Amino acid sequence around a hormonogenic tyrosine residue in the N-terminal region of human thyroglobulin after in vivo and in vitro iodination.

Reduced and S-alkylated human thyroglobulin (hTgb), normally iodinated, was previously shown by SDS/PAGE to contain a small peptide (Mr 26 000) rich in thyroxine. This peptide was not found when very poorly iodinated hTgb was treated under the same conditions but was present after iodination in vitro. Peptide 26 K was purified from in vivo and in vitro iodinated hTgb. The study of these peptides shows that: 1/ at the iodination levels studied (0.18 and 0.25% iodine) one of the preferential hormonogenic sites is the same in hTgb iodinated both in vivo and in vitro; 2/ the amino acid sequence around the thyroxine residue: Asn-Ile-Phe-Glu-T4-Gln-Val is identical with the previously described hormonogenic site of bovine thyroglobulin. Most probably the hormone-containing peptide 26 K is the N-terminal peptide of the hTgb chain.