Oxidation of rat liver phospholipids: comparison of pathways in homogeneous solution, in liposomal suspension and in whole tissue homogenates.

The oxidation of equivalent concentrations of phospholipids in homogeneous solution, in multilamellar liposomal suspension, and in rat liver homogenate was carried out under aerobic conditions at 37 degrees C in order to examine the biochemical fate of oxidized phospholipids. Rat liver phospholipids were extracted with chloroform and methanol, and oxidation in this homogeneous solution was initiated with a lipid-soluble radical initiator. The oxidation products were phosphatidylcholine hydroperoxide (PC-OOH) and phosphatidylethanolamine hydroperoxide (PE-OOH), which were quantified by HPLC separation using a hydroperoxide-specific chemiluminescence detector. Co-extracted alpha-tocopherol and ubiquinol-9 suppressed the formation of PC-OOH and PE-OOH until oxidatively exhausted. The oxidation of extracted rat liver phospholipids in multilamellar liposomal suspension initiated with the lipid-soluble initiator gave similar results, but with slower rates of antioxidant depletion and phospholipid hydroperoxide formation due to a lower efficiency of free radical production in liposomal membranes. In contrast, the oxidation of rat liver homogenate containing active tissue enzymes initiated by the addition of either free radical initiators or tert-butyl hydroperoxide gave phosphatidylcholine hydroxide, phosphatidylethanolamine hydroxide, and free fatty acid hydroxides as oxidation products. Exogenous PC-OOH added to the rat liver homogenate was reduced to phosphatidylcholine hydroxide with subsequent hydrolysis to its free fatty acid hydroxide. These results suggest that peroxidase and phospholipase enzymes play important roles in the repair of oxidatively damaged phospholipids in biomembranes.