Dynamics of binary mixtures of plasmenylcholine/arachidonic acid and phosphatidylcholine/arachidonic acid--a study using fluorescence and NMR spectroscopy

Structural characterization of plasmenylcholine photooxidation products

Oxidative damage to plasmenyl-type lipids contributes to decreased membrane barrier function, loss of membrane structure and formation of nonlamellar defects in membrane bilayers. Previous results from this laboratory have shown that membrane-soluble sensitizers (e.g. zinc phthalocyanine and bacteriochlorophyll a) mediate the photooxidation of palmitoyl plasmenylcholine (1-O-alk-1'-Z-enyl-2-palmitoyl-sn-glycero-3-phosphocholine; PPlsC) vesicles with the subsequent creation of lamellar defect structures, vesicle contents leakage and membrane-membrane fusion. Because plasmalogen lipids are significant components of sarcoplasma and myelin membranes, we sought to characterize the products of their photooxidation. This study focuses on the photooxidation of PPlsC vesicles in the presence of the water-soluble sensitizer, aluminum phthalocyanine tetrasulfonate (AlPcS4(4-)). Attack of photogenerated singlet oxygen on the 1-O-alkenyl ether linkage of PPlsC lipids was expected to generate dioxetane- and ene-type photoproducts. The products formed during continuous aerobic irradiation (28 mW/cm2, (610 nm) of PPlsC vesicles in the presence of AlPcS4(4-) were separated via reverse-phase high-performance liquid chromatography (HPLC) with electrochemical detection (ECD) or evaporative light-scattering detection (ELSD). Photooxidized dipalmitoyl-phosphatidylcholine-cholesterol vesicles (control) were used to optimize the HPLC-ECD conditions, using 7 alpha-hydroperoxy-cholesterol as standard. HPLC-ECD was found to be most sensitive for PPlsC hydroperoxides, whereas HPLC-ELSD was more sensitive for nonhydroperoxide photoproducts. The three major photoproducts formed during vesicle irradiation were isolated via preparative HPLC and then characterized by 1H-nuclear magnetic resonance and mass spectrometry. 1-Formyl-2-palmitoyl-sn-glycero-3-phosphocholine and 1-hydroxy-2-palmitoyl-sn-glycero-3-phosphocholine were identified as dioxetane cleavage products that coeluted at approximately 3 min. The second fraction (retention time [RT] = 48 min) was identified as a PPlsC allylic hydroperoxide. The third photoproduct, eluting at RT = 64 min, is tentatively identified as an oxidation product arising from allylic hydroperoxide degradation via Hock rearrangement or free radical decomposition.

A proton magnetic resonance spectroscopy study of aging and transformed human fibroblasts

Proton magnetic resonance spectroscopy (1H MRS) has been used to monitor changes occurring during aging and transformation in human lung fibroblasts. Aging was studied in MRC-5 cells from nonsenescent (early passage) to presenescent (late passage) and senescence. Nonsenescent cells infected with SV40 virus (pretransformed) were monitored through crisis and subsequent immortalization. Aging changes were observed with one- and two-dimensional MR spectra. Cholesterol and lipid resonances were significantly increased from nonsenescent cultures to senescence. These changes could be caused by chemical or structural changes in the plasma membrane or in intracellular lipid pools. In contrast, choline levels rose from nonsenescent to presenescent cells but at senescence dropped to that of nonsenescent cells. Increased choline levels are often associated with increased cellular proliferation. After SV40 infection of MRC-5 cells there was an increase of cholesterol and lipid levels that peaked at crisis. Newly immortalized cells exhibited a drop in cholesterol and lipid to nonsenescent cell levels, but these rose again in established immortalized cells. In contrast to presensescent cultures, the levels of choline gradually increased from pretransformed to crisis phase but still continued to rise after immortalization. Thus, 1H MRS illustrates similarities in lipid behavior at senescence and crisis, whereas the choline levels are different.

Ether lipids in biomembranes

Plasmalogens (1-O-1'-alkenyl-2-acylglycerophospholipids) and to a lesser extent the 1-O-alkyl analogs are ubiquitous and in some cases major constituents of mammalian cellular membranes and of anaerobic bacteria. In archaebacteria polar lipids of the cell envelope are either diphytanylglycerolipids or bipolar macrocyclic tetraether lipids capable of forming covalently linked 'bilayers'. Information on the possible role of ether lipids as membrane constituents has been obtained from studies on the biophysical properties of model membranes consisting of these lipids. In addition, effects of modified ether lipid content on properties of biological membranes have been investigated using microorganisms or mammalian cells which carry genetic defects in ether lipid biosynthesis. Differential utilization of ether glycerophospholipids by specific phospholipases might play a role in the generation of lipid mediators that are involved in signal transduction. A possible function of plasmalogens as antioxidants has been demonstrated with cultured cells and might play a role in serum lipoproteins. Synthetic ether lipid analogs exert cytostatic effects, most likely by interfering with membrane structure and by specific interaction with components of signal transmission pathways, such as phospholipase C and protein kinase C.