PHTHALOCYANINE AND NAPHTHALOCYANINE PHOTOSENSITIZED OXIDATION OF 2'-DEOXYGUANOSINE

Photophysics of the cationic 5,10,15,20-tetrakis (4-N-methylpyridyl) porphyrin bound to DNA, [poly (dA-dT)]2 and [poly (dG-dC)]2: interaction with molecular oxygen studied by porphyrin triplet-triplet absorption and singlet oxygen luminescence

Interaction between molecular oxygen and the cationic free-base 5,10,15,20-tetrakis (4-N-methylpyridyl) porphyrin (H2TMpyP4+) complexed with [poly (dA-dT)]2, [poly (dG-dC)]2 and calf thymus DNA, has been monitored in air-saturated heavy water solutions through porphyrin triplet-triplet absorption and singlet oxygen luminescence. Three different rate constants of porphyrin triplet state quenching have been found which correspond to different accessibilities of molecular oxygen to porphyrins embedded in the duplexes. The longest triplet state lifetime (30 microseconds), found for porphyrin bound to [poly (dG-dC)]2, corresponds to molecules well protected from oxygen. This supports the hypothesis of an intercalative binding mode of the porphyrin between GC base-pairs ('type A' sites). The fraction fT delta of the porphyrin triplet states quenched by molecular oxygen with singlet oxygen generation, is unity. In [poly (dA-dT)]2-porphyrin complexes, two sites ('type B' and 'C' sites of interaction) are involved, yielding very different triplet state lifetimes (5.5 microseconds and 20.5 microseconds) and efficiencies of singlet oxygen generation (fT delta = 0.50 and 0.82). The fT delta decreases can likely be explained in terms of competition between energy and electron transfer from the porphyrin excited triplet state to molecular oxygen. All three types (A, B and C) of interaction sites can be expected in porphyrin-DNA complexes.

Proposed mechanism for the photodynamic generation of 8-oxo-7,8-dihydro-2'-deoxyguanosine produced in cultured cells by exposure to lomefloxacin

In this study, lomefloxacin (LMX), a widely used quinolone antibiotic with a high frequency of clinical phototoxicity, was investigated by measuring the effects of several antioxidants on its ability to form of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) in cultured adult rat liver cells after exposure to UVA. In the current study the observed DNA damage, reflected by the formation of 8-oxo-dG, was almost completely inhibited by co-incubation of LMX and cultured cells with sodium azide (NaN3) that specifically quenches singlet oxygen. Vitamin E (alpha-tocopherol), known to quench both superoxide and singlet oxygen, inhibited 8-oxo-dG formation by approximately 54%. Mannitol, a hydroxyl radical scavenger, inhibited 8-oxo-dG formation by 64%. Butylated hydroxyanisole (BHA), a scavenger of hydroxyl, peroxy and alkoxy radicals, showed no inhibition of 8-oxo-dG formation but in fact enhanced levels of 8-oxo-dG by 169%. The results of this study suggest that the mechanism for the photodynamic generation of 8-oxo-dG by LMX is mediated, at least in part, by both singlet oxygen and hydroxyl radical and involves both type I and type II photosensitization.

Nuclear and non-nuclear targets of genotoxic agents in the induction of gene expression. Shared principles in yeast, rodents, man and plants

The interplay between environmental cues and the genetic response is decisive for the development, health and well-being of an organism. For some environmental factors a narrow margin separates beneficial and toxic impacts. With the increasing exposure to UV-B this dichotomy has reached public attention. This review will be concerned with the mechanisms that mediate a cellular genetic response to noxious agents. The toxic stimuli find access to the regulatory network inside cells by interacting at several points with cellular molecules - a process that converts the 'outside information' into 'cellular language'. As a consequence of such interactions, many adverse agents cause massive signal transduction and changes of gene expression. There is an interesting conservation of the mechanisms from yeast to man. An understanding of the genetic programs and of their phenotypic consequences is lagging behind.

