Photodynamic activity of cercosporin on plant tissues

Abstract Cercosporin, excited by incandescent light in the presence of oxygen, induced a loss of ions from the plant tissues tested. Silymarin, BHA (2(3)-tert-butyl-4-hydroxyanisole) and EDTA were effective in lowering the activity of cercosporin. Circumstantial evidence suggests that the toxic effect may be mediated by the singlet oxygen and free radical formation which induces a lipoperoxidative degradation of the polyunsaturated fatty acids of cell membranes.     

Photodynamic Activity of Compounds Structurally Related to Cercosporin

The photodynamic activity of three compounds structurally related to cercosporin (iso-cercosporin, phleichrome, iso-phleichrome) and rubellin, a pigment possessing an anthra-quinone-like structure, was studied. As previously reported for cercosporin, iso-cercosporin, phleichrome, iso-phleichrome and rubellin, when irradiated by an incandescent lamp, induced oxygen uptake and malondialdehyde (MDA) formation in pea stem and rat liver mitochondria and microsomes. All these compounds were also capable of inhibiting MDA formation induced by the peroxidizing system ascorbate-FeSO₄. Circumstantial evidence suggests that the compounds, as shown for cercosporin, induce a lipoperoxidative degradation of the polyunsaturated fatty acids of cell membranes when irradiated by light, but at the same time, they are able to lower the lipoperoxidation induced by ferrous ions.     

Are dioxetanes chemiluminescent intermediates in lipoperoxidation?

Ultraweak chemiluminescence arising from lipoperoxidation has been attributed by several authors to the radiative deactivation of singlet oxygen and triplet carbonyl products. The latter emitters have been suggested to come from annihilation of RO. and ROO. radicals as well as from the thermolysis of dioxetane intermediates formed by (2 + 2) cycloaddition of 1O2 to polyunsaturated fatty acids. This article questions possible dioxetane intermediacy in lipoperoxidation, as the literature clearly states that addition of 1O2 to alpha-hydrogen-containing alyphatic olefins yields only the corresponding allylic hydroperoxides. These compounds may undergo dark thermal or Lewis acid-assisted decomposition to the same product obtained from dioxetane cleavage. Here, reexamining the chemiluminescence properties of dioxygenated tetramethylethylene and linoleic acid and comparing them with those of tetraethyldioxetane, a hindered dioxetane, we corroborate the literature information that only steric hindrance leads to dioxetane formation upon singlet oxygen addition to electron-poor olefins, albeit in very low yields. Proton nuclear magnetic resonance (1H-NMR) analysis, quenching by dioxygen and energy transfer studies to 9,10-dibromoanthracene, as well as gas chromatography (GC) analysis of triphenylphosphine-treated and untreated photo- and chemically dioxygenated olefins support our final conclusion that dioxetane formation during lipoperoxidation can be safely excluded on the basis of the data presently available.     

Sterol metabolism. XL. On the failure of superoxide radical anion to react with cholesterol.

Superoxide radical anion (O-2) failed to react with cholesterol under a variety of conditions. In some instances products indicative of free radical oxidation by molecular oxygen (O2) were found, but no products of electronically excited (singlet) molecular oxygen (1O2) attack on cholesterol were detected. These results do not support a direct role of O-2 in lipid peroxidation of cholesterol-rich membranes or of the formation of 1O2 from O-2 dismutation.     

Cytometric quantification of singlet oxygen in the human malaria parasite Plasmodium falciparum

The malaria parasite Plasmodium falciparum proliferates within human erythrocytes and is thereby exposed to a variety of reactive oxygen species (ROS) such as hydrogen peroxide, hydroxyl radical, superoxide anion, and highly reactive singlet oxygen ((1)O(2)). While most ROS are already well studied in the malaria parasite, singlet oxygen has been neglected to date. In this study we visualized the generation of (1)O(2) by live cell fluorescence microscopy using 3-(p-aminophenyl) fluorescein as an indicator dye. While (1) O(2) is found restrictively in the parasite, its amount varies during erythrocytic schizogony. Since the photosensitizer cercosporin generates defined amounts of (1)O(2) we have established a new cytometric method that allows the stage specific quantification of (1)O(2). Therefore, the parasites were first classified into three main stages according to their respective pixel-area of 200-600 pixels for rings, 700-1,200 pixels for trophozoites and 1,400-2,500 pixels for schizonts. Interestingly the highest mean concentration of endogenous (1)O(2) of 0.34 nM is found in the trophozoites stage, followed by 0.20 nM (ring stage) and 0.10 nM (schizont stage) suggesting that (1)O(2) derives predominantly from the digestion of hemoglobin.     

