Interest of photochemical methods for induction of lipid peroxidation

Deuterated polyunsaturated fatty acids inhibit photoirradiation-induced lipid peroxidation in lipid bilayers

Lipid peroxidation (LPO) plays a key role in many age-related neurodegenerative conditions and other disorders. Light irradiation can initiate LPO through various mechanisms and is of importance in retinal and dermatological pathologies. The introduction of deuterated polyunsaturated fatty acids (D-PUFA) into membrane lipids is a promising approach for protection against LPO. Here, we report the protective effects of D-PUFA against the photodynamically induced LPO, using illumination in the presence of the photosensitizer trisulfonated aluminum phthalocyanine (AlPcS₃) in liposomes and giant unilamellar vesicles (GUV), as assessed in four experimental models: 1) sulforhodamine B leakage from liposomes, detected with fluorescence correlation spectroscopy (FCS); 2) formation of diene conjugates in liposomal membranes, measured by absorbance at 234 nm; 3) membrane leakage in GUV assessed by optical phase-contrast intensity observations; 4) UPLC-MS/MS method to detect oxidized linoleic acid (Lin)-derived metabolites. Specifically, in liposomes or GUV containing H-PUFA (dilinoleyl-sn-glycero-3-phosphatidylcholine), light irradiation led to an extensive oxidative damage to bilayers. By contrast, no damage was observed in lipid bilayers containing 20% or more D-PUFA (D2-Lin or D10-docosahexanenoic acid). Remarkably, addition of tocopherol increased the dye leakage from liposomes in H-PUFA bilayers compared to photoirradiation alone, signifying tocopherol's pro-oxidant properties. However, in the presence of D-PUFA the opposite effect was observed, whereby adding tocopherol increased the resistance to LPO. These findings suggest a method to augment the protective effects of D-PUFA, which are currently undergoing clinical trials in several neurological and retinal diseases that involve LPO.

Lipid Unsaturation Properties Govern the Sensitivity of Membranes to Photoinduced Oxidative Stress

Unsaturated lipid oxidation is a fundamental process involved in different aspects of cellular bioenergetics; dysregulation of lipid oxidation is often associated with cell aging and death. To study how lipid oxidation affects membrane biophysics, we used a chlorin photosensitizer to oxidize vesicles of various lipid compositions and degrees of unsaturation in a controlled manner. We observed different shape transitions that can be interpreted as an increase in the area of the targeted membrane followed by a decrease. These area modifications induced by the chemical modification of the membrane upon oxidation were followed in situ by Raman tweezers microspectroscopy. We found that the membrane area increase corresponds to the lipids' peroxidation and is initiated by the delocalization of the targeted double bonds in the tails of the lipids. The subsequent decrease of membrane area can be explained by the formation of cleaved secondary products. As a result of these area changes, we observe vesicle permeabilization after a time lag that is characterized in relation with the level of unsaturation. The evolution of photosensitized vesicle radius was measured and yields an estimation of the mechanical changes of the membrane over oxidation time. The membrane is both weakened and permeabilized by the oxidation. Interestingly, the effect of unsaturation level on the dynamics of vesicles undergoing photooxidation is not trivial and thus carefully discussed. Our findings shed light on the fundamental dynamic mechanisms underlying the oxidation of lipid membranes and highlight the role of unsaturations on their physical and chemical properties.

Benzophenone used as the photochemical reagent for pinpointing C=C locations in unsaturated lipids through shotgun and liquid chromatography-mass spectrometry approaches

Unsaturated lipids exhibit different physiological significances due to the different locations of the carbon-carbon double bond (C=C). Identifying lipid isomers with mass-based methods remains challenging. Xia's group has been successfully employed Paternò-Büchi (PB) reaction, a photochemical reaction with UV irradiation, coupled with tandem mass spectrometry (MS/MS) to identify and quantify unsaturated lipids in complex mixtures. However, the existing PB reagents possess certain demerits. In this regard, a new PB reagent that is compatible with various lipidomic analysis platforms must be screened. In this study, we comprehensively evaluated the conditions of the PB reaction and screened benzophenone as a new PB reagent. Benzophenone possesses unique advantages, such as a relative high PB yield; the PB products could be readily distinguished from the reacted lipids based on the added high molecular weight (182 Da); and the benzophenone does not affect the lipids appearance interval for reversed-phase column separation. Furthermore, we optimized the reaction conditions by using benzophenone as a PB reagent and summarized the molecular formulas of the diagnostic ions according to the fragment rules. The proposed PB method has been implemented in shotgun and LC-MS lipidomics. To our best knowledge, this work is the first to report the integrated PB reaction with LC-MS lipidomics for identification of lipid isomers.

