Hydroxyl radical-induced strand break formation of poly(U) in the presence of oxygen: comparison of the rates as determined by conductivity, e.s.r. and rapid-mix experiments with a thiol

Chemoprevention against arsenic-induced mutagenic DNA breakage and apoptotic liver damage in rat via antioxidant and SOD1 upregulation by green tea (Camellia sinensis) which recovers broken DNA resulted from arsenic-H2O2 related in vitro oxidant stress

Green tea (Camellia sinensis; CS) strongly reverses/prevents arsenic-induced apoptotic hepatic degeneration/micronecrosis and mutagenic DNA damage in in vitro oxidant stress model and in rat as shown by comet assay and histoarchitecture (HE and PAS staining) results. Earlier, we demonstrated a link between carcinogenesis and impaired antioxidant system-associated mutagenic DNA damage in arsenic-exposed human. In this study, arsenic-induced (0.6 ppm/100 g body weight/day for 28 days) impairment of cytosolic superoxide-dismutase (SOD1), catalase, xanthine-oxidase, thiol, and urate activities/levels led to increase in tissue levels of damaging malondialdehyde, conjugated dienes, serum necrotic-marker lactate-dehydrogenase, and metabolic inflammatory-marker c-reactive protein suggesting dysregulation at the transcriptional/signal-transduction level. These are decisively restrained by CS-extract (≥10 mg/ml aqueous) with a restoration of DNA/tissue structure. The structural/functional impairment of dialyzed and centrifugally concentrated (6-8 kd cutoff) hepatic SOD1 via its important Cys modifications by H2O2/arsenite redox-stress and that protection by CS/2-mercaptoethanol are shown in in vitro/in situ studies paralleling the present Swiss-Model-generated rSOD1 structural data. Here, arsenite(3+) incubation (≥10(-8) μM + 10 mM H2O2, 2 hr) is shown for the first time with this low-concentration to initiate breakage in rat hepatic-DNA in vitro whereas, arsenite/H2O2/UV-radiation does not affect DNA separately. Arsenic initiates Fe and Cu ion-associated free-radical reaction cascade in vivo. Here, 10 μM of Cu(2+)/Fe(3+)/As(3+) +H2O2-induced in vitro DNA fragmentation is prevented by CS (≥1 mg/ml), greater than the prevention of ascorbate or tocopherol or DMSO or their combination. Moreover, CS incubation for various time with differentially and already degraded DNA resulted from pre-incubation in 10 μM As(3+)-H2O2 system markedly recovers broken DNA. Present results decisively suggest for the first time that CS and its mixed polyphenols have potent SOD1 protecting, diverse radical-scavenging and antimutagenic activities furthering to DNA protection/therapy in arsenic-induced tissue necrosis/apoptosis.

Product and mechanistic analysis of the reactivity of a C6-pyrimidine radical in RNA

Nucleobase radicals are the major reactive intermediates produced when hydroxyl radical reacts with nucleic acids. 5,6-Dihydrouridin-6-yl radical (1) was independently generated from a ketone precursor via Norrish Type I photocleavage in a dinucleotide, single-stranded, and double-stranded RNA. This radical is a model of the major hydroxyl radical adduct of uridine. Tandem lesions resulting from addition of the peroxyl radical derived from 1 to the 5'-adjacent nucleotide are observed by ESI-MS. Radical 1 produces direct strand breaks at the 5'-adjacent nucleotide and at the initial site of generation. The preference for cleavage at these two positions depends upon the secondary structure of the RNA and whether O(2) is present or not. Varying the identity of the 5'-adjacent nucleotide has little effect on strand scission. In general, strand scission is significantly more efficient under anaerobic conditions than when O(2) is present. Strand scission is more than twice as efficient in double-stranded RNA than in a single-stranded oligonucleotide under anaerobic conditions. Internucleotidyl strand scission occurs via β-fragmentation following C2'-hydrogen atom abstraction by 1. The subsequently formed olefin cation radical ultimately yields products containing 3'-phosphate or 3'-deoxy-2'-ketouridine termini. These end groups are proposed to result from competing deprotonation pathways. The dependence of strand scission efficiency from 1 on secondary structure under anaerobic conditions suggests that this reactivity may be useful for extracting additional RNA structural information from hydroxyl radical reactions.

