Transthyretin binding heterogeneity and antiamyloidogenic activity of natural polyphenols and their metabolites

Transthyretin (TTR) is an amyloidogenic protein, the amy- loidogenic potential of which is enhanced by a number of spe- cific point mutations. The ability to inhibit TTR fibrillogenesis is known for several classes of compounds, including natural polyphenols, which protect the native state of TTR by specifi- cally interacting with its thyroxine binding sites. Comparative analyses of the interaction and of the ability to protect the TTR native state for polyphenols, both stilbenoids and flavonoids, and some of their main metabolites have been carried out. A main finding of this investigation was the highly preferential binding of resveratrol and thyroxine, both characterized by neg- ative binding cooperativity, to distinct sites in TTR, consistent with the data of x-ray analysis of TTR in complex with both ligands. Although revealing the ability of the two thyroxine binding sites of TTR to discriminate between different ligands, this feature has allowed us to evaluate the interactions of poly- phenols with both resveratrol and thyroxine preferential bind- ing sites, by using resveratrol and radiolabeled T4 as probes. Among flavonoids, genistein and apigenin were able to effec- tively displace resveratrol from its preferential binding site, whereas genistein also showed the ability to interact, albeit weakly, with the preferential thyroxine binding site. Several glucuronidated polyphenol metabolites did not exhibit signifi- cant competition for resveratrol and thyroxine preferential binding sites and lacked the ability to stabilize TTR. However, resveratrol-3-O-sulfate was able to significantly protect the pro- tein native state. A rationale for the in vitro properties found for polyphenol metabolites was provided by x-ray analysis of their complexes with TTR.