Thyroid Hormone Binding by Human Serum Prealbumin (TBPA). Electrophoretic Studies of Triiodothyronine-TBPA Interaction

Thyroxine-binding prealbumin (TBPA) in normal human serum has been shown in a polyacrylamide gel electrophoresis system to bind 7-9% of tracer level purified [(125)I]triiodothyronine (T3), and more than 30% of T3 in serum deficient in thyroxinebinding globulin (TBG). The T3-TBPA interaction has been confirmed at pH 9.0 and pH 7.4 in this electrophoretic demonstration of TBPA binding of T3 in serum. Purified human TBPA has also been shown to bind T3. Progressive additions of unlabeled thyroxine (T4) to serum containing tracer [(125)I]T3 displace T3 from TBG, its principal carrier, to TBPA and albumin; however, T4 loading does not lead to significant T3 displacement from TBPA even at T4 levels known to saturate TBPA. Loading of serum with unlabeled T3 results in displacement of more than 50% of [(125)I]T3 from TBPA, as well as from TBG, to albumin. Studies carried out with serum containing diphenylhydantoin (DPH) or MK-185, known inhibitors of T4 binding by TBG, also showed T3 displacement from TBG to TBPA and albumin. Although salicylate and tetraiodothyroacetic acid (TETRAC) displace T4 from sites on TBPA, they have only minimal effects on T3-TBPA interaction.