title: Exposure of thyroxine residues in human thyroglobulin. Two-site binding studies.
abstract: Human thyroglobulin (Tg) could be adsorbed through one of its thyroxine (T4) residues by either of two T4-binding antibodies which had been covalently attached to Sepharose- CL4B . The antibodies used were (i) a purified human autoantibody specific for a T4-containing epitope in human Tg, or (ii) a rabbit antibody raised against T4 conjugated to bovine albumin side chains. Tg adsorbed by either immobilized antibody could then itself adsorb either type of antibody free in solution on to a further T4 residue. At least two T4 residues in human Tg are therefore sufficiently exposed to interact with T4-binding antibodies. Furthermore, these T4 residues are sufficiently far apart to allow the binding of two immunoglobulin molecules simultaneously. Previous observations of a marked preference by human autoantibodies for one of the T4-containing epitopes in Tg therefore reflect a higher binding energy with that epitope rather than an inability to interact with others. The T4-containing epitope which preferentially reacts with human Tg autoantibodies must therefore have a distinctive topography.
mesh_terms: D000327:Adsorption; D001323:Autoantibodies / Q000276:immunology*; D001665:Binding Sites; D002846:Chromatography, Affinity; D006801:Humans; D013954:Thyroglobulin / Q000276:immunology*; D013974:Thyroxine / Q000276:immunology*
chemical_list: D001323:Autoantibodies; D013954:Thyroglobulin; D013974:Thyroxine