Cellular thyroxine-binding proteins in rabbit skeletal muscle and liver were studied by gel filtration, ion-exchange chromatography and polyvinyl chloride electrophoresis. Soluble protein fractions with affinity for thyroxine were detected in muscle and liver extracts by gel filtration.
The fraction with the highest relative affinity for thyroxine was further purified. In both tissues this fraction was found to consist mainly of two cellular thyroxine-binding proteins when analysed by ion-exchange chromatography.
Electrophoresis showed that these fractions were not completely homogeneous.
The cellular thyroxine-binding proteins thus isolated had different electrophoretic mobilities than rabbit serum thyroxine-bindingprotein. Thethyroxinebinding capacities of the isolated cellular thyroxine-binding proteins were studied by electrophoresis.
The maximal binding capacities varied from 3.5 to 10.5 ,ug thyroxine per mg protein. The total binding capacity of both muscle and liver extract was so large that it was not possible to completely saturate it with the electrophoretic technique.
It was over 600 pg thyroxine per 100 ml supernatant. The easy release of thyroxine from cellular thyroxine-binding proteins during the purification procedure indicates a rather low affinity of cellular thyroxine-binding proteins for thyroxine.
The physiological function of cellular thyroxine-binding proteins is not known.