Analysis of melphalan adducts of 2'-deoxynucleotides in calf thymus DNA hydrolysates by capillary high-performance liquid chromatography-electrospray tandem mass spectrometry

Investigation of Melphalan interaction as an alkylating agent with nucleotides by using surface enhanced Raman spectroscopy (SERS)

SERS (Surface Enhanced Raman Spectroscopy) is a new Raman spectroscopy which relies on Surface Plasmon Resonance (SPR) of metal nanoparticles. We have applied colloidal silver and gold nanoparticles as amplifier agents to enhance nucleotide Raman signals. It is observed that without these enhancing agents, it is impossible to investigate nucleotide spectrum due to weak Raman signals. Interaction mechanism of Melphalan, an anticancer drug with four nucleotides (Adenine, Cytosine, Guanine, Thymine) was investigated using SERS to detect and identify changes due to alkylating process in Raman spectra. After incubating Melphalan drug with nucleotides for 24 h at 37 °C, some changes occurred in SERS spectrum and interpretation of SERS spectra revealed the influence of the alkyl substitution on peaks and Raman shifts. After incubation of Melphalan with each nucleotide, intensity of relevant SERS signals assigned to Amid III group of Cytosine and Amid I of Thymine decreased significantly, confirming alkylating taking place. In this study, we also investigated the effect of nanoparticles type on nucleotide spectrum. We could not obtain useful information in the cases of guanine nucleotide. The SERS spectrum of Cytosine as an example of nucleotides in aqueous solution compared to solid state and results demonstrated that in solid state better signals were obtained than in liquid state.

Characterization of nitrogen mustard formamidopyrimidine adduct formation of bis(2-chloroethyl)ethylamine with calf thymus DNA and a human mammary cancer cell line

A robust, quantitative ultraperformance liquid chromatography ion trap multistage scanning mass spectrometric (UPLC/MS(3)) method was established to characterize and measure five guanine adducts formed by reaction of the chemotherapeutic nitrogen mustard (NM) bis(2-chloroethyl)ethylamine with calf thymus (CT) DNA. In addition to the known N7-guanine (NM-G) adduct and its cross-link (G-NM-G), the ring-opened formamidopyrimidine (FapyG) monoadduct (NM-FapyG) and cross-links in which one (FapyG-NM-G) or both (FapyG-NM-FapyG) guanines underwent ring-opening to FapyG units were identified. Authentic standards of all adducts were synthesized and characterized by NMR and mass spectrometry. These adducts were quantified in CT DNA treated with NM (1 μM) as their deglycosylated bases. A two-stage neutral thermal hydrolysis was developed to mitigate the artifactual formation of ring-opened FapyG adducts involving hydrolysis of the cationic adduct at 37 °C, followed by hydrolysis of the FapyG adducts at 95 °C. The limit of quantification values ranged between 0.3 and 1.6 adducts per 10(7) DNA bases when the equivalent of 5 μg of DNA hydrolysate was assayed on column. The principal adduct formed was the G-NM-G cross-link, followed by the NM-G monoadduct; the FapyG-NM-G cross-link adduct; and the FapyG-NM-FapyG was below the limit of detection. The NM-FapyG adducts were formed in CT DNA at a level ∼20% that of the NM-G adduct. NM-FapyG has not been previously quanitified, and the FapyG-NM-G and FapyG-NM-FapyG adducts have not been previously characterized. Our validated analytical method was then applied to measure DNA adduct formation in the MDA-MB-231 mammary tumor cell line exposed to NM (100 μM) for 24 h. The major adduct formed was NM-G (970 adducts per 10(7) bases), followed by G-NM-G (240 adducts per 10(7) bases), NM-FapyG (180 adducts per 10(7) bases), and, last, the FapyG-NM-G cross-link adduct (6.0 adducts per 10(7) bases). These lesions are expected to contribute to NM-mediated toxicity and genotoxicity in vivo.

Analysis of modified nucleosides in the urine of patients with malignant cancer by liquid chromatography/electrospray ionization mass spectrometry

As modified nucleosides reflect altered tRNA turnover which seems to be impaired in the body of cancer patients, they have been evaluated as potential tumor markers. High-performance liquid chromatography/electrosprary ionization quadrupole time-of-flight mass spectrometry (HPLC/ESI-Q-TOFMS) was used to identify nucleosides purified from urine in positive ionization mode. Potential nucleosides were assessed by their evident UV absorbance in HPLC and then further examined by mass spectrometric techniques. In this manner, 21 nucleosides were detected in the urine of a patient with lymphoid cancer including three modified nucleosides 5'-dehydro-2-deoxyinosine, N1,N2,N7-trimethylguanosine and N1-methyl-N2-ethylguanosine, which had never been reported previously.

