Aggregation of rhodopsin molecules during damaging exposure of photoreceptor membranes to light

Injuring light induced structural changes in rod outer segment (ROS) membranes are studied using "ST EST spectroscopy" for spin labelled rhodopsin, ESR of lipid spin label and SDS gel-electrophoresis. Free SH-group content of rhodopsin and lipid peroxidation level were simultaneously determined as well. A decrease of rotational mobility of rhodopsin in ROS induced by prolonged illumination is shown to result from irreversible protein aggregation caused by disulfide bond formation between "hydrophobic" SH-groups of rhodopsin. Some decrease of lipid microviscosity and degree of order are found, in contrast to considerable rise in microviscosity due to Fe2+-ascorbate induced lipid peroxidation of ROS membranes. Lipid oxidation is found to accelerate protein aggregation which in its turn influences the state of lipid bilayer.     

Partitioning and membrane disordering effects of dopamine antagonists: influence of lipid peroxidation, temperature, and drug concentration.

The effect of four dopamine antagonists (spiperone, haloperidol, pimozide, and domperidone) on the lipid order of caudate nucleus microsomal membranes and on liposomes from membrane lipid extracts was evaluated and related to the partition coefficients (Kp) of the drugs. Lipid membrane order was determined by fluorescence polarization using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe of the membrane core and 1-[4-(trimethylammonium)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH) as a probe of the membrane surface. Dopamine antagonists decrease the fluorescence polarization of both probes, indicating that they disorder the membrane lipids at different depths. Pimozide and domperidone, the drugs with higher Kp values, are more effective at decreasing the polarization of DPH, a probe of the membrane core, than that of TMA-DPH. In contrast, spiperone and haloperidol, which have lower values for Kp, induce more significant decreases in TMA-DPH depolarization, a probe of the membrane surface. These findings indicate that higher partition coefficients of the drugs are directly correlated with an increase of fluidity in the hydrophobic core of brain membranes. Ascorbate/Fe(2+)-induced membrane lipid peroxidation increases membrane order. Membrane lipid peroxidation decreases the partition coefficients of the dopamine antagonists tested. Increasing temperature (4-37 degrees C) decreases membrane order, but temperature effect is less evident after lipid peroxidation. The disordering effect of dopamine antagonists increases with increasing drug concentrations (1-15 microM), a maximum being observed at 10 microM. However, this effect is also less evident after membrane lipid peroxidation. We can conclude that dopamine antagonists and membrane lipid peroxidation affect membrane lipid order and that the action of these drugs is dependent on initial bilayer fluidity. Membrane lipid peroxidation increases membrane order while dopamine antagonists show a disordering effect of membrane phospholipids. This disordering effect can indirectly influence the activity of membrane proteins and it is one of the mechanisms through which membrane function can be altered by these drugs.     

An overview of lipid peroxidation with emphasis in outer segments of photoreceptors and the chemiluminescence assay

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. This article provides current information on the origin and function of polyunsaturated fatty acids in nature, lipid peroxidation of cellular membranes: enzymatic (lipoxygenases) and non-enzymatic. The latest knowledge on in vivo biomarkers of lipid peroxidation including isoprostanes, isofurans and neuroprostanes are discussed. A further focus is placed on analytical methods for studying lipid peroxidation in membranes with emphasis in chemiluminescence and its origin, rod outer segments of photoreceptors, the effect of antioxidants, fatty acid hydroperoxides and lipid protein modifications. Since rhodopsin, the major integral protein of rod outer segments is surrounded by phospholipids highly enriched in docosahexaenoic acid, the author proposes the outer segments of photoreceptors as an excellent model to study lipid peroxidation using the chemiluminescence assay since these membranes contain the highest concentration of polyunsaturated fatty acids of any vertebrate tissue and are highly susceptible to oxidative damage.     

