Albumin-bound quercetin repairs vitamin E oxidized by apolipoprotein radicals in native HDL3 and LDL

In the minor fraction of HDL3 containing α-tocopherol (αTocOH), selective one-electron oxidation of Trp and Tyr residues of apolipoproteins A-I and A-II by •Br2- radical-anions produces the corresponding semioxidized species, TyrO• and •Trp. Repair of TyrO• by endogenous αTocOH generates the α-tocopheroxyl radical (αTocO•). Fast spectroscopic studies show that two populations representing 80% of αTocO• initially formed are repaired over several seconds with rate constants of 3.0 × 106 and 1.5 × 105 M-1 s-1 by quercetin bound to human serum albumin (HSA) at physiologically relevant concentration. Formation of HSA-bound quercetin radicals (•Qb) is observed. In the major fraction of HDL3 particles lacking αTocOH, TyrO• and •Trp are repaired by free and HSA-bound quercetin. In LDL particles which all contain αTocOH, αTocO• radicals are formed in the millisecond time scale by repair of TyrO• radicals produced in apolipoprotein B. Then, 75% of initial αTocO• are repaired over seconds by HSA-bound quercetin (rate constant: 2.0 × 106 M-1 s-1). HSA-bound quercetin can also repair •Trp radicals. In O2-saturated solutions, the fraction of αTocO• radicals (more than 50%) not repaired by superoxide radical-anions can be repaired by HSA-bound quercetin with formation of •Qb but to a much lesser extent in LDL than in HDL.