Separation and identification of trinucleotide-melphalan adducts from enzymatically digested DNA using HPLC-ESI-MS

Melphalan is a bifunctional alkylating agent that covalently binds to the nucleophilic sites present in DNA. In this study we investigated oligonucleotides prepared enzymatically from DNA modified with melphalan. Calf thymus DNA was incubated in-vitro with melphalan and the resulting modifications were enzymatically cleaved by means of benzonase and nuclease S1. Efficient sample preconcentration was achieved by solid-phase extraction, in which phenyl phase cartridges resulted in better recovery of the modified species than C(18). The applied enzymatic digestion time resulted in production of trinucleotide adducts which were efficiently separated and detected by use of reversed-phase HPLC coupled to an ion-trap mass spectrometer with electrospray ionization. It was assumed that melphalan could act as both a monofunctional and bifunctional alkylating agent. Mono-alkylated adducts were much more abundant, however, and the alkylation site was located on the nucleobases. On the other hand, we unequivocally identified cross-link formation in DNA, even though at low abundance and only a few adduct types were detected.

Investigation of enzymatic behavior of benzonase/alkaline phosphatase in the digestion of oligonucleotides and DNA by ESI-LC/MS

We have developed an ion-pairing HPLC-MS method that has sufficient separation power, selectivity, and sensitivity to investigate the enzymatic behavior of benzonase/alkaline phosphatase upon digestion of oligonucleotides and DNA. Mass spectrometry revealed that this enzyme pair can nonspecifically digest oligonucleotides and DNA into fragments ranging from 2 to 10 nucleotides, i.e., sizes suitable for routine mass spectrometric measurements. Trimers, tetramers, and pentamers are the most prominent digested products. This makes benzonase/alkaline phosphatase a promising choice for DNA and DNA adduct related studies that require a nonspecific enzyme. A computer software program developed in-house was critical in automating the processing of mass spectral data. The methodology described here provides a systematic approach for evaluating the behavior of DNA-cleaving enzymes by mass spectrometry.

Quantitative determination of melphalan DNA adducts using HPLC - inductively coupled mass spectrometry

For the quantification of Melphalan DNA adducts, an analytical approach based on the detection of phosphorus using liquid chromatography combined with inductively-coupled-plasma mass spectrometry (ICP-MS) was developed. In reaction mixtures of native 2'-deoxynucleotides-5'-monophosphates and Melphalan, which were separated using reversed phase chromatography, phosphate adducts were found as the most abundant modifications. Besides the phosphate adducts, several base alkylated adducts were observed. In calf thymus DNA incubated with Melphalan and enzymatically digested using Nuclease P1, the phosphate adducts as well as monoalkylated dinucleotides were found. The most abundant single Melphalan adduct observed in DNA was a ring-opened adenosine monophosphate. Some dinucleotide adducts and the adenosine adduct were identified using electrospray ionization mass spectrometry (ESI-MS).

Simultaneous determination of O6-methyl-2'-deoxyguanosine, 8-oxo-7,8-dihydro-2'-deoxyguanosine, and 1,N6-etheno-2'-deoxyadenosine in DNA using on-line sample preparation by HPLC column switching coupled to ESI-MS/MS

