[Proceedings: In vitro thyroid hormone formation (author's transl)].

Iodination of a non halogenated goiter thyroglobulin and the resulting thyroxinogenesis was studied in vitro with purified thyroid peroxidase, H2O2 generating system and various concentrations of iodide. The rate of iodination was linear during the first minutes of incubation but thyroxine synthesis only began after a lag period whatever the iodide concentration in the incubation medium was. With high iodide concentrations a highly iodinated thyroglobulin (40-50 iodine atoms) containing no thyroxine was obtained after 3 minutes of incubation. If this highly iodinated goiter thyroglobulin was purified and reincubated with peroxidase and H2O2, thyroxine synthesis was again observed only after a lag period (2-3 min). In the absence of iodide the enzyme to elicit thyroxine synthesis. Depending of its concentration free diiodotyrosine exerts two opposite effects on the reaction catalyzed by thyroid peroxidase : at high concentration (10(-4) M) in inhibition of thyroglobulin iodination, and at low concentration (10(-7), 10(-8) M) a stimulating effect on thyroid hormones biosynthesis.