Singlet Oxygen Induces Oxidation of Cellular DNA

The aim of the present work was to evaluate the potential for 1O2 to induce oxidation of cellular DNA. For this purpose cells were incubated in the presence of a water-soluble endoperoxide whose thermal decomposition leads to the formation of singlet oxygen. Thereafter, DNA was extracted and the level of several modified DNA bases was determined by HPLC analysis coupled to a tandem mass spectrometric detection. A significant increase in the level of 8-oxo-7,8-dihydro-2′-deoxyguanosine was observed upon incubation of the cells with the chemical generator of1O2, whereas the level of the other DNA bases measured remained unchanged. To demonstrate that singlet oxygen is directly involved in the formation of 8-oxo-7,8-dihydro-2′-deoxyguanosine, the corresponding18O-labeled endoperoxide was used. Incubation of the cells with such a generator of 18O-labeled singlet oxygen results in the formation of 18O-labeled 8-oxo-7,8-dihydro-2′-deoxyguanosine in the nuclear DNA. This result clearly demonstrates that singlet oxygen, when released within cells, is able to directly oxidize cellular DNA. The aim of the present work was to evaluate the potential for 1O2 to induce oxidation of cellular DNA. For this purpose cells were incubated in the presence of a water-soluble endoperoxide whose thermal decomposition leads to the formation of singlet oxygen. Thereafter, DNA was extracted and the level of several modified DNA bases was determined by HPLC analysis coupled to a tandem mass spectrometric detection. A significant increase in the level of 8-oxo-7,8-dihydro-2′-deoxyguanosine was observed upon incubation of the cells with the chemical generator of1O2, whereas the level of the other DNA bases measured remained unchanged. To demonstrate that singlet oxygen is directly involved in the formation of 8-oxo-7,8-dihydro-2′-deoxyguanosine, the corresponding18O-labeled endoperoxide was used. Incubation of the cells with such a generator of 18O-labeled singlet oxygen results in the formation of 18O-labeled 8-oxo-7,8-dihydro-2′-deoxyguanosine in the nuclear DNA. This result clearly demonstrates that singlet oxygen, when released within cells, is able to directly oxidize cellular DNA. 2′-deoxyguanosine 8-oxo-7,8-dihydro-2′-deoxyguanosine N,N′-di(2,3-dihydroxypropyl)-1,4-naphthalenedipropanamide 1,4-endoperoxide of DHPN 18O21,4-endoperoxide of DHPNO2 high performance liquid chromatography coupled to tandem mass spectrometry Modification of cellular DNA upon exposure to reactive oxygen and nitrogen species is the likely initial event involved in the induction of the mutagenic and lethal effects of various oxidative stress agents (1Basu-Modak S. Tyrrell R.M. Cancer Res. 1993; 53: 4505-4510PubMed Google Scholar, 2Piette J. J. Photochem. Photobiol. B Biol. 1990; 4: 335-342Crossref PubMed Scopus (51) Google Scholar, 3Epe B. Chem.-Biol. Interact. 1992; 80: 239-260Crossref Scopus (174) Google Scholar). As an example, the deleterious effects of UVA radiation are, at least partly, explained in terms of photooxidation of cellular DNA (4Kielbassa C. Epe B. Carcinogenesis. 1997; 18: 811-817Crossref PubMed Scopus (476) Google Scholar, 5Alapetite C. Wachter T. Sage E. Moustacchi E. Int. J. Radiat. Biol. 1996; 69: 359-369Crossref PubMed Scopus (143) Google Scholar). The mechanism of UVA-mediated photooxidative damage to DNA is not completely established. Evidence has been accumulated over the years for the significant implication of singlet oxygen, as the result of UVA activation (4Kielbassa C. Epe B. Carcinogenesis. 1997; 18: 811-817Crossref PubMed Scopus (476) Google Scholar, 6Cadet J. Berger M. Douki T. Morin B. Raoul S. Ravanat J.-L. Spinelli S. Biol. Chem. 1997; 378: 1275-1286PubMed Google Scholar) of endogenous photosensitizers (porphyrins, flavins, … ) not yet characterized. However, a type I mechanism involving the initial formation of a DNA radical cation, that could be predominantly located at guanine sites due the lowest ionization potential of the latter base and/or to the possibility of charge transfer in DNA (7Hall D.B. Holmlin R.E. Barton J.K. Nature. 