A model membrane system to investigate antioxidants in bovine rod outer segments.

The antioxidant activities of compounds endogenous to bovine rod outer segments (ROS) were investigated by measuring the loss of polyunsaturated fatty acids (PUFA's) from membranes exposed to the water-soluble oxidant 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH). Osmotically intact ROS, ROS membranes, and unilamellar liposomes prepared from ROS phospholipids (PL) were compared. Intact ROS were most resistant to oxidative loss of PUFA's, followed by ROS membranes and then PL liposomes. The development of a model membrane system allowed the investigation of putative antioxidants singly and in combination. These lipid-soluble compounds were incorporated into PL liposomes, and it was found that normal physiological concentrations of alpha-tocopherol (vitamin E) and free fatty acids (16:0, 18:0, 18:1, 22:6) significantly decreased oxidative loss of PUFA's. When all the major free fatty acids were added to PL liposomes at the same concentrations found when ROS phospholipase A is stimulated, the oxidative loss of PUFA's was reduced by 31%. The antioxidant effect of free fatty acids suggests that endogenous phospholipase A's may act to protect membranes by releasing esterified fatty acids in proportions and concentrations that afford protection to membrane lipids.