Auto-oxidation-induced fusion of lipid vesicles

Stability and release performance of curcumin-loaded liposomes with varying content of hydrogenated phospholipids

The aim of this study was to substitute part of soybean phospholipid (SPC) with hydrogenated soybean phospholipid (HSPC) in curcumin-loaded liposomes (Cur-LP), in order to further enhance stability and release performances of curcumin. When the SPC/HSPC mass ratio changed from 10:0 to 5:5, vesicle size, encapsulation efficiency and alkali resistance of curcumin increased, although a small decrease in centrifugal stability was observed. Salt stability became worse as more HSPC was used (3:7 and 0:10). Owing storage at 4 °C and 25 °C, Cur-LP at a SPC/HSPC mass ratio of 5:5 performed well considering vesicle size, lipid oxidation and curcumin retention. These vesicles displayed also the best sustained-release performance in simulated digestion, attributed to the tighter lipid packing in membranes as indicated by fluorescence probes, DSC and FTIR. This study can guide the development of a Cur-LP product with improved shelf-life stability by using HSPC.

Copper-induced peroxidation of liposomal palmitoyllinoleoylphosphatidylcholine (PLPC), effect of antioxidants and its dependence on the oxidative stress

In an attempt to deepen our understanding of the mechanisms responsible for lipoprotein peroxidation, we have studied the kinetics of copper-induced peroxidation of the polyunsaturated fatty acid residues in model membranes (small, unilamellar liposomes) composed of palmitoyllinoleoylphosphatidylcholine (PLPC). Liposomes were prepared by sonication and exposed to CuCl(2) in the absence or presence of naturally occurring reductants (ascorbic acid (AA) and/or alpha-tocopherol (Toc)) and/or a Cu(I) chelator (bathocuproinedisulfonic acid (BC) or neocuproine (NC)). The resultant oxidation process was monitored by recording the time-dependence of the absorbance at several wavelengths. The observed results reveal that copper-induced peroxidation of PLPC is very slow even at relatively high copper concentrations, but occurs rapidly in the presence of ascorbate, even at sub-micromolar copper concentrations. When added from an ethanolic solution, tocopherol had similar pro-oxidative effects, whereas when introduced into the liposomes by co-sonication tocopherol exhibited a marked antioxidative effect. Under the latter conditions, ascorbate inhibited peroxidation of the tocopherol-containing bilayers possibly by regeneration of tocopherol. Similarly, both ascorbate and tocopherol exhibit antioxidative potency when the PLPC liposomes are exposed to the high oxidative stress imposed by chelated copper, which is more redox-active than free copper. The biological significance of these results has yet to be evaluated.

Sickle hemoglobin is more fusogenic than normal hemoglobin at physiological pH and ionic strength conditions

We used electron microscopy, quasi-elastic light scattering and static light scattering to show that human hemoglobin (Hb) interacts with bovine brain phosphatidylserine lipid vesicles and promotes vesicle fusion in an isotonic buffer at pH 7.4. The fusogenic properties of Hb were observed in both small unilamellar vesicles (SUVs) and large unilamellar vesicles (LUVs). A simple turbidity measurement method was used to follow increases in vesicle size (scattering diameter) as a function of time. For the first 3 h, upon incubation with oxygenated Hb, the scattering diameters of vesicles increased at a rate of 7.8 nm/h for LUVs. Continuous incubation with Hb led to complicated vesicle fusion, probably due to the oxidation products of Hb and lipid molecules. In the absence of both Hb and lipid oxidation, using Hb liganded with carbon monoxide, we obtained, for the entire 20 h incubation period, a fusion rate of 2.9 nm/h for LUVs. We also studied interactions between sickle Hb and vesicles under the same conditions and found that the vesicle fusion rates for sickle Hb were about 2 times faster than those for normal Hb. These results showed that sickle Hb exhibited more extensive interactions with lipid bilayer than normal Hb at physiological pH and ionic strength conditions, and provide insights toward understanding the molecular mechanisms in sickle cell abnormalities.