A survey of photogeochemistry

The participation of sunlight in the natural chemistry of the earth is presented as a unique field of study, from historical observations to prospects for future inquiry. A compilation of known reactions shows the extent of light-driven interactions between naturally occurring components of land, air, and water, and provides the backdrop for an outline of the mechanisms of these phenomena. Catalyzed reactions, uncatalyzed reactions, direct processes, and indirect processes all operate in natural photochemical transformations, many of which are analogous to well-known biological reactions. By overlaying photochemistry and surface geochemistry, complementary approaches can be adopted to identify natural photochemical reactions and discern their significance in the environment.

Challenges of utilizing healthy fats in foods

Over the past few decades, the Dietary Guidelines for Americans has consistently recommended that consumers decrease consumption of saturated fatty acids due to the correlation of saturated fatty acid intake with coronary artery disease. This recommendation has not been easy to achieve because saturated fatty acids play an important role in the quality, shelf life, and acceptability of foods. This is because solid fats are critical to producing desirable textures (e.g., creaminess, lubrication, and melt-away properties) and are important in the structure of foods such as frozen desserts, baked goods, and confectionary products. In addition, replacement of saturated fats with unsaturated fats is limited by their susceptibility to oxidative rancidity, which decreases product shelf life, causes destruction of vitamins, and forms potentially toxic compounds. This article will discuss the fundamental chemical and physical properties in fats and how these properties affect food texture, structure, flavor, and susceptibility to degradation. The current sources of solid fats will be reviewed and potential replacements for solid fats will be discussed.

Benzophenone Suppression of Quercetin Antioxidant Activity towards Lipids under UV-B Irradiation Regime: Detection by HPLC Chromatography

Quercetin, a well-known flavonoid antioxidant, has been employed to control benzophenone-sensitized peroxidation of the lipid mixture in methanol solution, induced by continuous UV-B irradiation. Surprisingly, the detected quercetin antioxidant activity was almost negligible. The presented data suggests that the reason is not in its own UV-B-induced degradation but rather in its interrelationship with benzophenone during UV-B stress. On the other side of this relationship, benzophenone anticipated sensitizing role towards lipids; that is, the initiation of lipid peroxidation has been affected as well. These results, obtained by HPLC chromatography, partly confirm but partly relativize to some extent recent results obtained with the same system by spectrophotometric method.

Bioassay technique using seed shrimps for comparative studies regarding the aquatic acute lethality of biodegradable lubricants

To evaluate the environmental load resulting from the spillage of biodegradable lubricants in aquatic systems, a comparative acute lethality test wherein an oil-water interfacial area could be examined was considered. In this study, oleic acid was employed as a model biodegradable lubricant. Measurements of the pH value and dissolved oxygen (DO) level of water during the exposure tests indicate that water degradation depends on the oil-water interfacial area, exposure duration, and water temperature. Furthermore, 72 h acute lethality tests were performed using two types of freshwater ostracods (seed shrimps) as test organisms: the large species Stenocypris hislopi and the small species Cypretta seurati. The longevity of the small species, which was physically more active, was strongly affected by water pollution. During the exposure test, the DO in water was significantly consumed by the degradation of the lubricant floating on it. Water exposed to a lubricant containing copper (Cu) demonstrated strong toxicity even after the recovery of the pH value and DO level by aging. The decrease in the DO level of water and increase in the concentration of metal compounds are dominant factors responsible for the mortality of aquatic organisms.

Light-induced oxidation of unsaturated lipids as sensitized by flavins

Triplet-excited riboflavin ((3)RF*) was found by laser flash photolysis to be quenched by polyunsaturated fatty acid methyl esters in tert-butanol/water (7:3, v/v) in a second-order reaction with k approximately 3.0 x 10(5) L mol(-1) s(-1) at 25 degrees C for methyl linoleate and 3.1 x 10(6) L mol(-1) s(-1), with DeltaH(double dagger) = 22.6 kJ mol(-1) and DeltaS(double dagger) = -62.3 J K(-1) mol(-1), for methyl linolenate in acetonitrile/water (8:2, v/v). For methyl oleate, k was <10(4) L mol(-1) s(-1). For comparison, beta-casein was found to have a rate constant k approximately 4.9 x 10(8) L mol(-1) s(-1). Singlet-excited flavin was not quenched by the esters as evidenced by insensitivity of steady-state fluorescence to their presence. Density functional theory (DFT) calculations showed that electron transfer from unsaturated fatty acid esters to triplet-excited flavins is endergonic, while a formal hydrogen atom transfer is exergonic (DeltaG(o)(HAT) = -114.3, -151.2, and -151.2 kJ mol(-1) for oleate, linoleate, and linolenate, respectively, in acetonitrile). The reaction is driven by acidity of the lipid cation radical for which a pK(a) approximately -0.12 was estimated by DFT calculations. Absence of electrochemical activity in acetonitrile during cyclic voltammetry up to 2.0 V versus NHE confirmed that DeltaG(o)(ET) > 0 for electron transfer. Interaction of methyl esters with (3)RF* is considered as initiation of the radical chain, which is subsequently propagated by combination reactions with residual oxygen. In this respect, carbon-centered and alkoxyl radicals were detected using the spin trapping technique in combination with electron paramagnetic resonance spectroscopy. Moreover, quenching of (3)RF* yields, directly or indirectly, radical species which are capable of initiating oxidation in unsaturated fatty acid methyl esters. Still, deactivation of triplet-excited flavins by lipid derivatives was slower than by proteins (factor up to 10(4)), which react preferentially by electron transfer. Depending on the reaction environment in biological systems (including food), protein radicals are expected to interfere in the mechanism of light-induced lipid oxidation.

