Effects of Amino Acids on the Photosensitized Oxidation of Linoleic Acid in an Aqueous Ethanol Solution

Abstract

The antioxidative activities of l-amino acids on the methylene blue sensitized photooxidation of linoleic acid (LA) were studied in a 40% aqueous ethanol solution at pH 7.0 and 33°C. His and CysH inhibited the photosensitized oxidation of LA, and the antioxidative activity of His was larger than that of CysH. The ratio of the chemical reaction rate constant for His and singlet oxygen (1O2) to that for LA and 1O2 was similar to the ratio of the 1O2 quenching rate constant for His to that for LA. This fact shows that His inhibits the photosensitized oxidation of LA by quenching 102 chemically. On the other hand, CysH reacted with linoleic acid hydroperoxides (LAHPO) or with and caused the decomposition of LAHPO or the formation of Cys. This fact suggests that CysH inhibits the photosensitized oxidation of LA by decomposing LAHPO or by quenching 1O2 chemically.