Metal Binding of Flavonoids and Their Distinct Inhibition Mechanisms Toward the Oxidation Activity of Cu2+–β-Amyloid: Not Just Serving as Suicide Antioxidants

The accumulation of plagues of β-amyloid (Aβ) peptides in the brain is a hallmark of Alzheimer’s disease (AD). The redox-active Cu and Fe complexes of Aβ can cause damage to the neurons potentially via reactive oxygen species (ROS). The significant metal-mediated oxidative activity of CuAβ suggests that its presence can be chemically devastating regardless whether it is a cause or a result of AD. Flavonoids exhibit various benefits to human health, attributable to their metal-binding and antioxidation activities to certain extents. Despite broad interests and extensive studies of their metal-binding properties and anti/pro-oxidation activities, these properties and the mechanisms of the activities toward metal-centered oxidation reactions have not been fully revealed and concluded. We report herein distinctive antioxidation mechanisms between two flavonoid families toward the oxidation reactions by CuAβ1–20, wherein the flavonols quercetin (Qr) and myricetin (Mr) competitively inhibit the oxidation of catechol by CuAβ1–20 with K i of 11.2 and 32.6 μM, respectively, whereas the flavanols catechin (Ct) and epicatechin (Et) are substrates with k cat = 1.01 × 10–2 and 1.55 × 10–3 s–1 and K m = 0.94 and 0.55 mM, respectively. Qr has a nearly 10-fold higher antioxidative efficacy than Ct against the oxidation activity of CuAβ, while Ct is effectively oxidized, which further decreases its antioxidant capacity. Similar inhibition patterns are observed toward oxidation of the catecholamine neurotransmitter dopamine by CuAβ1–20. Metal ions and CuAβ bind Qr with a 1:1 ratio under our experimental conditions through the α-ketoenolate moiety as determined by the use of Co2+ and Yb3+ as paramagnetic NMR probes. Unlike flavanols, which are merely suicide antioxidative substrates, flavonols bind to the metal center and prevent metal-mediated redox reactions. We suggest flavonols may serve as leads for drug discovery and/or as agents toward preventing metal-mediated oxidative stress due to AD and other disorders. Moreover, CuAβ shows 8.6- and 4.2-fold higher kinetic regioselectivity in terms of k cat and k cat/K m, respectively, toward the peroxidation of Ct than that of the enantiomer Et, suggesting potential development of metallo-catalysts in regioselective catalysis by the use of metallopeptides as templates.