Low‐density lipoprotein is oxidized by phospholipase A 2 and lipoxygenase in xanthoma lesions

Oxidized LDL has been obtained by incubation with copper ions (Cu-LDL) or various kinds of cells. LDL incubated with xanthoma tissues (x-LDL) is considered a model of in vivo oxidized LDL that has extravasated into xanthoma lesions. To investigate the mechanism of x-LDL formation, we studied the effects of various enzyme inhibitors or antioxidants on the oxidation process of LDL. Thiobarbituric acid-reactive substance (TBARS) levels, electrophoretic mobility and spectrophotometric pattern of the oxidized LDL were examined. Antioxidants suppressed TBARS formation in both x-LDL and Cu-LDL. Enzyme inhibitors inhibited TBARS levels in x-LDL, but not in Cu-LDL. All the enzyme inhibitors and antioxidants, except for the cyclooxygenase inhibitor, inhibited the anodic electrophoretic mobility of x-LDL. The anodic electrophoretic mobility of Cu-LDL was suppressed only with antioxidants. Spectrophotometry indicated that an increase in the absorbance at 240 nm was observed in Cu-LDL, but not in x-LDL. x-LDL oxidation is primarily catalyzed by phospholipase A 2 , and subsequently generated polyunsaturated free fatty acids propagate the peroxidation. Fatty acid hydroperoxides conjugated with dienes are not synthesized in x-LDL. On the other hand, non-enzymatic oxidants, such as superoxide anion and hydroxyl radicals generate Cu-LDL with diene-conjugated fatty acid hydroperoxides.