Free radical-induced alterations of myocardial membrane proteins

Rat myocardial membranes exposed to the free radical-generating systems, Fe2+/ascorbate, Cu2+/t-butylhydro-peroxide, linoleic acid hydroperoxide, and soybean lipoxygenase (Type I) undergo lipid peroxidation. This is evidenced by the accumulation of thiobarbituric acid-reactive substances and the loss of both extractable phospholipids and their polyunsaturated acyl groups. Lipid peroxidation is accompanied by alterations of membrane proteins including the general loss of polypeptides and accumulation of high-molecular weight material. The most sensitive protein is a polypeptide with a molecular weight of 28 kDa. At low levels of oxidation, this protein moves incrementally to slightly higher apparent molecular weight. At higher oxidant levels or longer periods of oxidation, the protein disappears completely from the SDS-PAGE gel. The "28K reaction" occurs prior to the massive, oxidant-induced lipid alterations and may thus indicate specific adduct formation between this protein and certain peroxidized membrane phospholipids.     

Lipid peroxidation in purified plasma membrane fractions of rat liver in relation to the hepatoxicity of carbon tetrachloride

Preparations of rat liver sinusoidal plasma membrane have been tested for their ability to metabolize the hepatotoxin carbon tetrachloride (CCl4) to reactive free radicals in vitro and compared in this respect with standard preparations of rat liver microsomes. The sinusoidal plasma membranes were relatively free of endoplasmic reticulum-associated activities such as the enzymes of the cytochrome P450 system and glucose-6-phosphatase. CCl4 metabolism was measured as (i) covalent binding of [14C]-CCl4 to membrane protein, (ii) electron spin resonance spin-trapping of CCl3. radicals and (iii) CCl4-induced lipid peroxidation. By all of these tests, purified sinusoidal plasma membranes were found unable to metabolize CCl4. The fatty acid composition of the plasma membranes was almost identical to that of the microsomal preparation and both membrane fractions exhibited similar rates of the lipid peroxidation that was stimulated non-enzymically by gamma-radiation or incubation with ascorbate and iron. The absence of CCl4-induced lipid peroxidation in the plasma membranes seems to be due, therefore, to an absence of CCl4 activation rather than an inherent resistance to lipid peroxidation. We conclude that damage to the hepatocyte plasma membrane during CCl4 intoxication is not due to a significant local activation of CCl4 to CCl3. within that membrane.     

Structural and dynamic effects of oxidatively modified phospholipids in unsaturated lipid membranes.

Phospholipid hydroperoxides and phospholipid alcohols are two of the major forms of oxidatively modified phospholipids produced during oxidant stress and lipid peroxidation. The process of lipid peroxidation is known to affect the physiological function of membranes. We, therefore, investigated the effects of lipid peroxidation products on the molecular interactions in membranes. Our study was specifically focused on the effects of lipid peroxidation products on static membrane structure (molecular orientational order) and on the reorientational dynamics of the probe molecules in lipid bilayers. The study was done by performing angle-resolved fluorescence depolarization measurements (AFD) on the fluorescent probe diphenylhexatriene (DPH) and by performing angle-resolved electron spin resonance (A-ESR) measurements on cholestane (CSL) nitroxide spin probes embedded in macroscopically oriented planar bilayers consisting of 2-10% 1-palmitoyl-2-(9/13-hydroperoxylinoleoyl)phosphatidylcholine (PLPC-OOH) or 1-palmitoyl-2-(9/13-hydroxylinoleoyl)phosphatidylcholine (PLPC-OH) in 1-palmitoyl-2-linoleoylphosphatidylcholine (PLPC) or dilinoleoylphosphatidylcholine (DLPC). Both probe molecules have rigid cylindrical geometries and report on the overall molecular order and dynamics. However, being more polar, the nitroxide spin probe CSL is preferentially located near the surface of the membrane, while the less polar fluorescent probe DPH reports preferentially near the central hydrophobic region of the lipid bilayers. The results show that the presence of relatively small amounts of oxidatively modified phospholipids within the PLPC or DLPC membranes causes pronounced structural effects as the molecular orientational order of the probe molecules is strongly decreased. In contrast, the effect on membrane reorientational dynamics is minimal.     