Biflavonoids from Brazilian pine Araucaria angustifolia as potentials protective agents against DNA damage and lipoperoxidation

A biflavonoid fraction (BFF) obtained from Araucaria angustifolia needles was effective to quench singlet oxygen (1O2), to protect plasmid DNA against single strand break (ssb) caused by 1O2 or Fenton reaction and to inhibit Fenton or UV radiation-induced lipoperoxidation in phosphatidylcholine liposomes. The activity of the biflavonoid fraction (BFF) was compared with quercetin, rutin (flavonoids), ginkgetin, amentoflavone (biflavonoids), alpha-tocopherol and Trolox. The BFF displayed a higher quenching rate constant compared to flavonoids and biflavonoids and protected against ssb induced by 1O2. Although the BFF was not as efficient as either flavonoids, alpha-tocopherol or Trolox in protection against ssb induced by Fenton-reaction or lipoperoxidation, these scavenging properties suggest that BFF is still an excellent candidate for successful employment as an antioxidant and photoprotector.     

Specificity of oxygen radical scavengers and assessment of free radical scavenger efficiency using luminol enhanced chemiluminescence

The selective scavenging capacities of 19 important oxygen radical scavengers were determined by adding them individually to each of the four oxy radical standards, (superoxide, hydroxy, alkoxy and hydroperoxy, and singlet O2), calculating the percent chemiluminescence inhibited, and extrapolating O2 equivalents neutralized from baseline. The sensitivity (0.01 nm/ml) and selectivity of this method not only allows identification of individual oxygen free radical species but also quantitates the efficiency of free radical scavengers.     

Photosensitized formation of ascorbate radicals by chloroaluminum phthalocyanine tetrasulfonate: an electron spin resonance study.

The chloroaluminum phthalocyanine tetrasulfonate sensitized photooxidation of ascorbic acid to ascorbate radical (A.-) was followed by electron spin resonance (ESR) spectroscopy. In air saturated aqueous media, steady-state amounts of A.- are rapidly established upon irradiation. The ESR signal disappears within a few seconds after the light is extinguished--more slowly under constant irradiation as oxygen is depleted. No photooxidation was observed in deaerated media. The effect of added superoxide dismutase, catalase, desferrioxamine, and singlet oxygen scavengers (NaN3 and tryptophan) was studied, as was replacement of water by D2O and saturation with O2. The results are indicative of free radical production by direct reaction between ascorbate ion and sensitized phthalocyanine (a Type I mechanism) in competition with the (Type II) reaction of HA- with singlet oxygen, a reaction which does not produce ascorbate radical intermediates.     

Photoilluminated riboflavin/riboflavin-Cu(II) inactivates trypsin: Cu(II) tilts the balance

Riboflavin (RF) upon irradiation with fluorescent light generates reactive oxygen species like superoxide anion, singlet and triplet oxygen, flavin radicals and substantial amounts of hydrogen peroxide (H2O2). H2O2 can freely penetrate cell membrane and react with a transition metal ion like Cu(ll), generating hydroxyl radical via the modified metal-catalyzed Haber-Weiss reaction. Earlier, it was reported that trypsin-chymotrypsin mixture served as an indirect antioxidant and decreased free radical generation. Thus, in the present study, we used photoilluminated RF as a source of ROS to investigate the effect of free radicals on the activity of trypsin. We also compared the damaging effect of photoilluminated RF and RF-Cu(ll) system using trypsin as a target molecule. RF caused fragmentation of trypsin and the effect was further enhanced, when Cu(II) was added to the reaction. Results obtained with various ROS scavengers suggested that superoxide radical, singlet and triplet oxygen were predominantly responsible for trypsin damage caused by photoilluminated RF. On the other hand, when Cu(ll) was added to the reaction, hydroxyl radical was mainly responsible for trypsin damage. A mechanism of generation of various ROS in the reaction is also proposed. Trypsin did not show any antioxidant effect with RF alone or with RF-Cu(II) combination.     

