INDUCTION OF DIRECT AND INDIRECT SINGLE-STRAND BREAKS IN HUMAN CELL DNA BY FAR- AND NEAR-ULTRAVIOLET RADIATIONS: ACTION SPECTRUM AND MECHANISMS

Abstract— An action spectrum for the immediate induction in DNA of single-strand breaks (SSBs, frank breaks plus alkali-labile sites) in human P3 teratoma cells in culture by monochromatic 254-, 270-, 290-, 313-, 334-, 365-, and 405-nm radiation is described. The cells were held at +0.5C during irradiation and were Iysed immediately for alkaline sedimentation analysis following the irradiation treatments. Linear fluence responses were observed over the fluence ranges studied for all energies. Irradiation of the cells in a D₂O environment (compared with the normal H₂O environment) did not alter the rate of induction of SSBs by 290-nm radiation, whereas the D₂O environment enhanced the induction of SSBs by 365- and 405-nm irradiation. Analysis of the relative efficiencies for the induction of SSBs, corrected for quantum efficiency and cellular shielding, revealed a spectrum that coincided closely with nucleic acid absorption below 313 nm. At longer wavelengths, the plot of relative efficiency vs . wavelength contained a minor shoulder in the same wavelength region as that observed in a previously obtained action spectrum for stationary phase Bacillus subtilis cells. Far-UV radiation induced few breaks relative to pyrimidine dimers, whereas in the near-UV region of radiation, SSBs account for a significant proportion of the lesions relative to dimers, with a maximum number of SSBs per lethal event occurring at 365-nm radiation.     

PHOTOSENSITIZATION OF SINGLE-STRAND BREAKS IN pBR322 DNA BY ROSE BENGAL

Rose bengal photosensitized the formation of frank single-strand breaks (SSBs) in double-stranded, supercoiled pBR322 DNA as measured by neutral agarose electrophoresis. The yield of SSBs followed first order kinetics with respect to light fluence and dye concentration. The efficiency of cleavage was more than 20 times greater in an argon atmosphere than in an oxygen atmosphere. The quantum yield in an air atmosphere was 1.7 (+/- 0.3) X 10(-8). Sodium azide quenched the cleavage more efficiently in an oxygen atmosphere than when the oxygen concentration was reduced. Isopropanol and mannitol were poor quenchers; ribose-5-phosphate and guanosine-5'-monophosphate did not quench the cleavage. Substituting D2O for H2O increased the yield of SSBs in both oxygen and oxygen-depleted atmospheres. The results are consistent with initiation of cleavage by reaction of the triplet state of rose bengal (or a radical derived from it) with DNA. In the presence of oxygen, an additional mechanism is introduced.     

Camellia Seed Cake Extract Supports Hair Growth by Abrogating the Effect of Dihydrotestosterone in Cultured Human Dermal Papilla Cells

Autocrine and paracrine factors play key roles in the process of Androgenetic alopecia (AGA), which are secreted by balding dermal papilla cells (DPCs) after dihydrotestosterone (DHT) induction. Camellia seed cake is an oriental oil extraction byproduct, and its extract has been traditionally used to wash hair in China. This study elucidated the hair growth-promoting effects of Camellia seed cake extract (CSCE) in DHT-treated cultured DPCs and its underlying mechanisms. The effect of CSCE on cell viability and release of inflammatory factors IL-6 and IL-1α was performed on human dermal papilla cells (DPCs) incubated with DHT. Relative expression of bax, bcl-2, p53, androgen receptor (AR) and 5α- reductase type II (SRD5A2) was determined by PCR. Senescence-associated was examined by β-galactosidase (SA-β-Gal) assays. CSCE restored DHT-induced cell damage in a dose-dependent manner, and effectively reduced the production of IL-6 and IL-1α in DHT-treated DPCs. CSCE exhibited an anti-apoptotic effect, which increased the expression of bcl-2, and decreased the expressions of bax and p53 in DHT-incubated DPCs. CSCE also showed an anti-androgenic effect reversing the increase in AR and SRD5A2 expressions in DPCs driven by DHT incubation. In addition, CSCE inhibited the β-galactosidase enzyme activity and slowed down the cell senescence of DPCs which is crucial for AGA progression. In this study, we found that CSCE may have the potential to prevent and alleviate AGA by abrogating the effect of DHT in cultured DPCs.     

