Specific labeling of the thyroxine binding site in thyroxine-binding globulin: determination of the amino acid composition of a labeled peptide fragment isolated from a proteolytic digest of the derivatized protein.
[125I] Thyroxine has been covalently bound to the thyroxine binding site in thyroxine-binding globulin by reaction with the bifunctional reagent, 1,5-difluoro-2,4-dinitrobenzene. An average of 0.47 mol of [125I] thyroxine was incorporated per mol protein; nonspecific binding amounted to 8%. A labeled peptide fragment was isolated from a proteolytic digest of the derivatized protein by HPLC and its amino acid composition was determined. Comparison with the amino acid sequence of thyroxine-binding globulin indicated partial correspondence of the labeled peptide with two possible regions in the protein. These regions also coincide with part of the barrel structure present in the closely homologous protein, alpha 1-antitrypsin.
D000596:Amino Acids / Q000032:analysis; D001665:Binding Sites; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007457:Iodine Radioisotopes; D007700:Kinetics; D010446:Peptide Fragments / Q000032:analysis; D011402:Pronase; D013974:Thyroxine / Q000378:metabolism*; D013975:Thyroxine-Binding Proteins / Q000378:metabolism*
D000596:Amino Acids; D007457:Iodine Radioisotopes; D010446:Peptide Fragments; D013975:Thyroxine-Binding Proteins; D011402:Pronase; D013974:Thyroxine