Glabridin protects paraoxonase 1 from linoleic acid hydroperoxide inhibition via specific interaction: A fluorescence-quenching study

The enzyme paraoxonase 1 (PON1) binds to high-density lipoprotein (HDL) and is responsible for many of HDL's antiatherogenic properties. We previously showed that recombinant PON1 is inhibited by linoleic acid hydroperoxide (LA-OOH) present in the lipid fraction of the human carotid plaque (LLE) via oxidation of the enzyme's Cys284 thiol. Here we explore the effect of glabridin, an isoflavan isolated from licorice root, on preventing LA-OOH's inhibitory effect on rePON1 using the tryptophan-fluorescence-quenching technique and modeling calculations. Glabridin significantly prevented rePON1 inhibition by LLE or oxidized linoleic acid (by 22% and 15%, respectively), whereas ascorbic acid and Trolox, strong antioxidants, had no effect. Glabridin quenched the intrinsic fluorescence of rePON1 in a concentration-dependent manner. Binding parameters and modeling calculations demonstrated a major role for hydrophobic forces in the rePON1-glabridin interaction, indicating that it is not the antioxidant capacity of glabridin that protects rePON1 from LA-OOH inhibition, but rather its specific interaction with the enzyme.