Liposomal membranes. XX. Autoxidation of unsaturated fatty acids in liposomal membranes

Lipid peroxidation inhibition capacity assay for antioxidants based on liposomal membranes

An in vitro antioxidant assay has been developed to better reflect the in vivo conditions of antioxidants interacting with membrane and lipid surfaces. The lipid peroxidation inhibition capacity (LPIC) method measures the ability of both lipophilic and hydrophilic antioxidants to protect a lipophilic fluorescent probe 4, 4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-undecanoic acid, incorporated in the membrane, from 2,2'-azobis(2-amidinopropane)hydrochloride generated radicals in the surrounding aqueous solution. Antioxidant activities of test compounds were measured either after they were mixed with preformed liposomes (LPIC(Mixed)) or after they were incorporated into liposomes (LPIC(Inco)) as they were made. The results were analysed to determine how the method of mixing and the structures of the antioxidants influenced their protection of the membrane from free radical attack. The LPIC(Mixed) values were larger than the LPIC(Inco) values for a range of 12 structurally diverse antioxidant compounds. However, there was no linear correlation between the lipophilicities, as measured by their partition coefficient, log P and either LPIC(Inco) or LPIC(Mixed) values. A strong correlation was found between LPIC(Inco) and LPIC(Mixed) values.

Possible involvement of free radical scavenging properties in the action of tumor necrosis factor-alpha

Constitutive production of hydroxyl radicals from four established cancer cell lines was detected as spin adducts of 5,5-dimethyl-l-pyroline-N-oxide (DMPO), using an electron spin resonance spectrometer. The generated hydroxyl radicals was decreased in three out of four cancer cell lines when incubated in vitro for 3 h with TNF-alpha No direct scavenging effect of TNF-alpha on hydroxyl radicals or superoxide anions was observed in the in vitro radical generation system. The modulation of intracellular reactive oxygen species of these cancer cells by adding menadione or CuDIPS to the culture medium changed the antiproliferative effect of TNF-alpha on the cells. The ultrastructural localization of the radical-generating sites in cancer cells was visualized using the diaminobenzidine/horseradish peroxide histochemical system at the electron microscopic level. The hydrogen peroxide-dependent formation of electron-dense materials localized at the mitochondrial membranes was decreased after the treatment of the cancer cells with TNF-alpha. These data indicate that the reduction of radical generation in cancer cells by TNF-alpha may be an early mechanism that contributes to the antiproliferative effect of this cytokine on some cancer cells.

Monitoring by cis-parinaric fluorescence of free radical induced lipid peroxidation in aqueous liposome suspensions

Cis-parinaric acid is fluorescent when partioned into a lipid environment and its fluorescence is destroyed upon reaction with free radicals. In our study 1-palmitoyl-2-parinoyl-phosphatidylcholine (cis-PnA) has been used to monitor the time-course of liposomal lipid peroxidation, using reverse-phase evaporation vesicles (REV) of different composition exposed to oxidative stress in various conditions. This methodology allowed us to estimate the potential damage produced by two different oxidizing systems, namely hydrogen peroxide (H2O2), a water soluble oxidant, and t-butyl hydroperoxide (t-BHP), a hydrophobic hydroperoxide. Furthermore, we evaluated the protective effects of bilayer-associated antioxidants, namely alpha-tocopherol acetate (alpha-THA), vitamin K1 and beta-carotene, as well as of two antioxidants dissolved in the aqueous bulk solution, that is, biverdin and uric acid. Under our experimental conditions, the results suggest that (i) both oxidizing compounds were able to interact with liposomal PnA leading to decay either of the excitation and of emission spectra of the probe; (ii) hydrogen peroxide seemed to be of most effective among the two stressing agents, when employed at similar concentrations; (iii) the alpha-THA appeared to be a stronger antioxidant than vitamin K1 and beta-carotene, resulting in a decrease of the liposomal membrane stress caused by those two oxidizing agents; (iv) among the water soluble antioxidant compounds, biliverdin displayed a protective effect at least 10 x higher than uric acid; (v) the overall damage, as well as the protection mechanisms, seemed to be dependent either on the lipid composition of the vesicles and on the pH of the liposomal suspension. This relatively easy experimental approach suggests the validity of the use of the bilayer associated fluorescent probe PnA in the monitoring of spontaneous and/or chemically induced liposomal lipid damage.

0/w-emulsion of alpha-linolenic acid stabilized with hydrophobized polysaccharide. Its effect on the growth of human colon cancer cells

To pursue a systemic administration of alpha-linolenic acid (ALA), which is a selective cytotoxic agent, we formulated an ALA o/w-emulsion stabilized by cholesterol-bearing pullulan (CHP-55-2.1) and trioctanoylglyceride (TriC8). This emulsion was stable even in the presence of bovine serum albumin (BSA). Peroxidation of ALA was drastically depressed by the emulsification using CHP. In addition, cytotoxic effect of the CHP/ALA/TriC8-emulsion against human colon cancer cell (RPM14788) was much higher than that of free ALA. However, no significant difference was observed in cell internalization efficiency of ALA between the two. These results suggest that difference in the cytotoxicity between the CHP/ALA/TriC8-emulsion and free ALA may come from difference in the intracellular behavior of ALA.

