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

Partition of T-4 to plasma proteins is classically attributed only to TBGC approximate to70%), transthyretin (approximate to15%) and albumin (approximate to10%), based on zone electrophoresis. Since TBG migration spans the alpha1 and alpha2 regions, and since HDL, which have alpha1 migration, transport approximate to4% of circulating T-4, other alpha-globulins could bind T-4 and "contaminate" the TBG area. Hence, we determined the association of [I-125]T-4 to TBG and, for comparison, to transthyretin and albumin. Sera from 50 normolipidemic individuals were equilibrated with [I-125] T4 and analyzed by both zone electrophoresis and radioimmunoprecipitation with specific antisera. Transthyretin-T-4 or albumin-T-4 bindings as assessed by the two methods agreed, while TBG-T-4 did not, because other alpha-globulins carrying T-4 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. (C) 2002, Editrice Kurtis.