1996; 382: 731-735Crossref PubMed Scopus (856) Google Scholar), could not be excluded. To our knowledge, no clear evidence has been provided to demonstrate that singlet oxygen is able to oxidize cellular DNA. It should be added, however, that1O2 is known to be mutagenic and genotoxic (2Piette J. J. Photochem. Photobiol. B Biol. 1990; 4: 335-342Crossref PubMed Scopus (51) Google Scholar, 3Epe B. Chem.-Biol. Interact. 1992; 80: 239-260Crossref Scopus (174) Google Scholar, 8Piette J. J. Photochem. Photobiol. B Biol. 1991; 11: 241-260Crossref PubMed Scopus (175) Google Scholar). In addition, singlet oxygen has been identified as the reactive oxygen species involved in numerous biological processes. Among others we may cite neutrophils phagocytosis (9Steinbeck M.J. Khan A.U. Karnovsky M.J. J. Biol. Chem. 1992; 267: 13425-13433Abstract Full Text PDF PubMed Google Scholar) and enzymatic processes (10Cadenas E. Sies H. Hoppe-Seyler's Z. Physiol. Chem. 1983; 364: 515-528Crossref PubMed Scopus (82) Google Scholar).Reactions of singlet oxygen with nucleosides and isolated DNA are well documented. Interestingly, was that1O2 the the guanine oxygen with and to to the formation of the and of with a of J.-L. J. Chem. Res. PubMed Scopus Google Scholar, J. Ravanat J.-L. PubMed Scopus Google Scholar, C. J. Chem. 1993; Scopus Google Scholar, J. Douki T. Ravanat J.-L. PubMed Google Scholar, J. Berger M. Ravanat J.-L. Res. 1992; PubMed Scopus Google Scholar). In was to be the oxidation upon exposure of isolated DNA J. Berger M. Douki T. Morin B. Raoul S. Ravanat J.-L. Spinelli S. Biol. Chem. 1997; 378: 1275-1286PubMed Google Scholar, J.-L. J. Chem. Res. PubMed Scopus Google Scholar, J. Photochem. Photobiol. PubMed Scopus Google Scholar). However, be as a biological of1O2, this DNA could be various of oxidative by H. Z. Berger M. J. J. Chem. 1992; Scopus Google Scholar), radical Int. J. Radiat. Biol. 1993; PubMed Scopus Google Scholar), and H. S. Res. PubMed Scopus Google Scholar). this could be as an of DNA oxidation S. PubMed Scopus Google Scholar, S. J. Scopus Google Scholar, S. J. 1996; PubMed Scopus Google Scholar). the other the formation of in cellular DNA could not be to the initial formation of the water-soluble of1O2, that of C. S. C. J. Photochem. Photobiol. B Biol. 1996; PubMed Scopus Google Scholar, H. H. Res. PubMed Scopus Google Scholar, Sies H. J. Chem. Scopus Google Scholar). decomposition of the latter is to singlet oxygen C. S. C. J. Photochem. Photobiol. B Biol. 1996; Scopus Google Scholar). Interestingly, the water-soluble endoperoxide DHPNO2 C. S. C. J. Photochem. Photobiol. B Biol. 1996; PubMed Scopus Google Scholar) was to be cells upon incubation C. C. Sies H. J. PubMed Scopus Google Scholar). such a chemical generator of singlet oxygen, C. C. Sies H. J. PubMed Scopus Google Scholar) were able to the activation of upon of singlet purpose of the present work was to DHPNO2 to singlet oxygen is able to oxidize nuclear DNA. was the of the mechanism of DNA For this the 18O-labeled endoperoxide was to the formation of oxidative damage results the within cellular DNA. It was clearly that formation of singlet oxygen is able to of for at with DHPNO2 results in the formation of in cellular DNA as determined the The level of in cellular DNA was to be to DNA bases Incubation of the cells with of DHPNO2 and results in a significant increase to in the level of significant increase in the level of was observed when cells were incubated with of DHPNO2 when the incubation was at of in DNA of cells for of in the presence of in of were by of DHPNO2 at for at for and the of the endoperoxide the is to endogenous and to by as the cells are incubated with DHPNO2 an increase in the level of is observed as no could be in cells in the cells were incubated with the endoperoxide a increase of is and 18O-labeled is The results of the cells with of the