Specific binding proteins of thyroxine and triiodothyronine in liver soluble proteins

Abstract

1. 1. The binding of l-thyroxine (T 4) and 3, 5, 3′-triiodo- l-thyronine (T 3) by rat serum and liver soluble proteins was studied using DEAE-cellulose chromatography and gel filtration on Sephadex G-100. 2. 2. The methods employed provided clear evidence for specific T 4- and T 3- binding proteins, which differed from each other, in the liver soluble fraction. 3. 3. Gel filtration studies indicated that both cellular T 4-binding protein and T 3-binding protein were excluded by Sephadex G-100 gel in contrast with serum T 4-binding protein, and therefore, their molecular weights were considered greater than 100 000. 4. 4. The sorption effect of Sepahdex for the thyroid hormones permitted comparison of the relative binding affinities of proteins for the hormones. The liver soluble proteins showed considerably lower for T 4, but nearly the same order of affinity for T 3 as compared with serum proteins. Cellular T 4-binding protein and T 3-binding protein had the highest affinities for corresponding hormones in the liver soluble proteins. 5. 5. The results suggested that the cellular T 4-binding protein and T 3-binding protein could play a major role in the hepatic uptake and intracellular transport of separate hormones, and that an intracellular mechanism of T 3 transport may differ somewhat from that of T 4. 6. 6. It was demonstrated that serum T 4-binding protein of rats was different from albumin, and its molecular weights is probably lower than that of albumin.