THYROXINE-BINDING BY HUMAN TRANSTHYRETIN VARIANTS - MUTATIONS AT POSITION-119, BUT NOT POSITION-54, INCREASE THYROXINE-BINDING AFFINITY

A mutation at codon 119 in the transthyretin (TTR) gene leads to a substitution of methionine for threonine at this position in the circulating protein. As the amino acid at position 119 is located in the T-4 binding channel, mutations here may affect the binding of T-4 by TTR. A previous study has shown an increase in the amount of hormone carried by the TTRMet(119) variant. To determine whether this increase in binding was due to a change in affinity or capacity, TTR was partially purified from normal individuals and those with the TTRMet(119) mutation. The isolation procedure was a rapid, single step passage through Blue Sepharose. With normal serum, the resulting protein bound T-4 With a Single site of intermediate affinity (K-a, 1.63 +/- 0.36 x 10(7) L/mol). No sites of higher or lower affinity were detected. Comparisons of binding capacity and immunoreactive TTR concentrations showed that the preparations bound T-4 With 8 molar ratio between 1-2. With TTRMet(119) serum, the T-4 affinity was approximately doubled [K-a, 3.40 +/- 0.76 x 10(7) L/mol (+/-SD); P < 0.001] with no change in binding capacity. This doubling in affinity explains the observed T-4 levels of about 120 nmol/L in individuals with this mutation. Binding of rT(3) to TTRMet(119) was increased approximately 5-fold over normal. Identical experiments with TTRGly(54), in which glycine is substituted for glutamine, showed that the T-4 affinity of this variant was unchanged from normal. These results suggest that the TTRMet(119) mutation leads to secretion of a normal concentration of TTR that has a raised affinity for T-4 Depending on their location, mutations in the TTR gene may lead to an increase or no change in T-4 binding by the secreted protein.