endoperoxide are in for the of in cellular DNA. were not with of the endoperoxide of the corresponding18O-labeled endoperoxide For the mass spectrometric was to endogenous the as well as 18O-labeled of and measured in cellular DNA upon of cells with of the and of was to evaluate the formation of is due to the of singlet oxygen. For this the cells were incubated with at to the of the endoperoxide Thereafter, the cells were by to the of in incubation at the formation of could be However, the of endoperoxide an of the is of in cellular DNA upon incubation in the presence of no and cells were with of were incubated at for whereas was at For to the incubation at cells were incubated with the endoperoxide at Thereafter, of was added, and cells were by to the of endoperoxide has not of the for at in the presence of DHPNO2 results in a significant formation of increase is observed the endoperoxide is the of singlet oxygen in the formation of the The level of was determined to be to DNA This is in with J. Douki T. Ravanat J.-L. PubMed Google Scholar, S. PubMed Scopus Google Scholar) and that to an of the level of in cellular DNA due to an oxidation the J. C. Douki T. Ravanat J.-L. S. Res. PubMed Scopus Google Scholar). increase in the formation of other DNA and not was observed upon incubation of the cells in the presence of DHPNO2 as determined by S. Douki T. Ravanat J.-L. C. J. Chem. Res. PubMed Scopus Google Scholar). The results clearly that 1O2 is able to in cellular in with the known the guanine of isolated was to demonstrate that is not For this cells were incubated in the presence of DHPNO2 at the endoperoxide is at the of singlet oxygen the incubation is and the of DHPNO2 is no increase in was observed This demonstrates that is not and DNA This could be at least partly, by the that the of the DNA in the of the J. Douki T. Ravanat J.-L. PubMed Google Scholar), is by the of a of the In addition, a well known singlet oxygen was to the to singlet oxygen and to DNA oxidation due to the presence of the generator The results that the of 1O2 the thermal decomposition of DHPNO2 is able to induce oxidation to the guanine base of cellular such an oxidation could result the of 1O2 within cellular DNA the oxidative stress by the of singlet oxygen. a chemical generator of singlet oxygen Ravanat J.-L. J. J. Chem. Scopus Google Scholar) was to the For this the was incubated with DHPNO2 and the of was by the and The latter the of endogenous with The latter is by the of 1O2 by the thermal decomposition of cells are incubated with DHPNO2 an increase in the level of is observed as no could be in cells in the A in the level of and an formation of 18O-labeled is measured when cells were incubated with the The in as observed with the the formation of is with the of endoperoxide used. Interestingly, that the of the endoperoxide is Ravanat J.-L. J. J. Chem. Scopus Google Scholar), the result that the formation of observed when cells are with the endoperoxide could be to the of singlet oxygen within nuclear the in of 18O-labeled singlet oxygen within the guanine of cellular was to evaluate the formation of is due to the of singlet oxygen. For this the of endoperoxide was by incubation of the cells with the However, the of endoperoxide an of the is This that could be by as well as singlet oxygen. However, the in the formation of could be explained by the that an the and of the results in a of that may the of In addition, of the endoperoxide cells within a at is not may be that the formation of is due to the formation of This is in with the that the endoperoxide has been to predominantly an due to chemical C. C. Sies H. J. PubMed Scopus Google could be to the of formation of the in DNA bases could be when cells were in of with of DNA to the of was and the was may be that of were that of the oxygen by the thermal decomposition of the endoperoxide is in singlet C. S. C. J. Photochem. Photobiol. B Biol. 1996; PubMed Scopus Google Scholar, C. Sies H. PubMed Google Scholar), this the