Preferential formation of the hydroperoxide of linoleic acid in choline glycerophospholipids in human erythrocytes membrane during peroxidation with an azo initiator

The formation of phospholipid hydroperoxides was monitored in human red blood cell (RBC) membranes that had been peroxidized with an azo initiator. Peroxidation of RBC membranes caused a profound decrease in the amount of polyunsaturated fatty acids and concomitantly hydroperoxides, as primary products of peroxidation, appeared in the phospholipids. Hydroperoxides were predominantly generated in choline glycerophospholipid (CGP), while the extent of formation of ethanolamine glycerophospholipid (EGP) hydroperoxides was low and their presence was transient. Hydroxy and hydroperoxy moieties in CGP were identified as 9-hydroxy and 13-hydroxy octadecanoic acid, derived from linoleic acid, by gas chromatography-mass spectrometric analysis. No consistent generation of hydroperoxide from arachidonic acid was evident in CGP. The CGP-hydroperoxide accounted for approximately 76% of linoleic acid consumed during peroxidation of RBC membranes. The prominent generation of phospholipid hydroperoxides was observed in the linoleic acid-rich membranes from rabbit RBC, indicating that the level of linoleic acid in phospholipids determines, in part, the extent of formation of phospholipid hydroperoxides. Aldehydic phospholipids, as secondary products of peroxidation, were detected in oxidized membranes. EGP was the most prominent aldehydic phospholipid, while negligible amounts of aldehydic CGP were formed. This study indicates that the process of oxidation of individual phospholipids clearly differs among phospholipids and depends on the structure of each.

The rabbit pulmonary cytochrome P450 arachidonic acid metabolic pathway: characterization and significance

Cytochrome P450 metabolizes arachidonic acid to several unique and biologically active compounds in rabbit liver and kidney. Microsomal fractions prepared from rabbit lung homogenates metabolized arachidonic acid through cytochrome P450 pathways, yielding cis-epoxyeicosatrienoic acids (EETs) and their hydration products, vic-dihydroxyeicosatrienoic acids, mid-chain cis-trans conjugated dienols, and 19- and 20-hydroxyeicosatetraenoic acids. Inhibition studies using polyclonal antibodies prepared against purified CYP2B4 demonstrated 100% inhibition of arachidonic acid epoxide formation. Purified CYP2B4, reconstituted in the presence of NADPH-cytochrome P450 reductase and cytochrome b5, metabolized arachidonic acid, producing primarily EETs. EETs were detected in lung homogenate using gas chromatography/mass spectroscopy, providing evidence for the in vivo pulmonary cytochrome P450 epoxidation of arachidonic acid. Chiral analysis of these lung EETs demonstrated a preference for the 14(R),15(S)-, 11(S),12(R)-, and 8(S),9(R)-EET enantiomers. Both EETs and vic-dihydroxyeicosatrienoic acids were detected in bronchoalveolar lavage fluid. At micromolar concentrations, methylated 5,6-EET and 8,9-EET significantly relaxed histamine-contracted guinea pig hilar bronchi in vitro. In contrast, 20-hydroxyeicosatetraenoic acid caused contraction to near maximal tension. We conclude that CYP2B4, an abundant rabbit lung cytochrome P450 enzyme, is the primary constitutive pulmonary arachidonic acid epoxygenase and that these locally produced, biologically active eicosanoids may be involved in maintaining homeostasis within the lung.

Lipid peroxidation, low-level chemiluminescence and regulation of secretion in the mammary gland

In mammary explants of lactating mice, changes in the intensity of chemiluminescence (CL) were observed after the addition to the incubation medium of hormones and mediators that are involved in the regulation of secretion: oxytocin, acetylcholine, epinephrine and norepinephrine. A 15-min period of treatment with oxytocin, epinephrine or norepinephrine changed the level of thiobarbituric acid-reactive substances (TBARS). Two mammary explants, one of which was treated with oxytocin, acetylcholine, epinephrine or norepinephrine, were found to interact even when separated by a quartz glass wall. Analysis of the level of TBARS formation in these two explants showed that the observed interactions might be connected with light emission resulting from lipid peroxidation (LP) processes. The possible role of LP and low-level CL in the regulation of mammary gland secretion is discussed.

The inverted hexagonal phase is more sensitive to hydroperoxidation than the multilamellar phase in phosphatidylcholine and phosphatidylethanolamine aqueous dispersions

The effect of phase behaviour (hexagonal II phase and lamellar phase) on the peroxidation of membrane phospholipids has been investigated in dilinoleoyl phosphatidylcholine (DLPC)/dilinoleoyl phosphatidylethanolamine (DLPE) aqueous dispersions. Peroxidation was initiated with a water-soluble radical inducer 2,2'-azobis (2-amidino-propane) dihydrochloride (AAPN). The phospholipid morphology was monitored by 31P-nuclear magnetic resonance (NMR). Phospholipid hydroperoxides (PCOOH and PEOOH) were determined by chemiluminescence high-performance liquid chromatography (CL-HPLC). In pH-induced phase transition systems, DLPE in the bilayer state was much less oxidized than in the hexagonal II state. In composition-induced phase transition systems, the formation of total hydroperoxides and the consumption of alpha-tocopherol in the hexagonal II phase were greater than in the bilayer phase. These data suggest that the hexagonal II phase is more sensitive to hydroperoxidation than the bilayer phase in phospholipid aqueous dispersions.

