Characterization and primary structures of bovine and porcine thyroxine-binding globulin.

Thyroxine-binding globulin (TBG) is the major serum transport protein for iodothyronines in most of the large, omni- or herbivorous mammals. Characterization of human TBG (hTBG), including its 20 known natural variants, allowed the identification of the ligand-binding site and a correlation of diminished synthesis or loss of function with mutations in the TBG gene. Further refinement of the structure-function correlation, especially the high binding affinity and heat stability, requires characterization of other mammalian TBGs, of which only rat and sheep TBG were available.