Va. Tyurin et al., PEROXIDASE-CATALYZED OXIDATION OF BETA-CAROTENE IN HL-60 CELLS AND INMODEL SYSTEMS - INVOLVEMENT OF PHENOXYL RADICALS, Lipids, 32(2), 1997, pp. 131-142
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.