Methylene blue plus light mediates 8-hydroxyguanine formation in DNA

Exposure to methylene blue (MB) plus light mediates formation of large levels of 8-hydroxyguanine in DNA. The amount of 8-hydroxy-2'-deoxyguanosine (8-OHdG) present in DNA increased as the amount of MB concentration increased throughout the 2 to 200 microM range studied and was dependent on light exposure. As the time of light exposure increased so did the 8-OHdG content to levels of about 750 8-OHdG/10(5) deoxyguanosine after 15 min of light exposure when MB was at 20 microM. Even though previous research has demonstrated that hydroxyl free radicals formed from a variety of sources mediate 8-OHdG formation in DNA, inclusion of mannitol, superoxide dismutase, catalase, and desferal in the MB plus light experiments demonstrated that these scavengers of oxygen free radical intermediates or precursors caused either no change or an increase in the 8-OHdG content of DNA exposed to MB plus light. These results appear to rule out the direct role of oxygen free radical intermediates in the primary events involved in the MB plus light mediated formation of 8-OHdG in DNA. Oxygen was essential to cause MB plus light mediated 8-OHdG formation in DNA. It was noted that when the reaction was carried out where the deuterium oxide content had been increased to 100%, the amount of 8-OHdG formed in DNA increased about threefold over that observed when comparable reactions were carried out in pure H2O. Use of the singlet oxygen scavenger 2,5-dimethylfuran has yielded variable results on the MB plus light mediated formation of 8-OHdG in DNA. The data taken collectively clearly indicate that MB plus light mediates 8-OHdG formation in DNA. The D2O data and the requirement for oxygen suggest that singlet oxygen may be an intermediate.     

Chemical and biochemical aspects of superoxide radicals and related species of activated oxygen

The spectrum of biological processes in which oxygen is used by living systems is quite large, and the products include some damaging species of activated oxygen, particularly the superoxide radical (O-.2) and hydrogen peroxide (H2O2). Superoxide radicals and hydrogen peroxide, in turn, can lead to the formation of other damaging species: hydroxyl radicals (.OH) and singlet oxygen (1O2). Hydroxyl radicals react with organic compounds to give secondary free radicals that, in the presence of oxygen, yield peroxy radicals, peroxides, and hydroperoxides. Formation, interconversion, and reactivity of O-.2 and related activated oxygen species, methods available for their detection, and the basis of their biological toxicity are briefly reviewed.     

Effect of proline on the production of singlet oxygen

Molecular oxygen in electronic singlet state is a very powerful oxidant. Its damaging action in a variety of biological processes has been well recognized. Here we report the singlet oxygen quenching action of proline. Singlet oxygen (1O2) was produced photochemically by irradiating a solution of sensitiser and detected by following the formation of stable nitroxide radical yielded in the reaction of 1O2 with the sterically hindered amine (2,2,6,6-tetramethylpiperidine, TEMP). Illumination of a sensitiser, toluidine blue led to a time dependent increase in singlet oxygen production as detected by the formation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) by EPR spectrometry. Interestingly, the production of TEMPO was completely abolished by the presence of proline at concentration as low as 20mM. These results show that proline is a very effective singlet oxygen quencher. Other singlet oxygen generating photosensitizer like hematopophyrin and fluorescein also produced identical results with proline. Since proline is one of the important solutes which accumulate in many organisms when they are exposed to environmental stresses, it is likely that proline accumulation is related to the protection of these organisms against singlet oxygen production during stress conditions. A possible mechanism of singlet oxygen quenching by proline is discussed.     

Singlet oxygen formation by a peroxidase, H2O2 and halide system.

Evidence for singlet oxygen formation has been obtained for the lactoperoxidase, H2O2 and bromide system by monitoring 2,3-diphenylfuran and diphenylisobenzofuran oxidation, O2 evolution, and chemiluminescence. This could provide an explanation for the cytotoxic and microbicidal activity of peroxidases and polymorphonuclear leukocytes. Evidence for singlet oxygen formation included the following. (a) Chemiluminescence accompanying the enzymic reaction was doubled in a deuterated buffer and inhibited by singlet oxygen traps. (b) The singlet oxygen traps, diphenylfuran and diphenylisobenzofuran, were oxidized to their known singlet oxygen oxidation products in the presence of lactoperoxidase, hydrogen peroxide and bromide. (c) The rate of oxidation of diphenylfuran and diphenylisobenzofuran was inhibited when monitored in the presence of known singlet oxygen traps or quenchers. (d) Oxygen evolution from the enzymic reaction was inhibited by singlet oxygen traps but not by singlet oxygen quenchers. (e) The traps or quenchers which were effective inhibitors in the experiments above did not inhibit peroxidase activity, were not competitive peroxidase substrates and did not react with the hypobromite intermediate since they did not inhibit hydrogen peroxide consumption by the enzyme. Using these criteria, various biological molecules were tested for their reactivity with singlet oxygen. Furthermore, by studying their effect on oxygen release by the enzymic reaction, it could be ascertained whether they were acting as singlet oxygen traps or quenchers.     

