title: Threonine for alanine substitution at position 109 of transthyretin differentially alters human transthyretin's affinity for iodothyronines.
abstract: The heterozygous substitution of threonine for alanine at amino acid 109 of human transthyretin (TTR) increases its affinity for T4. We compared the affinity of recombinant wild-type (WT) and Thr109-TTRs for various iodothyronines in an attempt to elucidate how this mutation alters the T4-binding site. Homozygous WT and Thr109-TTRs were expressed recombinantly in Escherichia coli, and heterozygous Thr109-TTR was purified from plasma. The affinities of the iodothyronines for TTR were determined by measuring [125I]T4 bound by TTR in the presence of increasing concentrations of unlabeled iodothyronines. Homozygous Thr109-TTR bound T4 with an affinity slightly, but not significantly, greater than that of heterozygous Thr109-TTR. The affinity of Thr109-TTR for all iodothyronines was higher than that of WT TTR. However, the Thr109 mutation increased TTR's affinity for T4, Triac (triiodothyroacetic acid), and T3 to a greater extent than it did for Tetrac (tetraiodothyroacetic acid), EMD21388 (3',5'-dibromo-4',6'-dihydroxy-3-methylflavone), and dextro-T4. These data demonstrate that a subtle change in the structure of the T4-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.
authors.affiliation: Diabetes Unit, Charles A. Dana Research Institute, Beth Israel Hospital, Boston, Massachusetts 02215.
authors.forename: H N
authors.initials: HN
authors.lastname: Rosen
authors.forename: J R
authors.initials: JR
authors.lastname: Murrell
authors.forename: J J
authors.initials: JJ
authors.lastname: Liepnieks
authors.forename: M D
authors.initials: MD
authors.lastname: Benson
authors.forename: V
authors.initials: V
authors.lastname: Cody
authors.forename: A C
authors.initials: AC
authors.lastname: Moses
mesh_terms: D000409:Alanine; D000595:Amino Acid Sequence; D001667:Binding, Competitive; D004926:Escherichia coli / Q000378:metabolism; D005419:Flavonoids / Q000378:metabolism; D006801:Humans; D017354:Point Mutation; D011228:Prealbumin / Q000235:genetics* / Q000378:metabolism*; D011994:Recombinant Proteins; D013237:Stereoisomerism; D013912:Threonine; D013974:Thyroxine / Q000031:analogs & derivatives* / Q000378:metabolism*; D014284:Triiodothyronine / Q000031:analogs & derivatives* / Q000378:metabolism*
chemical_list: D005419:Flavonoids; D011228:Prealbumin; D011994:Recombinant Proteins; D014284:Triiodothyronine; C057452:3-methyl-4',6-dihydroxy-3',5'-dibromoflavone; D013912:Threonine; D000409:Alanine; D013974:Thyroxine