Radiation chemistry comes before radiation biology

PURPOSE

This article seeks to illustrate some contributions of radiation chemistry to radiobiology and related science, and to draw attention to examples where radiation chemistry is central to our knowledge of specific aspects. Radiation chemistry is a mature branch of radiation science which is continually evolving and finding wider applications. This is particularly apparent in the study of the roles of free radicals in biology generally, and radiation biology specifically. The chemical viewpoint helps unite the spatial and temporal insight coming from radiation physics with the diversity of biological responses. While historically, the main application of radiation chemistry of relevance to radiation biology has been investigations of the free-radical processes leading to radiation-induced DNA damage and its chemical characterization, two features of radiation chemistry point to its wider importance. First, its emphasis on quantification and characterization at the molecular level helps provide links between DNA damage, biochemical repair processes, and mutagenicity and radiosensitivity. Second, its central pillar of chemical kinetics aids understanding of the roles of 'reactive oxygen species' in cell signalling and diverse biological effects more generally, and application of radiation chemistry in the development of drugs to enhance radiotherapy and as hypoxia-specific cytotoxins or diagnostic agents. The illustrations of the broader applications of radiation chemistry in this article focus on their relevance to radiation biology and demonstrate the importance of synergy in the radiation sciences.

CONCLUSIONS

The past contributions of radiation chemistry to radiation biology are evident, but there remains considerable potential to help advance future biological understanding using the knowledge and techniques of radiation chemistry.

Monitoring of dithiothreitol clearance by means of micellar electrokinetic chromatography

A new method for specific determination of dithiothreitol (DTT) using micellar electrokinetic chromatography and on-column reaction with reactive disulfide-2,2'-dipyridyldisulfide is described. DTT in this reaction is quantitatively transformed into a mixed disulfide concomitantly with formation of equimolar amount of the 2-thiopyridone that is further separated by micellar electrokinetic chromatography and determined spectrophotometrically at 343 nm. The concentration of DTT is thus estimated indirectly from the result of 2-thiopyridone determination. The linear detection range for concentration versus peak area for the assay is from 0.05 to 2.5 mM (correlation coefficient 0.993) with a detection limit of 2.5 microM. The inter-day reproducibility of the peak area was 1.35% and the inter-day reproducibility of the migration time 0.56%. The method can be applied for DTT monitoring both in chemical and biological systems.

Hydroperoxide-induced DNA damage and mutations

Hydroperoxides (ROOH) are believed to play an important role in the generation of free radical damage in biology. Hydrogen peroxide (R=H) is produced by endogenous metabolic and catabolic processes in cells, while alkyl hydroperoxides (R=lipid, protein, DNA) are produced by free radical chain reactions involving molecular oxygen (autooxidation). The role of metal ions in generating DNA damage from hydroperoxides has long been recognized, and several distinct, biologically relevant mechanisms have been identified. Identification of the mechanistic pathways is important since it will largely determine the types of free radicals generated, which will largely determine the spectrum of DNA damage produced. Some mechanistic aspects of the reactions of low valent transition metal ions with ROOH and their role in mutagenesis are reviewed with a perspective on their possible role in the biological generation of DNA damage. A survey of hydroperoxide-induced mutagenesis studies is also presented. In vitro footprinting of DNA damage induced by hydroperoxides provides relevant information on sequence context dependent reactivity, and is valuable for the interpretation of mutation spectra since it represents the damage pattern prior to cellular repair. Efforts in this area are also reviewed.

