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    Photooxidation and beta-carotene: Effects in membrane models

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    Author
    Stratton, Steven Paul, 1968-
    Issue Date
    1996
    Keywords
    Health Sciences, Toxicology.
    Chemistry, Biochemistry.
    Advisor
    Liebler, Daniel C.
    
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    Show full item record
    Publisher
    The University of Arizona.
    Rights
    Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
    Abstract
    Intake and serum levels of β-carotene have inverse associations with cancer risk. Previous research indicates chemopreventive actions may be due to antioxidant properties. Photooxidation reactions are an important source of reactive oxygen species. Photosensitizers can damage tissue by catalyzing the formation of oxyradicals and singlet oxygen (¹O₂). β-Carotene efficiently quenches ¹O₂ catalytically via a physical reaction. However, concomitant chemical reactions during photosensitized oxidations consume β-carotene. This dissertation is a study of β-carotene antioxidant mechanisms in solution and phospholipid membrane models of photooxidation. Photosensitized oxidation of β-carotene in solution yielded products analyzed by reverse-phase HPLC, UV-vis spectrophotometry, and mass spectrometry. These products were identified as β-ionone, β-apo-14'-carotenal, β-apo-10'-carotenal, β-apo-8'-carotenal, and the novel product β-carotene-5,8-endoperoxide, which was determined to be a specific marker for ¹O₂ oxidation of β-carotene. To study the effect of β-carotene and other agents on photooxidation, an isotope dilution GC-MS assay was developed which quantitatively distinguishes ¹O₂-mediated and radical-mediated lipid peroxidation products resulting from photosensitized oxidation of dilinoleoylphosphatidylcholine liposomes. This unique assay utilizes quantitative standards of 9- and 10-hydroxyoctadecadienoate and was used to generate "photooxidation profiles" of the photosensitizers methylene blue, Rose Bengal, and tetraphenylporphine. These profiles indicate a shift from Type II to Type I photooxidation mechanisms in later stages. In the liposome system, β-carotene successfully inhibited both ¹O₂-mediated and radical-mediated lipid peroxidation at early stages but was less effective at later stages. Production of radical-mediated products increased faster than ¹O₂-mediated products at later stages even though 40% of the initial β-carotene was present after 4 h. β-Carotene-5,8-endoperoxide was not detected in this system. Equimolar α-tocopherol was ineffective in inhibiting lipid peroxidation, however, a 10-fold increase in α-tocopherol concentration inhibited almost all radical-mediated lipid peroxidation as well as early-stage ¹O₂-mediated lipid peroxidation. Cumene hydroperoxide stimulated radical-mediated lipid peroxidation. Type II photooxidation products may enhance Type I mechanisms. β-Carotene was shown quantitatively to suppress photooxidation by inhibiting Type II mechanisms alone. Since β-Carotene may prevent tissue damage due to photooxidation, an understanding of the mechanisms involved will be important in maximizing its protective effects.
    Type
    text
    Dissertation-Reproduction (electronic)
    Degree Name
    Ph.D.
    Degree Level
    doctoral
    Degree Program
    Graduate College
    Pharmacology & Toxicology
    Degree Grantor
    University of Arizona
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