An approach to the structure of thyroglobulin. Hormone-forming sequences in porcine thyroglobulin.
The mixture of CNBr peptides obtained by treatment of porcine thyroglobulin with cyanogen bromide was separated into three fractions by Sephadex G-200 gel filtration in 1 M propionic acid. When one of these fractions was reduced and S-alkylated, a hormone-containing CNBr peptide was purified by filtration on Biogel A-1.5 m and ion-exchange chromatography on DEAE-Sephadex. Similarly, two other hormone-containing CNBr peptides were separated from another reduced and S-alkylated fraction by Sephacryl S-200 gel filtration and DEAE-Sephadex chromatography. The three purified CNBr peptides have the same Mr (15000), differ in amino acid composition and contain 60-70% of the hormones present in the initial thyroglobulin. Fragmentation of these peptides into smaller hormone peptides was carried out by trypsin digestion followed by gel filtration. This resulted in the purification of eight discrete hormone-containing tryptic peptides whose homogeneity was established by the study of their N-terminal and C-terminal sequences. At least four sites for the synthesis of thyroxine and two sites for the synthesis of triiodothyronine have been identified in the three CNBr peptides. A possible additional site was recovered in the form of the peptides seryl-triiodothyronine and probably seryl-thyroxine. To gain information on the number of hormone-forming sites in thyroglobulin iodinated in vivo and on the distribution of these sites in the different hormone-containing peptides isolated, estimation of thyroxine and triiodothyronine content of batches of thyroglobulin of iodine content comprised between 0.73% and 1.6% was carried out, together with the estimation of the hormone content of all the CNBr peptides separated from two preparations of thyroglobulin of different iodine contents (0.73% and 1.23%). It was shown that the number of thyroxine and triiodothyronine residues increased linearly with increasing thyroglobulin iodine content in the range 0.73-1.6% and reached six residues of thyroxine and two residues of triiodothyronine/mol protein for a thyroglobulin iodine content of 1.6% with no indication of saturation. This result is at variance with previous findings on porcine thyroglobulin iodinated in vivo which suggested that the thyroxine content reached a plateau of three to four residues/molecule for an iodine content comprised between 0.7% and 1.1%.