Strand breakage in poly(C), poly(A), single- and double-stranded DNA induced by nanosecond laser excitation at 193 nm

Single- and double-stranded calf thymus DNA and two polynucleotides (0.4 mM) were studied in aqueous solution at pH approximately 7 using pulsed, 20 ns laser excitation at 193 nm. Monophotonic ionization of the nucleic acids is suggested from the linear dependences of the concentration of ejected electrons and the number of single- and double-strand breaks (ssb, dsb, respectively) on laser intensity (IL) in the range (0.2-3) x 10(6) W cm-2. The quantum yields of formation of hydrated electrons (phi e-) and ssb and dsb (phi ssb and phi dsb) are therefore independent of IL. In contrast, under 248 nm excitation these quantum yields increase linearly with IL under otherwise comparable conditions. Nevertheless, several effects and mechanistic implications are analogous using lambda exc = 193 and 248 nm. For polycytidylic acid, poly(C), in Ar-saturated solution for example, the efficiency of ssb per radical cation (eta RC = phi ssb/phi e-) is similar to the efficiency of ssb per OH radical (eta OH). For polyadenylic acid, poly(A), and single- and double-stranded DNA eta RC (lambda exc = 193 nm) is significantly smaller than eta OH. The ratio phi ssb (N2O)/phi ssb (Ar) is approximately 2 for poly(C), approximately 4 for poly(A) approximately 10 for DNA; the conversion of hydrated electrons into OH radicals in N2O-saturated solution and smaller eta RC than eta OH values in the case of DNA account for these results. For double-stranded DNA phi dsb does not depend on IL but increases linearly with the dose, indicating an accumulative effect of two ssb to generate one dsb. The critical distance for this event is 60-85 phosphoric acid diester bonds.

Kinetics of radiation-induced strand break formation in single-stranded pyrimidine polynucleotides in the presence and absence of oxygen; a time-resolved light-scattering study

Time-resolved reductions in the light-scattering intensity (LSI) of aqueous oxic and anoxic solutions of poly-C and poly-U at pH 7.8, following pulse-irradiation, have been studied as indices of strand break formation. With doses of 3-24 Gy per pulse, a number of kinetically distinct strand breakage components have been detected. A comparison of the LSI responses obtained from irradiations conducted under N2O with those conducted under air or O2 show no marked difference in the overall extent of LSI change. However, the immediate and fast (t 1/2 less than or equal to 50 microseconds) reduction in LSI, accounting for about 18-19% of the pyrimidine polynucleotide's total LSI response in oxic solution, is reduced in the absence of oxygen, to about 12% of the total LSI response found with poly-C and to about 9% for poly-U. For poly-C there is a five-fold enhancement in the rate of major strand breakage under anoxia [k1(N2O) = 7.9s-1] whereas for poly-U a more modest enhancement (about two-fold) is observed. These enhanced rates are mirrored by those for the losses of the principal optical anoxic absorptions (observed pulse radiolytically) that are assigned to the pyrimidine 6-yl base radicals. Such findings support a proposal that the rate-limiting step of major strand breakage for pyrimidine polynucleotides is a base radical mediated hydrogen atom abstraction reaction (Lemaire et al. 1987, Hildenbrand and Schulte-Frohlinde 1989). Irradiation of poly-C and poly-U in N2O/O2 (4:1, v/v) saturated solutions yields LSI changes much larger than those noted under N2O and air (or O2), which are in turn approximately double the responses observed under N2. This indicates that the major strand breaking species of water radiolysis is the OH-radical and that there is an oxygen enhancement of single strand breakage of about 1.9 for poly-C and 1.6 for poly-U.

