Further studies on the antioxidant role of pyrophosphate in model membranes

Our previous studies (G. Cervato et al., Chem. Phys. Lipids 56 (1990), 91-99) demonstrated that 100 microM pyrophosphate (PPi) inhibits Fe2+/ascorbate-induced peroxidation of arachidonic acid (AA) inserted in unilamellar liposomes (SUVs) of dipalmitoylphosphatidylcholine (DPPC), by chelating ferrous ions in the aqueous phase. In this work we demonstrate that the kinetics of AA peroxidation in DPPC SUVs are strongly affected by PPi, also at very low concentration (1 microM). In fact at low PPi concentration there is a longer lag-phase, while the maximum of thiobarbituric acid reactive substances (TBARS) is similar in both the presence and absence of 1 microM PPi. The lag-phase of peroxidation of AA in lysophosphatidylcholine (palmitoyl) (LysoPC) micelles is also prolonged. The AA peroxidation in membrane models without choline as the polar headgroup (dipalmitoylphosphatidylethanolamine (DPPE) SUVs, or Triton X-100 micelles), or in which AA is not free but esterified with glycerol (stearoyl-arachidonyl-phosphatidylcholine (SAPC)), is unaffected by the presence of 1 microM PPi.