Low-energy electron-induced single strand breaks in 2'-deoxycytidine-3'-monophosphate using the local complex potential based time-dependent wave packet approach

Recent experimental and theoretical investigations on resonant electron scattering off DNA and DNA fragments using low-energy electrons (LEEs), to propose the mechanism for single strand breaks (SSBs) and double strand breaks (DSBs), have received considerable attention. It is our purpose here to understand theoretically the comprehensive route to SSB in a selected DNA fragment, namely, 2'-deoxycytidine-3'-monophosphate (3'-dCMPH), induced by LEE (0-3 eV) scattering using the local complex potential based time-dependent wave packet (LCP-TDWP) approach. To the best of our knowledge, there is no time-dependent quantum mechanical study that has been reported in the literature for this DNA fragment to date. Initial results obtained from our calculation in the gas phase provide a good agreement with experimental observation and show the plausibility of SSB at 0.75 eV, which is very close to the highest SSB yield reported from the experimental measurement (0.8 eV) on plasmid DNA in the condensed phase.     

Low energy electron induced damage to plasmid DNA pQE30

Low energy electrons (LEEs) are produced in copious amounts by the primary radiation used in radiation therapy. The damage caused to the DNA by these secondary electrons in the energy range 5-22 eV has been studied to understand their possible role in radiation induced damage. Electrons are irradiated on dried films of plasmid DNA (pQE30) and analysed using agarose gel electrophoresis. Single strand breaks (SSBs) induced by LEE to supercoiled plasmid DNA show resonance structures at 7, 12, and 15 eV for low doses and 6, 10, and ～18 eV at saturation doses. The present measurements have an overall agreement with the literature that LEEs resonantly induce SSBs in DNA. Resonant peaks in the SSBs induced by LEEs at 7, 12, and 15 eV with the lowest employed dose in the current study are somewhat different from those reported earlier by two groups. The observed differences are perhaps related to the irradiation dose, conditions and the nature of DNA employed, which is further elaborated.     

EFFECTS OF INTENSITY AND FLUENCE UPON DNA SINGLE-STRAND BREAKS INDUCED BY EXCIMER LASER RADIATION

Abstract— A Xenon-chloride excimer laser emitting energy at 308 nm was used to induce single-strand breaks (SSBs, frank breaks plus alkali-labile lesions as assayed by alkaline sucrose sedimentation techniques) in purified DNA from Bacillus subtilis . A fluence response study and a peak pulse intensity study were performed. At a pulse energy of 0.1 mJ/pulse, the radiation induced SSBs in a linear fashion (91 SSB/10⁸ Da per MJ/m²) to a maximum exprimental fluence of 1.28 MJ/m². The pulse intensity study showed that there were no significant changes in DNA breakage (105 SSB/10⁸ Da) between 2.93 times 10⁹ and 5.86 times 10¹¹ W/m² (0.11 and 22.0 mJ/pulse) at a constant total fluence of 1.1 MJ/m² (27000 mJ dose). This study has verified and extended previous work by quantifying the yield of SSBs induced in DNA by this laser radiation.     

OBSERVATIONS ON THE PHOTOSENSITIZED BREAKAGE OF DNA BY 2-THIOURACIL AND 334-nm ULTRAVIOLET RADIATION

Abstract— Breaks induced in purified DNA by 334-nm ultraviolet (UV) radiation are enhanced 30 times when 2-thiouracil (s²Ura) is present during aerobic irradiation. This enhancement by s²Ura is maximally effective at a concentration of about 1 m M. Anoxic irradiation reduces the s²Ura-enhanced breakage by 90%, indicating a Type II photosensitization. Benzoate, glycerol, diazabicyclo[2.2.2.]octane (DABCO) and histidine all inhibit formation of s²Ura photosensitized breaks, unlike diethylenetriaminepenta-acetic acid (DETAPAC) and catalase, which do not. The relationships between the concentration of DABCO. benzoate and histidine and their protection against induction of single strand breaks (SSBs) were similar, with little inhibition below 10 m M and maximal inhibition near 0.1 M for all compounds. Irradiation of the DNA-s²Ura mixture dissolved in D₂O instead of H₂O enhanced the rate of induction of SSBs in DNA by 334-nm light almost five times. Addition of superoxide dismutase (40, 80 and 200 μg/ml) decreased the rate of induction of breaks in DNA by 334-nm radiation plus s²Ura (in H₂O) by about 40%. Boiled superoxide dismutase had no effect.     

