Inhibition of the peroxidation of linoleic acid by the flavonoid quercetin within their complex with human serum albumin

This work provides a quantitative kinetic analysis of oxidative pathways involving linoleic acid and the common dietary antioxidant quercetin (flavonoid), both bound to human serum albumin (HSA). In particular, it is shown that quercetin, although embedded in drug site I, is oxidized as quickly as free quercetin under a flux of hydrophilic peroxyl radicals. This observation suggests that efficient charge relays are established between the periphery of HSA and bound quercetin. Moreover, the peroxidation of HSA-bound linoleic acid is shown to take place at some specific fatty acid binding sites once one to two critical HSA residues are themselves oxidized. Quercetin efficiently delays the onset of lipid peroxidation. The inhibition persists long after the total consumption of quercetin, in agreement with some quercetin oxidation products exerting a residual antioxidant activity. Consistently, HSA markedly increases the maximal concentration of a two-electron oxidation product of quercetin that is accumulated and then consumed in the course of the peroxidation. The additional observation of the faster consumption of the single Trp residue in the presence of quercetin suggests that HSA enhances the antioxidant activity of quercetin by regenerating some of its oxidation products retaining a H-donating activity.