Plasmalogens (Plg) are phospholipids containing vinyl ether linkage at the sn-1 position of the glycerophospholipid backbone. In spite of being quite abundant in humans, the biological role of plasmalogens remains speculative. It has been postulated that plasmalogens are physiological antioxidants with the vinyl ether functionality serving as sacrificial trap for free radicals and singlet oxygen. However, no quantitative data on the efficiency of plasmalogens to scavenge these reactive species are available. In this study, rate constants of quenching of singlet oxygen, generated by photosensitized energy transfer, by several plasmalogens and, for comparison, by their diacyl analogs, were determined by time-resolved detection of phosphorescence at 1270 nm. Relative rates of the interaction of singlet oxygen, with plasmalogens and other lipids in solution and liposomal membranes were measured by electron paramagnetic resonance oximetry and product analysis, employing HPLC-EC detection of cholesterol hydroperoxides and iodometric assay of lipid hydroperoxides. Results show that singlet oxygen interacts with plasmalogens significantly faster than with the other lipids, with he corresponding rate constants being by one-two orders of magnitude greater. The quenching of singlet oxygen by plasmalogens is mostly reactive in nature and results from its preferential interaction with the vinyl ether bond. The data suggest that plasmalogens could protect unsaturated membrane lipids against oxidation induced by singlet oxygen, providing that the oxidation products are not excessively cytotoxic.

We have studied the interaction of trifluoperazine (TFP) with monolayers of various glycerophospholipids at 37 degrees C. TFP (1-10 microM) had little effect on surface pressure/molecular area isotherms in monolayers (on pure water) of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine but greatly increased the mean molecular area (mma) of dipalmitoylphosphatidylserine; the increment was greatest between 0 and 1 microM, and a further increase to 10 microM TFP gave only a slight increase in mma. With phosphatidylserine (PS)-containing stearoyl and varying acyls in the sn-1 and -2 positions, respectively, TFP increased the mma in a manner that depended on the number of double bonds and chain length. In mixtures of DPPC with two of these PS species the TFP-induced mma of the monolayers (on buffer, pH 7.4) increased linearly with the proportion of PS. Both PS and TFP have ionizable groups, and the TFP-induced mma increase had optima at pH 5.0 and 7.0. We conclude that the TFP-PS interaction is mainly, but not entirely, driven by electrostatic interactions between the TFP cation and PS headgroup anion, with an insertion of the phenothiazine moiety among the acyls in the monolayer that depends on the packing of the acyls.

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