The chemistry of free-radical-mediated DNA damage

In the living cell, ionizing radiation can cause DNA damage by the direct effect (ionization of DNA) and the indirect effect (reaction of radicals formed in the neighborhood of DNA with DNA, e.g., OH, eaq-, H, protein- and glutathione-derived radicals). Properties of the base radical cations have been studied in model systems using SO4- radical to oxidize the nucleobases in aqueous solution. The pKa values of some nucleobase radical cations are reported, so are the ensuing reactions of the thymidine radical cation with water. The products of reactions are compared with those formed by OH radical attack. The reaction of eaq- with the nucleobases yields radical anions. Protonation at heteroatom sites and at carbon are discussed, and some recent results regarding the electron transfer to adjacent nucleobases as well as to 5-bromouracil are reported. A brief account is given on the reaction of carbon-centered radicals with the nucleobases. These reactions may mimic the reactions of protein-derived radicals with DNA. Glutathione is present in cells at rather high concentrations and is expected to act as an H- or electron-donor in repairing radiation-induced DNA damage (chemical repair). As thiyl radicals are known to also undergo the reverse reaction, i.e., H-abstraction from suitable solutes, some experiments are reported which probe this type of reaction with dilute DNA solutions. In some polynucleotides radical transfer from the base radical to the sugar moiety occurs with the consequence of strand breakage and base release. Some currently held mechanistic concepts are discussed. Attention is drawn to some important open questions which should be addressed in the near future.

Determination of the constants of the Alper formula for single-strand breaks from kinetic measurements on DNA in aqueous solution and comparison with data from cells

On the basis of rate constants measured in aqueous solution for (i) DNA single-strand break (ssb) formation induced by OH radicals, (ii) prevention of ssb formation by reaction of DNA radicals with glutathione, and (iii) addition of O2 to DNA radicals, oxygen enhancement ratios (OER) and K values of the Alper equation have been calculated. The values obtained were compared with OER and K values determined for ssb formation in lambda DNA irradiated in Escherichia coli as a function of the oxygen concentration. Without adjustment of any parameter the two sets of data are similar when the corrected Alper formula is used. The results support the oxygen fixation-thiol repair model of Howard-Flanders and Alper, and indicate that under selected conditions DNA in aqueous solution may serve as a model system for DNA in cells.

The influence of glutathione on single-strand breakage in single-stranded DNA irradiated in aqueous solution in the absence and presence of oxygen

The yields of strand break formation (Gssb) in single-stranded DNA (ssDNA) initiated by radiation-generated OH radicals have been determined using the method of low-angle laser light scattering (LALLS). The irradiations were carried out in aqueous, N2O-saturated solutions in the absence and presence of oxygen and at different concentrations of glutathione (GSH). GSH exhibits a protective effect, which is shown to be mainly due to OH radical scavenging. To quantify this, the rate constants for the reactions of OH with GSH and DNA have been redetermined under our experimental conditions. The values obtained were 9.0 x 10(9) and 4.5 x 10(8) dm3 mol-1 s-1, respectively. From the Gssb values obtained under anoxic conditions it is concluded that GSH protects against strand breakage (in addition to OH scavenging) by reacting with DNA radicals in competition to strand break formation. The rate constant of the repair reaction is 8.1 x 10(4) dm3 mol-1 s-1 at room temperature. For irradiations carried out in the presence of oxygen the rate of strand break formation is determined by the decay of DNA peroxyl radicals. Under these conditions we observed no protective effect of GSH apart from OH radical scavenging. The results are compatible with those that are expected from the oxygen-fixation hypothesis.