Changes in the fluorescence parameters of bound N-(1-pyrene) maleimide by lipid peroxidation of intestinal brush-border membranes

Using a fluorogenic thiol reagent, N-(1-pyrene)maleimide (NPM), we have examined of lipid peroxidation on the microenvironment around SH groups of the membrane proteins in porcine intestinal brush-border membrane vesicles. The lipid peroxidation of the membranes was performed with various concentrations of t-butylhydroperoxide (t-BuOOH) in the presence of 100 microM ascorbic acid and 10 microM Fe2+. Treatment of NPM-labeled membranes with these oxidizing agents resulted in a decrease of the fluorescence lifetime, suggesting modification of the environmental properties around the bound dye. Measurement of the steady-state fluorescence anisotropy of the labeled membranes indicated restriction of the motion of the bound dye by the lipid peroxidation membranes. This interpretation was further supported by an elevation of the transition temperature of the anisotropy, a decrease in the quenching rate constant of the fluorescence with acrylamide and a decrease in the SH reactivity of the membrane proteins for NPM by lipid peroxidation. Based on these results, the possibility of conformation changes in the vicinity of SH groups in the membrane proteins associated with lipid peroxidation has been discussed.     

Increase of the molecular rigidity of the protein conformation in the intestinal brush-border membranes by lipid peroxidation

The effect of lipid peroxidation on the protein conformation of the porcine intestinal brush-border membranes was studied using a fluorogenic thiol reagent, N-[7-dimethylamino-4-methylcoumarinyl]maleimide (DACM). By a kinetic analysis of the reaction of the membranes with DACM, it was shown that the reaction rate of the SH groups (SHf) of the membrane proteins, whose reaction with the dye is very fast, decreases in proportion to the extent of thiobarbituric acid-reactive substance formation. The difference in the rate of the reaction of the SHf groups for DACM between the control and peroxidized membranes completely disappeared after denaturation of the proteins by treatment with guanidine hydrochloride. The reaction of DACM with the SHf groups of the control membranes accelerated when the temperature was increased with an apparent transition temperature between 25 degrees C and 30 degrees C. On the other hand, no transition was observed in the peroxidized membranes over the temperature range 20-43 degrees C. These results suggest that the conformation around the SHf groups of the proteins in the peroxidized membranes is apparently different from that in the control membranes. A modification of the conformation around the SH groups in the membrane proteins associated with lipid peroxidation was further demonstrated by finding that the quenching efficiency of the fluorescence of the DACM-labeled membranes by Tl+ was markedly decreased after lipid peroxidation. Based on these results, changes in the protein conformation of the porcine intestinal brush-border membranes by lipid peroxidation are discussed.     

Lipid order and composition of synaptic membranes in experimental diabetes mellitus

The effect of diabetes in rats on lipid composition and order of synaptosomal membranes (SM) was determined in streptozotocin-induced diabetic rats after 6 weeks of chronic hyperglycemia. The cholesterol content was slightly, but not significantly, higher in diabetic SM (0.287±0.042 vs. 0.209±0.061 mol/mg protein). The phospholipid concentration in diabetic SM was significantly increased (0.515±0.042 vs. 0.305±0.041 mol/mg protein;P<0.005). Neither the molar ratios of cholesterol to phospholipids in the SM nor the fatty acid composition of the SM was significantly altered with diabetes. Diabetes did not affect membrane order or the thermotropic transition temperature of the SM as determined fluorometrically. On the other hand, the SM of diabetic rats had significantly increased concentration of lipid peroxidation products, namely conjugated dienes (the calculated O.D./mol phospholipids was 11.56±1.83 in controls and 19.95 ±4.1 in diabetic ratsP<0.01). Despite the accumulation of lipid peroxidation byproducts in SM of diabetic rats the overall membrane order and the cholesterol to phospholipid molar ratio do not appear to be significantly altered.     

