Oxygen-Copper (II) Interplay in the Repair of Semi-Oxidized Urate by Quercetin Bound to Human Serum Albumin

Abstract

The 1:1 complex of copper (II) and human serum albumin (HSA) slowly reacts with radiolytically generated ^•O_2- radical-anion at a rate constant of 6.1×10⁶ M^-1 s^-1. Absorbance and fluorescence spectroscopies demonstrate that addition of an equimolar portion of quercetin (QH²) to the solution of the copper (II)-HSA complex induces a relocalization of the copper resulting in a ternary copper (II)-QH²-HSA complex. This form of quercetin slowly oxidizes in air-saturated solutions. A 10-fold excess urate, a plasma antioxidant, cannot displace copper (II) bound to HSA. In N²O-saturated solutions the ternary complex form of QH² can repair the urate radical with a rate constant of 2.7×10⁶ M^-1 s^-1 by an electron transfer reaction similar to that observed in the absence of copper (II). In O²-saturated solutions and in the absence of copper, HSA-bound QH² fails to repair the urate radical because of the fast competitive reaction of ^•O_2- with urate radicals. However, addition of equimolar copper (II) restores the electron transfer from QH² to the urate radical. These contrasting results are tentatively explained either by an enhanced reactivity of copper (II) with ^•O_- in the ternary complex or by direct production of quercetin radicals via a copper-catalyzed reduction of the ^•O_- radicals by QH².

• Copper (II)-quercetin chelate
• Pulse radiolysis
• Absorption spectrophotometry
• Fluorescence
• Electron transfer
• Superoxide radical-anion