THYROXINE-BINDING TO TRANSTHYRETIN (TTR) VARIANTS 2 VARIANTS (TTR PRO-55 AND TTR MET-111) WITH A PARTICULARLY LOW BINDING-AFFINITY

Thyroxine (T-4) binding is a well-characterized function of transthyre tin (TTR) and has been used to assess structural alterations of TTR va riants. Most TTR variants deposit as amyloid fibrils originating diffe rent forms of hereditary amyloidosis. It has been hypothesized that am ino acid substitutions in the TTR variants induce structural alteratio ns that may be responsible for the amyloidogenicity of the mutant prot eins. To test for structural alterations in TTR, ive studied T-4 bindi ng to TTR variants both in whole serum and in isolated form obtained f rom heterozygotic carriers of amyloidogenic and nonamyloidogenic varia nts and compared the binding with the corresponding homozygotic recomb inant variants. The results obtained indicated a normal T-4 binding af finity for heterozygotic TTR Ala 491 TTR Leu 68, TTR Ala 71 and TTR Ar g 102. Concerning TTR Pro 55 and TTR Met 111, we found a consistent de crease in binding affinity. These results were more pronounced for the equivalent recombinant proteins. Recombinant TTR Pro 55 did not show specific binding to T-4 and recombinant TTR Met 111 presented very low binding affinity. Neither TTR Pro 55 nor TTR Met 111 are localized in the T-4, binding channel, thus structural alterations induced by thes e mutations should be transmitted to the binding channel interfering w ith T-4 binding, The results now reported, together with ongoing struc tural data by X-ray analyses on mutants Pro 55 and Met 111 and future binding studies with other ligands, will contribute to the elucidation of the structure and function of TTR, particularly in thyroid hormone metabolism.