ascorbic-acid and 1-2-linoleoylphosphatidylcholine

Free radical damage to cellular membranes appears to underlie alterations in function in aging and various pathological processes, including cardiovascular disease. The objective of this study was to directly characterize changes in the molecular structure of membrane lipid bilayers resulting from oxidative stress. Membrane samples reconstituted from either synthetic or cardiac phospholipids enriched with polyunsaturated fatty acids were examined at high resolution using small-angle x-ray diffraction methods. In this study, Fe2+/ascorbate-induced lipid peroxidation produced significant and dose-dependent alterations in the basic physical structure of the phospholipid bilayer. Electron density profiles (A vs. electrons/A3) calculated from the x-ray diffraction data showed a marked reduction in the hydrocarbon core width of dilinoleoyl phosphatidylcholine (DLPC) bilayers from 36 A to 32 A, and a decrease in overall membrane width, including surface hydration, from 48.7 A to 44.6 A. In addition, a broad decrease in molecular volume was observed +/-3-10 A from the center of the membrane bilayer, along with interdigitation of the terminal methyl segments. Pronounced changes in the lipid bilayer structure following oxidative stress were also observed in membranes reconstituted from cardiac lipids, including a 4 A reduction in hydrocarbon core width from 40 A to 36 A and interdigitation of the terminal methyl segments. These data provide direct evidence for changes in membrane hydrocarbon core width and molecular volume resulting from phospholipid peroxidation, which may contribute to perturbations in membrane structure/function relationships associated with aging and cardiovascular disease.

Topics: Aging; Ascorbic Acid; Electrons; Fatty Acids, Unsaturated; Free Radicals; Iron; Lipid Bilayers; Lipid Peroxidation; Membrane Lipids; Myocardium; Oxidative Stress; Particle Size; Phosphatidylcholines; X-Ray Diffraction

Thermally labile azo-initiators, dissolved in either the aqueous or lipid phase, have been used to generate peroxyl radicals at a known, steady rate in an aqueous dispersion of dilinoleoylphosphatidylcholine multilamellar liposomes at 37 degrees C in order to study the antioxidant behaviour of ascorbate itself and ascorbate in combination with either alpha-tocopherol or a water-soluble alpha-tocopherol analogue (TROLOX(-]. It is found that ascorbate is an effective inhibitor of peroxidations initiated in the aqueous phase, with each ascorbate terminating 0.6 radical chains (i.e., n = 0.6), but it is a very poor inhibitor of peroxidations initiated in the lipid phase. Peroxidations initiated in the lipid-phase in the presence of either alpha-tocopherol or TROLOX(-) indicate that ascorbate is an excellent synergist with both phenolic antioxidants (n = 0.4). In peroxidations initiated in the aqueous phase ascorbate acts as a co-antioxidant with TROLOX(-) (n = 0.7), but the interpretation of the approximately additive effect obtained in the presence of alpha-tocopherol is complicated by the fact that under the experimental conditions employed alpha-tocopherol alone does not give a distinct, measurable inhibition period. The latter problem is shown to be due to a non-uniform distribution of the water-soluble initiator within the liposome. Other examples of the complicating effects of non-uniform distributions of reactants in kinetic studies of the autoxidation of organic substrates dispersed in water are described.

Topics: Antioxidants; Ascorbic Acid; Benzopyrans; Chromans; Kinetics; Liposomes; Models, Biological; Oxidation-Reduction; Oxygen Consumption; Phosphatidylcholines; Vitamin E