Hematin- and peroxide-catalyzed peroxidation of phospholipid liposomes

The effect of hydroperoxides on hematin-catalyzed initiation and propagation of lipid peroxidation was examined utilizing soybean phosphatidylcholine liposomes as model membranes. Polarographic and spectrophotometric methods revealed a bimodal pseudocatalytic activity for hematin. A slow initiation phase of peroxidation was observed in the presence of low peroxide concentrations, whereas a fast propagative phase was observed at higher peroxide levels. Peroxide levels were manipulated enzymatically by the combination of phospholipase A2 and lipoxidase or by the direct addition of linoleic acid hydroperoxide, cumene hydroperoxide, or hydrogen peroxide. In addition, the effect of two different techniques for liposome preparation, i.e., sonication and extrusion, were compared on the basis of peroxidation kinetics. High pressure liquid chromatography analysis showed that sonicated liposomes contained higher levels of endogenous peroxides than the extruded ones. These sonicated liposomes also exhibited more rapid peroxidation following hematin addition. Extruded liposomes were more resistant to hematin-catalyzed peroxidation but became better substrates when exogenous hydroperoxides were added. All three peroxides reacted with hematin during which decomposition of peroxide and irreversible oxidation of hematin took place. Spectral analysis of hematin indicated that a higher oxidation state of hematin iron may be transiently formed during reaction with hydroperoxides and accounts for the propagation of lipid peroxidation when reactions proceed in the presence of soybean phosphatidylcholine liposomes. Of the three peroxides studied, linoleic acid hydroperoxide was most efficient in supporting hematin-catalyzed lipid peroxidation. The relevance of our findings is discussed in terms of the concentration dependence for lipid peroxides in determining the rate and extent of radical propagation chain reactions catalyzed by heme-iron catalysts such as hematin. Variation of hematin and linoleic hydroperoxide concentrations may provide an efficient and reproducible method for inducing and manipulating the rates and extent of lipid peroxidation through facilitation of the propagative phase of lipid peroxidation. In addition, we address a problem inherent to in vitro studies of heme-catalyzed lipid peroxidation where preparations of peroxide-free membranes should be of concern.