Minor contribution of direct ionization to DNA base damage inducedby heavy ions.
OBJECTIVE
The deleterious processes triggered by heavy ions on DNA were studied through the determination of the yield of a series of oxidized bases. Emphasis was placed on the estimation of the respective contribution of direct ionization and indirect effects, mostly by comparison with low linear energy transfer (LET) gamma-rays.


METHODS
DNA samples and human monocytes were exposed either to gamma-rays emitted by a (60)Co source or to (12)C(6+) or (36)Ar(18+) ions. The levels of thymidine and 2'-deoxyguanosine oxidation products were determined by liquid chromatography coupled to tandem mass spectrometry subsequently to DNA digestion into nucleosides.


RESULTS
The yields of thymidine lesions were similar to those of 8-oxo-7,8-dihydro-2'-deoxyguanosine within isolated DNA exposed either to gamma-rays or argon ions. Addition of spermine and Tris aimed at minimizing the indirect effect modified this ratio to the same extent with both types of radiation. In cells, the level of radiation-induced base damage was found to be correlated with the radiolytic yield of degrees OH that depends on the LET of the particle. In addition, radiation-induced thymidine and 2'-deoxyguanosine lesions were produced in similar amounts. In contrast, oxidation of 2'-deoxyguanosine was the main process when ionization was triggered in cellular DNA by ultraviolet laser-induced biphotonic processes.


CONCLUSIONS
Predominant oxidation of 2'-deoxyguanosine is expected to be the hallmark of direct DNA ionization. The observation that thymidine and 2'-deoxyguanosine are equally damaged rules out a major contribution of the direct ionization in radiation-induced base damage to both isolated and cellular DNA by heavy ions. Dependence of the yield of lesions on the LET provides further support for this conclusion.
Laboratoire 'Lésions des Acides Nucléiques', Service de Chimie Inorganique et Biologique UMR-E 3 CEA-UJF, CEA/DSM/Département de Recherche Fondamentale sur la Matière Condensée, CEA-Grenoble, Grenoble, France. tdouki@cea.fr
Thierry
T
Douki
Jean-Luc
JL
Ravanat
Jean-Pierre
JP
Pouget
Isabelle
I
Testard
Jean
J
Cadet
D004247:DNA / Q000737:chemistry* / Q000528:radiation effects*; D004249:DNA Damage; D004307:Dose-Response Relationship, Radiation; D005720:Gamma Rays; D020450:Heavy Ions; D009711:Nucleotides / Q000737:chemistry* / Q000528:radiation effects*; D011829:Radiation Dosage
D009711:Nucleotides; D004247:DNA