Peroxidase-catalyzed oxidation of beta-carotene in HL-60 cells and in model systems: involvement of phenoxyl radicals.

Recent studies provide extensive evidence for the importance of carotenoids in protecting against oxidative stress associated with a number of diseases. In particular, reactions of carotenoids with phenoxyl radicals generated by peroxidase-catalyzed one-electron metabolism of phenolic compounds may represent an important antioxidant function of carotenoids. To further our understanding of the antioxidant mechanisms of carotenoids, we used in the present work two different phenolic compounds, phenol and a polar homologue of vitamin E (2,2,5,7,8-pentamethyl-6-hydroxychromane, PMC), as representatives of two different types of phenols to study reactions of their respective phenoxyl radicals with carotenoids in cells and in model systems. We found that phenoxyl radicals of PMC did not oxidize beta-carotene in either HL-60 cells or in model systems with horseradish peroxidase (HRP)/H2O2. In contrast, the 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 electron spin resonance-detectable formation of semidehydroascorbyl radicals) prevented HRP/H2O2-induced oxidation of beta-carotene. PMC, but not phenol, protected beta-carotene against oxidation induced by a lipid-soluble azo-initiator of peroxyl radicals. No adducts of peroxidase/phenol/H2O2-induced beta-carotene oxidation intermediates with phenol were detected by high-performance liquid chromatography-mass spectrometry analysis of the reaction mixture. Since carotenoids are essential constituents of the antioxidant defenses in cells and biological fluids, 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 oxidative stress.