Site-specific Inhibitory Mechanism for Amyloid β42 Aggregation by Catechol-type Flavonoids Targeting the Lys Residues

The aggregation of the 42-residue amyloid beta-protein (A beta 42) is involved in the pathogenesis of Alzheimer disease (AD). Numerous flavonoids exhibit inhibitory activity against A beta 42 aggregation, but their mechanism remains unclear in the molecular level. Here we propose the site-specific inhibitory mechanism of (+)-taxifolin, a catechol-type flavonoid, whose 3',4'-dihydroxyl groups of the B-ring plays a critical role. Addition of sodium periodate, an oxidant, strengthened suppression of A beta 42 aggregation by (+)-taxifolin, whereas no inhibition was observed under anaerobic conditions, suggesting the inhibition to be associated with the oxidation to form o-quinone. Because formation of the A beta 42-taxifolin adduct was suggested by mass spectrometry, A beta 42 mutants substituted at Arg(5), Lys(16), and/or Lys(28) with norleucine (Nle) were prepared to identify the residues involved in the conjugate formation. (+)-Taxifolin did not suppress the aggregation of A beta 42 mutants at Lys(16) and/or Lys(28) except for the mutant at Arg(5). In addition, the aggregation of A beta 42 was inhibited by other catechol-type flavonoids, whereas that of K16Nle-A beta 42 was not. In contrast, some non-catechol-type flavonoids suppressed the aggregation of K16Nle-A beta 42 as well as A beta 42. Furthermore, interaction of (+)-taxifolin with the beta-sheet region in A beta 42 was not observed using solid-state NMR unlike curcumin of the non-catechol-type. These results demonstrate that catechol-type flavonoids could specifically suppress A beta 42 aggregation by targeting Lys residues. Although the anti-AD activity of flavonoids has been ascribed to their antioxidative activity, the mechanism that the o-quinone reacts with Lys residues of A beta 42 might be more intrinsic. The Lys residues could be targets for Alzheimer disease therapy.