Induction of DNA strand breaks and DNA-protein cross-links in normal human skin fibroblasts following exposure to 254 nm UV radiation

Levels of DNA strand breaks and DNA-protein cross-links (DPCs) were measured using the alkaline elution assay in normal human skin fibroblasts irradiated with 0-200 J m-2 of 254 nm UV radiation and incubated for 0-24 h. On incubation, the yields of both single-strand breaks (SSBs) and DPCs increased with similar kinetics and remained elevated. In addition, when SSBs were measured under conditions in which DPCs were not eliminated by treatment with proteinase K, a measurable yield of SSBs could not be detected. Hence, the SSBs that form in the UV-irradiated cells following incubation appear to be associated with the DPCs.     

DNA polymerase I-mediated repair of 365 nm-induced single-strand breaks in the DNA of Escherichia coli.

Abstract. Irradiation of closed circular phage Λ DNA in vivo at 365 nm results in the induction of single-strand breaks and alkali-labile lesions at rates of 1.1 × 10^-14, and 0.2 × 10^-14/dalton/J/m², respectively. The sum of the induction rates is similar to the rate of induction of single-strand breaks plus alkali-labile lesions (1 × 10^-14/dalton/J/m²) observed in the E. coli genome. Postirradiation incubation of wild-type cells in buffer results in rapid repair of the breaks (up to 80% repaired in 10 min). No repair was observed in a DNA polymerase I-deficient mutant of E. coli.     

KINETIC ANALYSIS OF THE PHOTOREACTION (365 nm) BETWEEN PSORALEN AND DNA

Abstract— Psoralen complexed with DNA under irradiation at 365 nm forms monofunctional and bifunctional adducts with pyrimidine bases of DNA. while the fraction of psoralen which remains free in the aqueous solution forms photodimer and photooxidation products. We havc studied the photoreaction betwecn psoralen and DNA from a kinetic point of view. Only some products of this photoreaction could be measured quantitatively. Thus. in order to present a picture of the overall photoreaction. we have represented this by a mathematical model consisting of a system of seven differential equations. Utilizing all the experimental data that it was possible to obtain. we were able to estimate rate constants for formation of all photoproducts. From the general picture thus obtained. we conclude that monofunctional adducts are formed in a four to one ratio to bifunctional adducts. Among the monofunctional photoproducts. 3,4-cycloadducts occur with a higher yield than 4',5'-cycloadducts. The combined rate constant of formation of photodimer and photooxidation products of psoralen is small. indicating that they are marginal by-products relative to the cycloadducts formed in the photoreactions with DNA.     

Protection of plasmid pBR322 DNA by flavonoids against single-strand breaks induced by singlet molecular oxygen

Flavonoids, the dominant colouring pigments of plants, as well as the related polyphenol tannic acid significantly inhibit single-strand breaks in plasmid pBR322 DNA induced by singlet molecular oxygen (¹O₂). This reactive species of oxygen was generated in an aqueous buffer system by the thermal dissociation of the endoperoxide of 3,3′-(1,4-naphthylene)dipropionate. Among the antioxidants examined, myricetin showed the highest protective ability, followed by tannic acid, (+) catechin, rutin, fisetin, luteolin and apigenin, when the inhibitory abilities were compared at 90 min after incubation. The protective abilities of these compounds were both time and concentration dependent. At equimolar concentrations (100 μM) the antioxidant effect of myricetin was better than that of other known antioxidants such as lipoate, -tocopherol and β-carotene. Data, when analysed in relation to the structures of various compounds, showed a rough correlation with protective abilities. Owing to the abundance of these compounds in our normal diet, they may play significant roles in preventing oxidative damage resulting from potentially deleterious ¹O₂.     

Induction and repair of cyclobuta-dithymidine photoproducts in hamster skin by solar-simulated UV radiation

The induction and repair of cyclobuta-dithymidine (T less than greater than T) photoproducts in hamster skin was measured following exposure to artificial solar radiation. DNA extracted from irradiated hamster epidermis was analyzed by enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody specific for T less than greater than T photoproducts. Between 49% and 58% of T less than greater than T antibody binding sites were eliminated from DNA by 24 h after radiation exposures comparable with approximately 60 - 120 min of midday sunlight, whereas all detectable T less than greater than T binding sites were eliminated by 24 h after a radiation exposure comparable with 30 min of sunlight.     

Non-coherent visible and infrared radiation increase survival to UV (254 nm) in Escherichia coli K12

Interactions between visible or infrared (IR) and ultraviolet (UV, 254 nm) radiation have been studied in E. coli. Pre-illumination with non-coherent monochromatic 446, 466, 570 and 685 nm radiation, as well as with polychromatic red and IR radiation at room temperature, leads to increased cell survival after a subsequent irradiation with UV light. In the thermic range of the spectrum (red and IR), IR but not red light pre-treatment is able to increase cell survival to a subsequent lethal heat (51 degrees C) challenge, suggesting that increased UV survival may be due to IR-induced heat-shock response. On the other hand, visible-light-induced resistance may be due to a different mechanism, possibly involved with unknown bacterial light receptors.     

