Similar Hormone-Rich Peptides from Thyroglobulins of Five Vertebrate Classes*

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Abstract
Thyroglobulins (Tg) were purified from 5 species (rat, chicken, turtle, frog and goldfish), each representing a different vertebrate class. On reduction with mercaptoethanol, each Tg produced 5 major iodopeptides, designated A-E, with ranges of estimated molecular mass, in kilodaltons (K), as follows: A, more than 300K; B, 210-280K; C, 30-42K; D, 19-28K; and E, 10-23K. Of these, the 2 smallest, D and E, had 40-80% of their iodines as iodothyronine, compared with 15-20% for the parent Tg. They contained 25-63% of Tg total iodothyronines but only a few percent of its peptide material. Calculations from amino acid analyses and I contents showed approximately 1 mol each of D and E/mol 660,000 dalton Tg. In comparisons of amino acid compositions by cluster analysis, iodopeptides D and E resembled each other and their counterparts in other species more than they resembled their parent Tg. Also, the Tg from different species were more similar to each other and to iodopeptides D and E than to nonthyroidal proteins randomly selected from the literature. 125I was injected into rats and turtles, and its distribution among the iodopeptides was compared to that of 127I. These dual isotope experiments showed that as Tg was iodinated in vivo, iodopeptide B decreased both in molecular size and in its share of Tg iodine, while the sum of iodopeptides D and E increased, indicating that B may be the precursor of D and E. In vivo iodination of rat Tg with 125I for different periods of time suggested that iodopeptide E and its iodothyronines are derived from a larger portion of the Tg molecule, perhaps iodopeptide B. The amount of 125I in iodopeptide D also increased with time. These 2 discrete hormone-rich peptide fractions of small MW are a consistent feature of Tg structure throughout the vertebrate classes. Both are probably derived from a larger iodopeptide fraction (iodopeptide B) during the course of Tg maturation and iodination. Their conservation during evolution indicates an important role for them in thyroid hormone biosynthesis.