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.     

Effects of scavengers of reactive oxygen and radical species on cell survival following photodynamic treatment in vitro: comparison to ionizing radiation

The effects of various scavengers of reactive oxygen and/or radical species on cell survival in vitro of EMT6 and CHO cells following photodynamic therapy (PDT) or gamma irradiation were compared. None of the agents used exhibited major direct cytotoxicity. Likewise, none interfered with cellular porphyrin uptake, and none except tryptophan altered singlet oxygen production during porphyrin illumination. The radioprotector cysteamine (MEA) was equally effective in reducing cell damage in both modalities. In part, this protection seems to have been induced by oxygen consumption in the system due to MEA autoxidation under formation of H2O2. The addition of catalase, which prevents H2O2 buildup, reduced the effect of MEA to the same extent in both treatments. Whether the remaining protection was due to MEA's radical-reducing action or some remaining oxygen limitation is unclear. The protective action of MEA was not mediated by a doubling of cellular glutathione levels, since addition of buthionine sulfoximine, which prevented glutathione increase, did not diminish the observed MEA protection. The hydroxyl radical scavenger mannitol also afforded protection in both kinds of treatment, but it was approximately twice as effective in gamma irradiation as in PDT. This is consistent with the predominant role of OH radicals in ionizing radiation damage and their presumed minor involvement in PDT damage. Superoxide dismutase, a scavenger of O2, acted as a radiation protector but was not significantly effective in PDT. Catalase, which scavenges H2O2, was ineffective in both modalities. Tryptophan, an efficient singlet oxygen scavenger, reduced cell death through PDT by several orders of magnitude while being totally ineffective in gamma irradiation. These data reaffirm the predominant role of 1O2 in the photodynamic cell killing but also indicate some involvement of free radical species.     

Influence of neopterin on generation of reactive species by myeloperoxidase in human neutrophils

Increased neopterin concentrations in human serum indicate activation of cell-mediated immune response. Earlier we have shown that neopterin enhanced generation of singlet oxygen, hydroxyl radical and nitric oxide in human peripheral blood neutrophils by NADPH-independent pathways. To further investigate a participation of neopterin in reactive species production by neutrophils, we studied its influence on myeloperoxidase (MPO) activity. MPO was isolated from human peripheral blood neutrophils from healthy donors. Generation of reactive species by MPO/H(2)O(2) in Earl's solution (pH=7.2) at 37 degrees C was investigated by monitoring of chemiluminescence using luminol as light emitter. In the MPO/H(2)O(2) system, neopterin increased singlet oxygen in a concentration-dependent manner, but it decreased formation of other oxidizing species. Comparing several oxygen scavengers, formation of reactive species was totally blocked by sodium azide (NaN(3)), both in the presence and in the absence of neopterin. Superoxide dismutase (SOD) and d-mannitol insignificantly decreased chemiluminescence of this reaction, but diazabicyclo[2.2.2]octane (DABCO) strongly inhibited it. We conclude that the effects of neopterin on neutrophils' MPO are directed to increase singlet oxygen and to decrease other reactive species via inhibition of MPO and/or scavenging of reactive species.     

Singlet oxygen quenching and the redox properties of hydroxycinnamic acids.

The singlet oxygen quenching rate constants (kq) for a range of hydroxycinnamic acids in acetonitrile and D2O solutions were measured using time resolved near infrared phosphorescence in order to establish their antioxidant activity. The magnitude of kq observed depends on both the nature of the substituent groups and solvent polarity. The variations in kq depend on the energy of the hydroxycinnamic acid/molecular oxygen charge transfer states, (O2delta- ...HCAdelta+). In D2O the values of kq range from 4x10(7) M(-1) s(-1) to 4x10(6) M(-1) s(-1) for caffeic acid and o-coumaric acid respectively. In acetonitrile, the charge transfer energy levels are raised and this is reflected in lower singlet oxygen quenching rate constants with a kq value of 5x10(6) M(-1) s(-1) for caffeic acid. The phenoxyl radical spectra derived from the hydroxycinnamic acids were determined using pulse radiolysis of aqueous solutions and the reduction potentials were found to range from 534 to 596 mV. A linear correlation is observed between reduction potential, and hence free energy for electron transfer, and log kq. These correlations suggest a charge transfer mechanism for the quenching of singlet oxygen by the hydroxycinnamic acids.