Peroxide modification of skeletal muscle sarcoplasmic reticulum in antioxidant deficiency and under the effect of ionol. II. Physico- chemical properties of the sarcoplasmic reticulum membrane

Physico-chemical parameters of membranes of skeletal muscles' sarcoplasmic reticulum in antioxidant insufficiency, which was modelled by excluding alpha-tocopherol from the animals ration, and after treatment with phenol antioxidant ionol were studied. It was shown that activation of lipid peroxidation in vitamin E insufficiency results in a significant lowering of microviscosity of lipid bilayer membranes of sarcoplasmic reticulum. Using polarography significant changes in membrane protein conformation were revealed, which were characterized by lowering of integrity and by disorganization of protein globules. Treatment of animals with antioxidant insufficiency with ionol led to certain normalization of changes of physico-chemical characteristics of the learned membrane structures caused by lipid peroxidation.     

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.     

Comparing beta-carotene,vitamin E and nitric oxide as membrane antioxidants

Singlet oxygen initiates lipid peroxidation via a nonfree radical mechanism by reacting directly with unsaturated lipids to form lipid hydroperoxides (LOOHs). These LOOHs can initiate free radical chain reactions leading to membrane leakage and cell death. Here we compare the ability and mechanism by which three small-molecule membrane antioxidants (beta-carotene, alpha-tocopherol and nitric oxide) inhibit lipid peroxidation in membranes. We demonstrate that beta-carotene provides protection against singlet oxygen-mediated lipid peroxidation, but does not slow free radical-mediated lipid peroxidation. Alpha-Tocopherol does not protect cells from singlet oxygen, but does inhibit free radical formation in cell membranes. Nitric oxide provides no direct protection against singlet oxygen exposure, but is an exceptional chain-breaking antioxidant as evident from its ability to blunt oxygen consumption during free radical-mediated lipid peroxidation. These three small-molecule antioxidants appear to have complementary mechanisms for the protection of cell membranes from detrimental oxidations.     

The effect of α tocopherol,all-trans retinol and retinyl palmitate on the non enzymatic lipid peroxidation of rod outer segments

The effect of tocopherol, all-trans retinol and retinyl palmitate on the non enzymatic lipid peroxidation induced by ascorbate-Fe²⁺ of rod outer segment membranes isolated from bovine retina was examined. The inhibition of light emission (maximal induced chemiluminescence) by tocopherol, all-trans retinol and retinyl palmitate was concentration dependent. All trans retinol showed a substantial degree of inhibition against ascorbate-Fe²⁺ induced lipid peroxidation in rod outer segment membranes that was 10 times higher than the observed in the presence of either tocopherol or retinyl palmitate. Inhibition of lipid peroxidation of rod outer segment membranes by tocopherol and retinyl palmitate was almost linear for up to 0,5 mol vitamin/mg membrane protein, whereas all-trans retinol showed linearity up to 0,1 mol vitamin/mg membrane protein. Incubation of rod outer segments with increasing amounts of low molecular weight cytosolic proteins carrying 1-[¹⁴C] linoleic acid, [³H] retinyl palmitate or [³H] all-trans retinol during the lipid peroxidation process produced a net transfer of ligand from soluble protein to membranes. Linoleic acid was 4 times more effectively transferred to rod outer segment membranes than all-trans retinol or retinyl palmitate. Incubation of rod outer segments with delipidated low molecular weight cytosolic proteins produced inhibition of lipid peroxidation. The inhibitory effect was increased when the soluble retinal protein fraction containing a tocopherol was used. These data provide strong support for the role of all-trans retinol as the major retinal antioxidant and open the way for many fruitful studies on the interaction and precise roles of low molecular weight cytosolic retinal proteins involved in the binding of antioxidant hydrophobic compounds with rod outer segments.     