Photochemical and photobiological studies of a furonaphthopyranone as a benzo-spaced psoralen analog in cell-free and cellular DNA

Photobiological activities of the benzo-spaced psoralen analog furonaphthopyranone 3 have been investigated in cell-free and cellular DNA. The molecular geometry parameters of 3 suggest that it should not form interstrand crosslinks with DNA. With cell-free DNA no evidence for crosslinking but also not for monoadduct formation was obtained; rather, the unnatural furocoumarin 3 induces oxidative DNA modifications under near-UVA irradiation. The enzymatic assay of the photosensitized damage in cell-free PM2 DNA revealed the significant formation of lesions sensitive to formamidopyrimidine DNA glycosylase (Fpg protein). In the photooxidation of calf thymus DNA by the furonaphthopyranone 3, 0.29 +/- 0.02% 8-oxo-7,8-dihydroguanine (8-oxoGua) was observed. With 2'-deoxyguanosine (dGuo), the guanidine-releasing photooxidation products oxazolone and oxoimidazolidine were formed predominately, while 8-oxodGuo and 4-HO-8-oxodGuo were obtained in minor amounts. The lack of a significant D2O effect in the photooxidation of DNA and dGuo reveals that singlet oxygen (type II process) plays a minor role; control experiments with tert-butanol and mannitol confirm the absence of hydroxyl radicals as oxidizing species. The furonaphthopyranone 3 (Ered = -1.93 +/- 0.03V) should act in its singlet-excited state as electron acceptor for the photooxidation of dGuo (delta GET ca -6 kcal/mol), which corroborates photoinduced electron transfer (type I) as a major DNA-oxidizing mechanism. A comet assay in Chinese hamster ovary (CHO) AS52 cells demonstrated that the psoralen analog 3 damages cellular DNA upon near-UVA irradiation; however, no photosensitized mutagenicity was observed in CHO AS52 cell cultures.

Peroxynitrite mediated oxidation of purine bases of nucleosides and isolated DNA

Reaction of nitric oxide with superoxide anion produces the highly reactive species peroxynitrite (ONOO-). This compound has been shown to be a strong oxidant of lipids and proteins. However, no data are available on its effect on DNA, with the exception of the induction of strand breaks. We report the result of studies on the reactions of peroxynitrite with the adenine and guanine moieties of nucleosides and isolated DNA. The samples were analyzed for 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dGuo),2,2-diamino-4-[(2-deoxy-beta-D-erythro-pentofuranosyl) amino]-5-(2H)-oxazolone (oxazolone) and 8-oxo-7,8-dihydro-2'-deoxyadenosine (8-oxo-dAdo). The effects of peroxynitrite treatment were compared with those of ionizing radiation in aerated aqueous solution, chosen as a source of hydroxyl radicals. At the nucleoside level, both oxidizing conditions led to the formation of oxazolone and 8-oxo-dAdo. In addition, evidence was provided for the formation of the 4R* and 4S* diastereoisomers of 4-hydroxy-8-oxo-4, 8-dihydro-2'-deoxyguanosine. The latter dGuo oxidation products were chosen as markers of the release of singlet oxygen (1O2) upon reaction of peroxynitrous acid with hydrogen peroxide. Oxidation of purine bases was then studied within isolated DNA. A significant increase in the level of 8-oxo-dGuo, oxazolone and 8-oxo-dAdo was observed within double stranded DNA upon exposure to gamma-radiation. Oxazolone and 8-oxo-dAdo were formed upon peroxynitrite treatment but no significant increase in the amount of 8-oxo-dGuo was detected. These results showed that peroxynitrite exhibits oxidizing properties toward purine moieties both in nucleosides and isolated DNA. However, the significant differences in the oxidative damage distribution within DNA observed after exposure to gamma radiation by comparison with peroxynitrite treatment questions the involvement of hydroxyl radicals as the main oxidizing species released by decomposition of peroxynitrous acid.

