[In vitro synthesis of 3,5,3'-triiodothyronine catalyzed by thyroid peroxidase (author's transl)]

3,5,3'-triiodotyronine formation was studied in vitro after iodination of goiter thyroglobulin catalyzed by purified thyroid peroxidase. A fractionnal number of T3 is always obtained per mole of thyroglobulin in both in vitro and in vivo. This result cannot be explained by a heterogeneity in thyroglobulin iodination or in a partial conversion of T3 to T4. It is suggested that thyroglobulin is heterogenous either in its primary sequence or in its teritiary configuration. Thyroglobulin contains tyrosine residues which are specific for the T3 formation and moreover T3 is not the precursor of T4. The efficiency of T3 formation was studied: the maximal number of T3 molecules is obtained with 30 iodine atoms per mole of thyroglobulin. In addition the results suggest that the tyrosines which are coupled with a high efficiency are iodinated sequentially.