Influence of serum albumin and the flavonol quercetin on the peroxidase activity of metmyoglobin

Metmyoglobin (MbFeIII), a major form of dietary iron, is an efficient inducer of lipid and protein oxidation. Indeed, MbFeIII is able to cleave hydrogen peroxide and lipid hydroperoxides with subsequent formation of ferrylmyoglobin (MbFeIV = O) and lipid oxyl and peroxyl radicals. In the first part of this work, the mechanism of the reaction between MbFeIII and H2O2 is revisited with an emphasis on the influence of bovine serum albumin (BSA). BSA does not affect the rate of MbFeIV = O formation but inhibits the formation of a redox-inactive green pigment (heme-protein cross-link species). Although tightly bound to BSA, the flavonol quercetin is still able to reduce MbFeIV = O as a likely result of long-range electron transfers within a protein–protein complex. In the second part, BSA is shown to strongly slow down the metmyoglobin-catalyzed consumption of linoleic acid hydroperoxides with formation of ketones as the main products. In the process, only low concentrations of ferrylmyoglobin are slowly accumulated. A catalytic mechanism is proposed that involves a one-electron-oxidized metmyoglobin species distinct from ferrylmyoglobin.