PEROXIDASE-CATALYZED OXIDATION OF BETA-CAROTENE IN HL-60 CELLS AND INMODEL SYSTEMS - INVOLVEMENT OF PHENOXYL RADICALS

Recent studies provide extensive evidence for the importance of carote noids in protecting against oxidative stress. associated with a number of diseases. In particular, reactions of carotenoids with phenoxyl ra dicals generated by peroxidase-catalyzed one-electron metabolism of ph enolic compounds may represent an important antioxidant function of ca rotenoids. To further our understanding of the antioxidant mechanisms of carotenoids, we used in the present work two different phenolic com pounds, phenol and a polar homologue of vitamin E (2,2,5,7,8-pentameth yl-6-hydroxychromane PMC), as representatives of two different types o f phenols to study reactions of their respective phenoxyl radicals wit h carotenoids in cells and in model systems. We found that phenoxyl ra dicals of PMC did not oxidize beta-carotene in either HL-60 cells or i n model systems with horseradish peroxidase (HRP)/H2O2. In contrast, t he phenoxyl radicals generated from phenol (by native myeloperoxidase in HL-60 cells or HRP/H2O2 in model systems) effectively oxidized beta -carotene and other carotenoids (canthaxanthin, lutein, lycopene). One -electron reduction of the phenoxyl radical by ascorbate (assayed by e lectron spin resonance-detectable formation of semidehydroascorbyl rad icals) prevented HRP/H2O2-induced oxidation of beta-carotene. PMC, but not phenol, protected beta-carotene against oxidation induced by a li pid-soluble azo-initiator of peroxyl radicals. No adducts of peroxidas e/phenol/H2O2-induced beta-carotene oxidation intermediates with pheno l were detected by highperformance liquid chromatography-mass spectrom etry analysis of the reaction mixture. Since carotenoids are essential constituents of the antioxidant defenses in cells and biological flui ds, their depletion through the reaction with phenoxyl radicals formed from endogenous, nutritional and environmental phenolics, as well as phenolic drugs, may be an important factor in the development of oxida tive stress.