Dynamics of iron-ascorbate-induced lipid peroxidation in charged and uncharged phospholipid vesicles

Peroxidation of egg yolk phosphatidylcholine (egg PC) liposomes was induced by addition of ascorbic acid (AsA) and Fe(II) in the presence of a trace of autoxidized egg PC (PC-OOH), but not in the absence of PC-OOH. PC-OOH was degraded upon addition of AsA and Fe(II) but not of either one alone. The results suggest that PC-OOH is necessary to initiate lipid peroxidation by AsA/Fe(II). AsA oxidation in the bulk water phase was also associated with an increase in lipid peroxidation by AsA/Fe(II) in the presence of PC-OOH, but not in the absence of PC-OOH. Furthermore, the spin probe 12-NS [12-(N-oxyl-4,4'-dimethyloxazolidin-2-yl)stearic acid], which labels the hydrophobic region of dimyristoyl phosphatidylcholine (DMPC) liposomal membranes, was degraded upon addition of AsA and Fe(II) in the presence of PC-OOH, but not in the absence of PC-OOH. These results indicate that the "induction message" that is associated with decreases of PC-OOH and AsA in the initiation step of lipid peroxidation must be transferred from the membrane surface to the inner hydrophobic membrane region. AsA in the bulk phase was oxidized faster and more extensively upon its addition together with Fe(II) to egg PC liposomes than to DMPC liposomes, though the initial content of PC-OOH in the former was 5-10 times lower than in the latter. This suggests that, in egg PC liposomes, the OOH-groups of new PC-OOH generated in the inner membrane regions must become accessible from the surface, enabling reaction with AsA/Fe(II) which in turn would result in an extensive decrease in AsA.(ABSTRACT TRUNCATED AT 250 WORDS)