Enhanced amyloidogenicity of sulfonated transthyretin in vitro, a hypothetical etiology of senile amyloidosis.

Genetic variants of transthyretin (TTR) cause systemic amyloidosis and wild-type TTR may also in some situations produce amyloid fibrils. We have analyzed wild-type and variant TTRs by mass spectrometry and found that TTR preparations from all individuals demonstrated free TTR, TTR conjugated with thiol compounds and several minor components. We previously described a component which had a molecular mass 80 Da larger than free TTR and was proved to be TTR conjugated with sulfite. Here, the amyloid fibril formation of the TTR isoforms was monitored by the turbidity at 330 nm, and by a Congo red-binding assay as a function of pH, according to the method of Lai et al. The S-sulfonated TTR showed the highest level of amyloid fibril formation. In contrast, TTR reduced by dithiothreitol, which was free of the S-sulfonated component, showed neither elevation of the turbidity nor the Congo red binding. Commercially purchased TTR without further treatment containing free, S-sulfonated and other species of TTR molecules showed an intermediate elevation. These results suggested that the S-sulfonated wild-type TTR is highly amyloidogenic. Although further experiments are needed to apply the observation to in vivo phenomenon, exogenous sulfite may be a cause of senile systemic amyloidosis.