Supramolecular cationic tetraruthenated porphyrin induces single-strand breaks and 8-oxo-7,8-dihydro-2'-deoxyguanosine formation in DNA in the presence of light

The aim of this investigation is the evaluation of DNA interaction of with tetraruthenated porphyrin (TRP) and of DNA damage in the presence of light. Direct-fluorescence and electronic absorption measurements after incubation of DNA with TRP indicate strong binding between pBR322 DNA or calf thymus DNA with the modified porphyrin. Exposure of pBR322 DNA to TRP (up to 3 microM) and light leads to single-strand break formation as determined by the conversion of the supercoiled form (form I) of the plasmid into the nicked circular form (form II). Oxidative DNA base damage was evaluated by the detection of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) after irradiation of calf thymus DNA in the presence of the TRP. The data demonstrated a dose and time dependence with each type of DNA damage. These data indicate (1) a specificity of the binding mode and (2) type I and II photoinduced mechanisms leading to strand scission activity and 8-oxodGuo formation. Accordingly, singlet molecular oxygen formation, after TRP excitation, was confirmed by near-infrared emission. From these investigations a potential application of TRP in photodynamic therapy is proposed.

Competitive reactions of peroxynitrite with 2'-deoxyguanosine and 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG): relevance to the formation of 8-oxodG in DNA exposed to peroxynitrite

We have examined the formation of 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG) in reactions of peroxynitrite with 2'-deoxyguanosine (dG) and calf-thymus DNA. Peroxynitrite reacts with dG at neutral pH, but this reaction does not result in the buildup of 8-oxodG. We also do not find any evidence for the formation of 8-oxodG in calf-thymus DNA upon exposure to peroxynitrite. When 8-oxodG is mixed with 1000-fold excess dG and then allowed to react with peroxynitrite, about 50% of the 8-oxodG is destroyed. The preferential reaction of 8-oxodG is also evident when dG in calf-thymus DNA is partially oxidized in an Udenfriend system and then allowed to react with peroxynitrite. We suggest that 8-oxodG is not produced in peroxynitrite-mediated oxidations of dG and DNA or that it is produced but then is rapidly consumed in further reactions with peroxynitrite. Oxidized DNA bases frequently can be more oxidation sensitive than their corresponding progenitors and, therefore, may be present at] low steady-state concentrations and not represent stable markers of oxidative stress status. The importance of the 8-oxodG/peroxynitrite reaction is discussed in relation to the formation of more stable, secondary oxidation products that might be more useful markers of DNA damage.

Formation of 7,8-dihydro-8-oxoguanine in the 1,2-dioxetane-induced oxidation of calf thymus DNA: evidence for photosensitized DNA damage by thermally generated triplet ketones in the dark