Asymmetric oxidation of giant vesicles triggers curvature-associated shape transition and permeabilization

Oxidation of unsaturated lipids is a fundamental process involved in cell bioenergetics as well as in cell death. Using giant unilamellar vesicles and a chlorin photosensitizer, we asymmetrically oxidized the outer or inner monolayers of lipid membranes. We observed different shape transitions such as oblate to prolate and budding, which are typical of membrane curvature modifications. The asymmetry of the shape transitions is in accordance with a lowered effective spontaneous curvature of the leaflet being targeted. We interpret this effect as a decrease in the preferred area of the targeted leaflet compared to the other, due to the secondary products of oxidation (cleaved-lipids). Permeabilization of giant vesicles by light-induced oxidation is observed after a lag and is characterized in relation with the photosensitizer concentration. We interpret permeabilization as the opening of a pore above a critical membrane tension, resulting from the budding of vesicles. The evolution of photosensitized giant vesicle lysis tension was measured and yields an estimation of the effective spontaneous curvature at lysis. Additionally photo-oxidation was shown to be fusogenic.

Quantitative analysis of beta-sitosterol oxides induced in vegetable oils by natural sunlight, artificially generated light, and irradiation

UV radiation is able to induce lipid peroxidation. Photooxidation-induced beta-sitosterol oxides were monitored in four vegetable oils exposed to sunlight for 10, 20, and 30 days during May 2005 (northeastern France), exposed to artificial light generated by a high-pressure Hg lamp for 21, 42, and 63 h at room temperature, and exposed to a 10 MeV electron beam at 0.93, 2.69, and 9.30 kGy at 8 degrees C. Quantification was performed by capillary gas chromatography-mass spectrometry according to the total ion current mode and using a reconstructed ion trace chromatogram with specific ion fragments. Sunlight induced the formation of higher amounts of oxides than UV light, while no significant oxidizing effect was observed with electron beam irradiation. However, data suggested that the amount of the main oxides formed was strongly dependent on the dose rate (length of exposure). Accordingly, shorter but more intense treatments had lower oxidizing effects.

Polyunsaturated but not Conjugated Linoleic Acid Supplementation of Leukemic U937 Cells Can Act as an Amplification Factor for Photofrin-mediated Photodynamic Therapy

Polyunsaturated fatty acids located in leukemia cell membranes are excellent targets for peroxidation. They can significantly enhance the effectiveness of Photofrin-mediated photodynamic therapy (PDT)-induced cell killing. In this study, the peroxidizability of conjugated fatty acid isomers (9c,11t-linoleic acid and 9c,11c-linoleic acid) and polyunsaturated fatty acids (PUFAs; linoleic acid, gamma-linolenic acid and arachidonic acid) with 2,2'-azo-bis(2-amidinpropane)dihydrochloride, soybean lipoxygenase and photomediated peroxidation are compared with each other. Peroxidation was determined using different methods: by means of gas chromatography to estimate the fatty acid (FA) consumption, by photometry for the level of FA peroxides or phospholipid peroxides and by definition of the content of malondialdehyde for thiobarbituric acid reactive substances (TBARS). The results suggest that the generation of oxidation products from individual FAs indicate a different formation rate of oxidation products. Radical FA peroxides were produced most by polyunsaturated arachidonic acid, followed by linoleic acid and gamma-linolenic acid, whereas conjugated FA isomers did not generate peroxides. Accordingly, the levels of lipid peroxides and TBARS were substantially increased after incorporation and oxidation of polyunsaturated FAs into U937 cells and could significantly enhance the effectiveness of Photofrin-PDT-induced cytotoxicity. The results showed that PUFA, but not conjugated FA supplementation of U937 cells, can act as a PDT amplification factor.

