Partial amino acid sequence of human thyroxine-binding globulin. Further evidence for a single polypeptide chain.

The NH₂-terminal amino acid of highly purified thyroxine-binding globulin has been identified by dansyl chloride, cyanate and Edman degradation methods. All three gave alanine as the only amino terminal residue. Carbamylation and Edman degradation of the denatured protein yielded 0.86 and 0.98 – 1.05 mole of alanine per mole of protein, respectively. These data further indicate that thyroxine-binding globulin is composed of a single polypeptide chain. Automated Edman degradation gave the partial sequence as: Ala-Ser-Pro-Glu-Gly-Lys-Val-Thr-Ala-Asp-Ser-Ser-Ser-Gln-(Pro)-X-Ala-(Ser)-Leu-Tyr- A computer search revealed no homology of the NH₂-terminal segment of thyroxine-binding globulin with human prealbumin. The NH₂-terminal portion of prealbumin contains part of the thyroxine binding site.