Triplet diphenylcarbenes protected by (trimethylsilyl)ethynyl groups

(2,4,6-Tribromophenyl)(4-tert-butyl-2,6-dimethylphenyl)diazomethane (1a) was shown to be stable enough to survive Sonogashira coupling reaction conditions at an elevated temperature and gave not only a para-monosubstituted product, (4-trimethylsilylethynyl-2,6-dibromophenyl)(4-tert-butyl-2,6-dimethylphenyl)diazomethane (1b), but also a disubstituted one, [2,4-bis(trimethylsilylethynyl)-6-bromophenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethane (1c), and a trisubstituted product, [2,4,6-tris(trimethylsilylethynyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethane (1d). Triplet diphenylcarbenes (DPCs) generated by photolysis of those ethynylated diphenyldiazomethanes were characterized by ESR and UV-vis spectroscopies at low temperature and laser flash photolysis techniques in solution at room temperature. Although ESR data indicated that ethynyl groups at the ortho positions are likely to stabilize triplet DPCs both sterically and electronically more effectively than o-bromine groups, kinetic studies suggested that the stability of triplet DPCs is not increased by o-ethynyl groups, as opposed to o-bromine groups. It is likely that triplet DPCs decay by interacting with the o-ethynyl groups.     

Quantitation of ultraviolet-induced single-strand breaks using oligonucleotide chip

A simple, accurate and robust methodology was established for the direct quantification of ultraviolet (UV)-induced single-strand break (SSB) using oligonucleotide chip. Oligonucleotide chips were fabricated by covalently anchoring the fluorescent-labeled ssDNAs onto silicon dioxide chip surfaces. Assuming that the possibility of more than one UV-induced SSB to be generated in a small oligonucleotide is extremely low, SSB formation was investigated quantifying the endpoint probe density by fluorescence measurement upon UV irradiation. The SSB yields obtained based on the highly sensitive laser-induced fluorometric determination of fluorophore-labeled oligonucleotides were found to coincide well with that predicted from a theoretical extrapolation of the results obtained for plasmid DNAs using conventional agarose gel electrophoresis. The developed method has the potential to serve as a high throughput, sample-thrifty, and time saving tool to realize more realistic, and direct quantification of radiation and chemical-induced strand breaks. It will be especially useful for determining the frequency of SSBs or lesions convertible to SSBs by specific cleaving reagents or enzymes.     

THE PHOTOLABILITY OF DNA CONTAINING 5-BROMOURACIL-I. SINGLE-STRAND BREAKS AND ALKALI-LABILE BONDS*

Abstract— The proportions of single-strand breaks and alkali-labile bonds produced by UV-light were investigated in covalently-closed circular 5-bromouracil (BrUra)-containing λ-phage DNA. When BrUra DNA was irradiated in 001 M Tris-0–001 M EDTA (pH 8-1) buffer, the D₀ was 11-7 J/m² for single-strand breaks, 2–25 J/m² for total breaks, and 2–8 J/m² for alkali-labile bonds. Thus, alkali-labile bonds were the predominant photochemical products. No double-strand breaks were observed after exposure to 7-7 times the D₀ for neutral breakage. The photolability measured under both neutral and alkaline conditions was affected by the NaCl concentration in the irradiation solvent, with the greatest resistance to breakage exhibited at the lowest concentrations. The composition of the irradiation buffer also affected sensitivity. Exposure in 1/10 SSC yielded 4-4 (neutral) and 5–7 (alkaline) times the breakage produced in Tris-EDTA.     

Structure determination of triplet diphenylcarbenes by in situ X-ray crystallographic analysis

Crystalline-state photoreactions of the following diphenyldiazomethanes were investigated by in situ X-ray crystallography, spectroscopy, and theoretical calculations: bis(2,4,6-trichlorophenyl)diazomethane (1-N2), bis(2,4,6-tribromophenyl)diazomethane (2-N2), bis(2,6-dibromo-4-methylphenyl)diazomethane (3-N2), bis(2,6-dibromo-4-tert-butylphenyl)diazomethane (4-N2), (2,4,6-tribromophenyl)-(2,6-dimethyl-4-tert-butylphenyl)diazomethane (5-N2), bis(4-bromophenyl)diazomethane(6-N2), and diazofluorene (7-N2). Crystal structures of photoinduced triplet diphenylcarbenes (DPCs) of 1, 2, and 4 were determined. We found remarkable differences between their structural information obtained in the crystalline state and that previously obtained spectroscopically in a glass matrix. Although the triplet DPCs of 1, 2, and 4 have significantly different stabilities in solution, only subtle differences in their structural parameters, except for their C(:)-Ar bond lengths, are observed. It is noteworthy that the average bond length of C(:)-Ar for 4 (1.374 A) is considerably shorter than those for (3)1 and (3)2 (1.430 and 1.428 A, respectively), provided that the two C(:)-Ar bonds being compared were chemically equivalent. The most likely explanations for the small and large differences in bond lengths in 1, 2, and 4 may be derived from the packing effect. The packing patterns of 1 and 2 are identical, but that of 4 is totally different from those of 1 and 2. Moreover, these results are interpreted as indicating that triplet DPCs undergo relaxation upon softening of the environments. Theoretical calculations indicate that the potential energy surface of triplet DPCs in terms of the carbene angle is extremely flat and changes in the angles have little effect on the energies. Triplet DPCs with a sterically congested carbene center are trapped in a structure dictated by the precursor structure in a rigid matrix, even if this is not the thermodynamically most stable geometry, but undergo geometrical relaxation upon softening the matrix to relieve steric compression. ESR studies indicate that the interplanar angles are more flexible than the bond angles.     