Illumination of the malaria parasite Plasmodium falciparum alters intracellular pH. Implications for live cell imaging

Live cell fluorescence microscopy has been widely used to study physiological processes in the human malarial parasite Plasmodium falciparum, including pH homeostasis, Ca(2+) signaling and protein targeting. However, the reproducibility of the data is often poor. Controversial statements exist regarding cytosolic and vacuolar baseline pH, as well as regarding the subcellular localization of some of the fluorochromes used. When trying to reproduce published baseline values, we observed an unexpected light sensitivity of P. falciparum, which manifests itself in the form of a strong cytoplasmic acidification. Even short exposure times with moderate to low light intensities caused the parasite cytosol to acidify. We show that this effect arises from the selective disruption of the parasite's acidic food vacuole, brought about by lipid peroxidation initiated by light-induced generation of hydroxyl radicals. Our data suggest that heme serves as a photosensitizer in this process. Our findings have major implications for the use of live cell microscopy in P. falciparum and add a cautionary note to previous studies where live cell fluorometry has been used to determine physiological parameters in P. falciparum.

Phototoxic process after rapid photosensitive membrane damage of 5,5"-bis(aminomethyl)-2,2':5',2"-terthiophene dihydrochloride

We report a new aspect of rapid (<30 s) light-induced cell membrane damage photosensitized by 5,5"-bis(aminomethyl)-2,2':5',2"-terthiophene dihydrochloride (BAT), which is a water-soluble alpha-terthienyl analogue, using a high-power laser (light intensity 1.6 W cm(-2)). In this paper, we will discuss the relationship between the exposure time of the cells to the photosensitizer and the phototoxic process. Three toxic processes can be identified: first, a non-light-mediated toxicity dependent on BAT-cell incubation; second, a phototoxicity independent of BAT exposure time when the BAT concentration is in the 2-10-microM range; third, a phototoxicity dependent on BAT exposure time when BAT concentration becomes 20 microM. The cytotoxicity decreases when alpha-tocopherol, an antioxidant, is added to a cell membrane. This pattern of phototoxicity is the typical of a phospholipid peroxidation chain reaction and oxidative damage of membrane proteins triggered by a reactive oxygen species generated by a triplet state of BAT. The BAT exposure time is clearly correlated with the partition of the photosensitizer in the cell membrane and inside the cell.

Chlorophyllin as an effective antioxidant against membrane damage in vitro and ex vivo

Chlorophyllin (CHL), the sodium-copper salt and the water-soluble analogue of the ubiquitous green pigment chlorophyll, has been attributed to have several beneficial properties. Its antioxidant ability, however, has not been examined in detail. Using rat liver mitochondria as model system and various sources for the generation of reactive oxygen species (ROS) we have examined the membrane-protective properties of CHL both under in vitro and ex vivo conditions. Oxidative damage to proteins was assessed as inactivation of the enzymes, cytochrome c oxidase and succinic dehydrogenase besides formation of protein carbonyls. Damage to membrane lipids was measured by formation of lipid hydroperoxides and thiobarbituric acid reactive substances. The effect of this compound on the antioxidant defense system was studied by estimating the level of glutathione and superoxide dismutase. ROS were generated by gamma-radiation, photosensitization, ascorbate-Fe(2+), NADPH-ADP-Fe(3+) and the peroxyl radical generating agent, azobis-amidopropane hydrochloride. Our results show that CHL is highly effective in protecting mitochondria, even at a low concentration of 10 microM. The antioxidant ability, at equimolar concentration, was more than that observed with ascorbic acid, glutathione, mannitol and tert-butanol. When CHL was fed to mice at a dose of 1% in drinking water, there was a significant reduction in the potential for oxidative damage in cell suspensions from liver, brain and testis. To examine the possible mechanisms responsible for the observed antioxidant ability we have studied the reaction of CHL with the potent ROS in the form of hydroxyl radical and singlet oxygen. The compound shows a fairly high rate constant with singlet oxygen, in the order of 1.3x10(8) M(-1) s(-1). In conclusion, our studies showed that CHL is a highly effective antioxidant, capable of protecting mitochondria against oxidative damage induced by various ROS.

Vanillin as an antioxidant in rat liver mitochondria: inhibition of protein oxidation and lipid peroxidation induced by photosensitization

Using rat liver mitochondria, as model systems, we have examined the ability of the natural compound and the food-flavoring agent, vanillin to protect membranes against oxidative damage induced by photosensitization at concentrations normally used in food preparations. Vanillin, at a concentration of 2.5 mmol/L, has afforded significant protection against protein oxidation and lipid peroxidation in hepatic mitochondria induced by photosensitization with methylene blue plus light. The effect observed was both time- and concentration-dependent. The inhibitory effect is similar to ascorbic acid and the singlet oxygen quencher, diazabicyclo[2.2.2]octane (DABCO) but less effective than sodium azide and glutathione. Examination of possible mechanisms responsible for the observed protection, showed that vanillin has a significant ability to quench singlet oxygen (1O2), a reactive species responsible for damage induced during photosensitization by Type II mechanism. Hence, this flavoring compound, due to its antioxidant ability, may have potential to prevent oxidative damage to membranes in mammalian tissues and thereby the ensuing diseased states.