O(6)-Methyl-2'-deoxyguanosine (O(6)-mdGuo), 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo), and 1,N(6)-etheno-2'-deoxyadenosine (epsilondAdo) are promutagenic DNA lesions originating from both endogenous and exogenous agents and actions (methylation, hydroxylation, lipid peroxidation products). A highly sensitive quantitative method was developed to measure these DNA adducts simultaneously, using liquid chromatography tandem mass spectrometry with column switching. Deuterated O(6)-[(2)H(3)]mdGuo was synthesized and used as internal standard. The limits of quantification for O(6)-mdGuo, 8-oxodGuo, and epsilondAdo were 24, 98, and 48 fmol on column, respectively. The method showed linearity in the range 0.24-125 pmol/ml, 0.98-125 pmol/ml, and 0.49-62.5 pmol/ml for the three adducts, respectively. The inter-day precision in the linear concentration range was between 1.7 and 9.3% for O(6)-mdGuo, 10.6 and 28.7% for 8-oxodGuo, and 6.2 and 10.4%, for epsilondAdo. In DNA isolated from liver of untreated 12-week-old female F344 rats, O(6)-mdGuo was above the limit of detection (37 adducts per 10(9) normal nucleosides) but could not be quantified. 8-oxodGuo and epsilondAdo showed background levels of 500 and 130 adducts per 10(9) normal nucleosides, respectively. DNA analyzed 1h after treatment of rats with dimethylnitrosamine by oral gavage of 50 microg/kg b.wt. did not affect the levels of 8-oxodGuo and epsilondAdo but resulted in 200 O(6)-mdGuo adducts per 10(9) normal nucleosides. The method developed will be of use to study the biological significance of exogenous DNA adducts as an increment to background DNA damage and the role of modulating factors, such as DNA repair.

Liquid chromatography-electrospray ionization-mass spectrometry: the future of DNA adduct detection

Over the past 40 years considerable emphasis has been placed on the development of accurate and sensitive methods for the detection and quantitation of DNA adducts. The formation of DNA adducts resulting from the covalent interaction of genotoxic carcinogens with DNA, derived from exogenous and endogenous sources, either directly or following metabolic activation, can if not repaired lead to mutations in critical genes such as those involved in the regulation of cellular growth and subsequent development of cancer. The major analytical challenge has been to detect levels of DNA adducts at the level of 0.1-1 adducts per 10(8) unmodified DNA bases using only low microgram amounts of DNA, and with high specificity and accuracy, in humans exposed to genotoxic carcinogens derived from occupational, environmental, dietary and life-style sources. In this review we will highlight the merits as well as discuss the progress made by liquid chromatography coupled to electrospray ionization mass spectrometry as a method for DNA adduct detection.

First results of a quantitative study of DNA adducts of melphalan in the rat by isotope dilution mass spectrometry using capillary liquid chromatography coupled to electrospray tandem mass spectrometry

Rats were intravenously injected with a single high dose (10 mg/kg) of the alkylating agent melphalan in order to study DNA-adduct formation. Quantitation of a dGuo-melphalan adduct was done by isotope dilution mass spectrometry using capillary liquid chromatography/mass spectrometry (LC/MS) and [15N5]-labeled dGuo-melphalan as internal standard. DNA-adduct levels were studied in bone marrow, liver and kidney. The instrumental detection limit of the method was determined to be 900 fg (S/N 3, pure standard). These first results clearly show a 10 times higher adduct level in bone marrow compared to kidney and a 6 times higher level compared to liver. More experiments will be necessary to gather more information on the pharmacokinetics of melphalan-DNA adducts under in vivo conditions.

Implications of enzymatic, acidic and thermal hydrolysis of DNA on the occurrence of cross-linked melphalan DNA adducts

Calf thymus DNA was treated with melphalan, a nitrogen mustard, and the formation of melphalan cross-linked DNA adducts was investigated. These cross-linked adducts could not be detected either in the enzymatically or in the thermally generated DNA hydrolysates. However, a search for DNA cross-linked adducts in the hydrolysates obtained under acidic conditions revealed the presence of different types of cross-links, mainly containing an adenine moiety. These results are very important because they show that the detection of cross-links is dependent on the hydrolytic procedure used and that these cross-linked adducts are formed under totally different reaction conditions from those in in vivo situations. This can explain the very low abundance or even the absence of cross-linked adducts in nitrogen mustard treated animals. The generally accepted theory that the anti-cancer activity of bifunctional mustards such as melphalan is due to cross-linking of DNA strands remains therefore from our point of view questionable.

Structural characterization of melphalan modified 2'-oligodeoxynucleotides by miniaturized LC-ES MS/MS

In this study a miniaturized LC coupled to electrospray tandem mass spectrometry was used to analyze modifications originating from the interaction between the chemotherapeutic agent melphalan and 2'-oligodeoxynucleotides. Low energy CAD product ion spectra gave information about the specificity of melphalan alkylation with regard to certain DNA sequences. These data can be very useful to estimate the risk in the development of secondary leukaemia as a result of a melphalan cure. In the study of the interaction between melphalan and d(GG), differentiation could be made between alkylation on the 5'-side and alkylation on the 3'-side, because of the presence or absence of the alkylated w1 fragment in the low energy CAD spectra. In the other di-mers alkylation specificity for the different bases could be observed. Melphalan alkylation occurs in the sequence G > A > C > T. The study of the alkylated d(GGGG) revealed the presence of mainly 5'-end alkylation. Furthermore studies were performed which investigated other melphalan treated di-, tetra-, hepta-, and octa-mers.

