Generation of free radicals by photoexcitation of pheophorbide alpha, haematoporphyrin and protoporphyrin

Photophysical properties of pheophorbide a in solution and in model membrane systems

Pheophorbide a (Pheo) is a well-known hydrophobic photosensitizer used for photodynamic treatment of various diseases. The influence of the surroundings on the electronic properties of photosensitizers mainly accumulating in membrane structures is of relevance for their photoactivity. In this paper the current knowledge about the electronic properties of Pheo in different microheterogeneous environments is summarized and new findings about its incorparation in different model membranes are discussed.

Naturally occurring chlorophyll derivatives inhibit aflatoxin B1-DNA adduct formation in hepatoma cells

The inhibitory effects of four chlorophyll derivatives (chlorophyllide [Chlide] a and b and pheophorbide [Pho] a and b) on aflatoxin B1 (AFB1)-DNA adduct formation, and on the modulation of hepatic glutathione S-transferase (GST) were evaluated in murine hepatoma (Hepa-1) cells. Enzyme-linked immunosorbent assay showed that pretreatment with Chlide or Pho significantly reduced the formation of AFB1-DNA adducts, and that Pho was the most potent inhibitor. However, wash-out prior to adding AFB1 totally eliminated inhibition by Childe and partially eliminated inhibition by Pho, indicating that the inhibitory effect of Chlide, and to some extent Pho, was mediated through direct trapping of AFB1. Furthermore, spectrophotometric analysis showed that Pho treatment could increase GST activity in Hepa-1 cells. These observations indicate that the chlorophyll derivatives studied may attenuate AFB1-induced DNA damage in the Hepa-1 cell by direct trapping of AFB1. Pho provided additional protection not only by direct trapping, but also by increasing GST activity against hepatic AFB1 metabolites.

A reporter system for the individual detection of hydrogen peroxide and singlet oxygen: its use for the assay of reactive oxygen species produced in vivo

A reporter system for the assay of reactive oxygen species (ROS) was developed in Chlamydomonas reinhardtii, a plant model organism well suited for the application of inhibitors and generators of various types of ROS. This system employs various HSP70A promoter segments fused to a Renilla reniformis luciferase gene as a reporter. Transformants with the complete HSP70A promoter were inducible by both hydrogen peroxide and singlet oxygen. Constructs that lacked upstream heat-shock elements (HSEs) were inducible by hydrogen peroxide, indicating that this induction does not require such HSEs. Rather, downstream elements located between positions -81 to -149 with respect to the translation start site appear to be involved. In contrast, upstream sequences are essential for the response to singlet oxygen. Thus, activation by singlet oxygen appears to require promoter elements that are different from those used by hydrogen peroxide. ROS generated endogenously by treatment of the alga with metronidazole, protoporphyrin IX, dinoterb or high light intensities were detected by this reporter system, and distinguished as production of hydrogen peroxide (metronidazole) and singlet oxygen (protoporphyrin IX, dinoterb, high light). This system thus makes it possible to test whether, under varying environmental conditions including the application of abiotic stress, hydrogen peroxide or singlet oxygen or both are produced.

Chemistry and reactions of reactive oxygen species in foods

Reactive oxygen species (ROS) are formed enzymatically, chemically, photochemically, and by irradiation of food. They are also formed by the decomposition and the inter-reactions of ROS. Hydroxy radical is the most reactive ROS, followed by singlet oxygen. Reactions of ROS with food components produce undesirable volatile compounds and carcinogens, destroy essential nutrients, and change the functionalities of proteins, lipids, and carbohydrates. Lipid oxidation by ROS produces low molecular volatile aldehydes, alcohols, and hydrocarbons. ROS causes crosslink or cleavage of proteins and produces low molecular carbonyls from carbohydrates. Vitamins are easily oxidized by ROS, especially singlet oxygen. The singlet oxygen reaction rate was the highest in ss-carotene, followed by tocopherol, riboflavin, vitamin D, and ascorbic acid.

In Vivo Oxygen Radical Generation in the Skin of the Protoporphyria Model Mouse with Visible Light Exposure: An L-Band ESR Study

Although oxygen radicals are thought to play a key role in the skin injury that is caused by protoporphyria, there is no direct evidence of generation of these radicals in vivo. This study measured the generation of oxygen radicals caused by visible light non-invasively in the skin of griseofulvin-induced protoporphyria model mice, using an in vivo electron spin resonance spectrometer equipped with a surface-coil-type resonator that could detect radicals within about 0.5 mm of the skin surface. A durable nitroxyl radical was administered intravenously as a probe. Light irradiation enhanced the decay of the nitroxyl signal in griseofulvin-treated mice, whereas light irradiation did not enhance the signal decay in control mice. The enhanced signal decay was completely suppressed by intravenous administration of hydroxyl radical scavengers, superoxide dismutase or catalase, or the intraperitoneal administration of desferrioxamine. The enhanced signal decay with illumination was reversible, and quickly responded to turning the light on and off. These observations suggest that the hydroxyl radical is generated via an iron-catalyzed reaction in the skin. This paper demonstrates, for the first time, the specific generation of oxygen radicals in response to light irradiation of the skin of protoporphyria model mice.