Energy deposition mechanisms and biochemical aspects of DNA strand breaks by ionizing radiation

A theoretical model based on physical, chemical, and biochemical mechanisms has been presented to evaluate the yields of DNA strand breaks (single and double) as a function of linear energy transfer (LET ) or ?dE/dx. Energetic heavy charged particles are considered explicitly to provide a general theory for low- as well as for high-LET radiation. There are three main features of the calculation: (a) track structure considerations for the energy deposition pattern, (b) three-dimensional structure of DNA molecules to provide information on the exact location of damage, and (c) a Monte-Carlo scheme to simulate the diffusion processes of water radicals. To avoid the complexities of a cellular medium, an aqueous solution of DNA is considered in the calculation. When the results of the calculations are compared with experimental measurements of the yields of strand breaks in mammalian DNA (exposed in a cellular complex), reasonable agreement is obtained. However, only those experimental data have been compared where there were no enzyme repair processes. The method of calculation has also been extended to study breaks in higher-order structures of DNA molecules such as chromatin. Specific limitations of the present model have been pointed out for making further improvements.     

Degradation of a model naphthenic acid, cyclohexanoic acid, by vacuum UV (172 nm) and UV (254 nm)/H2O2

The mechanism of hydroxyl radical initiated degradation of a typical oil sands process water (OSPW) alicyclic carboxylic acid was studied using cyclohexanoic acid (CHA) as a model compound. By use of vacuum ultraviolet irradiation (VUV, 172 nm) and ultraviolet irradiation in the presence of hydrogen peroxide UV(254 nm)/H(2)O(2), it was established that CHA undergoes degradation through a peroxyl radical. In both processes the decay of the peroxyl radical leads predominantly to the formation of 4-oxo-CHA, and minor amounts of hydroxy-CHA (detected only in UV/H(2)O(2)). In UV/H(2)O(2), additional 4-oxo-CHA may also have been formed by direct reaction of the oxyl radical with H(2)O(2). The oxyl radical can be formed during decay of the peroxyl-CHA radical or reaction of hydroxy-CHA with hydroxyl radical. Oxo- and hydroxy-CHA further degraded to various dihydroxy-CHAs. Scission of the cyclohexane ring was also observed, on the basis of the observation of acyclic byproducts including heptadioic acid and various short-chain carboxylic acids. Overall, the hydroxyl radical induced degradation of CHA proceeded through several steps, involving more than one hydroxyl radical reaction, thus efficiency of the UV/H(2)O(2) reaction will depend on the rate of generation of hydroxyl radical throughout the process. In real applications to OSPW, concentrations of H(2)O(2) will need to be carefully optimized and the environmental fate and effects of the various degradation products of naphthenic acids considered.     

The dose-response relationship for tumourigenesis by UV radiation in the region 311-312 nm

Groups of hairless mice were irradiated daily with Philips TL01 UVB sources. This type of lamp has become available recently and was developed for UVB phototherapy of psoriasis. The TL01 emits radiation in a narrow band around 311-312 nm. Tumours developed on all animals. The dose-response relationship had practically the same shape as that found in a similar experiment with Westinghouse FS40 sunlamps; the tumour induction time appeared to be proportional to the daily dose to a power of -0.58. An additional experiment was performed with a TL01 from which the shorter wavelengths were filtered away. This reduced the carcinogenic effectiveness by a factor of 2.3. The potential of the filtered lamp for phototherapy of psoriasis is discussed.     

Angular distribution of photo electrons from above-threshold-ionization (ATI) of xenon by 532 nm, 355 nm and 266 nm radiation

A time-of-flight electron energy spectrometer has been used to measure the angular distributions of photoelectrons emitted after the absorption of up to four excess photons above the ionization threshold of Xenon at 532 nm. For shorter wavelength less efficient ATI is observed. The shape of the angular distributions and the branching ratios for the two ionic fine structure states Xe⁺(² P _3/2) and Xe⁺(² P _1/2) may be plausibly attributed to the influence of excited states of the atom.     

