Artifact formation during transmethylation of lipid peroxides.

By-product formation during base-catalyzed transesterification of lipid peroxides was observed in model experiments. A partially oxidized linoleic acid methyl ester as well as purified hydroxylinoleates served as test compounds. By-products formed during a simulated transmethylation of purified hydroxylinoleates were separated from the main components by means of thin-layer chromatography. The loss attributable to these by-products amounted to 10%. Oxygenated fatty acid derivatives were completely destroyed by acid-catalyzed transmethylation. Catalytic hydrogenation prior to base-catalyzed transmethylation proved to be a simple means to minimize side-product formation. By using this technique the yield of hydroxystearates from a partially autoxidized linoleic acid methyl ester preparation was improved significantly.     

Uranyl acetate induces gel phase formation in model lipid and biological membranes.

The effect of uranyl acetate on the mesomorphic phase state of lipids in model membranes as well as in isolated biological membranes has been examined. As little as 0.8 mM (0.03% [wt/vol]) uranyl acetate induces a liquid crystal-to-gel phase transformation in egg phosphatidic acid, bovine brain phosphatidylserine, and in lysed chromaffin granule membranes. These results along with others in the literature indicate that the uranyl acetate used in samples for electron microscopy could alter membrane morphology.     

Transbilayer coupling mechanism for the formation of lipid asymmetry in biological membranes. Application to the photoreceptor disc membrane.

An equilibrium transmembrane asymmetry in charged lipids is shown to arise as a result of oriented, bipolar proteins in the membrane. The basic interaction giving rise to the asymmetry is between a lipid molecule and a transbilayer potential generated by the asymmetric charge distribution in the protein. Thus, a protein can generate a lipid asymmetry without a direct binding interaction between lipid and protein. The generation of an asymmetry in charged lipid by this mechanism can also lead to a concomitant asymmetry in neutral lipids if deviations from ideality in the lipid mixture are taken into account. It is shown that regular solution theory applied to the lipid phase predicts an asymmetry in all components of a ternary mixture as long as one component is electrostatically oriented according to the mechanism mentioned above. The resulting asymmetry is not strongly salt dependent. The mechanism quantitatively accounts for the experimentally determined phospholipid asymmetry in the rod outer segment disc membrane of the vertebrate photoreceptor.     

Alamethicin-induced pore formation in biological membranes.

The effects of alamethicin on the membrane barrier function of rabbit erythrocytes, human platelets and sarcoplasmic reticulum vesicles, as well as on that of brain microsomes and liver mitochondria of the rat were compared. An upset of the barrier function was observed for plasma membranes of brain microsomes as well as for erythrocyte and platelet membranes at alamethicin concentrations ranging between 25-80 micrograms/ml. The membrane barrier functions of sarcoplasmic reticulum vesicles, of endoplasmic reticulum vesicles of rat brain microsomes, and of liver mitochondria were disturbed at 3-7 micrograms/ml alamethicin. The different sensitivities of plasma and intracellular membranes to alamethicin were supposed to be due to the presence of considerable quantities of cholesterol in plasma membranes as well as to peculiarities of their protein compositions.     

Photo-voltages of bilayer lipid membranes in the presence of cyanine dyes.

The transmembrane photo-voltage waveforms induced by 10 different cyanine dyes absorbed to one side of bilayer lipid membranes are described. The membranes were prepared from lecithin, oxidized cholesterol, and mixed lecithin and oxidized cholesterol. An 8-mus flash illumination was used. Three dyes induced a photo-voltage which developed in a few milliseconds, then discharged in less than the membranes' resistance-capacitance time. Five dyes induced a photo-voltage which increased for much longer than the membranes' resistance-capacitance time. Two dyes did not induce any photo-electric effects. Models are presented which correlate the dye structure with the type of photo-voltage waveform induced.     

Cobaltous ion inhibition of lipid peroxidation in biological membranes.

The effect of cobalt on lipid peroxidation in biological membranes, phospholipid liposomes and fatty acid micelles was investigated. Cobaltous ion, at micromolar concentrations, inhibited iron-ascorbate induced lipid peroxidation in erythrocyte ghosts, microsomes and phosphatidylserine liposomes at pH 7.4. The pH seemed to be important for the anti-peroxidative effect of cobalt, because under slightly acidic conditions cobalt did not inhibit peroxidation. Cobalt was less effective in inhibiting peroxidation stimulated by organic hydroperoxides. Iron-ascorbate induced lipid peroxidation was also inhibited by EDTA. However, certain ratios of EDTA: cobalt in the reaction mixture stimulated peroxidation. Cobalt did not inhibit lipid peroxidation in linoleic acid micelles and phosphatidylethanolamine liposomes. The presence of phosphatidylserine, however, rendered these micelles and liposomes to cobalt inhibition. We conclude that the cobaltous ion is a potent inhibitor of lipid peroxidation in biological membranes and that the binding of cobalt to phosphatidylserine is necessary for the inhibitory effect of this metal ion.     

