 and T3 in the human liver cytosol, though their physiological roles remain to be elucidated.