The effects and mechanism of flavonoid-rePON1 interactions. Structure-activity relationship study

Flavonoids are plant phenolic secondary metabolites that are widely distributed in the human diet. These antioxidants have received much attention because of their neuroprotective, cardioprotective, and chemopreventive actions. While a major focus has been on the flavonoids' antioxidant properties, there is an emerging view that many of the potential health benefits of flavonoids and their in vivo metabolites are due to modulatory actions in cells through direct interactions with proteins, and not necessarily due to their antioxidant function. This view relies on the observations that flavonoids are present in the circulation at very low concentrations, which are not sufficient to exert effective antioxidant effects. The enzyme paraoxonase 1 (PON1) is associated with high-density lipoprotein (HDL), and is responsible for many of HDLs' antiatherogenic properties. We previously showed that the flavonoid glabridin binds to rePON1 and affects the enzyme's 3D structure. This interaction protects the enzyme from inhibition by an atherogenic component of the human carotid plaque. Here, we broadened our study to an investigation of the structure-activity relationships (SARs) of 12 flavonoids from different subclasses with rePON1 using Trp-fluorescence quenching, modeling calculations and Cu2+-induced low-density lipoprotein (LDL) oxidation methods. Our findings emphasize the 'protein-binding' mechanism by which flavonoids exert their beneficial biological role toward rePON1. Flavonoids' capacity to interact with the enzyme's rePON1 hydrophobic groove mostly dictates their pro/antioxidant behavior.