The free iodotyrosines of the rat thyroid gland

Abstract

The free monoiodotyrosine of I131 -labelled rat thyroid glands has been determined by column and paper chromatography of thyroid diffusates at different times after the administration of [I131]iodide. An estimate of the specific activity of free monoiodotyrosine at 0.5 to 72 hr. after the injection of [Il31]iodide was made. It rose more sharply than the specific activities of thyroglobulin-bound labelled monoiodotyrosine and of intrathyroidally formed I- ion. It is suggested that both the iodotyrosines are likely precursors of this I- ion. The effect of graded doses of propylthiouracil administered in vivo was examined. This goitrogen inhibited the appearance of free monoiodotyrosine much more markedly than that of thyroglobulin-bound monoiodotyrosine. The rate of hydrolysis of I131 -labelled rat thyroglobulin with crude preparations of calf- and sheep-thyroid protease was studied at different times of labelling. For short time-intervals, up to 4 hr., the thyroglobulin contained a fraction of monoiodotyrosine that was more labile to the protease than the average. Labile di-iodotyrosine was not demonstrated. It was concluded that both the free iodotyrosines in the thyroid are the precursors of intrathyroidally formed I- ion (second I pool). The free monoiodotyrosine may come in part from a moiety of thyroglobulin that is more labile than the average. Other sources of free iodotyrosines are discussed.