Natural anti-HIV agents. Part IV. Anti-HIV constituents from Vatica cinerea

In a continuing search for anti-HIV compounds from plants of Vietnam, 19 compounds, including a new triterpene, were isolated from an extract of the leaves and stem of Vatica cinerea. The new triterpene was determined to be a cycloartane triterpenoid with 29 skeletal carbons and was assigned the name vaticinone (1). The known triterpenes included three cycloartanes, a lanostane, two dammaranes, three lupanes, an ursane, and an oleanane. A chlorophyll isolate was identified as pheophorbide a (13). The majority of the triterpenes, the sesquiterpene, 1-hydroxycyclocolorenone, and pheophorbide a showed anti-HIV activity, with the chlorophyll being the most active, demonstrating an IC(50) value of 1.5 microgram/mL (2.5 microM), while being completely devoid of toxicity up to a concentration of 20 microgram/mL (33.8 microM). Vaticinone (1) was found to inhibit the replication of HIV-1, with an IC(50) value of 6.5 microgram/mL (15.3 microM; selective index = 1.4). The structures of these isolates were determined by spectral data including 1D and 2D NMR spectra.

Expression of uroporphyrinogen decarboxylase or coproporphyrinogen oxidase antisense RNA in tobacco induces pathogen defense responses conferring increased resistance to tobacco mosaic virus

Transgenic tobacco plants with reduced activity of either uroporphyrinogen decarboxylase or coproporphyrinogen oxidase, two enzymes of the tetrapyrrole biosynthetic pathway, are characterized by the accumulation of photosensitizing tetrapyrrole intermediates, antioxidative responses, and necrotic leaf lesions. In this study we report on cellular responses in uroporphyrinogen decarboxylase and coproporphyrinogen oxidase antisense plants, normally associated with pathogen defense. These plants accumulate the highly fluorescent coumarin scopolin in their leaves. They also display increased pathogenesis-related protein expression and higher levels of free and conjugated salicylic acid. Upon tobacco mosaic virus inoculation, the plants with leaf lesions and high levels of PR-1 mRNA expression show reduced accumulation of virus RNA relative to wild-type controls. This result is indicative of an increased resistance to tobacco mosaic virus. We conclude that porphyrinogenesis as a result of deregulated tetrapyrrole synthesis induces a set of defense responses that resemble the hypersensitive reaction observed after pathogen attack.

Inactivation of cell surface receptors by pheophorbide a, a green pigment isolated from Psychotria acuminata

The inhibition of cytokine and monoclonal antibody binding cell surfaces caused by an extract of Psychotria acuminata, a medicinal plant used in the traditional medicine of the people of Belize (Central Africa), was attributed to the presence of pheophorbide a and pyropheophorbide a. Since the binding of tumor necrosis factor-alpha, interleukin-8, complement factor 5a as well as epidermal growth factor to target cells was dramatically reduced, the inhibition was not receptor or cytokine specific. In addition, the respective binding of radiolabeled monoclonal antibodies CL203 and R15.7 to the cell surface antigens intracellular cell adhesion molecule-1 and lymphocyte function-associated antigen-1 beta-chain was decreased by pretreatment of cells with pheophorbide a as well. In all cases, the inhibition by pheophorbides was dependent on the simultaneous presence of light, indicating causative involvement of a photodynamic process. These observations are not unique to pheophorbides and can be extended to porphyrins as well as to other photodynamic agents. Cytotoxicity resulting from photodynamic therapy (PDT) has been documented by many studies. Our investigations suggest that the inactivation of cell surface receptors contributes not only to an antitumor effect of PDT but also to the systemic immunosuppression, a serious side effect of PDT.

