Partition of purified human thyroxine-binding globulin in aqueous two-phase systems in response to reactive dyes

Thyroxine-binding globulin was isolated from pooled human serum by a two-step method involving affinity chromatography on thyroxine-Sepharose and preparative disc-electrophoresis. The final product was found to be homogeneous by polyacrylamide gel electrophoresis and has a molecular weight of 59,000. Isoelectric focusing resolved the protein into seven bands with isoelectric points ranging from 3.9 to 4.3. The isolated protein showed affinity to a number of different dyes as recognized by affinity phase partitioning. The interaction of the protein with the dye Cibacron Blue F3G-A was found to be strongly competitive with the natural ligand thyroxine.