Thyroxine binding to members and non-members of the serine protease inhibitor family.

Partition of T4 to plasma proteins is classically attributed only to TBGC approximately/= 70%), transthyretin (approximately/= 15%) and albumin (approximately/= 10%), based on zone electrophoresis. Since TBG migration spans the alpha1 and alpha2 regions, and since HDL, which have alpha1 migration, transport approximately/= 4% of circulating T4, other alpha-globulins could bind T4 and "contaminate" the TBG area. Hence, we determined the association of [125I]T4 to TBG and, for comparison, to transthyretin and albumin. Sera from 50 normolipidemic individuals were equilibrated with [125I] T4 and analyzed by both zone electrophoresis and radioimmunoprecipitation with specific antisera. Transthyretin-T4 or albumin-T4 bindings as assessed by the two methods agreed, while TBG-T4 did not, because other alpha-globulins carrying T4 with low affinity co-migrated with TBG. Some, but not all, of these alpha-globulins belong to the same superfamily of TBG and also bind steroid hormones.