Lack of correlation between DNA strand breakage and p53 protein levels in human fibroblast strains exposed to ultraviolet lights

The contribution of DNA strand breaks accumulating in the course of nucleotide excision repair to upregulation of the p53 tumor suppressor protein was investigated in human dermal fibroblast strains after treatment with 254 nm ultraviolet (UV) light. For this purpose, fibroblast cultures were exposed to UV and incubated for 3 h in the presence or absence of l-beta-D-arabinofuranosylcytosine (araC) and/or hydroxyurea (HU), and then assayed for DNA strand breakage and p53 protein levels. As expected from previous studies, incubation of normal and ataxia telangiectasia (AT) fibroblasts with araC and HU after UV irradiation resulted in an accumulation of DNA strand breaks. Such araC/HU-accumulated strand breaks (reflecting nonligated repair-incision events) following UV irradiation were not detected in xeroderma pigmentosum (XP) fibroblast strains belonging to complementation groups A and G. Western blot analysis revealed that normal fibroblasts exhibited little upregulation of p53 (approximately 1.2-fold) when incubated without araC after 5 J/m2 irradiation, but showed significant (three-fold) upregulation of p53 when incubated with araC after irradiation. AraC is known to inhibit nucleotide excision repair at both the damage removal and repair resynthesis steps. Therefore, the potentiation of UV-induced upregulation of p53 evoked by araC in normal cells may be a consequence of either persistent bulky DNA lesions or persistent incision-associated DNA strand breaks. To distinguish between these two possibilities, we determined p53 induction in AT fibroblasts (which do not upregulate p53 in response to DNA strand breakage) and in XP fibroblasts (which do not exhibit incision-associated breaks after UV irradiation). The p53 response after treatment with 5 J/m2 UV and incubation with araC was similar in AT, XPA, XPG and normal fibroblasts. In addition, exposure of XPA and XPG fibroblasts to UV (5, 10 or 20 J/m2) followed by incubation without araC resulted in a strong upregulation of p53. We further demonstrated that HU, an inhibitor of replicative DNA synthesis (but not of nucleotide excision repair), had no significant impact on p53 protein levels in UV irradiated and unirradiated human fibroblasts. We conclude that upregulation of p53 at early times after exposure of diploid human fibroblasts to UV light is triggered by persistent bulky DNA lesions, and that incision-associated DNA strand breaks accumulating in the course of nucleotide excision repair and breaks arising as a result of inhibition of DNA replication contribute little (if anything) to upregulation of p53.     

Investigation of dissociative electron attachment to 2'-deoxycytidine-3'-monophosphate using DFT method and time dependent wave packet approach

Effect of electron correlation on single strand breaks (SSBs) induced by low energy electron (LEE) has been investigated in a fragment excised from a DNA, viz., 2'-deoxycytidine-3'-monophosphate [3'-dCMPH] molecule in gas phase at DFT-B3LYP/6-31+G(d) accuracy level and using local complex potential based time dependent wave packet (LCP-TDWP) approach. The results obtained, in conjunction with our earlier investigation, show the possibility of SSB at very low energy (0.15 eV) where the LEE transfers from π? to σ? resonance state which resembles a S(N)2 type mechanism. In addition, for the first time, an indication of quantum mechanical tunneling in strand breaking is seen from the highest anionic bound vibrational state (χ(5)), which may have a substantial role during DNA damage.     