Spontaneous vesicle formation at lipid bilayer membranes.

Unilamellar vesicles are observed to form spontaneously at planar lipid bilayers agitated by exothermic chemical reactions. The membrane-binding reaction between biotin and streptavidin, two strong transmembrane neutralization reactions, and a weak neutralization reaction involving an "antacid" buffer, all lead to spontaneous vesicle formation. This formation is most dramatic when a viscosity differential exists between the two phases bounding the membrane, in which case vesicles appear exclusively in the more viscous phase. A hydrodynamic analysis explains the phenomenon in terms of a membrane flow driven by liberated reaction energy, leading to vesicle formation. These results suggest that energy liberated by intra- and extracellular chemical reactions near or at cell and internal organelle membranes can play an important role in vesicle formation, membrane agitation, or enhanced transmembrane mass transfer.     

A theoretical approach to cluster formation in biological membranes

A theoretical analysis of cluster formation within the lipid matrix of biological membranes is presented. Various models are analysed: (a) one-dimensional monolayer, (b) two-dimensional monolayer and (c) one dimensional bilayer. Furthermore, lipid-protein interactions are considered. The model is based on differential equations for the probabilities a_i and b₁ which characterize the occupation of the lattice site i by the lipids A and B, respectively. These differential equations are an approximation of the Master-equation. Steady states as well as time-dependent variations are analysed. Depending on the interaction energies of the two lipids, different stationary lipid distributions are obtained, including clusters of lipids A or B and alternating structures. The distributions may be dynamically stable or unstable. It is shown that phase transitions within the lipid matrix may be induced by alteration of the composition of the membrane, by changing the interaction energies of the lipids, by variation of the temperature or by lipid-protein interactions. The transitions between different stationary distributions are studied by use of bifurcation diagrams. The analysis of time-dependent states reveals that unstable structures of the membrane may be important for certain time periods. Consideration of the lipid bilayer leads to a great number of possible distributions, which may be symmetric or asymmetric with respect to the outer and inner leaflets of the membrane.     

The ability of melatonin to counteract lipid peroxidation in biological membranes

This paper reviews recent data relevant to the antioxidant effects of melatonin with special emphasis on the changes produced in polyunsaturated fatty acids located in the phospholipids of biological membranes. The onset of lipid peroxidation within cellular membranes is associated with changes in their physicochemical properties and with the impairment of protein functions located in the membrane environment. All cellular membranes are especially vulnerable to oxidation due to their high concentration of polyunsaturated fatty acids. These processes combine to produce changes in the biophysical properties of membranes that can have profound effects on the activity of membrane-bound proteins. This review deals with aspects for lipid peroxidation of biological membranes in general, but with some emphasis on changes of polyunsaturated fatty acids, which arise most prominently in membranes and have been studied extensively in our laboratory. The article provides current information on the effect of melatonin on biological membranes, changes in fluidity, fatty acid composition and lipid-protein modifications during the lipid peroxidation process of photoreceptor membranes and modulation of gene expression by the hormone and its preventive effects on adriamycin-induced lipid peroxidation in rat liver. Simple model systems have often been employed to measure the activity of antioxidants. Although such studies are important and essential to understand the mechanisms and kinetics of antioxidant action, it should be noted that the results of simple in vitro model experiments cannot be directly extrapolated to in vivo systems. For example, the antioxidant capacity of melatonin, one of the important physiological lipophilic antioxidants, in solution of pure triglycerides enriched in omega-3 polyunsaturated fatty acids is considerably different from that in subcellular membranes.     

Relay model of lipid peroxidation in biological membranes

The present study examines the evidence for the important role of free radicals, localized on carbon atoms of the hydrocarbon chains, in lipid peroxidation. These radicals show a great inter- and intramolecular mobility in membranes by the way of relay-transfer (isomerisation). The sequence of intermediate steps of shift of free radicals in membranes with correction for molecular organization of the hydrocarbon zone of membranes, the intramembrane localization of unsaturated links and the gradient of mobility of the hydrocarbon chains are described. The effect of inhibitors in lipid peroxidation are interpreted in terms of decay of hydrocarbon free radicals as a result of its interaction with the antioxidant molecules. The natural antioxidants having a side chain (such as tocopherols) may be regarded as a some kind of "channel" through which free radicals leave the hydrocarbon moiety of the membrane. The processes of lipid peroxidation in membranes are subjected to a great extent to the requirements of the theory of oxidation of solid polymers and hydrocarbons.