Thyroid hormone synthesis is under the control of thyrotropin (TSH), which also regulates the sulfation of tyrosines in thyroglobulin (Tg). We hypothesized that sulfated tyrosine (Tyr[S]) might be involved in the hormonogenic process, since the consensus sequence required for tyrosine sulfation to occur was observed at the hormonogenic sites. Porcine thyrocytes, cultured with TSH but without iodide in the presence of [(35)S]sulfate, secreted Tg which was subjected to in vitro hormonosynthesis with increasing concentrations of iodide. A 63% consumption of Tyr[S] (1 residue) was observed at 40 atoms of iodine incorporated into Tg, corresponding to a 40% hormonosynthesis efficiency. In addition, hyposulfated Tg secreted by cells incubated with sodium chlorate was subjected to in vitro hormonosynthesis. With 0.5 Tyr[S] residue (31% of the initial content), the efficiency of the hormonosynthesis was 29%. In comparison, when hormonosynthesis was performed by cells, with only 0.25 Tyr[S] residue (16% of the initial content), the hormonosynthesis efficiency fell to 18%. These results show that there exists a close correlation between the sulfated tyrosine content of Tg and the production of thyroid hormones.
Copyright 1999 Academic Press.