Nicotinamide (vitamin B3) as an effective antioxidant against oxidative damage in rat brain mitochondria

Nicotinamide (vitamin B3) an endogenous metabolite, showed significant inhibition of oxidative damage induced by reactive oxygen species (ROS) generated by ascorbate-Fe2+ and photosensitization systems in rat brain mitochondria. It protected against both protein oxidation and lipid peroxidation, at millimolar concentrations. Inhibition was more pronounced against oxidation of proteins than peroxidation of lipids. Chemically related endogenous compounds, tryptophan and isonicotinic acid, showed comparable inhibitory properties. The protective effect observed, at biologically relevant concentrations, with nicotinamide was more than that of the endogenous antioxidants ascorbic acid and alpha-tocopherol. Hence our studies suggest that nicotinamide (vitamin B3) can be considered as a potent antioxidant capable of protecting the cellular membranes in brain, which is highly susceptible to prooxidants, against oxidative damage induced by ROS.

The photoreactivity of the retinal age pigment lipofuscin

The presence of the age pigment lipofuscin is associated with numerous age-related diseases. In the retina lipofuscin is located within the pigment epithelium where it is exposed to high oxygen and visible light, a prime environment for the generation of reactive oxygen species. Although we, and others, have demonstrated that retinal lipofuscin is a photoinducible generator of reactive oxygen species it is unclear how this may translate into cell damage. The position of lipofuscin within the lysosome infers that irradiated lipofuscin is liable to cause oxidative damage to either the lysosomal membrane or the lysosomal enzymes. We have found that illumination of lipofuscin with visible light is capable of extragranular lipid peroxidation, enzyme inactivation, and protein oxidation. These effects, which were pH-dependent, were significantly reduced by the addition of the antioxidants, superoxide dismutase and 1,4-diazabicyclo(2,2,2)-octane, confirming a role for both the superoxide anion and singlet oxygen. We postulate that lipofuscin may compromise retinal cell function by causing loss of lysosomal integrity and that this may be a major contributory factor to the pathology associated with retinal light damage and diseases such as age-related macular degeneration.

Photodynamic effects induced by meso-tetrakis[4-(carboxymethyleneoxy)phenyl] porphyrin on isolated Sarcoma 180 ascites mitochondria

Using mitochondria isolated from Sarcoma 180 ascites tumour in Swiss mice as a model system, we have evaluated the ability of a novel porphyrin, meso-tetrakis[4-(carboxymethyleneoxy)phenyl]porphyrin (H2T4CPP), to induce damage on photosensitization. Oxidative damage to mitochondria, one of the primary and crucial targets of the photodynamic effect, is assessed by measuring products of lipid peroxidation such as thiobarbituric acid reactive substances (TBARS) and lipid hydroperoxides (LOOH), besides the loss of activity of the mitochondrial marker enzyme succinate dehydrogenase (SDH). Analysis of product formation, the effect of deuteration and selective inhibition by scavengers of reactive oxygen species (ROS) show that the damage observed is due mainly to singlet oxygen (1O2) and to a minor extent to hydroxyl radicals (.OH). The 1O2 generation and triplet lifetime of this porphyrin have also been estimated. Fluorescence spectroscopy, used to ascertain the binding of this porphyrin to the mitochondrial proteins, shows a rapid association within 0-2 h and a decline thereafter. Confocal microscopy reveals intracellular localisation of this porphyrin in cells in vitro. Our overall results suggest that the porphyrin H2T4CPP, due to its ability to bind to mitochondrial protein components and to generate ROS upon photoexcitation, may have potential applications in photodynamic therapy.

Oxidative damage induced by the fullerene C60 on photosensitization in rat liver microsomes

We have examined the ability of a commonly used fullerene, C60, to induce oxidative damage on photosensitization using rat liver microsomes as model membranes. When C60 was incorporated into rat liver microsomes in the form of its cyclodextrin complex and exposed to UV or visible light, it induced significant oxidative damage in terms of (1) lipid peroxidation as assayed by thiobarbituric acid reactive substances (TBARS), lipid hydroperoxides and conjugated dienes, and (2) damage to proteins as assessed by protein carbonyls and loss of the membrane-bound enzymes. The oxidative damage induced was both time- and concentration-dependent. C60 plus light-induced lipid peroxidation was significantly inhibited by the quenchers of singlet oxygen ((1)O2), beta-carotene and sodium azide, and deuteration of the buffer-enhanced peroxidation. These observations indicate that C60 is an efficient inducer of peroxidation and is predominantly due to (1)O2. Biological antioxidants such as glutathione, ascorbic acid and alpha-tocopherol significantly differ in their ability to inhibit peroxidation induced by C60. Our studies, hence, indicate that C60, on photosensitization, can induce significant lipid peroxidation and other forms of oxidative damage in biological membranes and that this phenomenon can be greatly modulated by endogenous antioxidants and scavengers of reactive oxygen species.

