Preferential Synthesis of Thyroxine from Early Iodinated Tyrosyl Residues in Thyroglobulin

Abstract

Abstract 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 131I at relatively low levels of iodination (5 to 20 atoms of iodine per molecule) and afterward with 125I at higher levels of iodination (20 to 60 atoms of iodine per molecule). Iodinations were carried out either chemically with I3-, 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 was defined for assessing whether there was preferential use of the first labeled [131I]tyrosyl 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 structure of thyroglobulin plays an important role in facilitating thyroxine formation. It appears that thyroglobulin contains tyrosyl residues whose function is to form hormone at low levels of iodination.