Rapid diffusion of the lipid phosphorus of phosphatidylglycerol liposomes through polycarbonate membranes is caused by the oxidation of the unsaturated fatty acids

The lipid phosphorus of phosphatidylglycerol liposomes was found to diffuse extensively, after a lag time of 1 to 2 days, through a 0.1 micron pore size polycarbonate membrane in a two compartment system. Diffusion occurred when either multilamellar or large unilamellar vesicles were studied, even if they were sedimented to eliminate any smaller particles. The lipid of liposomes prepared under sterile conditions also diffused extensively. Diffusion appeared to be related to the age of the vesicles, and could be eliminated by incorporating antioxidants into the liposomes, or by using liposomes prepared from saturated phospholipids (C14 or larger). This indicated that diffusion was caused by phospholipid oxidation, which was confirmed by HPLC analysis. Phospholipid phosphorus that diffused through a membrane appeared more polar, as indicated by its capacity to distribute into the upper phase of a two phase extraction. Phospholipid phosphorus diffusion was preceded by the complete loss of liposomes contents, indicated by the complete diffusion of encapsulated carboxyfluorescein through the membrane. Oxidation of the lipid could be prevented by inclusion of either butylated hydroxytoluene or alpha-tocopherol in the membrane. The best retention of liposomal contents was achieved when both antioxidants and cholesterol were included in the liposome preparation. The antioxidant incorporated in the liposomes remained effective in protecting the phospholipids upon storage at 4 degrees C for 2 months. The inclusion of EDTA in the suspension medium retarded the rapid oxidation, suggesting that the presence of trace amounts of heavy metal ions in the buffer catalyzed the oxidation. Phospholipid oxidation was most effectively inhibited by the presence of serum or chemically defined medium, suggesting that oxidation of liposomal lipids in a biological environment may be minimized if appropriate steps are taken.

Is methyl-branching in alpha-tocopherol's "tail" important for its in vivo activity? Rat curative myopathy bioassay measurements of the vitamin E activity of three 2RS-n-alkyl-2,5,7,8-tetramethyl-6-hydroxychromans

The vitamin E activity of the acetates of three 2RS-n-alkyl-2,5,7,8-tetramethyl-6-hydroxychroman analogs of alpha-tocopherol have been measured and compared directly with all-rac-alpha-tocopheryl acetate, or indirectly via 2R,4'R,8'R-alpha-tocopheryl acetate, using the rat curative myopathy, plasma pyruvate kinase assay. The analogs with alkyl chain lengths of 11 and 13 carbons have activities which not only do not differ significantly (p greater than 0.05) from each other but also do not differ from that of all-rac-alpha-tocopheryl acetate. This finding indicates that methyl branching in the phytyl tail at the 4', 8', and 12' positions has little if any influence upon vitamin E activity. Thus physical interactions involving the methyl branches of the phytyl tail and the polyunsaturated moieties of membrane phospholipids are unimportant in vivo, insofar as this bioassay is concerned. However, the length of the hydrocarbon tail is important. This is indicated by the result obtained with the acetate of the analog with an alkyl chain length of 15 carbon atoms which had only 15% of the activity of 2R,4'R,8'R-alpha-tocopheryl acetate, i.e., 22% of the activity of all-rac-alpha-tocopheryl acetate since this form is 1.47 times less active than 2R,4'R,8'R-alpha-tocopheryl acetate in the curative myopathy bioassay (Weiser, Vecchi, & Schlachter, Internat. J. Vit. Nutr. Res. 55:149-158, 1985).

Activity of bile-salt-stimulated human milk lipase in the presence of liposomes and mixed taurocholate-phosphatidylcholine micelles

(1) The interaction of bile-salt-stimulated human milk lipase and liposomal membranes has been investigated in the presence or absence of sodium taurocholate. Freshly purified enzyme enhances the permeability of liposomal membranes but thermally inactivated enzyme does not. (2) The ability of the enzyme to catalyze the hydrolysis of a relatively hydrophilic substrate, 4-nitrophenyl acetate, and a more hydrophobic substrate, 4-nitrophenyl palmitate, has also been measured in media containing small unilamellar vesicles of egg phosphatidylcholine in both the absence and presence of taurocholate, and also in the presence of free taurocholate in the absence of liposomes. (3) The enzyme-catalyzed hydrolysis of 4-nitrophenyl acetate is enhanced in all of these systems, but 4-nitrophenyl palmitate is protected from enzymic attack in the phosphatidylcholine-bile salt systems. If free taurocholate be present in the system before 4-nitrophenyl palmitate is added, then, and only then, is enzymic activity observed. (4) These results have been interpreted in terms of the importance of the microenvironment around the substrate and the role played by the bile salt surfactant in stimulating the enzyme.

