Lipid peroxidation in rod outer segments. Role of hydroxyl radical and lipid hydroperoxides

Lipid peroxidation of rod outer segment (ROS) membranes has been implicated in the pathogenesis of numerous ocular disease processes. The hydroxyl radical might be involved in initiating the reaction. An in vitro system was developed to study lipid peroxidation of the ROS and the role of the hydroxyl radical. Bovine ROS were suspended in various concentrations of ferrous sulfate, incubated for 10 min at 37 degrees C, treated with diethylenetriamine pentaacetic acid to chelate the iron, and subjected to a thiobarbituric acid assay for malondialdehyde. A predictable increase in lipid peroxidation occurred in the presence of Fe+2. No effect was seen in the presence of Fe+3. Adding hydrogen peroxide, which would form the hydroxyl radical by reacting with Fe+2, had no effect at low concentrations. At higher concentrations, lipid peroxidation was inhibited, presumably from the oxidation of Fe+2 to Fe+3. Ethanol, a known hydroxyl radical scavenger, had no inhibitory effect in concentrations up to 0.50 mol/l. Conversely, cumene hydroperoxide and linoleic acid hydroperoxide, which form hydrophobic radicals, stimulated lipid peroxidation in the presence of Fe+2. These findings suggest that, under these experimental conditions, the hydroxyl radical is not an initiator of lipid peroxidation in ROS. They provide evidence that endogenous lipid radicals may initiate the reaction.