DNA Damage Induced by Late Spring Sunlight in Antarctica

Sunlight ultraviolet (UV) radiation constitutes an important environmental genotoxic agent that organisms are exposed to, as it can damage DNA directly, generating pyrimidine dimers, and indirectly, generating oxidized bases and single-strand breaks (SSBs). These lesions can lead to mutations, triggering skin and eye disorders, including carcinogenesis and photoaging. Stratospheric ozone layer depletion, particularly in the Antarctic continent, predicts an uncertain scenario of UV incidence on the Earth in the next decades. This research evaluates the DNA damage caused by environmental exposure to late spring sunlight in the Antarctic Peninsula, where the ozone layer hole is more pronounced. These experiments were performed at the Brazilian Comandante Ferraz Antarctic Station, at King’s George Island, South Shetlands Islands. For comparison, tropical regions were also analyzed. Samples of plasmid DNA were exposed to sunlight. Cyclobutane pyrimidine dimers (CPDs), oxidized base damage and SSBs were detected using specific enzymes. In addition, an immunological approach was used to detect CPDs. The results reveal high levels of DNA damage induced by exposure under the Antarctic sunlight, inversely correlated with ozone layer thickness, confirming the high impact of ozone layer depletion on the DNA damaging action of sunlight in Antarctica.     

Interfacial challenges in all-solid-state lithium batteries

This mini-review provides a current opinion on interfacial challenges in solid-state batteries (SSBs). We focus our discussion on the effects of mechanical instability, chemical instability, electrochemical instability, and space charge layer formation on the interfacial resistance of SSBs. We highlight that the resulting interfacial resistance of SSBs arises from the coupling of all these effects. Quantifying the exact contribution of each effect to the overall interfacial resistance can be very difficult, if not impossible, because all these effects occur simultaneously during battery operation, but the dominant source, which varies with different electrode/electrolyte combinations, will need to be determined for future development of SSBs.     

Analysis of a photosensitive lesion induced by sunlamp UV greater than 315 nm exposure of 254-nm-irradiated human cells

Normal human skin fibroblasts were exposed to 0-10 J m-2 of 254 nm UV, incubated 0-16 h and then treated with 0-150 kJ m-2 of sunlamp UV greater than 315 nm. For each treatment, the cells were subjected to alkaline elution in order to measure the yield of single strand breaks (ssb) produced. It was found that treatment of 254-nm-irradiated cells with sunlamp UV greater than 315 nm resulted in the production of a higher level of ssb than that produced by separate exposures. Hence, lesions are produced by the 254 nm irradiation that are photolyzed through exposure to sunlamp UV greater than 315 nm. Approximately 50% of these lesions are removed following a 2-4 h incubation of the 254-nm-irradiated cells and nearly complete removal is achieved by 16 h. In addition, the profiles for elutions performed at pH 12.8 with cells exposed to the combined treatment were indicative of the presence of alkali labile sites. The repair kinetics of this lesion and alkaline lability of the photolysis product suggest that this photosensitive lesion may represent pyrimidine(6-4)pyrimidone photoproducts. Hence, this approach may represent a relatively simple and sensitive assay for the measurement of this DNA damage.     

PHOTOSENSITIZED INACTIVATION OF DNA BY MONOCHROMATIC 334-nm RADIATION IN THE PRESENCE OF 2-THIOURACIL: GENETIC ACTIVITY AND BACKBONE BREAKS

Abstract Monochromatic 334-nm radiation delivered under aerobic conditions inactivates the genetic activity (ability to transform auxotrophic recipient cells to nutritional prototrophy) of isolated transforming Bacillus subtilis DNA. The presence of superoxide dismutase (SOD), catalase, and mannitol reduces the 334-nm inactivation. The rate of inactivation of the genetic activity by 334-nm radiation is enhanced fivefold by the sensitizer 2-thiouracil (s²Ura). This enhancement is substantially reversed when the irradiations are performed in the presence of mannitol, and, to a lesser extent, SOD. Catalase slightly reduces the s²Ura enhancement of 334-nm inactivation of transforming activity. Backbone breaks induced in the same DNA by aerobic 334-nm radiation were also enhanced markedly by the presence of s²Ura; this enhancement was reversed by the presence of mannitol and, to a lesser extent, SOD during irradiation. Catalase had no effect upon s²Ura-enhanced, 334-nm-induced SSBs. Whereas DNA breakage may be responsible for a portion of the inactivation of the DNA by the photosensitized reaction between s²-Ura and 334-nm radiation, it is not the only inactivating lesion, because the yield of SSBs per lethal hit per unit length of DNA is not constant for all the irradiation conditions studied. The results support a complex role for active oxygen species in inactivation of transforming activity and DNA breakage by s²Ura-enhanced 334-nm radiation. They are also consistent with the formation of superoxide anion, hydroxyl radical, and possibly also singlet molecular oxygen, generated from ground-state molecular oxygen by reactive s²Ura in both Type I and II reactions.     