Peroxidation and phospholipase A2 hydrolytic susceptibility of liposomes consisting of mixed species of phosphatidylcholine and phosphatidylethanolamine

The relationship between lipid peroxidation and phospholipase A2 (PLA2) hydrolytic activity was studied using unilamellar vesicles (liposomes) as model membranes. Hydrolytic specificity was examined using vesicles prepared with pure bovine heart phosphatidylcholine (PC), bovine heart phosphatidylethanolamine (PE), or mixtures of these phospholipids, using two preparative procedures, i.e., sonication or extrusion. Lipid peroxidation was induced by incubating vesicles with cumene hydroperoxide and hematin at 37 degrees C. Determinations of the extent of peroxidation by means of diene conjugate content derived from second derivative spectra or by polarographic measurement of oxygen consumption rates provided a basis for comparing the extent of peroxidation of each phospholipid species to their subsequent hydrolysis by PLA2 (from Crotalus adamanteus). The extent of hydrolysis was determined through the release of arachidonic acid from either PC or PE. The PE distribution among the outer vs. inner leaflet of the membrane bilayer was nearly equal in sonicated vesicles, whereas most of the phospholipid was incorporated into the inner leaflet in extruded vesicles. The proportion of PE found in the inner leaflet progressively increased as the ratio of PE to PC increased in both sonicated and extruded vesicle preparations. Lipid peroxidation had no effect on PE distribution under the conditions examined. There was a clear preference for PC peroxidation for all vesicle compositions tested and PC was preferentially hydrolyzed by PLA2. This effect is proposed to result from a perturbation of membrane structure following peroxidation with assimilation of PC into PLA2-susceptible domains whereas PE peroxidation and hydrolysis is less affected in mixed PC/PE vesicles. Lipid peroxidation imposes an additional hydrolytic susceptibility over the effects exerted through the mixing of these phospholipids which is based on structural changes rather than formation of specific substrates for PLA2.

Changes of the oligomeric structure of legumin from pea (Pisum sativum L.) after succinylation

The influence of various levels of succinylation on the structure of the legumin from pea seed has been studied by the techniques of sedimentation velocity, viscometry, fluorescence and circular dichroism spectroscopy, as well as dynamic light scattering. The protein dissociates gradually into the 3S subunit forming a 7S intermediate. At a level of 75-80% succinylation, sudden unfolding of the protein occurs characterized by drastic changes in viscometric and spectroscopic properties. The fluorescence spectra point to the formation of a novel organized structure at a moderate degree of modification before the molecular unfolding takes place. The succinylated subunit was shown to have a sedimentation coefficient of 3.2S, a diffusion coefficient of 5.03 x 10(-7) cm2 . s-1 a Stokes' radius of 4.24 nm, a partial specific volume of 0.703 ml/g, an intrinsic viscosity of 0.13 dl/g, a molar mass of 52.2 kDa and a frictional ratio of 1.74.

Biophysical consequences of lipid peroxidation in membranes

This article reviews the biophysical consequences of lipid peroxidation in biological membranes. In the lipid domain, lipid peroxidation (a) causes an increase in the order and "viscosity" of the membrane bilayer, particularly at the depth around acyl-carbon 12, (b) changes the thermotropic phase behaviour, (c) decreases the electrical resistance, and (d) facilitates phospholipid exchange between the two monolayers. Upon lipid peroxidation membrane proteins are crosslinked, and their rotational and lateral mobility is decreased. Studies with microsomal cytochrome P-450 suggest protein aggregation but not the increased lipid order to be the major cause of protein immobilization in peroxidized membranes.

