The hormonogenic sites of turtle thyroglobulin and their homology with those of mammals.

Thyroglobulin (Tg) from turtles previously injected with 125I was reduced, alkylated, and digested with trypsin. We purified the resultant peptides on HPLC columns, determined their amino acid sequences and the locations of [125I]T4 and [125I]T3 residues, and compared them with established sequences from humans, cows, rabbits, rats, and guinea pigs. We found five major T4 peptides, three of which were homologous with the major hormonogenic sites A, B, and D of mammalian Tg. Site A, the highly conserved major T4 site in mammals, had substitutions in three residues near the T4 residue and had much less of Tg's newly synthesized T4 than is found in mammalian Tg (25% in turtle vs. 44% in rabbit). Site B contained correspondingly more of Tg's new T4 (42% vs. 24% in rabbit). Turtle Tg contained little [125I]T3, and we did not find site C (Ser-T3/T4-Ser, the major T3 site in guinea pig and rabbit) in turtles, but did find Val-T4, a possible homolog. Site D was quantitatively less important than in mammals. The fifth turtle hormonogenic site, containing 12% of Tg's newly formed T4, had a tyrosyl residue substituted for the phenylalanine at residue 632 in the human sequence. We conclude that Tg's major hormonogenic sites are generally conserved across a considerable evolutionary distance, but that differences in primary structure occur and may contribute to changes in priority of hormone synthesis among these sites.