Characterisation of estrone-nucleic acid adducts formed by reaction of 3,4-estrone-o-quinone with 2'-deoxynucleosides/deoxynucleotides using capillary liquid chromatography/electrospray ionization mass spectrometry

Xenobiotic and endobiotic molecules can react with DNA leading to formation of so-called DNA adducts. This modified DNA can be repaired enzymatically, but, if not, these modifications are believed to be responsible for the initiation of carcinogenic processes. Hence, we studied the interaction of 2'-deoxynucleosides and 2'-deoxynucleotides with 3,4-estronequinone (3,4-E(1)Q), a metabolite of estrone (E(1)) and a supposed carcinogen. These estrone-nucleic acid adducts were analysed by capillary liquid chromatography (CapLC) coupled to electrospray ionization mass spectrometry (ESI-MS). Knowledge of their behaviour from in vitro studies is a prerequisite for detecting adducts in in vivo studies. Our initial attempts to synthesise nucleos(t)ide adducts of 3,4-E(1)Q in an aprotic solvent (dimethylformamide) yielded no adducts. However, under acidic aqueous conditions, adducts were obtained. With dGuo, a dGuo adduct was found in addition to a Gua adduct. Earlier publications on adduct formation in protic solvents failed to report formation of any adduct with dAdo. A N(3)-Ade adduct was reported upon reaction of 3,4-E(1)Q with Ade base and with DNA. With dAdo, we obtained two nucleoside adducts and six Ade adducts due to loss of 2'-deoxyribose. Thus, contrary to general belief that only 2,3-E(1)Q can form stable adducts, we showed formation of substantial amounts of intact DNA adducts with 3,4-E(1)Q in addition to deglycosylated adducts. Adducts were also obtained with dGMP and dAMP, but no phosphate alkylation was found. Adducts of dCyd, dCMP, dThd, and dTMP were not detected. Using chromatographic-MS data a structural relationship between the 2'-deoxynucleoside, 2'-deoxynucleotide and base adducts was found in the various reaction mixtures. The adducts of dGuo and dGMP reaction mixtures were alkylated at the same N(7)-position of the nucleobase, as indicated by the occurrence of a rapid deglycosylation reaction. In dAdo and dAMP reaction mixtures, 14 adducts were detected; their relationships from the LC and MS data reduced the number of structures to six adenine base alkylated adducts with respect to alkylation between N(1), N(3), N(7) and/or N(6) in the adenine and C(1), C(2) and/or C(6) in 3,4-E(1)Q. We could infer, in addition, whether they had an A ring attachment or a C(6) attachment on the estrone moiety.

Detection of estrogen DNA-adducts in human breast tumor tissue and healthy tissue by combined nano LC-nano ES tandem mass spectrometry

For the first time estrogen DNA-adducts were identified in DNA human breast tumor tissue using nano-LC coupled to nano-Electrospray Tandem Mass Spectrometry. Normal breast tissue was analyzed analogously. The data obtained in the five breast tumor and five adjacent normal tissue samples were compared qualitatively, but no straightforward difference was observed. Prior to LC-MS analysis the DNA was enzymatically hydrolyzed to a nucleoside pool. The DNA-hydrolysates were directly injected onto a column switching system developed for on-line sample clean-up and subsequent analysis of the DNA-adducts. In four patients using Premarin, DNA-adducts of 4-hydroxy-equilenin (4OHEN) were detected. All except three samples contained DNA-adducts from 4-hydroxy-estradiol or 4-hydroxy-estrone. Also DNA isolated from eight alcohol fixed and paraffin embedded breast tumor tissue showed the presence of different estrogen DNA-adducts. Worthwhile mentioning is the presence of adducts responding to m/z 570 > m/z 454 transition. This is a well-known SRM-transition indicative for the presence of the 2'-deoxyguanosine (dGuo) adduct of Benzo[a]pyrene.

