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A study of retinal antioxidants in a model membrane system
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 $2,2\sp\prime$-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. Lipid-soluble compounds (vitamin E, free fatty acids, retinol, retinaldehyde) were incorporated into PL liposomes. Water-soluble compounds (reduced glutathione, taurine) were dissolved in incubation buffer. It was found that normal physiological concentrations of alpha-tocopherol (vitamin E), free fatty acids (16:0, 18:0, 18:1, 22:6), retinol, and glutathione significantly decreased oxidative loss of PUFA's.^ When 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 fatty acids from phospholipids in proportions and concentrations that afford protection to membrane lipids.^ The antioxidant activities of the two major retinoid compounds involved in the visual cycle in ROS, all-trans retinaldehyde and all-trans retinol, were compared. The addition of retinol partially protected PUFA's in ROS PL liposomes, whereas retinaldehyde promoted lipid peroxidation. When isolated ROS were stimulated to produce endogenous retinol, PUFA loss was inhibited by up to 17%. These findings suggest that there is an antioxidant function for the enzymatic reduction of retinaldehyde to retinol during photoreception.^ Water-soluble antioxidants, taurine and reduced glutathione (GSH), were investigated individually and in combination with retinol in ROS PL liposomes. GSH significantly protected PUFA's in ROS PL liposomes. Taurine alone showed little antioxidant activity, but in combination with retinol protected lipids by up to 47.9% (an increase in antioxidant protection of 24.8% over retinol alone). These data support previous findings that taurine protects ROS lipids during exposure to cyclic light conditions. ^
Keys, Susan A, "A study of retinal antioxidants in a model membrane system" (1998). Doctoral Dissertations Available from Proquest. AAI9841885.