The role of superoxide and singlet oxygen in lipid peroxidation promoted by xanthine oxidase

The peroxidative oxidation of extracted rat liver microsomal lipid, assayed as malondialdehyde production, can be promoted by milk xanthine oxidase in the presence of 0.2 mM FeCl₃ and 0.1 mM EDTA. The reaction is inhibited by the superoxide dismutase activity of erythrocuprein. The reaction is also inhibited by 1,3-diphenylisobenzofuran, which reacts with singlet oxygen to yield dibenzoylbenzene. During inhibition of the lipid peroxidation reaction by 1,3-diphenylisobenzofuran, o-dibenzoylbenzene was produced. The rate of superoxide production by xanthine oxidase was not affected by 1,3-diphenylisobenzofuran. Lipid peroxidation promoted by ascorbic acid is not inhibited by either erythrocuprein or 1,3-diphenylisobenzofuran. Therefore it is suggested that the peroxidative oxidation of unsaturated lipid promoted by xanthine oxidase involves the formation of singlet oxygen from superoxide, and the singlet oxygen reacts with the lipid to form fatty acid hydroperoxides.