Influence of cholesterol on equilibrium and dynamic bilayer structure of unsaturated acyl chain phosphatidylcholine vesicles as determined from higher order analysis of fluorescence anisotropy decay

The influence of cholesterol on equilibrium and dynamic bilayer structure in minimally to highly unsaturated phosphatidylcholine (PC) vesicles has been examined by characterization of the dynamic fluorescence properties of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH). Large, unilamellar egg PC, palmitoyloleoyl-PC (POPC), dioleoyl-PC (DOPC), palmitoylarachidonoyl-PC (PAPC), and palmitoyldocosahexaenoyl-PC (P-22:6-PC) vesicles containing no cholesterol or approximately 15 or 30 mol % cholesterol have been examined. Equilibrium and dynamic DPH orientational properties were analyzed according to an orthogonal, bimodal orientational distribution function [Straume, M., & Litman, B.J. (1987) Biochemistry (preceding paper in this issue)]. The same mathematical formalism was applied to TMA-DPH except that probe orientational probability was permitted only in the distribution peak aligned parallel to the bilayer normal. TMA-DPH fluorescence lifetimes were consistently increased by incorporation of cholesterol into these vesicles. Greater acyl chain unsaturation and increasing temperature each promoted reduction of lifetimes in the presence or absence of cholesterol. DPH lifetimes were much less sensitive than those of TMA-DPH to changes in composition or temperature. This behavior is consistent with reduced water penetrability into liquid-crystalline bilayers as cholesterol content is increased and as acyl chain unsaturation and temperature are reduced. Cholesterol also induces substantial equilibrium ordering of the bilayer both at the hydrophobic core and at the bilayer-water interface. DPH orientational distributions were shifted in favor of alignment parallel to the acyl side chains. The distributions of both probes were narrowed in response to incorporation of cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Equilibrium and dynamic structure of large, unilamellar, unsaturated acyl chain phosphatidylcholine vesicles. Higher order analysis of 1,6-diphenyl-1,3,5-hexatriene and 1-[4-(trimethylammonio)phenyl]- 6-phenyl-1,3,5-hexatriene anisotropy decay

Equilibrium and dynamic structural properties of minimally to highly unsaturated acyl chain, large, unilamellar phosphatidylcholine (PC) vesicles have been characterized by the dynamic fluorescence properties of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH). Fluorescence lifetimes and equilibrium and dynamic rotational properties of these probes were analyzed by limited-frequency phase-modulation fluorometry in egg PC, palmitoyloleoyl-PC (POPC), dioleoyl-PC (DOPC), palmitoylarachidonoyl-PC (PAPC), and palmitoyldocosahexaenoyl-PC (P-22:6-PC) vesicles over a temperature range from 5 to 37 degrees C. DPH equilibrium orientational distributions were derived according to a model permitting bimodal orientational distributions in which the parallel probability maximum was aligned parallel to the bilayer normal and the orthogonal probability maximum was oriented parallel to the plane of the bilayer. TMA-DPH orientational distributions were derived according to the same model except that all probability was constrained to the parallel orientation. TMA-DPH fluorescence lifetimes were much more sensitive than those of DPH to variations in acyl chain composition and temperature although the same qualitative behavior was generally observed with both probes. Greater acyl chain unsaturation and higher sample temperatures each gave rise to shorter lifetimes consistent with increased water penetrability into the bilayers. Equilibrium order of the hydrocarbon core (as probed by DPH) and of the interfacial and head group regions of the bilayers (as probed by TMA-DPH) was reduced by increasing levels of unsaturation and by higher sample temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)

A change in the lipid fluidity of the porcine intestinal brush-border membranes by lipid peroxidation. Studies using pyrene and fluorescent stearic acid derivatives

The effect of lipid peroxidation on the lipid fluidity of porcine intestinal brush-border membranes was examined by measuring the rotational mobility and the accessibility to fluorescence quenchers (CH3COOT1, CuSO4 and KI) of pyrene or n-(9-anthroyloxy)stearic acid (n = 2 or 12) in the membranes. The harmonic mean of the rotational relaxation times of pyrene increased and the rate constants, kq, of the quenching reaction of pyrene and 2-(9-anthroyloxy)stearic acid incorporated in the membrane lipids decreased upon lipid peroxidation, indicating reduction of the lipid fluidity of the membranes by lipid peroxidation. In addition, the kq value of the reaction of 2-(9-anthroyloxy)stearic acid in the membranes with Cu2+ decreased in proportion to the amount of the products of lipid peroxidation. On the other hand, the kq value of the reaction of 12-(9-anthroyloxy)stearic acid with Cu2+ or I- was unaffected by lipid peroxidation. Based on these results, a localized change in the lipid fluidity of the membranes in association with lipid peroxidation has been discussed.