Biosynthesis of Thyroglobulin and Its Subunitsin Vivoin the Rat Thyroid Gland1

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Abstract
H-leucine and 125I were administered to rats to follow the course of labeling of thyroglobulin and its subunits in the thyroid gland. 3H-leucine was incorporated into proteins of 3-8S, 12S and 17-18S sedimentation constants before the label appeared in the 19S component. A time of 48 hr. was required in the normal rat for full maturation into 19S thyroglobulin. The chronic administration of goitro-genic drugs altered the protein pattern, with the result that the 3-8S peak became the predominant one, a 12S peak appeared, and the peak in the thyroglobulin region shifted to the 17-18S position. The uptake of leucine was increased in the presence of the goitrogen. The pattern of subunit labeling was similar to that of the control rats but 19S thyroglobulin was never formed. In I deficiency the incorporation of the leucine into the 19S component was greatly accelerated. A small amount of 12S protein was labeled with 125I in the normal rat at early times. The labeled protein in the thyroglobulin area was 18 S and did not become 19S until 48 hr after administration of the 125L In I deficiency the iodination of 19S protein was greatly accelerated, occurring as early as 1 min. after administration of the label. 18S and 12S 25I-labeled proteins of high specific activity were found upon withdrawal of goitro-genic drugs. After administration of thyroxine to either a goitrogen-treated or normal rat, the 12S protein which accumulated could be labeled with 125I. In goitrogen-treated or I deficient rats there is the accumulation of a protein with a sedimentation constant of 32S, rather than the normally appearing 27S. The former protein represents an aggregate of thyroglobulin and/or its subunits.

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