The kinetics of radiation-induced strand breakage in polynucleotides in the presence of oxygen: a time-resolved light-scattering study

The time-resolved light-scattering changes of aqueous, aerated solutions of poly-C, poly-U and poly-A at pH 7.8, following pulse irradiation, have been studied as indices of strand break formation. With doses of 4-24 Gy/pulse a number of kinetically distinct components have been detected. For the poly-pyrimidines an immediate and fast reduction (tau 1/2 less than or equal to 50 microseconds) in light-scattering intensity (LSI), accounting for approximately 20% of the total LSI change, is followed by a much slower loss (k1 approximately 1.6 s-1) which constitutes their major LSI change. For poly-A a similar fast component is observed, present to an extent equivalent to the one noted with poly-C; it constitutes, however, over 50% of the purine polynucleotide's total response, with the remainder of the change being a slower loss (tau 1/2 approximately 0.09 s). Optical pulse radiolysis studies of poly-C and poly-U, in support of the LSI investigations, show that transient absorbances in a region assigned to base peroxyl radicals decay in a complex fashion, with some at a rate equivalent to that for the slow (major) component of LSI loss. These observations support a proposal that the rate-limiting step of major strand breakage for these polynucleotides, in the presence of oxygen, is a base peroxyl radical-mediated abstraction of a H-atom from an adjacent sugar moiety (Bothe et al. 1986), with the resulting sugar peroxyl radicals then leading to strand break formation at a rate equivalent to that for loss of the initial, fast LSI components. These latter processes are attributed to strand breaks arising from the direct interaction of .OH with the polynucleotide sugar phosphate backbone.

Photosensitized reactions of poly(U) with tris(2,2'-bipyridyl)ruthenium(II) and peroxydisulfate

The reactions of polyuridylic acid [poly(U)] with Ru(bpy)3(3+) [Ru(III)] and SO4.-, following UV and visible light irradiation of Ru(bpy)3(2+) [Ru(II)] in the presence of S2O8(2-), were studied in an argon-saturated aqueous solution using time-resolved absorption and conductivity methods. The kinetics of the Ru(III) conversion to Ru(II) in the presence of poly(U) was monitored spectroscopically either in the absence of SO4.- [rapid mixing with Ru(III)] or in its presence (after laser flash excitation, lambda exc = 353 nm). The conversion of Ru(III) to Ru(II) is complete at a [nucleotide]/[sensitizer] (N/S) ratio greater than or equal to 10 (rate constant k = 12 s-1) for rapid mixing and at N/S greater than or equal to 6 (k = 15 s-1 at N/S = 10) after laser pulsing. Conductivity measurements following the laser pulse revealed a fast conductivity increase (risetime less than 10 micros), due to the formation of charged species and protons. A slower increase in the 0.1-0.5 s range was observed for poly(U) but it is considerably smaller for poly(dU) and absent in uracil containing monounits. The slow increase is unaffected by pH changes in the 3.5-7 range, markedly reduced in the 7-9 range and is replaced by a slight decrease in conductivity in buffered solutions. An explanation is that poly(U)-bound excited Ru(II) reacts with S2O8(2-) forming Ru(III) and SO4.- as oxidizing species both of which react with poly(U) bases. The resulting base radicals react with Ru(III) or the ligands in the ruthenium complex, producing protons which give rise to the slow conductivity increase (k = 15 s-1 at N/S = 10). The formation of single-strand breaks and the ensuing release of condensed counterions does not appear to contribute significantly to the slow conductivity signal. At N/S less than 10 the observed rate and extent of Ru(III)--Ru(II) conversion and of the slow proton production vary markedly with the N/S ratio.

Hydroxyl radical-induced strand break formation in single-stranded polynucleotides and single-stranded DNA in aqueous solution as measured by light scattering and by conductivity