GENETIC TOXICOLOGY OF THE PHOTOSENSITIZATION OF CHINESE HAMSTER CELLS BY PHTHALOCYANINES

Chloroaluminum phthalocyanine (CAPC) was recently shown to photosensitize cell killing in culture and tumor destruction in vivo. Because this compound is potentially useful in the photodynamic therapy of cancer, its properties as a genotoxic agent were evaluated. Applying the technique of alkaline elution to study DNA integrity, it was found that CAPC could produce single-strand breaks in the DNA of Chinese hamster cells after exposure to white fluorescent light. At equicytotoxic doses, the number of DNA strand breaks produced by CAPC photosensitization was about three times lower than that induced by X-irradiation. During incubation in growth medium after exposure to CAPC-plus-fluorescent light, cells rejoined DNA strand breaks at a rate similar to that observed after X-irradiation. Resistance to 6-thioguanine (6-TG') or to ouabain (OUA') were used as end points of mutagenic potential. Following a treatment that caused -90% cell killing, there was a slight mutagenic effect, i.e. the frequencies were increased by -40% above the background or spontaneous mutations. However, this enhancement was not statistically significant. Taken together, the foregoing, plus an earlier observation that there is no variation in the sensitivity of cells to CAPC + light through the cell cycle, lead to the inferences that DNA damage does not play a major role in cell killing and that the mutagenic potential of this treatment is small.     

Influence of organic ions on DNA damage induced by 1 eV to 60 keV electrons

We report the results of a study on the influence of organic salts on the induction of single strand breaks (SSBs) and double strand breaks (DSBs) in DNA by electrons of 1 eV to 60 keV. Plasmid DNA films are prepared with two different concentrations of organic salts, by varying the amount of the TE buffer (Tris-HCl and EDTA) in the films with ratio of 1:1 and 6:1 Tris ions to DNA nucleotide. The films are bombarded with electrons of 1, 10, 100, and 60?000 eV under vacuum. The damage to the 3197 base-pair plasmid is analyzed ex vacuo by agarose gel electrophoresis. The highest yields are reached at 100 eV and the lowest ones at 60 keV. The ratios of SSB to DSB are surprisingly low at 10 eV (～4.3) at both salt concentrations, and comparable to the ratios measured with 100 eV electrons. At all characteristic electron energies, the yields of SSB and DSB are found to be higher for the DNA having the lowest salt concentration. However, the organic salts are more efficient at protecting DNA against the damage induced by 1 and 10 eV electrons. DNA damage and protection by organic ions are discussed in terms of mechanisms operative at each electron energy. It is suggested that these ions create additional electric fields within the groove of DNA, which modify the resonance parameter of 1 and 10 eV electrons, namely, by reducing the electron capture cross-section of basic DNA units and the lifetime of corresponding transient anions. An interstrand electron transfer mechanism is proposed to explain the low ratios for the yields of SSB to those of DSB produced by 10 eV electrons.     

INDUCTION OF DNA-PROTEIN CROSS-LINKS IN CHINESE HAMSTER CELLS BY THE PHOTODYNAMIC ACTION OF CHLOROALUMINUM PHTHALOCYANINE AND VISIBLE LIGHT

Abstract— Chloroaluminum phthalocyanine (CAPC) is an efficient photosensitizer for the inactivation of Chinese hamster V79 cells. In order to investigate possible molecular mechanisms in the photo-dynamic action of CAPC and visible light, the induction and repair rate of two classes of DNA lesions have been determined, i.e. DNA single-strand breaks and DNA-protein cross-links. In cells pretreated with 1 μ.M CAPC, a fluence of 12 kJ/m² of red light (>600 nm) kills approximately 50% of the cells and induces 3 to 3.5 Gy-equivalents of single-strand breaks. The repair of these breaks was slower than the repair of single-strand breaks induced by -irradiation. The photodynamic action of CAPC also induces a large number of DNA-protein cross-links which, in contrast to -radiation-induced DNA-protein cross-links, do not appear to be repaired during 4 h of post-treatment incubation in fresh medium. These studies suggest that DNA may be an important target for the cytotoxicity of CAPC + red light.