THYROID HORMONE BIOSYNTHESIS IN AGNATHA AND PROTOCHORDATA

In the present review the problem of the evolutionary level at which the biosynthesis of thyroid hormones first appeared in the zoological scale is discussed. Thyroid hormones are certainly present in all Chordata, Urochordata, Cephalocordata, and Vertebrata. However, although concentration and organification of iodine are widespread processes occurring throughout the animal kingdom, the synthesis of thyroid hormones, i.e., iodothyronines, has not been clearly established below the Chordata. The biochemical mechanism leading to hormone synthesis follows the same general pattern from its first appearance in the Protochordata. The coupling of iodotyrosines to yield iodothyronines occurs within the polypeptide chains of specific proteins. In vertebrates, from the cyclostomes up to the mammals, the polymerization of similar, but perhaps not identical, subunits, leads to the formation of a family of thyroid proteins of increasing molecular size, i.e., 12 S, 19 S (thyroglobulin), and 27 S iodoproteins. © 1969.