Preferential Synthesis of Thyroxine Tyrosyl Residues in Thyroglobulin* from Early Iodinated

A double labeling procedure was employed to determine whether the tyrosine residues in thyroglobulin that are iodinated first are also those that are most readily converted to thyroxine. Goiter thyroglobulin containing less than 1 atom of iodine per molecule was iodinated first with 9 at relatively low levels of iodination (5 to 20 atoms of iodine per molecule) and afterward with lzjI at higher levels of iodination (20 to 60 atoms of iodine per molecule). Iodinations were carried out either chemically with 13-, or enzymatically with thyroid peroxidase. Three different sets of conditions were employed for double labeling: (a) first and second iodinations both performed chemically with I3-; (b) first iodination performed chemically, second enzymatically; and (c) both first and second iodinations performed enzymatically. The double enzymatic procedure was used in most experiments. An index of “preferential synthesis” was defined for assessing whether there was preferential use of the first labeled [‘Yltyrosyl residues for thyroxine synthesis. Evidence for preferential synthesis was obtained with all the above procedures, but most convincingly when both iodinations were performed enzymatically. When the double enzymatic procedure was applied to casein and fibrinogen, two proteins that were previously shown to form thyroxine upon enzymatic iodination with thyroid peroxidase, no evidence for preferential synthesis was obtained. Similarly, disruption of the native structure of thyroglobulin by exposure to 5 M guanidine resulted in failure to observe preferential synthesis. Preferential synthesis, therefore, is not a general characteristic of all proteins that are capable of forming thyroxine on iodination with thyroid peroxidase. Results of the present investigation support the view that the native