of 1O2 were able to to of of 1O2 are for the formation of results could be the is the results in the However, the of formation is when the of the endoperoxide is For example, the of in the in is that in the in In the of DHPNO2 were to cells in of in the the was in of of formation of the the for singlet oxygen to oxidize cellular DNA. This be at least partly, to the by the cellular to the formation of mutagenic DNA In this the of and singlet oxygen been A.U. T. S. 1992; PubMed Scopus Google present results that singlet oxygen is able to induce oxidation of cellular DNA. an oxidation results in the formation of The of a chemical generator of 18O-labeled singlet oxygen to demonstrate that the formation of the is due to the of 1O2 within cellular DNA. however, not the of the present in cellular DNA with is likely to oxygen Modification of cellular DNA upon exposure to reactive oxygen and nitrogen species is the likely initial event involved in the induction of the mutagenic and lethal effects of various oxidative stress agents (1Basu-Modak S. Tyrrell R.M. Cancer Res. 1993; 53: 4505-4510PubMed Google Scholar, 2Piette J. J. Photochem. Photobiol. B Biol. 1990; 4: 335-342Crossref PubMed Scopus (51) Google Scholar, 3Epe B. Chem.-Biol. Interact. 1992; 80: 239-260Crossref Scopus (174) Google Scholar). As an example, the deleterious effects of UVA radiation are, at least partly, explained in terms of photooxidation of cellular DNA (4Kielbassa C. Epe B. Carcinogenesis. 1997; 18: 811-817Crossref PubMed Scopus (476) Google Scholar, 5Alapetite C. Wachter T. Sage E. Moustacchi E. Int. J. Radiat. Biol. 1996; 69: 359-369Crossref PubMed Scopus (143) Google Scholar). The mechanism of UVA-mediated photooxidative damage to DNA is not completely established. Evidence has been accumulated over the years for the significant implication of singlet oxygen, as the result of UVA activation (4Kielbassa C. Epe B. Carcinogenesis. 1997; 18: 811-817Crossref PubMed Scopus (476) Google Scholar, 6Cadet J. Berger M. Douki T. Morin B. Raoul S. Ravanat J.-L. Spinelli S. Biol. Chem. 1997; 378: 1275-1286PubMed Google Scholar) of endogenous photosensitizers (porphyrins, flavins, … ) not yet characterized. However, a type I mechanism involving the initial formation of a DNA radical cation, that could be predominantly located at guanine sites due the lowest ionization potential of the latter base and/or to the possibility of charge transfer in DNA (7Hall D.B. Holmlin R.E. Barton J.K. Nature. 1996; 382: 731-735Crossref PubMed Scopus (856) Google Scholar), could not be excluded. To our knowledge, no clear evidence has been provided to demonstrate that singlet oxygen is able to oxidize cellular DNA. It should be added, however, that1O2 is known to be mutagenic and genotoxic (2Piette J. J. Photochem. Photobiol. B Biol. 1990; 4: 335-342Crossref PubMed Scopus (51) Google Scholar, 3Epe B. Chem.-Biol. Interact. 1992; 80: 239-260Crossref Scopus (174) Google Scholar, 8Piette J. J. Photochem. Photobiol. B Biol. 1991; 11: 241-260Crossref PubMed Scopus (175) Google Scholar). In addition, singlet oxygen has been identified as the reactive oxygen species involved in numerous biological processes. Among others we may cite neutrophils phagocytosis (9Steinbeck M.J. Khan A.U. Karnovsky M.J. J. Biol. Chem. 1992; 267: 13425-13433Abstract Full Text PDF PubMed Google Scholar) and enzymatic processes (10Cadenas E. Sies H. Hoppe-Seyler's Z. Physiol. Chem. 1983; 364: 515-528Crossref PubMed Scopus (82) Google Scholar). of singlet oxygen with nucleosides and isolated DNA are well documented. Interestingly, was that1O2 the the guanine oxygen with and to to the formation of the and of with a of J.-L. J. Chem. Res. PubMed Scopus Google Scholar, J. Ravanat J.-L. PubMed Scopus Google Scholar, C. J. Chem. 1993; Scopus Google Scholar, J. Douki T. Ravanat J.-L. PubMed Google Scholar, J. Berger M. Ravanat J.-L. Res. 1992; PubMed Scopus Google Scholar). In was to be the oxidation upon exposure of isolated DNA J. Berger M. Douki T. Morin B. Raoul S. Ravanat J.