THE ROLE OF FREE RADICALS IN SENESCENCE AND WOUNDING

Reactions involving free radicals are an inherent feature of plant senescence and appear to contribute to a process of oxidative deterioration that leads ultimately to cell death. Radical species derived from molecular oxygen are the primary mediators of this oxidative damage, but non-radical excited states of oxygen, specifically singlet oxygen, may also be involved. Several lines of evidence suggest that degradation of lipids in senescing membranes and the ensuing release of free fatty acids initiate oxidative deterioration by providing substrate for lipoxygenase. In some tissues, lipoxygenase activity increases with advancing senescence in a pattern that is consistent with its putative role in promoting oxidative damage. However, there are important exceptions to this which may be explained by the fact that the timing and extent of peroxidative reactions initiated by lipoxygenase are likely to be determined more by the availability of substrate for the enzyme than by changes in its activity. There are both membranous and cytosolic forms of lipoxygenase in senescing tissues, and peroxidation of membrane lipids appears to be initiated by the membranous enzyme once the appropriate fatty acid substrates, linoleic acid and linolenic acid, become available. Since lipid peroxidation is known to form alkoxy and peroxy radicals as well as singlet oxygen, these reactions in membrane bilayers are probably a major source of activated oxygen species in senescing tissues. Further-more, there are indications that activated oxygen from the lipoxygenase reaction can become substrate for the cytosolic form of the enzyme which, in turn, may raise the titre of activated oxygen during senescence. Additional possible sources of increased free radical production in senescing tissues include peroxidase, which shows greatly increased activity with advancing age, leakage of electrons from electron transport systems to oxygen, in particular from the photosynthetic electron transport system, and decompartmentalization of iron, which would facilitate formation of the highly reactive hydroxyl radical from the less reactive superoxide anion. A variety of macromolecules can be damaged by activated oxygen. Unsaturated fatty acids are especially prone to attack, and this implies that membranes are primary targets of free radical damage. The manifestations of this damage in senescing tissues range from altered membrane fluidity and phase properties to leakiness that can be attributed to a destabilized and highly perturbed membrane bilayer. There is also a progressive breakdown of cellular protein with advancing senescence. Free radicals can inactivate proteins by reacting with specific amino acid residues, and a number of in zitro studies have indicated that such alteration renders the proteins more prone to hydrolysis by proteases. Thus, although there is no direct evidence linking enhanced proteolysis during senescence to free radical damage, there is reason to believe that this may be a contributing factor. Wounding of certain plant tissues also initiates a series of reactions that revolve around the breakdown of membrane lipids and their peroxidation. Indeed, as in the case of senescence, membrane deterioration follokving wounding appears to be facilitated by a self-perpetuating wave of free radical production emanating from peroxidation within the lipid bilayer. There is also recent evidence for activation of an O₂ ^- -producing NADPH oxidase in plant tissues following fungal infection that may be analogous to the well-characterized O₂ ^- -generating NADPH oxidase associated with the plasma membrane of polymorphonuclear leukocytes. This raises the interesting possibility that plants and animals share a common defence response to invading organisms. Contents Summary 317 I. Introduction 318 II. Species of activated oxygen 319 III. Sites of activated oxygen production 319 IV. Free radical production during senescence 323 V. Targets of free radical damage in senescing tissues 330 VI. The role of free radicals in seed ageing 336 VII. The role of free radicals in wounding 337 VIII. Concluding remarks 338 Acknowledgement 338 References 338.

Comparison of the structure of ribosomal 5S RNA from E. coli and from rat liver using X-ray scattering and dynamic light scattering

The structure of eukaryotic ribosomal 5S RNA from rat liver and of prokaryotic 5S RNA from E. coli (A-conformer) have been investigated by scattering methods. For both molecules, a molar mass of 44,500 +/- 4,000 was determined from small angle X-ray scattering as well as from dynamic light scattering. The shape parameters of the two rRNAs, volume Vc, surface Oc, radius of gyration Rs, maximum dimension of the molecule L, thickness D, and cross section radius of gyration Rsq, agree within the experimental error limits. The mean values are Vc = 57 +/- 3 nm3, Oc = 165 +/- 10 nm2, Rs = 3.37 +/- 0.05 nm, L = 10.8 +/- 0.7 nm, D = 1.57 +/- 0.07 nm, Rsq = 0.92 +/- 0.01 nm. Identical structures for the E. coli 5S rRNA and the rat liver 5S rRNA at a resolution of 1 nm can be deduced from this agreement and from the comparison of experimental X-ray scattering curves and of experimental electron distance distribution functions. The flat shape model derived for prokaryotic and eukaryotic 5S rRNA shows a compact region and two protruding arms. Double helical stems are eleven-fold helices with a mean base pair distance of 0.28 nm. Combining the shape information obtained from X-ray scattering with the information about the frictional behaviour of the molecules, deduced from the diffusion coefficients D020, w = (5.9 +/- 0.2) X 10(-7) cm2 s-1 and (6.2 +/- 0.2) X 10(-7) cm2 s-1 for rat liver 5S rRNA and E. coli 5S rRNA, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of phospholipid peroxidation on the phase behavior of phosphatidylcholine and phosphatidylethanolamine in aqueous dispersions

