Mechanistic analyses of the suppression of amyloid beta 42 aggregation by apomorphine

(R)-Apomorphine (1) has the potential to reduce the accumulation of amyloid beta-protein (A beta 42), a causative agent of Alzheimer's disease (AD). Although the inhibition of A beta 42 aggregation by 1 is ascribable to the antioxidative effect of its phenol moiety, its inhibitory mechanism at the molecular level remains to be fully elucidated. LC-MS and UV analyses revealed that 1 is autoxidized during incubation to produce an unstable o-quinone form (2), which formed a Michael adduct with Lys 16 and 28 of A beta 42. A further autoxidized form of 1 (3) with o-quinone and phenanthrene moieties suppressed A beta 42 aggregation comparable to 1, whereas treating 1 with a reductant, tris(2-carboxyethyl) phosphine diminished its inhibitory activity. H-1-N-15 SOFAST-HMQC NMR studies suggested that 1 interacts with Arg5, His13,14, Gln15, and Lys16 of the A beta 42 monomer. These regions form intermolecular b-sheets in A eta 42 aggregates. Since 3 did not perturb the chemical shift of monomeric A beta 42, we performed aggregation experiments using 1,1,1,3,3,3-hexafluoro-2-propanol-treated A beta 42 to investigate whether 3 associates with A beta 42 oligomers. Compounds 1 and 3 delayed the onset of the oligomer-driven nucleation phase. Despite their cytotoxicity, they did not exacerbate A beta 42-mediated neurotoxicity in SH-SY5Y neuroblastoma cells. These results demonstrate that extension of the conjugated system in 1 by autoxidation can promote its planarity, which is required for intercalation into the beta-sheet of A beta 42 nuclei, thereby suppressing further aggregation. (C) 2018 Elsevier Ltd. All rights reserved.