Isolated calf thymus DNA was treated with the 1,2-dioxetanes 3-acetoxymethyl-3,4,4-tri-methyl-1,2-dioxetane, 2,3-dimethylbenzofuran dioxetane, 3-hydroxymethyl-3,4,4-trimethyl-1,2-dioxetane (HTMD), 3,3,4,4-tetramethyl-1,2-dioxetane and 3,4,4-trimethyl-1,2-dioxetane (TrMD), which on thermal decomposition generate triplet-excited carbonyl products. To monitor quantitatively the formation of the mutagenic oxidation product 7,8-dihydro-8-oxoguanine (8-oxoGua), a sensitive and selective HPLC electrochemical assay was used after acidic hydrolysis (HF/pyridine) of the dioxetane-treated DNA. High yields of 8-oxoGua (up to ca 4% of the available guanine) were obtained for HTMD and TrMD. Both were investigated in detail with respect to effects of concentration, time and temperature. The oxidative reactivity of 1,2-dioxetanes was compared with several type I (benzophenone and riboflavin) and type II (methylene blue and rose bengal) photooxidants and disodium 1,4-etheno-2,3-benzodioxin-1,4-dipropionate as a chemical source of singlet oxygen. The persistence of 8-oxoGua towards oxidation by HTMD was examined in the reaction with 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodGuo) and with oxidized DNA. It was shown that, indeed, 8-oxoGua is consumed in the oxidized DNA on prolonged exposure to an excess of HTMD. The reaction of 8-oxodGuo with HTMD afforded the two 4R* and 4S* diastereomers of 9-(2-deoxy-beta-D-erythropentofuranosyl)-4, 8-dihydro-4-hydroxy-8-oxoguanine as main oxidation products. Trapping experiments with tert-butanol confirmed that hydroxyl radicals are not involved, whereas the use of the triplet quenchers sodium 9,10-dibromo-anthracene-2-sulfonate and 2,3-diazabicyclo[2.2.1]hept-2-ene established that triplet-excited states are mainly responsible for the observed DNA oxidation through type I action (electron transfer chemistry). The role of singlet oxygen was tested by means of deuterium isotope effects in D2O versus H2O, but no definitive conclusion could be reached in regard to the involvement of 1O2 in these oxidations.(ABSTRACT TRUNCATED AT 250 WORDS)

Methylene blue-mediated photooxidation of 7,8-dihydro-8-oxo-2'-deoxyguanosine

One well known product of the methylene blue-mediated photosensitization of 2'-deoxyguanosine (dG) in oxygen saturated aqueous solution is 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG). We observed that the rate of 8-oxodG photodecomposition by methylene blue-mediated photosensitization is approx. 3-times faster than for dG. The primary products of the methylene blue-mediated photosensitization of 8-oxodG are 2-amino-5-((2-deoxy-beta-D-erythro-pentofuranosyl)amino)-4H-imidazol-4-o ne (dIz), 2,2-diamino-4-((2-deoxy-beta-D-erythro-pentofuranosyl)amino)-5(2H)-oxazo lone (dZ), the 4R* and 4S* diastereoisomers of 4,8-dihydro-4-hydroxy-8-oxo-2'-deoxyguanosine (dO), and an as yet unidentified product with a molecular weight of 287 (dX). Except for the latter product, these compounds have all been identified following the methylene blue-mediated photooxidation of dG. Methylene blue-mediated photooxidation of 8-oxodG in D2O instead of H2O leads to a 4-fold increase in the rate of 8-oxodG photodecomposition while the addition of sodium azide retards the reaction, observations which imply that the reaction occurs via a type II (singlet oxygen mediated) mechanism. Like 8-oxodG, dIz and dZ are sensitive to hot piperidine and likely contribute to strand breaks observed in double stranded DNA exposed to methylene blue plus light followed by hot piperidine. Because 8-oxodG generates predominately G-->T transversions, the photooxidation of 8-oxodG to dIz, dO, and dX may explain the predominance of G-->C transversions in single-stranded M13mp2 bacteriophage DNA exposed to methylene blue plus light and then transfected into SOS-induced Escherichia coli.

Role of oxygen in the phototoxicity of phthalocyanines

The presence of molecular oxygen is a determinant in the phototoxicity of phthalocyanines, and photosensitized oxidation is the accepted chemical mechanism for photo-dynamic action. However, it is difficult to establish whether the process is initiated by a type I electron transfer, or by a type II energy transfer reaction to form singlet oxygen. Usually, the involvement of singlet oxygen in photodamage has been indicated by the inhibition of the biological effect by a competitive physical or chemical singlet oxygen quencher, or by a rate increase in D2O, in which singlet oxygen has a longer lifetime than in H2O. Unfortunately, these techniques are not completely specific for singlet oxygen. Moreover, thermodynamic considerations suggest that photoinduced electron abstraction from appropriate biomaterials could compete with singlet oxygen production under in vivo conditions. This likely source of one electron-oxidized primary radicals, which can provide the precursors of the oxidative damage in phthalocyanine photosensitization, suggests the possibility of modulated toxicity by interaction with chemical additives. Examples of such additives recently studied are ascorbate, tocopherol and quercetin, all of which are natural antioxidants.