Investigation of the regio- and stereo-selectivity of deoxyguanosine linkage to deuterated 2-hydroxyestradiol by using liquid chromatography/ESI-ion trap mass spectrometry

From previous studies on the reactivity of estradiol 2,3-quinone towards deoxyribonucleosides, it was demonstrated that several isomeric adducts were formed. Although adduction on steroid ring A or B has been evidenced using sequential MS(n) experiments, in some cases attachment positions are difficult to identify unambiguously. In this work, 2-hydroxyestradiol labeled with deuterium at various positions [6beta (1); 6alpha-7alpha (2); 6alpha-6beta-7alpha (3)] have been used. Isomeric adduct differentiation could be achieved using LC-ESI-MS(n). The m/z shift of the quasi-molecular ions as well as the fragmentation pathways suggested that adduction could occur on both C6 and C9 sites of the steroid B ring: Nucleophilic attack of the base on the C6 position of the steroid led to major adducts and addition of the base on the activated C9 site gave minor adducts that were found to be unstable. LC-MS(n) experiments carried out under deuterated medium provided information about some fragmentation processes by studying the m/z shift of fragment ions: (1) the loss of deoxyribose from the quasi-molecular ions took place according to a process involving a deuterium transfer from the deoxyribose alcohol function; (2) the cleavage of the steroid-base linkage involved a deuterium transfer from the hydroxy group of the catechol and likely occurred via the formation of an ion-dipole complex. The model studies conducted in this work provide new information on the fragmentation mechanisms of covalent adducts formed from estrogen quinones and deoxyguanosine, the most reactive DNA base. Besides, the first unequivocal characterization of adducts involving the steroid C9 position is shown by using deuterium labeled estrogen quinones.

Oligonucleotide covalent modifications by estrogen quinones evidenced by use of liquid chromatography coupled to negative electrospray ionization tandem mass spectrometry

Liquid chromatography coupled to tandem mass spectrometry has been used for the detection and the structural characterization of T-rich model oligonucleotides covalently modified by estradiol-2,3-quinone. After separation by gradient elution, adducts were analyzed by negative electrospray mass spectrometry, enabling to evidence and localize the modifications in the oligonucleotide sequence. Modifications by one molecule of estrogen were evidenced on purines (A, G) whereas no reaction was observed on pyrimidic bases (T). Isomeric adducts were differentiated using tandem mass spectrometry, and energy resolved mass spectrometry allowed to underline differences in the behavior of the adducts towards collisional excitation into an ion trap device.

Applications of mass spectrometry for quantitation of DNA adducts

DNA adducts are formed when electrophilic molecules or free radicals attack DNA. 32P-postlabeling has been the most commonly used assay for quantitation of DNA adducts due mainly to its excellent sensitivity that allows quantitation at concentrations as low as approximately 1 adduct per 10(9) normal bases. Such methods, however, do not have the specificity desired for accurate and reliable quantitation, and are prone to produce false positives and artifacts. In the last decade, mass spectrometry in combination with liquid and gas chromatography has presented itself as a good alternative to these techniques since it can satisfy the need for specificity and reliability through the use of stable isotope-labeled internal standards and highly specific detection modes such as selected reaction monitoring and high-resolution mass spectrometry. In this article, the contribution of mass spectrometry to the quantitation of DNA adducts is reviewed with special emphasis on unique applications of mass spectrometry in the area of DNA adduct quantitation and recent applications with improvements in sensitivity.

The role of DNA repair in nitrogen mustard drug resistance

The nitrogen mustards are an important class of DNA cross-linking agents, which are utilized in the treatment of many types of cancer. Unfortunately, resistance often develops in the treatment of patients and the tumor either never responds to or becomes refractory to these agents. Resistance to the nitrogen mustards in murine and human tumor cells has been reported to be secondary to alterations in (i) the transport of these agents, (ii) their reactivity, (iii) apoptosis and (iv) altered DNA repair activity. In the present review, we will discuss the role of DNA repair in nitrogen mustard resistance in cancer. The nitrogen mustards' lethality is based on the induction of DNA interstrand cross-links (ICLs). Two DNA repair pathways are known to be involved in removal of ICLs: non-homologous DNA end-joining (NHEJ) and Rad51-related homologous recombinational repair (HRR). The reports discussed here lead us to hypothesize that low NHEJ activity defines a hypersensitive state, while high NHEJ activity, along with increased HRR activity, contributes to the resistant state in chronic lymphocytic leukemia. Studies on human epithelial tumor cell lines suggest that HRR rather than NHEJ plays a role in nitrogen mustard sensitivity.