Pyrene derivatives as markers of transbilayer effect of lipid peroxidation on neuronal membranes

Two different pyrene derivatives, namely 12-(1-pyrene)dodecanoic acid (P12-FA) and N-(12-(1-pyrene)dodecanoyl)-galactosylsphingosine I3-sulfate (P12-CS) have been used to follow lipid peroxidation both in model and natural membranes. The malondialdehyde (MDA) production in small unilamellar vesicles of dipalmitoylphosphatidylcholine/arachidonic acid (80:20, molar ratio), symmetrically labelled with both probes determined a progressive decrease of pyrene fluorescence due to an involvement of pyrene in the peroxidative reaction. Nervous membranes are particularly sensitive to lipid oxidation which differentially acts on the two layers of the membrane determining a greater rigidity of the exofacial one. Thus, we consider the possibility to asymmetrically introduce the pyrene ring, as P12-FA or P12-CS, in synaptosomes for monitoring lipid peroxidation in each layer of the membrane. The amount of the two probes incorporated in the membrane was 20 +/- 3 and 10 +/- 2 nmol/mg of protein for P12-FA and P12-CS, respectively. P12-FA was symmetrically distributed in the two layers, whereas 95% of P12-CS was incorporated in the exofacial layer of the membrane as determined by TNBS measurements. The decrease in fluorescence of synaptosome associated pyrene was, in the early stages of lipid peroxidation, greater for P12-CS than for P12-FA labelled membranes, indicating a greater susceptibility of the exofacial layer to iron-induced peroxidation.     

The effect of alpha-tocopherol in water-soluble form on human erythrocyte membranes while in contact with oxygen

The effect of water soluble tocopherol forms (dipotassium salt d,l-alpha-tocopheryl-phosphate) on lipid peroxidation induced by oxygen in membranes of human erythrocytes has been studied. An erythrocyte was considered as a structure reflecting the state of plasmatic membranes of organs and tissues. It has been established that this substance doses optimum for membrane protection against lipid peroxidation are not in excess of 1.5-5.0 mg per 1 kg of human body mass; the dose by an order higher may exert a toxic effect.     

Lipid peroxidation increases the molecular order of microsomal membranes

The effect of enzymatic lipid peroxidation on the molecular order of microsomal membranes was evaluated by ESR spectroscopy using the spin probes 5-, 12-, and 16-doxyl-stearic acid. Rat liver microsomal membranes were peroxidized by the NADPH-dependent reaction in the presence of the chelate ADP-Fe3+. Peroxidation resulted in a preferential depletion of polyenoic fatty acids and an increase in the percentage composition of shorter fatty acyl chains. There was no change in the cholesterol/phospholipid ratio of the peroxidized microsomes. The molecular order of both control and peroxidized membranes decreased toward the central region of the bilayer, and the order parameter (S) of each probe was temperature dependent. Peroxidation of the microsomal membrane lipids resulted in an increase in the order parameter determined with the three stearic acid spin probes. Of the three probes, 12-doxylstearic acid was the most sensitive to the changes in membrane organization caused by peroxidation. These data indicate that ESR spectroscopy is a sensitive method of detecting changes in membrane order accompanying peroxidation of membrane lipids.     

Association of lipid peroxidation and polymerization of membrane proteins with erythrocyte aging

The addition of malondialdehyde to erythrocytes in vitro causes a decrease in bands 1 and 2 of spectrin and an increase in high molecular weight protein polymers. Additionally, this agent causes the formation of fluorscent chromolipids characteristic of those produced during the peroxidation of endogenous membrane phospholipids. These same alterations in proteins and lipids are observed in the membranes of older cells fractionated from freshly drawn blood and in the membranes of reticulocytes induced by treatment of animals with phenylhydrazine, but not in reticulocytes induced by bleeding. The former reticulocytes have a much shorter half-life in the circulation than do either normal erythrocytes or reticulocytes produced consequent to bleeding. These experiments and the apparent paradox of "young" reticulocytes with short half-lives suggest that the in vivo polymerization of membrane proteins consequent to radical-induced peroxidation of membrane lipids may contribute to the altered rheological behavior and hence to the splenic sequestration of cells. They also suggest that increases in intrinsic membrane rigidity due to lipid peroxidation, malondialdehyde, and protein polymerization may be a common feature of both aging in normal erythrocytes and in the accelerated aging that accompanies the administration of radical-generating, hemolytic agents. However, it is cautioned that other polymerization reactions involving disulfides, calcium, or direct radical attack on protein monomers may also be important determinants of the visco-elastic properties of erythrocyte membranes.     