The influence of oxygen-induced phospholipid peroxidation on the phase behavior of aqueous dispersions of both egg phosphatidylcholine (egg-PC) and egg phosphatidylethanolamine (egg-PE) has been investigated. Phospholipid peroxidation was followed via malondialdehyde formation and analyses of acyl chain compositions. 13C nuclear magnetic resonance spectroscopy (NMR) and the amino-indicating probe trinitrobenzenesulfonic acid were used to study the effect of peroxidation on the chemical structure of hydrated egg-PE. The macroscopic organization of the phospholipids was monitored by 31P NMR and small-angle X-ray diffraction. Differential scanning calorimetry was employed to study the influence of peroxidation on the thermotropic behavior of egg-PE. The results show that egg-PE is more sensitive to the effects of peroxidation than egg-PC. In the latter, no changes in the macromolecular organization were observed. However, peroxidation strongly influenced the polymorphic phase behavior of PE. Initial peroxidation stabilized hydrated egg-PE in a lamellar system up to 70 degrees C, presumably by modification of the head group. Such modifications were confirmed by 13C NMR experiments, which indicated the formation of Schiff bases between PE head groups and aldehydes. Furthermore, quantitative analyses of trinitrobenzenesulfonic acid reactable egg-PE and the corresponding fatty acid compositions revealed the presence of cross-links between the ethanolamine head groups, likely involving the bifunctional malondialdehyde. Prolonged peroxidation of egg-PE resulted in a loss of order in the system, possibly by the formation of intermediate nonbilayer structures.

Incorporation of the cytochrome P-450 monooxygenase system into large unilamellar liposomes using octylglucoside, especially for measurements of protein diffusion in membranes

Cytochrome P-450 and NADPH cytochrome P-450 reductase were incorporated into large unilamellar lipid vesicles (200-300 nm in diameter) removing octylglucoside from mixed micelles by dialysis. The large size of the protein-containing liposomes guarantees a negligibly small vesicle tumbling. Such large vesicles are better suited for studies of protein rotation in reconstituted membranes than vesicles prepared by use of bile salts. At present the octyl-glucoside reconstituted monooxygenase system seems to be the most appropriate model for studying protein-protein and protein-lipid interactions in liver microsomes due to the similarity with respect to the main structural and functional properties, including size.

Shape, symmetry, hydration and secondary structure of the legumin from Vicia faba in solution

The following physical parameters of the legumin from Vicia faba were determined by means of small-angle X-ray scattering, quasi-elastic light scattering and circular dichroism: molar mass, M = 3.5 X 10(5) g/mol; radius of gyration, Rg = 4.45 nm; maximum dimension, L = 13 nm; translational diffusion coefficient, D0(20),w = 3.38 X 10(-7) cm2 X s-1; alpha-helix content about 15%; content of beta-sheets 10%; dihedral point group symmetry of the molecule 32.

Lipid oxidation and gel to liquid-crystalline transition temperatures of synthetic polyunsaturated mixed-acid phosphatidylcholines

Synthetic preparations of the polyunsaturated phosphatidylcholines 1-stearoyl-2-linoleoyl-sn-glycero-3-phosphocholine (SLPC) and 1-stearoyl-2-alpha-linolenoyl-sn-glycero-3-phosphocholine (SLnPC) were observed to undergo autooxidation sometimes during synthesis and also on storage. Oxidation was also induced by treatment of unoxidized SLPC with ultraviolet irradiation. Oxidation was estimated by thin layer chromatographic, fatty acid and ultraviolet spectral analyses. With limited oxidation, the gel to liquid-crystalline transition temperatures of aqueous dispersions of these lipids were seen to increase. Extensive oxidation led to a reduction in the enthalpies of the transitions. The increases in transition temperatures were consistent with the presence of conjugated double bonds, as shown by increased absorption at 230 nm, and trans double bonds, in the oxidized lipids leading to the creation of more rigid domains within the bilayer. Some of the changes in the transitions, especially the decreasing enthalpy after extensive oxidation, may have occurred because of the presence of small amounts of lysophosphatidylcholine and other oxidation intermediates or breakdown products seen by thin layer analysis. Thermograms of mixtures of unoxidized SLPC with amounts of lysostearoylPC found in the oxidized samples showed, however, that lysoPC likely did not contribute significantly to the increase in transition temperatures. Thin layer analysis suggested that the presence of cross-linked products could have contributed to the observed thermotropic properties.