Antioxidant adaptive response in human blood mononuclear cells exposed to UVB

Human blood mononuclear cells exposed to UVB radiation develop increased antioxidant enzyme activities. Catalase (5.50 +/- 0.65 pmol (mg protein)-1), CuZn-superoxide dismutase (16.7 +/- 2.1 pmol (mg protein)-1), Mn-superoxide dismutase (11.3 +/- 1.7 pmol (mg protein)-1), Se-dependent glutathione peroxidase (13.2 +/- 1.5 mU (mg protein)-1) and Se-independent glutathione peroxidase (3.30 +/- 0.52 mU (mg protein)-1) activities increase by 1.3-1.5-fold from the control activities after exposure to 0.3 W m-2 of 280-315 nm light for 15 min and a 3 h dark incubation period. DT-diaphorase activity (2.86 +/- 0.21 mumol DCPIP min-1 (mg protein)-1) increases threefold from the indicated control values. In contrast, cytochrome oxidase (0.36 +/- 0.04 min-1 (k') (mg protein)-1) and succinate dehydrogenase (3.06 +/- 0.25 mumol DCPIP min-1 (mg protein)-1) activities remain unchanged during the same irradiation and incubation period. The treatment of cells with cycloheximide prevents the response triggered by UVB exposure. These findings suggest that an inducible antioxidant defence mechanism operates on photo-oxidative stress and that both superoxide dismutase and DT-diaphorase may display a concerted antioxidant role.

Photo-initiated peroxidation of lipids in micelles by azaaromatics

The monoazaaromatics, pyridine (1), hexyl nicotinate (2), and quinoline (3) and diazaaromatics, pyrimidine (4) and purine (5), readily act as photo-initiators for the peroxidation of methyl linoleate in 0.50 M SDS at 37 degrees C giving free radical chain oxidations of linoleate. Quantitative kinetic runs on the order in substate, RH, and in the rate of chain initiation, Ri, showed that the classical rate law for autoxidation, -d[O2]/dt = (kp/(2 kt 1/2))[RH] x Ri 1/2, is applicable to these photo-initiated oxidations. The oxidizability of methyl linoleate under these conditions is 2.92 x 10(-2) M-1/2 s-1/2. These peroxidations were inhibited by chromanol phenolic antioxidants of the vitamin E class, such as lipid-soluble 2,2,5,7,8-pentamethyl-6-hydroxychroman (PMHC) and water-soluble 2-carboxy- 2,5,7,8-tetramethyl-6-hydroxychroman (Trolox) and derived rate constants for inhibition of peroxidation were kinh (PMHC) = 4.35 x 10(4) M-1 s-1 and k(inh) (Trolox) = 2.81 x 10(4) M-1 s-1 during inhibited oxidation of methyl linoleate photo-initiated by 4. The products from photo-initiated peroxidation of methyl linoleate by 1 through 5 were determined by reduction and high-performance liquid chromatography analyses to be the 9- and 13-positional hydroperoxides of the four geometrical isomers: cis-9, trans-11 (6), trans-10, cis-12 (7), trans-9, trans-11 (8), and trans-10, trans-12 (9)-octadecadienoates typical of the free radical chain mechanism of lipid peroxidation. Products from dye-sensitized oxidation by Methylene Blue or Rose Bengal of methyl linoleate gave a product distribution of six hydroperoxides typical of oxidation by singlet oxygen. Thermal or photo-initiated peroxidation of methyl linoleate in SDS gave some selectivity of oxidation at the 13-position of the linoleate chain. The ratio of 13- to 9-oxidation varied in the range 1.23 to 1.14 as the cis/trans to trans/trans ratio of geometrical isomers varied from 0.44 to 1.25 during photooxidation of increased amounts of linoleate in SDS. This selectivity is attributed to loss of the pseudo symmetry around the pentadienyl system in the lipid chain in the SDS system during the peroxidation.

