THREONINE FOR ALANINE SUBSTITUTION AT POSITION-109 OF TRANSTHYRETIN DIFFERENTIALLY ALTERS HUMAN TRANSTHYRETINS AFFINITY FOR IODOTHYRONINES

The heterozygous substitution of threonine for alanine at amino acid 109 of human transthyretin (TTR) increases its affinity for T-4 We compared the affinity of recombinant wild-type (WT) and Thr(109)-TTRs for various iodothyronines in an attempt to elucidate how this mutation alters the T-4-binding site. Homozygous WT and Thr(109)-TTRs were expressed recombinantly in Escherichia coli, and heterozygous Thr(109)-TTR was purified from plasma. The affinities of the iodothyronines for TTR were determined by measuring [I-125]T-4 bound by TTR in the presence of increasing concentrations of unlabeled iodothyronines. Homozygous Thr(109)-TTR bound T-4 With an affinity slightly, but not significantly, greater than that of heterozygous Thr(109)-TTR. The affinity of Thr(109)-TTR for all iodothyronines was higher than that of WT TTR. However, the Thr(109) mutation increased TTR's affinity for T-4, Triac (triiodothyroacetic acid), and T-3 to a greater extent than it did for Tetrac (tetraiodothyloacetic acid), EMD21388 (3',5'-dibromo-4',6'-dihydroxy-3-methylflavone), and dextro-T-4. These data demonstrate that a subtle change in the structure of the T-4-binding channel in TTR differentially alters the affinity of binding of various iodothyronines and suggests that site-directed mutagenesis of residues within the binding channel might clarify the relative importance of specific domains of this binding channel.