Benzophenone photosensitization of 2'-deoxyguanosine: characterization of the 2R and 2S diastereoisomers of 1-(2-deoxy-beta-D-erythro-pentofuranosyl)- 2-methoxy-4,5-imidazolidinedione. A model system for the investigation of photosensitized formation of DNA-protein crosslinks

Benzophenone-mediated photosensitization of 2'-deoxyguanosine and its 3',5'-di-O-acetyl derivative, used as DNA model compounds, in oxygen-saturated water-methanol (1:1) solution results in the nucleophilic addition of methanol to the guanine base. The resulting modified nucleosides have been isolated by reverse-phase high-performance liquid chromatography and characterized by extensive spectroscopic measurements including 13C and 1H nuclear magnetic resonance, fast atom bombardment mass spectrometry and circular dichroism as the 2R and 2S diastereoisomers of 1-(2-deoxy-beta-D-erythro-pentofuranosyl)-2-methoxy-4,5-imidazolinedione and their related 3',5'-di-O-acetyl derivates. Information concerning the absolute configuration of the two pairs of diastereoisomers was inferred from detailed nuclear Overhauser effect experiments. A reaction mechanism, involving guanine radical intermediates, is proposed to explain the generation of these new guanine photoproducts.

Photosensitized DNA damage in human cells is localized in chromatin sensitive to DNAse I digestion

Human NHIK 3025 cells were sensitized to light by incubation with Photofrin II (PII) and aluminum phthalocyanine tetrasulphonate (AlPCS4). Light exposure of sensitized cells at 1 degree C induced DNA strand breaks causing unwinding of DNA in alkali. Subsequent to light exposure, cells were treated with deoxyribonuclease I (DNAse I) which produces DNA strand breaks in active chromatin. The combination of photo-damage, killing > 95% of the cells, and treatment with DNAse I caused the same degree of DNA unwinding as DNAse I treatment alone. This was not the case for X-irradiation. Thus, the photosensitized DNA strand breaks were selectively induced in or close to DNAse I-sensitive sites.

Photochemical interactions of methylene blue and analogues with DNA and other biological substrates

The light-induced reactions of methylene blue and related phenothiazinium dyes with biological substrates are described. The properties of the excited states of the dyes, their reactions with nucleic acids and their photosensitised chemical modifications of nucleic acid bases are examined. Reports on phenothiazinium dye-induced damage to proteins, lipids, biological membranes, organelles, viruses, bacteria, mammalian cells and carcinomas are reviewed.

Upper excited state photochemistry of DNA

The quantum yields for cyclobutylpyrimidine dimers, alkali-labile sites, and frank strand breaks in double-stranded DNA have been measured using low-intensity radiation at 199.8, 217.8, and 239.5 nm from a Raman-shifted frequency quadrupled Nd:YAG laser. The quantum yield for cyclobutylpyrimidine dimers was also measured using 254 nm radiation from a low-pressure mercury lamp. The quantum yield for cyclobutylpyrimidine dimers is constant within a factor of two between 254 and 199.8 nm except for 239.5 nm, indicating that upper excited singlet states of bases convert efficiently to the lowest singlet state. The quantum yields for alkali-labile sites and frank strand breaks both increase as the wavelength decreases but follow different patterns. These results indicate that alkali-labile sites from a higher excited state of the base, whereas frank strand breaks form by excitation of the sugar-phosphate backbone.

