Consumption of carotenoids in photosensitized oxidation of human plasma and plasma low-density lipoprotein.

Human plasma and plasma low-density lipoprotein (LDL) were exposed to photoirradiation in the presence of methylene blue (water-soluble photosensitizer) or 12-(1-pyrene)dodecanoic acid (P-12, lipid-soluble photosensitizer). In methylene-blue-sensitized photooxidation of human plasma and LDL, endogenous carotenoids and tocopherols were consumed with the accumulation of cholesteryl ester hydroperoxides (CE-OOH). Xanthophylls (zeaxanthin and lutein) decreased faster than lycopene and carotenes in the case of human plasma. In P-12-sensitized photooxidation of human plasma and LDL, the decrease rate of xanthophylls was slower than that of lycopene and carotenes. A lower level of beta-carotene exerted the effective inhibition of lipid peroxidation and retarded the oxidative loss of alpha-tocopherol, when the phosphatidylcholine liposomes containing these two lipid-soluble antioxidants were subjected to methylene blue- or P-12-sensitized photooxidation. These results suggest that antioxidant activity of carotenoids in photosensitized oxidation (Type II) of human plasma LDL depends on the site of singlet oxygen (1O2) to be generated and that carotenoids can protect tocopherols from the oxidative loss by 1O2 in the plasma.