Formation of alpha-tocopherol radical and recycling of alpha-tocopherol by ascorbate during peroxidation of phosphatidylcholine liposomes. An electron paramagnetic resonance study.

The events accompanying the inhibitory effect of alpha-tocopherol and/or ascorbate on the peroxidation of soybean L-alpha-phosphatidylcholine liposomes, which are an accepted model of biological membranes, were investigated by electron paramagnetic resonance, optical and polarographic methods. The presence of alpha-tocopherol radical in the concentration range 10(-8)-10(-7) M was detected from its EPR spectrum during the peroxidation of liposomes, catalysed by the Fe3+-triethylenetatramine complex. The alpha-tocopherol radical, generated in the phosphatidylcholine bilayer, is accessible to ascorbic acid, present in the aqueous phase at physiological concentrations. Ascorbic acid regenerates from it the alpha-tocopherol itself. A kinetic rate constant of about 2 X 10(5) M-1 X s-1 was estimated from the reaction as it occurs under the adopted experimental conditions. The scavenging effect of alpha-tocopherol on lipid peroxidation is maintained as long a ascorbic acid is present.