Inhibitory effect of beta-carotene and astaxanthin on photosensitized oxidation of phospholipid bilayers.

Large unilamellar liposomes comprising of egg yolk phosphatidylcholine (PC) was exposed to photoirradiation in the presence of methylene blue (water-soluble photosensitizer) or 12-(1-pyrene)dodecanoic acid (P-12, lipid-soluble photosensitizer) to estimate the inhibitory effect of beta-carotene and astaxanthin on photosensitized oxidation of phospholipid bilayers. Without sensitizers, astaxanthin decreased much slower than beta-carotene and other hydrocarbon carotenoids (lycopene, alpha-carotene). Astaxanthin lasted longer than beta-carotene even in the presence of methylene blue or P-12. Decrease of astaxanthin was also much slower than that of beta-carotene when egg yolk PC was replaced by dimyristoyl PC. However, inhibitory effect of astaxanthin was lower than beta-carotene in the case of P-12 sensitized photooxidation. These results suggest that effectiveness of carotenoids as antioxidants on photosensitized oxidation (Type II) in phospholipid bilayers depends on the site of singlet oxygen to be generated, as well as their stability on photoirradiation.