The isolation of identical thyroxine containing amino acid sequences from bovine, ovine and porcine thyroglobulins.

Identical, thyroxine containing tryptic peptides have been isolated from digests of bovine, ovine and procine thyroglobulins. This 19 residue hormone containing sequence, NH2-Asn-Ile-Phe-Glu-T4-Gln-Val-Asp-Ala-Gln-Pro-Leu-Arg-Pro-Cys-Glu-Leu-G in-Arg- COOH, is completely conserved across these three species, and it represents a principal site of thyroxine synthesis. HPLC maps of tryptic digests of the thyroglobulins have been monitored at several wavelengths and suggest that, in each case, only a small number of tryptic peptides are iodinated in vivo and that an even smaller number of tryptic peptides contain thyroid hormone. These data are consistent with a high degree of selectivity in iodination of tyrosines within thyroglobulin and the subsequent coupling of these selected tyrosines to form thyroid hormone.