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.
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.