Hydroxyl radical quenching agents protect against DNA breakage caused by both 365-nm UVA and by gamma radiation

The ability of hydroxyl radical (.OH) scavengers to reduce DNA breakage in isolated DNA from Bacillus subtilis by either gamma radiation or monochromatic radiation in the UVA region (365 nm) was examined by comparing dose reduction factors (the ratio of dose required to induce n DNA breaks in the absence to the presence of quencher). Previous data have demonstrated that acetate, formate, azide, and mannitol protect supercoiled DNA against gamma-radiation-induced ssb (single-strand breaks-relaxation of supercoil by first nick) in close agreement with the rate at which their solutions quench .OH. Here we show that these quenchers also protect against 365-nm-induced ssb. The ratios for protection against 365-nm induced DNA ssb in isolated B. subtilis DNA by the four quenchers are also in proportion to their ability to quench .OH. In view of the diverse chemical nature of the quenchers and the wide range of concentrations involved, these findings are evidence that both these radiations may induce ssb in DNA via a common step that might involve .OH.     

Photodissociation dynamics of CH(2)Br(2) near 234 and 267 nm

The photodissociation dynamics of CH(2)Br(2) was investigated near 234 and 267 nm. A two-dimensional photofragment ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton (REMPI) ionization scheme was utilized to obtain the angular and translational energy distributions of the nascent Br ((2)P(3/2)) and Br* ((2)P(1/2)) atoms. The obtained translational energy distributions of Br and Br* are found consist of two components which should be come from the radical channel and secondary dissociation process, respectively. It is suggested that the symmetry reduction from C(2v) to C(s) during photodissociation invokes a non-adiabatic coupling between the 2B(1) and A(1) states. Consequently, the higher internal energy distribution of Br channel than Br* formation channel and the broader translational energy distribution of the former are presumed correlate with a variety of vibrational excitation disposal at the crossing point resulting from the larger non-adiabatic crossing from 2B(1) to A(1) state than the reverse crossing. Moreover, the measured anisotropy parameter beta indicate that fragments recoil along the Br-Br direction mostly in the photodissociation.     

Comparison of the erythemogenic effectiveness of ultraviolet-B (290-320 nm) and ultraviolet-A (320-400 nm) radiation by skin reflectance

–Based on optical properties of the skin, its constituent layers and the blood, and a previously experimentally verified quantitative model for the optical transfer properties in skin, noninvasive determinations of the amount of cutaneous blood in the superficial plexus are made from skin reflectance. This method is used to determine UVA and UVB fluences that cause cutaneous blood in the superficial plexus to increase to 1.5, 2 and 2.5 times the pre-radiation volume at various times after irradiation. Our results show that on an equal fluence basis, UVB is two to three thousand times as effective as UVA in inducing erythema, whether erythema is evaluated 8, 24 or 72 h after irradiation. This agrees with the result obtained in terms of minimal erythema dose by visual inspection.     

Detection of atomic hydrogen in flames by resonance four-photon ionization at 365 nm

The first observations of the four-photon ionization of H atoms at 364.7 nm via a three-photon resonance on the Lyman alpha transition are reported. The resonance ionization detection of H and D atoms in an H₂/D₂/O₂/Ar flame is demonstrated and H-atom density profiles are measured with good spatial resolution. A broadening of 2.2 cm⁻¹ /GW cm² of the four-photon resonance by ac Stark shifting is observed.     

Induction of strand breaks in DNA films by low energy electrons and soft X-ray under nitrous oxide atmosphere

Five-monolayer (5 ML) plasmid DNA films deposited on glass and tantalum substrates were exposed to Al K_α X-rays of 1.5 keV under gaseous nitrous oxide (N₂O) at atmospheric pressure and temperature. Whereas the damage yields for DNA deposited on glass are due to soft X-rays, those arising from DNA on tantalum are due to both the interaction of low energy photoelectrons from the metal and X-rays. Then, the differences in the yields of damage on glass and tantalum substrates, essentially arises from interaction of essentially low-energy electrons (LEEs) with DNA molecules and the surrounding atmosphere. The G-values (i.e., the number of moles of product per Joule of energy absorbed) for DNA strand breaks induced by LEEs (G_LEE) and the lower limit of G-values for soft X-ray photons (G_XL) were calculated and the results compared to those from previous studies under atmospheric conditions and other ambient gases, such as N₂ and O₂. Under N₂O, the G-values for loss of supercoiled DNA are 103±15 nmol/J for X-rays, and 737±110 nmol/J for LEEs. Compared to corresponding values in an O₂ atmosphere, the effectiveness of X-rays to damage DNA in N₂O is less, but the G value for LEEs in N₂O is more than twice the corresponding value for an oxygenated environment. This result indicates a higher effectiveness for LEEs relative to N₂ and O₂ environments in causing SSB and DSB in an N₂O environment. Thus, the previously observed radiosensitization of cells by N₂O may not be only due to OH radicals but also to the reaction of LEE with N₂O molecules near DNA. The previous experiments with N₂ and O₂ and the present one demonstrate the possibility to investigate damage induced by LEEs to biomolecules under various types of surrounding atmospheres.