Thyroid hormone binding in rat liver cytosol.

Binding of 3,5,3'-tri-iodothyronine (T3) and thyroxine (T4) to components of perfused rat liver supernatant fraction and isolated liver cell cytosol was studied. Of the four binding fractions in supernatant (X, A, Y and Z) separable by gel chromatography, both T3 and T4 bound preferentially to the A-fraction, which was shown to contain albumin as the major binding protein. When cytosol prepared from isolated cells was examined, T4 was again bound mainly in the A-fraction; however, T3 was observed to bind predominantly in the Y-region. Hormone binding to soluble protein in the latter system is thought to reflect the pattern in vivo, better than does binding in supernatant, although the possibility exists that the concentration of albumin observed in cytosol may be artificially high due to transfer of membrane-bound albumin during cell disruption. Nevertheless, albumin (possibly derived from more than one intracellular source) is capable of binding T4 in vivo. The presence of this protein within the hepatocyte may thus contribute to the high T4 binding capacity of the liver compared to other tissues.