Photooxidation and beta-carotene: Effects in membrane models

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.