Thyroxine binding to transthyretin Met 119. Comparative studies of different heterozygotic carriers and structural analysis.

The majority of the known transthyretin (TTR) variants are associated with amyloidosis, but there are also variants associated with euthyroid hyperthyroxinemia and others are apparently nonpathogenic. TTR Met 119 is a nonpathogenic variant found to be frequent in the Portuguese population. Previous studies on thyroxine (T4) binding to semi-purified TTR from heterozygotic carriers of TTR Met 119, reported by us and other groups, revealed different results. Therefore, to further characterize T4 binding to TTR Met 119 we performed T4-TTR binding studies in homotetrameric-recombinant TTR Met 119 variant and normal TTR. We also studied T4 binding to TTR purified from serum of different heterozygotic carriers of TTR Met 119 including compound heterozygotic individuals carriers of a TTR mutation in the other allele. We observed an increased T4 binding affinity to TTR Met 119 from heterozygotic individuals and compound heterozygotes and this effect of increasing T4 binding affinity was consistent and independent from the mutation present in the other allele. Recombinant homotetrameric TTR Met 119 and heterotetrameric protein from heterozygotic carriers of TTR Met 119 presented similar T4 binding affinity demonstrating the increased T4 binding affinity of TTR Met 119. X-ray crystallography studies performed on the recombinant TTR Met 119 variant revealed structural alterations mainly at the level of residue Leu 110 allowing a closer contact between the hormone and the mutant protein. These results are consistent with the observed T4 binding results.