-L. Spinelli S. Biol. Chem. 1997; 378: 1275-1286PubMed Google Scholar, J.-L. J. Chem. Res. PubMed Scopus Google Scholar, J. Photochem. Photobiol. PubMed Scopus Google Scholar). However, be as a biological of1O2, this DNA could be various of oxidative by H. Z. Berger M. J. J. Chem. 1992; Scopus Google Scholar), radical Int. J. Radiat. Biol. 1993; PubMed Scopus Google Scholar), and H. S. Res. PubMed Scopus Google Scholar). this could be as an of DNA oxidation S. PubMed Scopus Google Scholar, S. J. Scopus Google Scholar, S. J. 1996; PubMed Scopus Google Scholar). the other the formation of in cellular DNA could not be to the initial formation of the water-soluble of1O2, that of C. S. C. J. Photochem. Photobiol. B Biol. 1996; PubMed Scopus Google Scholar, H. H. Res. PubMed Scopus Google Scholar, Sies H. J. Chem. Scopus Google Scholar). decomposition of the latter is to singlet oxygen C. S. C. J. Photochem. Photobiol. B Biol. 1996; Scopus Google Scholar). Interestingly, the water-soluble endoperoxide DHPNO2 C. S. C. J. Photochem. Photobiol. B Biol. 1996; PubMed Scopus Google Scholar) was to be cells upon incubation C. C. Sies H. J. PubMed Scopus Google Scholar). such a chemical generator of singlet oxygen, C. C. Sies H. J. PubMed Scopus Google Scholar) were able to the activation of upon of singlet oxygen. The purpose of the present work was to DHPNO2 to singlet oxygen is able to oxidize nuclear DNA. was the of the mechanism of DNA For this the 18O-labeled endoperoxide was to the formation of oxidative damage results the within cellular DNA. It was clearly that formation of singlet oxygen is able to of for at with DHPNO2 results in the formation of in cellular DNA as determined the The level of in cellular DNA was to be to DNA bases Incubation of the cells with of DHPNO2 and results in a significant increase to in the level of significant increase in the level of was observed when cells were incubated with of DHPNO2 when the incubation was at the of the endoperoxide the is to endogenous and to by as the cells are incubated with DHPNO2 an increase in the level of is observed as no could be in cells in the cells were incubated with the endoperoxide a increase of is and 18O-labeled is The results of the cells with of the endoperoxide are in for the of in cellular DNA. were not with of the endoperoxide of the corresponding18O-labeled endoperoxide For the mass spectrometric was to endogenous the as well as 18O-labeled of and measured in cellular DNA upon of cells with of the and of was to evaluate the formation of is due to the of singlet oxygen. For this the cells were incubated with at to the of the endoperoxide Thereafter, the cells were by to the of in incubation at the formation of could be However, the of endoperoxide an of the is of in cellular DNA upon incubation in the presence of no and cells were with of were incubated at for whereas was at For to the incubation at cells were incubated with the endoperoxide at Thereafter, of was added, and cells were by to the of endoperoxide has not Incubation of for at with DHPNO2 results in the formation of in cellular DNA as determined the The level of in cellular DNA was to be to DNA bases Incubation of the cells with of DHPNO2 and results in a significant increase to in the level of significant increase in the level of was observed when cells were incubated with of DHPNO2 when the incubation was at In the of the endoperoxide the is to endogenous and to by as the cells are incubated with DHPNO2 an increase in the level of is observed as no could be in cells in the cells were incubated with the endoperoxide a increase of is and 18O-labeled is The results of the cells with of the endoperoxide are in was to evaluate the formation of is due to the of singlet oxygen. For this the cells were incubated with at to the of the endoperoxide Thereafter, the cells were by to the of in incubation at the formation of could be However, the of endoperoxide an of the is of the for at in the presence of DHPNO2 results in a significant formation of increase is observed the endoperoxide is the of singlet oxygen in the formation of the The level of was determined to be to DNA This is in with J. Douki T. Ravanat J.