Combining conductivity measurements and molecular weight determination by means of low-angle laser light scattering, we have found for the polyribonucleotides (polyuridylic acid (poly(U], polyadenylic acid (poly(A], polycytidylic acid (poly(C] and polyguanylic acid (poly(G] and for single-stranded DNA (ssDNA) that, on average, 8.5 counterions per single-strand break (ssb) are liberated under salt-free conditions. This relationship allows us to estimate, from conductivity measurements alone, G-values of single-strand break formation (G(ssb] for the polydeoxyribonucleotides (polydeoxyriboadenylic acid (poly(dA], polydeoxyribocytidylic acid (poly(dC], polydeoxyribothymidylic acid (poly(dT], polydeoxyribouridylic acid (poly(dU] and polydeoxyriboguanylic acid (poly(dG]. The following G(ssb) values (units of mumol J-1) have been obtained for anoxic conditions: poly(dA), 0.23; poly(dC), 0.14; poly(dT), 0.06; poly(dU), 0.046 and poly(dG), 0.009. Time-resolved conductivity measurements in pulse radiolysis enable us to measure the rate of strand break formation. The rate has been found to be similar for poly(dA) and ssDNA over a range of pH values. Poly(dC) and poly(dU) exhibit conductivity increase components with half-lives similar to those of poly(dA) and ssDNA at corresponding pH values. The implications of these results are discussed.

Dithiothreitol pretreatment and inducible repair in UV-irradiated Escherichia coli K12 cells

The UV radiation survival of several Escherichia coli K12 strains was measured after pretreatment of the cells with dithiothreitol (DTT). In DNA repair-competent cells (AB1157), UV survival was enhanced (ER = 1.2) after pretreating cells for 1.0 h using 10 mmol dm-3 DTT and then incubating the cells for 1.5 h in buffer before UV irradiation. Similar experiments using the excision repair mutant, AB1886uvrA6, or the recombination repair and SOS-deficient mutant, AB2462recA, strains did not show enhanced UV survival. None of the E. coli strains tested were protected against UV killing by simultaneous treatment with DTT (10 mmol dm-3). These results, and the fact that incubation in chloramphenicol removed the wild-type response in DTT-pretreated, UV-irradiated cells, suggest that the observed UV radioprotection was a result of inducible enzymatic repair processes such as recA-dependent repair. The proposed stimulus for inducible repair in these cells is DNA damage caused by intracellular hydroxyl radicals arising from thiol oxidation. The involvement of oxygen radicals in the induction pathway is supported by results that showed superoxide dismutase and catalase could inhibit a portion (one-third) of the inducible repair.

Radiolysis of poly(U) in oxygenated solution

Aqueous N2O/O2-saturated solutions of poly(U) were irradiated at 0 degrees C and the release of unaltered uracil determined. Immediately after irradiation G(uracil release) was 1.5 which increased to a value of 5.3 +/- 0.3 upon heating to 95 degrees C. Thereby all of the organic hydroperoxides (G = 6.8 +/- 0.7) and some of the hydrogen peroxide (G = 1.7 +/- 0.2) was destroyed leaving G(peroxidic material; mainly hydrogen peroxide) = 1.0 +/- 0.7. G(chromophore loss) = 8-11 was measured immediately after irradiation, but no increase was observed upon heating. Addition of iodide destroyed the hydroperoxides and caused immediate base release to rise to G = 4 and further heating brought the value to that observed in the absence of iodide. In contrast, on reducing the hydroperoxides with NaBH4, immediate uracil release rose to only G = 2.8 and no further increase was observed on heating. A major product (G = 2.7) is carbon dioxide. There are also osazone-forming compounds produced (G = 2.7), all of which are originally bound to poly(U). Heating in acid solutions, as is required for this test, releases glycoladehyde-derived osazone (G = 0.8) and further unidentified low molecular weight material (G = 0.9). It is concluded that the primary radicals which cause these lesions are the base OH adduct radicals. In the presence of oxygen these are converted into the corresponding peroxyl radicals which abstract an H atom from the sugar moiety. In the course of this reaction base-hydroperoxides are formed. However, such base hydroperoxides cannot be the only organic hydroperoxides, but some (G congruent to 2.5) sugar-hydroperoxides must be formed as indicated by the increase in base release by the addition of iodide. It is speculated that a sugar-hydroperoxide located at C(3') is reduced by iodide to a carbonyl function at C(3'), a lesion that releases the base, while reduction with NaBH4 reduces it to an alcohol function at C(3') thus preventing base release.