Equilenin-2'-deoxynucleoside adducts: analysis with nano-liquid chromatography coupled to nano-electrospray tandem mass spectrometry

The interaction of 4-hydroxy metabolites of estrogens with DNA leads to the formation of DNA adducts. These adducts are believed to play an important role in the incidence of breast and endometrial cancer. In order to be able to analyze these adducts in in vivo samples a method based upon the coupling of miniaturized liquid chromatography (LC) to electrospray tandem mass spectrometry (ES-MS/MS) was developed for the analysis of the adducts formed with 4-hydroxyequilenin. In vitro synthesized adducts obtained by the reaction of 4-hydroxyequilenin with the main 2'-deoxynucleosides were separated on a Hypersyl C(18) BDS nano-HPLC column (15 cm x 75 microm i.d.) at a flow-rate of 300 nl min(-1) using gradient elution with CH(3)OH--0.2% CH(3)COOH in H(2)O. The column was coupled, in combination with a column switching system, to a nano-electrospray interface. Analysis of the low- and high-resolution low-energy collision-activated dissociation product ion spectra of normal and deuterated adducts supported earlier data demonstrating equilenin to form different isomeric adducts, except with thymidine, for which no adducts were found. The nano-HPLC column-switching ES-MS system was tested for its sensitivity on a triple-quadrupole instrument, and detection limits down to 197 fg in the single reaction monitoring mode were obtained for semi-preparatively isolated equilenin--2'-deoxyguanosine adduct.

Comparison between capillary and nano liquid chromatography-electrospray mass spectrometry for the analysis of minor DNA-melphalan adducts

Nano liquid chromatography (nanoLC) coupled to electrospray mass spectrometry (ES-MS) was evaluated for the analysis of DNA adducts in melphalan-treated Jurkat cells. The detection limit of the nanoLC-ES-MS-MS system was assessed using a dAMP-melphalan adduct. Compared to capillary liquid chromatography (capLC) ES-MS the absolute detection limit could be improved by a factor 10, leading to the detection of 395 fg dAMP-melphalan adduct under single-ion monitoring conditions at a S/N of 14. Minor adducts such as cross-linked adducts could be detected in in vitro solutions of 2'-deoxynucleotides (dNMP) treated with melphalan using column-switching nanoLC-ES-MS. These adducts were not found using capLC-ES-MS. More detailed structural information of the alkylation sites was obtained by examining the nanoLC-ES-MS-MS data. Jurkat cells were treated with melphalan, the modified DNA was isolated and enzymatically hydrolyzed. Several modified dinucleotides were identified, the most abundant adducts were pdG(Mel(Cl))pdC (m/z=453, t(r)=17.0 min) and pdG(Mel(OH)) pdC ring opened (m/z=453, t(r)=39.5 min).

High-performance liquid chromatography--tandem mass spectrometry measurement of radiation-induced base damage to isolated and cellular DNA

A method involving high-performance liquid chromatography (HPLC) separation associated with tandem mass spectrometry (MS/MS) detection in the multiple-reaction monitoring mode was set up for the assessment of radiation-induced degradation products of DNA bases. This sensitive and specific assay is aimed at assessing six oxidized 2'-deoxyribonucleosides and two modified purine bases within both isolated and cellular DNA. For this purpose, stable isotopically labeled internal standards were prepared and used for isotope dilution mass spectrometry measurements. The latter method was validated through a comparison with two other assays, including HPLC associated with electrochemical detection and gas chromatography coupled to mass spectrometry. Using the specific and sensitive HPLC-MS/MS approach, 5,6-dihydroxy-5,6-dihydrothymidine, 5-hydroxy-2'-deoxyuridine, 5-(hydroxymethyl)-2'-deoxyuridine, 5-formyl-2'-deoxyuridine, 8-oxo-7,8-dihydro-2'-deoxyadenosine, 8-oxo-7,8-dihydro-2'-deoxyguanosine, 4, 6-diamino-5-formamidopyrimidine, and 2, 6-diamino-4-hydroxy-5-formamidopyrimidine were quantified within both isolated and cellular DNA upon exposure to gamma-radiation.