-L. PubMed Google Scholar, S. PubMed Scopus Google Scholar) and that to an of the level of in cellular DNA due to an oxidation the J. C. Douki T. Ravanat J.-L. S. Res. PubMed Scopus Google Scholar). increase in the formation of other DNA and not was observed upon incubation of the cells in the presence of DHPNO2 as determined by S. Douki T. Ravanat J.-L. C. J. Chem. Res. PubMed Scopus Google Scholar). The results clearly that 1O2 is able to in cellular in with the known the guanine of isolated was to demonstrate that is not For this cells were incubated in the presence of DHPNO2 at the endoperoxide is at the of singlet oxygen the incubation is and the of DHPNO2 is no increase in was observed This demonstrates that is not and DNA This could be at least partly, by the that the of the DNA in the of the J. Douki T. Ravanat J.-L. PubMed Google Scholar), is by the of a of the In addition, a well known singlet oxygen was to the to singlet oxygen and to DNA oxidation due to the presence of the generator The results that the of 1O2 the thermal decomposition of DHPNO2 is able to induce oxidation to the guanine base of cellular such an oxidation could result the of 1O2 within cellular DNA the oxidative stress by the of singlet oxygen. a chemical generator of singlet oxygen Ravanat J.-L. J. J. Chem. Scopus Google Scholar) was to the For this the was incubated with DHPNO2 and the of was by the and The latter the of endogenous with The latter is by the of 1O2 by the thermal decomposition of cells are incubated with DHPNO2 an increase in the level of is observed as no could be in cells in the A in the level of and an formation of 18O-labeled is measured when cells were incubated with the The in as observed with the the formation of is with the of endoperoxide used. Interestingly, that the of the endoperoxide is Ravanat J.-L. J. J. Chem. Scopus Google Scholar), the result that the formation of observed when cells are with the endoperoxide could be to the of singlet oxygen within nuclear the in was to evaluate the formation of is due to the of singlet oxygen. For this the of endoperoxide was by incubation of the cells with the However, the of endoperoxide an of the is This that could be by as well as singlet oxygen. However, the in the formation of could be explained by the that an the and of the results in a of that may the of In addition, of the endoperoxide cells within a at is not may be that the formation of is due to the formation of This is in with the that the endoperoxide has been to predominantly an due to chemical C. C. Sies H. J. PubMed Scopus Google could be to the of formation of the in DNA bases could be when cells were in of with of DNA to the of was and the was may be that of were that of the oxygen by the thermal decomposition of the endoperoxide is in singlet C. S. C. J. Photochem. Photobiol. B Biol. 1996; PubMed Scopus Google Scholar, C. Sies H. PubMed Google Scholar), this the of 1O2 were able to to of of 1O2 are for the formation of results could be the is the results in the However, the of formation is when the of the endoperoxide is For example, the of in the in is that in the in In the of DHPNO2 were to cells in of in the the was in of of formation of the the for singlet oxygen to oxidize cellular DNA. This be at least partly, to the by the cellular to the formation of mutagenic DNA In this the of and singlet oxygen been A.U. T. S. 1992; PubMed Scopus Google present results that singlet oxygen is able to induce oxidation of cellular DNA. an oxidation results in the formation of The of a chemical generator of 18O-labeled singlet oxygen to demonstrate that the formation of the is due to the of 1O2 within cellular DNA. however, not the of the present in cellular DNA with is likely to oxygen Incubation of the for at in the presence of DHPNO2 results in a significant formation of increase is observed the endoperoxide is the of singlet oxygen in the formation of the The level of was determined to be to DNA This is in with J. Douki T. Ravanat J.