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Biochemistry. 1997 Oct 21;36(42):12911-20.

Determination of singlet oxygen-specific versus radical-mediated lipid peroxidation in photosensitized oxidation of lipid bilayers: effect of beta-carotene and alpha-tocopherol.

Author information

1
Department of Pharmacology and Toxicology, College of Pharmacy, University of Arizona, Tucson, Arizona 85721, USA.

Abstract

Photosensitized oxidation reactions damage tissue by catalyzing the formation of oxyradicals and singlet oxygen. beta-Carotene is hypothesized to exert photoprotective effects by quenching singlet oxygen formed by Type II reactions and by scavenging free radicals formed by Type I reactions. beta-Carotene antioxidant mechanisms were studied in a phospholipid membrane model of photooxidation with a new isotope dilution gas chromatography-mass spectrometry (GC-MS) assay that quantitatively distinguishes singlet oxygen-mediated and radical-mediated lipid peroxidation. This assay measures 9- and 10-hydroxylinoleate methyl esters and was used to generate photooxidation profiles for the photosensitizers methylene blue, Rose Bengal, and tetraphenylporphine. These profiles indicate a shift from Type II to Type I photooxidation mechanisms in later stages of photooxidation. beta-Carotene (0.45 mol %) inhibited singlet oxygen-mediated lipid peroxidation at early stages of methylene blue-sensitized photooxidation. Production of radical-mediated products increased faster than singlet oxygen-mediated products at later stages. beta-Carotene-5,8-endoperoxide, a specific marker for singlet oxygen oxidation of beta-carotene in solution, was unstable under the incubation conditions and was not detected in this system. alpha-Tocopherol (0.45 mol %) was ineffective in inhibiting photosensitized lipid peroxidation, whereas 4.5 mol % alpha-tocopherol inhibited almost all radical-mediated lipid peroxidation as well as early-stage singlet oxygen-mediated lipid peroxidation. Cumene hydroperoxide stimulated radical-mediated lipid peroxidation, indicating that accumulation of hydroperoxides from Type II photooxidation may enhance Type I reactions. These data suggest that singlet oxygen quenching, rather than radical scavenging reactions, accounts for the photoprotective actions of beta-carotene.

PMID:
9335550
DOI:
10.1021/bi9708646
[Indexed for MEDLINE]

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