Isolation and characterization of a new product produced by ionizing irradiation and type I photosensitization of 2'-deoxyguanosine in oxygen-saturated aqueous solution: (2S)-2,5'-ANHYDRO-1-(2'-deoxy-beta-D-erythro-pentofuranosyl)-5-guanidin ylidene- 2-hydroxy-4-oxoimidazolidine

A major product of the radiation-induced decomposition of 2'-deoxyguanosine in oxygen-saturated aqueous solution has been isolated by reverse phase high performance liquid chromatography and characterized by carbon and proton NMR spectroscopy, fast-atom bombardment mass spectrometry, and chemical analysis as (2S)-2,5'-anhydro-1-(2'-deoxy-beta-D-erythro-pentofuranosyl)-5-guanid inylidene- 2-hydroxy-4-oxoimidazolidine(d < G). This compound is stable in aqueous solution at room temperature but decomposes upon heating (45 degrees C). The lesion is also observed following type I (riboflavin, benzophenone, and acetophenone) photosensitized irradiation of 2'-deoxyguanosine at 350 nm in oxygen-saturated aqueous solution. A similar reaction mechanism, involving a neutral guanine radical intermediate, is proposed to explain the generation of d < G following both types of irradiation.

Photobiological activities of 1,6-dioxapyrene in pro- and eukaryotic cells

The photobiological effect of a new pyrene derivative, 1,6-dioxapyrene (1,6-DP), was studied in Salmonella typhimurium (strain TA100) and in the diploid strain D7 of the yeast Saccharomyces cerevisiae. In Salmonella, 1,6-DP shows little mutagenicity in the dark in comparison to benzo[a]pyrene (B[a]P). This mutagenic activity decreases in the presence of liver S9 homogenates from Aroclor induced XVIInc/Z mice. However, in combination with 365 nm (UVA) radiation and in the absence of S9 mix, 1,6-DP behaves as an effective photodynamic compound inducing lethal and mutagenic effects in both organisms. In yeast, its activity, like that of B[a]P, is highly dependent on the presence of oxygen. For the same incident dose of UVA, 1,6-DP is, however, at least 6 times more effective than B[a]P in inducing cytotoxic and mutagenic effects. At equitoxic doses, 1,6-DP is as photomutagenic as B[a]P, suggesting that in both cases mutagenicity is due to similar mechanisms. Spectrophotometric measurements indicate physical interaction of 1,6-DP with DNA in the dark. Laser flash photolysis experiments show that 1,6-DP generates singlet oxygen with a quantum yield of 0.17. In vitro 1,6-DP produces oxidative damage to guanine bases specific for singlet oxygen mediated reactions. Alkaline step elution analysis of 1,6-DP plus UVA treated yeast cells indicates a decrease in average molecular weights in DNA and an induction of single strand breaks (ssb) originating from alkali labile sites. This effect is enhanced by D2O and is thus likely to be due to the production of singlet oxygen. The strand breaks appear to differ from those induced by gamma-rays because little, if any, repair of these ssb occurs during 30 min of post-treatment incubation in complete growth medium. These results suggest that the photobiological effects of 1,6-DP are due to oxidative damage in DNA mostly induced by singlet oxygen.

Chemical and biochemical postlabeling methods for singling out specific oxidative DNA lesions

A survey of the main available chemical and biochemical postlabeling assays for measuring oxidative DNA damage is reported. Two main approaches, radio and fluorescent postlabeling, have been used in order to reach a high level of sensitivity of detection. This is required for the measurement of DNA damage within cells and tissues upon exposure to agents of oxidative stress. Most of the methods are based on liquid chromatographic separation of defined DNA modifications following either acidic hydrolysis or enzymic digestion of DNA. In a subsequent step, the isolated base or sugar damages are either radiolabeled or made fluorescent by chemical or enzymatic reactions. Emphasis is placed on the recently developed high performance liquid chromatographic 32P-postlabeling assay, which allows the specific and sensitive measurement of various base damages including adenine N-1 oxide and 5-hydroxymethyluracil at the level of one modification per 10(7) normal bases in a sample size of 1 microgram of DNA. Examples of application of radioactive postlabeling to the measurement of DNA base damage following exposure of human cells to oxidizing agents including hydrogen peroxide and UVA radiation are provided.