Further Studies on the Incubation of Tyrosine-UL-C 14 with Beef Thyroid Tissue Slices. ∗

Summary

1. Beef thyroid tissue slices were incubated in the presence of 100 microcuries UL-C¹⁴-tyrosine with 3 variations: a) no additions, b) propylthiouracil addition, c) non-radioactive thyronine. 2. Following digestion, analysis proceeded through counter current, paper and thin layer chromatography. The results for each of the experimental designs were the same. 3. Of the 3 major areas of radioactivity seen after counter current, only one could be further subdivided by the systems usually used for separation of iodotyro-sines, and iodothyronines, and this yielded at least 7 peaks in the systems used. 4. No evidence was found for in vitro formation of thyroxine or any of its precursors using tyro-sine as the labeled substrate.

Since this work was completed, Nunez. Mauchamp, Macchia and Roche (Biochim. Biophys. Acta, 1965, vl07, 247) have also reported that they were unable to demonstrate incorporation of tritiated tyrosine into the iodotyrosines of thyroglobulin formed by sheep thyroid slices. Since the tritium label of tyrosine is stable only on the alanine side-chain, this observation is specific only for the alanine chain on the tyrosyl residue attached to the thyroglobulin skeleton. The use of UL¹⁴C-tyrosine in our experiments also affords information about the ether-linked distal iodophenol.