Comparison between in vitro lipid peroxidation in fresh sheep platelets and peroxidative processes during sheep platelet ageing under storage at 4 degrees C.

Incubation of sheep platelet crude membranes with xanthine oxidase (XO)/hypoxanthine/Fe(2+)-ADP revealed: (i) a fast peroxidative response - with a maximal linear rate of 14 nmol malondialdehyde (MDA) equivalents/mg protein, as evidenced by the thiobarbituric acid test - and a decrease in the polyunsaturated fatty acid (PUFA) content of the platelet crude membranes; (ii) a decrease in the lipid fluidity in the deep lipid core of the membranes but not at the membrane surface; (iii) a dramatic inhibitory effect on glucose 6-phosphatase (Glc-6-Pase) but not on acetylcholinesterase activity. Platelets were also aged by storage at 4 degrees C in their own plasma or in Seto additive solution. In these media, platelet aggregates were visible and the effects on platelet phospholipids, PUFA, lipid extract fluorescence, crude membrane fluidity and membrane-bound enzyme activities were assessed for comparison with those observed in in vitro lipid peroxidation. The sensitivity of membranes from stored platelets to lipid peroxidation was also assessed. Storage of platelets in plasma for 5 days was associated with different changes in their crude membranes such as decreases in arachidonic acid contents, the decrease not being avoided by the presence of phospholipase A(2) inhibitors, increases in MDA equivalents, conjugated dienes and lipid extract fluorescence, decreases in the amounts of MDA equivalents formed by platelet crude membranes treated with the oxidizing agents, changes in membrane fluidity and inhibition of Glc-6-Pase. All these alterations were less pronounced or even abolished after platelet storage in Seto. These findings suggest that platelet lipid peroxidation due to XO/hypoxanthine/Fe(2+)-ADP and platelet membrane alterations observed after platelet ageing under storage at 4 degrees C share common features. Also, as regards the prevention of peroxidative processes, Seto solution permits better storage of sheep platelets than plasma.     

A spin-label study of the effect of gamma radiation on erythrocyte membrane. Influence of lipid peroxidation on membrane structure.

Gamma-irradiation of bovine erythrocyte membranes (0.1-4 Mrad) resulted in a decrease in the degree of order of membrane lipids, as measured by spin-labelled fatty acid esters, at the depth of C12 but not at the depth of C5. Dose dependence of this phenomenon corresponded to dose dependence of malondialdehyde formation in the membranes. On this basis a mechanism for the effect of lipid peroxidation on the membrance structure is proposed. Membrane proteins underwent radiation-induced conformational transitions revealed by maleimide spin label which could be also connected with lipid peroxidation.     

脂质过氧化对人红细胞膜脂流动性的影响

研究枯稀过氧化氢/高铁血红素体系所产生的烷基过氧自由基对红细胞的损伤。测定了脂质过氧化的产物——丙二脂的生成,并证明阿魏酸钠对脂质过氧化的抑制。荧光偏振的结果指出,膜脂过氧化以后降低了膜脂的流动性。人红细胞用5DSA和16DSA标记并用ESR检测膜脂流动性,结果表明,序参数S几乎没有发生变化,旋转相关时间τ值的增加证明膜脂过氧化以后,疏水尾部的物理状态发生了改变。经脂质过氧化以后,红细胞膜中的不饱和脂防酸的减少,可能是降低膜脂流动性的原因之一。