ABSTRACT
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.
Subject(s)
Peptide Fragments/analysis , Thyroglobulin , Thyroxine/analysis , Amino Acid Sequence , Animals , Cattle , Chromatography, High Pressure Liquid , Electrophoresis, Polyacrylamide Gel , Sheep , Species Specificity , Swine , TrypsinABSTRACT
A 19-residue, thyroxine (T4)-containing peptide, Tryp-T4, has been isolated from the tryptic digest of a low molecular weight, iodine-enriched fragment derived from 19S bovine thyroglobulin. This tryptic peptide represents the only site of significant iodination in the parent polypeptide fragment. The amino acid sequence of the tryptic peptide has been determined and is NH2-Asn-Ile-Phe-Glu-T4-Gln-Val-Asp-Ala-Gln-Pro-Leu-Arg-Pro-Cys-Glu-Leu-Gln- Arg-COOH. The carboxyl-terminal sequence of this peptide shows a high probability of a beta-turn. These findings establish the involvement of at least a single unique sequence within thyroglobulin in thyroxine biosynthesis and the general nature of a hormonogenic site within this protein. This sequence contains at least 30% of the thyroxine present in 19 S bovine thyroglobulin.