Photodynamic effects induced by meso-tetrakis[4-(carboxymethyleneoxy)phenyl]porphyrin using rat hepatic microsomes as model membranes

Porphyrins, in combination with light, offer an alternate approach to the treatment of cancer, in the form of photodynamic therapy (PDT). With a view to locate new porphyrins for use in PDT, we evaluated the ability of a novel water-soluble porphyrin, meso-tetrakis[4-(carboxymethyleneoxy)phenyl]porphyrin (T4CPP) to induce photodamage in membranes, using rat hepatic microsomes as a model system. Hepatic microsomes treated with T4CPP and exposed to visible light showed significant lipid peroxidation, as assessed by the formation of conjugated dienes, lipid hydroperoxides, and thiobarbituric acid-reactive substances. The peroxidation induced was both time- and concentration-dependent. T4CPP plus light also resulted in the destruction of the microsomal enzymes adenosine triphosphatase and glucose-6-phosphatase. Analysis of the products of peroxidation and selective inhibition by specific inhibitors showed that the oxidative damage induced was mainly due to singlet oxygen and partly due to hydroxyl radical. The porphyrin T4CPP was efficiently labeled with 99mTc. When this 99mTc-labeled porphyrin was injected into a mammary-tumor-bearing rat, it accumulated in the tumor. Our studies suggest that T4CPP, due to its potential to localize in tumors and to induce membrane damage as exemplified by alteration in rat liver microsomes, may have possible applications in this new modality of cancer treatment.

Lipid peroxidation induced by a novel porphyrin plus light in isolated mitochondria: possible implications in photodynamic therapy

With a view to locate porphyrins for use in photodynamic therapy (PDT), the new modality of cancer treatment we have evaluated the ability of a novel water soluble porphyrin meso-tetrakis[4-(carboxymethyleneoxy)phenyl]porphyrin (T4CPP) to induce damage to mitochondria during photosensitization. T4CPP, when exposed to visible light, induced lipid peroxidation in rat liver mitochondria as assessed by the formation of thiobarbituric acid reactive substances (TBARS), conjugated dienes (CD) and lipid hydroperoxides (LOOH). The effect on mitochondrial function was assessed by estimating the activity of succinate dehydrogenase (SDH). The peroxidation induced was observed to be time- and concentration- dependent. Analysis of product formation and selective inhibition by scavengers of reactive oxygen species showed that the oxidative damage observed was mainly due to singlet oxygen ((1)O2) and partly due to other reactive species. T4CPP plus light also caused significant lipid peroxidation in Sarcoma 180 ascites tumour mitochondria. Our studies indicate that T4CPP has the potential to photoinduce damage in hepatic and ascites mitochondria, a crucial site of damage in PDT.

Caffeine as an antioxidant: inhibition of lipid peroxidation induced by reactive oxygen species

Caffeine (1,3,7-trimethyl xanthine), an ingredient of coffee, has been investigated for its potential antioxidant activity against oxidative damage to rat liver microsomes. Such damage was induced by three reactive oxygen species of cardinal importance in causing membrane damage in vivo namely hydroxyl radical (.OH), peroxyl radical (ROO.) and singlet oxygen (1O2). The results obtained showed that caffeine was an effective inhibitor of lipid peroxidation, at millimolar concentrations, against all the three reactive species. The extent of inhibition was high against peroxidation induced by .OH, medium against 1O2 and low against ROO. In general, the antioxidant ability of caffeine was similar to that of the established biological antioxidant glutathione and significantly higher than ascorbic acid. Investigations into the possible mechanisms involved in the observed antioxidant effect reveal that the quenching of these reactive species by caffeine may be one of the possible factor responsible. The rate constant of caffeine with .OH was 7.3 x 10(9) M-1 s-1 and with 1O2 it was 2.9 x 10(7) M-1 s-1. Considering their potential for damage, half-life estimates and generation in biological systems, the ability of caffeine to inhibit oxidative damage induced by these reactive species in membranes suggest one more positive attribute of caffeine, whose daily intake as coffee may be considerable in most populations.

Contrary effects of lightly and strongly oxidized LDL with potent promotion of growth versus apoptosis on arterial smooth muscle cells, macrophages, and fibroblasts