-L. PubMed Google Scholar, S. PubMed Scopus Google Scholar) and that to an of the level of in cellular DNA due to an oxidation the J. C. Douki T. Ravanat J.-L. S. Res. PubMed Scopus Google Scholar). increase in the formation of other DNA and not was observed upon incubation of the cells in the presence of DHPNO2 as determined by S. Douki T. Ravanat J.-L. C. J. Chem. Res. PubMed Scopus Google Scholar). The results clearly that 1O2 is able to in cellular in with the known the guanine of isolated DNA. was to demonstrate that is not For this cells were incubated in the presence of DHPNO2 at the endoperoxide is at the of singlet oxygen the incubation is and the of DHPNO2 is no increase in was observed This demonstrates that is not and DNA This could be at least partly, by the that the of the DNA in the of the J. Douki T. Ravanat J.-L. PubMed Google Scholar), is by the of a of the In addition, a well known singlet oxygen was to the to singlet oxygen and to DNA oxidation due to the presence of the generator The results that the of 1O2 the thermal decomposition of DHPNO2 is able to induce oxidation to the guanine base of cellular DNA. However, such an oxidation could result the of 1O2 within cellular DNA the oxidative stress by the of singlet oxygen. a chemical generator of singlet oxygen Ravanat J.-L. J. J. Chem. Scopus Google Scholar) was to the For this the was incubated with DHPNO2 and the of was by the and The latter the of endogenous with The latter is by the of 1O2 by the thermal decomposition of cells are incubated with DHPNO2 an increase in the level of is observed as no could be in cells in the A in the level of and an formation of 18O-labeled is measured when cells were incubated with the The in as observed with the the formation of is with the of endoperoxide used. Interestingly, that the of the endoperoxide is Ravanat J.-L. J. J. Chem. Scopus Google Scholar), the result that the formation of observed when cells are with the endoperoxide could be to the of singlet oxygen within nuclear the in was to evaluate the formation of is due to the of singlet oxygen. For this the of endoperoxide was by incubation of the cells with the However, the of endoperoxide an of the is This that could be by as well as singlet oxygen. However, the in the formation of could be explained by the that an the and of the results in a of that may the of In addition, of the endoperoxide cells within a at is not may be that the formation of is due to the formation of This is in with the that the endoperoxide has been to predominantly an due to chemical C. C. Sies H. J. PubMed Scopus Google Scholar). A could be to the of formation of the in DNA bases could be when cells were in of with of DNA to the of was and the was may be that of were that of the oxygen by the thermal decomposition of the endoperoxide is in singlet C. S. C. J. Photochem. Photobiol. B Biol. 1996; PubMed Scopus Google Scholar, C. Sies H. PubMed Google Scholar), this the of 1O2 were able to to of of 1O2 are for the formation of results could be the is the results in the However, the of formation is when the of the endoperoxide is For example, the of in the in is that in the in In the of DHPNO2 were to cells in of in the the was in of The of formation of the the for singlet oxygen to oxidize cellular DNA. This be at least partly, to the by the cellular to the formation of mutagenic DNA In this the of and singlet oxygen been A.U. T. S. 1992; PubMed Scopus Google Scholar). The present results that singlet oxygen is able to induce oxidation of cellular DNA. an oxidation results in the formation of The of a chemical generator of 18O-labeled singlet oxygen to demonstrate that the formation of the is due to the of 1O2 within cellular DNA. however, not the of the present in cellular DNA with is likely to oxygen