Immunoelectron microscopy of the hormonogenic sites of the thyroglobulin molecule

Hog thyroglobulin was treated with the monoclonal anti-thyroglobulin antibody, 16.3.2, which recognizes the thyroxine residues of thyroglobulin. Size-exclusion high-performance liquid chromatography of the immune complexes indicated that there were 3.5 immunoreactable thyroxine residues/mol of 19 S thyroglobulin. The immune complexes were negatively stained and observed under an electron microscope. Under antigen excess conditions, the complexes were demonstrated as branched chains in which thyroglobulin molecules were linked by antibody molecules. When the antibody was in excess, we found many single thyroglobulin molecules each connecting to 2-3 antibody molecules. Based on the morphology of the immune complexes, we conclude that thyroxine residues are formed preferentially on the molecular surface and localized near both ends of the elongated molecule of 19 S thyroglobulin and possibly at one side of the middle of the molecule.