Use of un-derivatized thyroid hormones for photoaffinity labeling of binding proteins

Irradiation of the thyroid hormones thyroxine and 3,5,3'-triiodothyronine in the near UV (greater than 300 nm) causes homolytic fission of C--I bonds in both rings. In the presence of hormone-binding proteins, the phenyl radical thus formed, and possibly also the iodine radical, can establish a covalent bond with certain amino acid residues in the binding site. Most if not all of the iodine radicals, however, appear to be reduced to iodide. Incubation of purified carrier proteins for the thyroid hormones in human serum as well as of an extract of rat liver nuclei or of whole nuclei with trace amounts of 125I- or 14C-labeled hormone, followed by irradiation, resulted in covalent binding. This was proven by gel filtration after boiling with guanidine:HCl and by sodium dodecyl sulfate/polyacrylamide gel electrophoresis of the irradiated solutions or of the excluded-peak material obtained after gel filtration. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of 125I-labeled irradiated nuclear extracts showed a prominent peak (Mr approximately equal to 45,000), sometimes with a shoulder or small peak at Mr approximately equal to 56,000, and a fast-moving peak (Mr approximately equal to 12,000). Similar patterns were obtained with N-bromoacetylthyroxine or N-bromoacetyltriiodothyronine without irradiation. When a suspension of whole nuclei was irradiated instead of nuclear extracts, the shoulder also became a prominent peak.