The inhibition of experimental atherosclerosis by antioxidants and the presence of oxidized LDL (oxLDL) in atherosclerotic lesions indicate that oxLDL may play what is perhaps a primary role in atherogenesis. LDL promotes the growth of arterial smooth muscle cells (SMCs), and oxLDL has cytotoxic effects. Since excessive intimal growth alternating with necrosis is typical of atherosclerotic lesions, we wondered whether these extreme changes in the lesions could be related to the extreme effects of LDL and oxLDL on cells. We therefore examined the effects of increasing LDL oxidation on its capacity to induce cell growth or cell death and whether the latter could be due to apoptosis. Cells of the types present in the atherosclerotic artery used, ie, SMCs (human arterial), macrophages (human macrophage-like cell line THP-1), and human fibroblasts. Growth was evaluated by measuring the synthesis of DNA and culture size (MTT method) and apoptosis by using the in situ labeling of internucleosomally degraded DNA and, in the case of SMCs, the appearance of chromatin condensation. The oxidation of LDL was by UV or Fe ions. Shortly oxidized LDL had a markedly increased growth-promoting effect on all cell types. With prolonged exposure to UV, but not to Fe, LDL became increasingly cytotoxic, and this toxicity was paralleled by the appearance of apoptosis in all cell types. After prolonged UV treatment, low-molecular-weight material from the partially degraded LDL was responsible for the induction of apoptosis. The dual effect of oxLDL, ie, its strong growth-promoting effect or the induction of cell death by apoptosis, depending on the degree of change by oxidation, is compatible with the notion that oxLDL plays a role not only in atherogenesis but also more extensively in the development of the structure typical of the atherosclerotic lesion, with focal excessive growth alternating with necrosis.

Methylene blue plus light-induced lipid peroxidation in rat liver microsomes: inhibition by nicotinamide (vitamin B3) and other antioxidants

Methylene blue plus visible light, in the presence of oxygen, induced lipid peroxidation in rat liver microsomes, as assessed by the formation of thiobarbituric acid reactive substances (TBARS), lipid hydroperoxides and the loss of membrane-bound enzymes. Peroxidation was enhanced by deuteration of the buffer and inhibited by scavengers of singlet oxygen (1O2) and superoxide (O2.-). The damage induced seemed to be mainly due to Type II involving 1O2 and to a lesser extent Type I reactions with O2.- and hydroxyl radical (.OH) as intermediates. Nicotinamide or vitamin B3, an endogenous metabolite occurring at high concentrations in tissues, had a relatively high rate constant of 1.8 x 108 M-1 s-1 with 102 and had a significant inhibitory effect on lipid peroxidation induced by photosensitization. This effect was both time- and concentration-dependent, high inhibition being associated with millimolar concentrations. Chemically related endogenous compounds like tryptophan and isonicotinic acid also had significant inhibitory properties. Similar protective effects were observed with natural antioxidants such as beta-carotene, canthaxanthin, lipoic acid, glutathione, alpha-tocopherol and to a lesser extent ascorbic acid. Nicotinamide was a more effective antioxidant than ascorbic acid. It also showed a similar inhibitory effect against NADPH-ADP-FE3(+)-induced lipid peroxidation. Our results suggest that nicotinamide had significant ability to protect against photosensitization-induced cytotoxicity and cell damage and that it may do so by its ability to react with 102 and other reactive oxygen species.

Tocotrienols from palm oil as potent inhibitors of lipid peroxidation and protein oxidation in rat brain mitochondria

The tocotrienol-rich-fraction (TRF) from palm oil, being tried as a more economical and efficient substitute for alpha-tocopherol, significantly inhibited oxidative damage in vitro to both lipids and proteins in rat brain mitochondria induced by ascorbate-Fe2+, the free radical initiator azobis(2-amidopropane)dihydrochloride (AAPH) and photosensitisation. The observed inhibitory effect was both time- and concentration-dependent. At a low concentration of 5 microM, TRF can significantly inhibit oxidative damage to both lipids and proteins. The inhibitory effect of TRF seems to be mainly due to gamma-tocotrienol and to a lesser extent alpha- and delta-tocotrienols. TRF was significantly more effective than alpha-tocopherol. This fraction from palm oil can be considered a natural antioxidant supplement capable of protecting the brain against oxidative damage and thereby from the ensuing adverse alterations.

Dual role of melanins and melanin precursors as photoprotective and phototoxic agents: inhibition of ultraviolet radiation-induced lipid peroxidation

Ultraviolet radiation (UVR) is one of the risk factors for skin cancer and the main inducer of melanin pigmentation, the major protective mechanism of mammalian skin against radiation damage. The melanin pigments, eumelanin and pheomelanin, are likely to be important in protection against UVR, but their precursors are generally considered as phototoxic. The available data suggest DNA damage as the mechanism of phototoxicity. However, the effect of melanin precursors on membrane damage through lipid peroxidation, another important and probably more relevant (from the point-of-view of the melanosomal confinement of these molecules) mechanism of phototoxicity, not known. As a model system for UVR-melanin-membrane interactions, we irradiated liposomes in the presence of eumelanin, pheomelanin and two of their major precursors, 5,6-dihydroxyindole (DHI) and 5-S-cysteinyldopa (SCD). The presence of the two melanin precursors substantially reduced the formation of lipid peroxidation products resulting from UVR exposure. The antioxidant activity of the melanin precursors was diminished under strong prooxidant conditions (presence of Fe3+). These results suggest that melanin precursors may have an important role in the protection of skin against the harmful effects of UVR including photocarcinogenesis.