Spin trapping using 2,2-dimethyl-2H-imidazole-1-oxides

The ability of novel cyclic nitrones, 4-substituted 2,2-dimethyl-2H-imidazole-1-oxides (IMO's) to trap a variety of short-lived free radicals has been investigated using ESR spectroscopy. IMO's scavenge oxygen-, carbon- and sulfur-derived free radicals to give persistent nitroxides. Compared to the spin trap 5,5-dimethyl-pyrroline-1-oxide, a higher lifetime of hydroxyl radical adducts and a higher selectivity related to the trapping of carbon-centered radicals was found. A reaction between IMO's and superoxide was not observed. ESR parameters of 4-carboxyl-2,2-dimethyl-2H-imidazole-1-oxide (CIMO) spin adducts are highly sensitive to the structure of the trapped radical, e.g., different spectra were detected with radicals derived from Na2SO3 and NaHSO3. From the data obtained, a successful application of these new spin traps in biological systems can be expected.

Reactions of copper complexes with oxygen radicals generated by human neutrophils

The intensity of the chemiluminescence of unstimulated human neutrophils in the presence of luminol was used to investigate the effects of low-molecular-weight copper complexes at the cellular level. In different models (superoxide dismutase mimetic activity, inhibition of haematoporphyrin derivative/light-induced lysis of cells), the biological activity of the complexes exceeded the activity of the ligands alone.

Pheophorbide a-induced photo-oxidation of cytochrome c: implication for photodynamic therapy

Pheophorbide a-induced photo-oxidation, in vitro, of cytochrome c oxidase and cytochrome c results in irreversible modifications to both protein components. Photo-oxidation of cytochrome c, as exhibited by change in its heme oxidation state, displays exponential kinetics and is detected with a lag period. Both the photo-induced inactivation of the enzyme, and destruction of the substrate ability of cytochrome c occur as complex multi-process events. Under similar experimental conditions, the loss of the substrate capability of cytochrome c develops approximately three times faster than inactivation of the enzyme. The slight lag in the photo-oxidation of cytochrome c is due to pheophorbide a-induced superoxide production. However, the relative amount of photo-oxidant produced is considerably more effective than the cytochrome c reducing capacity of the superoxide. Neither hydroxyl radical nor hydrogen peroxide are involved in the photo-oxidation of the heme function. The possibilities of heme oxidation by a singlet oxygen mediated pathway or direct electron abstraction involving the heme or apoprotein are not excluded. It is proposed that a multi-site oxidation of numerous reduced energy cofactors within cells may augment collateral enzyme inactivation in maximizing photosensitizer-induced cytotoxicity. Accordingly, amphipathic photosensitizers, capable of accessing both lipid and aqueous compartments containing reduced cofactors, may be more effective agents for photodynamic therapy than those which exhibit a high specificity of subcellular localization.

Preclinical examination of first and second generation photosensitizers used in photodynamic therapy

Numerous photosensitizers with absorption peaks spanning the 600-800 nm "therapeutic window" have been and continue to be synthesized. Structural modifications of the dyes can then be made in order to improve tumor deliverability and retention. Chemical alterations can also enhance the yields of light generated reactive oxygen species. Utilization of lipoproteins, emulsions and antibody conjugates can enhance the selectivity of drug localization. Most cell types and subcellular structures are highly photosensitive and biochemical analysis indicates that cellular target sites associated with PDT correlate with photosensitizer location. In vivo data suggest that vascular and direct tumor cell damage as well as systemic and local immunological reactions are involved in PDT responsiveness. Additional mechanistic, synthetic and developmental studies are required in order to fully appreciate the potentials of PDT. However, continued enthusiasm and support for basic PDT research (as observed during the past 8 years) will depend to a large extent on the outcome of the current clinical trials.

Porphyrin sensitizers in tumour phototherapy. Novel sensitizers of the chlorin and bacteriochlorin class with amphiphilic properties

The requirements for activity in a tumour-photosensitizing drug are outlined. A series of metallo tetrasulphonatophthalocyanines are shown to be inactive in an in vivo assay of tumour photosensitization; however, some less water-soluble compounds (hydroxylated derivatives of octaethylchlorin and octaethylbacteriochlorin) are shown to possess promising activity.

Hydroxyl radical scavenging and antipsoriatic activity of benzoic acid derivatives

The hydroxyl radical scavenging and antipsoriatic activity of a number of lipophilic and hydrophilic benzoic acid derivatives was investigated. To quantify antioxidative effects, a newly introduced test system based on the diminution of the ESR signal of DMPO-OH (generated by Fenton's reagent) by the tested compounds was applied. It was found that the in vitro antioxidative (toward hydroxyl radical) activity of benzoic acid esters decreases with increasing chain length whereas the antipsoriatic activity increases. This effect is discussed in terms of a larger lipophilicity of long-chain esters. Propyl gallate was found to be the most active OH scavenger since it is some orders of magnitude more efficient than "model" antioxidants like alpha-tocopherol or mannitol. The highest antipsoriatic activity was exhibited by hydroxy benzoic acid decyl ester.