Dynamics of xanthine oxidase- and Fe(3+)-ADP-dependent lipid peroxidation in negatively charged phospholipid vesicles

Superoxide (O2-)-dependent lipid peroxidation on addition of xanthine oxidase (XO) and Fe(3+)-ADP was induced in egg phosphatidylcholine (PC) liposomes containing dicetylphosphate (DCP), which are negatively charged like biological membranes, but not in uncharged egg PC liposomes. Positively charged Fe(3+)-ADP interacted more with negatively charged egg PC-DCP liposomes than with uncharged egg PC liposomes. The activities of Fe(3+)-chelates for initiating O(2-)-dependent lipid peroxidation were in the order Fe(3+)-ADP > Fe(3+)-citrate > Fe(3+)-oxalate = Fe(3+)-malonate > Fe(3+)-EDTA = 0. This order was the same as that for the reduction rates of these Fe(3+)-chelates to Fe(2+)-chelates by O(2-)-generated by XO. Lineweaver-Burk plots showed that the chelators inhibited XO by different mechanisms: uncompetitively by ADP and adenosine and non-competitively by organic acid chelators (citrate and oxalate) and EDTA. These results suggest that ADP interacts with XO in a manner different from the other chelators. Lipid peroxidation by XO-xanthine and Fe(3+)-ADP was induced in egg PC liposomes containing a trace (0.31-0.35 mol%) of peroxidized egg PC (PC-OOH), but not in PC-OOH-free liposomes of egg PC obtained by their pretreatment with triphenylphosphine. PC-OOH incorporated into dimyristoyl phosphatidylcholine (DMPC) liposomes was degraded on addition of both XO-xanthine and Fe(3+)-chelate, but not of either one alone. alpha-Tocopherol in DMPC liposomes was oxidized on addition of XO-xanthine and Fe(3+)-chelates in the presence, but not in the absence of PC-OOH. Furthermore, PC-OOH was required for decrease of the ESR spectrum of the spin probe 12-(N-oxyl-4,4'-dimethyloxazolidin-2-yl)stearic acid, which labels the hydrophobic region of egg PC liposome membranes, on addition of XO-xanthine and Fe(3+)-chelates. These results indicate that the "induction message of lipid peroxidation," which is associated with reduction of Fe(3+)-ADP by O2- and concurrent degradation of PC-OOH, must be transferred from the membrane surface to the inner hydrophobic region of the membranes.