Favored sites for thyroid hormone formation on the peptide chains of human thyroglobulin.

Gel electrophoresis of reduced thyroglobulin from normal human thyroids showed five major iodinated components, with estimated molecular masses of >300,000 daltons (14% of thyroglobulin's iodine), 230,000 (30%), 45,000 (4.5%), 26,000 (17%), and 18,000 (4%).The 26,000-and 18,000-dalton components had over 50% of their iodine as thyroxine.The 26,000-dalton iodopeptide had a single NHz-terminal, contained 29% of thyroglobulin's thyroxine, and was present in a ratio of approximately 1 mol/mol of 19 S thyroglobulin.In vitro addition of iodine to low iodine thyroglobulin, in increments of 0 to 120 mol of elemental iodine/mol of protein, showed the 230,000-and 45,000-dalton components to be iodinated preferentially, followed by the 26,000-dalton component with low to moderate additions of iodine, and by the 18,000-dalton component at higher iodine levels.Iodination on the 230,000-and 45,000-dalton peptides produced chiefly iodotyrosines, while that on the 26,000-and 18,000-dalton components formed iodothyronines.On initial iodination, most of thyroglobulin's iodothyronine was in the 26,000-dalton iodopeptide.Pulse-labeling experiments with iodination of low iodine thyroglobulin suggested a transfer of iodine from the 230,000-dalton component to the 26,000dalton one, and further transfer from these or other precursors to the 18,000-and >300,000-dalton component at high iodine levels.We conclude: 1) the 26,000-dalton iodopeptide contains the most favored site for thyroid hormone synthesis; 2) iodotyrosine formation first occurs on the 230,000-dalton component, and this iodotyrosyl is a probable precursor to the iodothyronine of the 26,000dalton iodopeptide; 3) the 18,000-dalton iodopeptide also contains an effective iodothyronine forming site, but is only utilized when high levels of iodine are available.The data raise the possibility that iodination may cleave peptide bonds, and this in turn might have a role in the physiologic degradation of thyroglobulin to release thyroid hormones.Many proteins can form iodotyrosyl residues in uitro, but thyroglobulin is much more efficient than others in forming iodothyronines (1).In nature, the thyroid hormones, thyroxine and 3,5,3'-triiodothyronine, occur only in thyroglobulin.The proposed mechanism for thyroxine biosynthesis is by diphenyl