DNA and Double-Stranded Oligonucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 384

antitumor antibiotics, see Sect. 12.9.3). The limited f flexibility of DNA may favor
rather large loop sizes. In dsDNA, this type of process could, in principle, con-
nect not only two sites of the same strand but also two opposite sites (at a dis-
tance). Some of the tandem lesions mentioned above (Sect. 12.5.1) are a kind of
intrastrand mini-cross-link.
In cells, DNA is always associated with proteins, in eukaryotic cells with the
nucleoproteins. This 'packaging' will further reduce the mobility of DNA seg-
ments that may carry a free radical, and their recombination with concomitant
cross-linking is further slowed down. This will allow chemical repair processes
such as H-donation by GSH to take place (Sect. 12.12).

12 .6. 2
DNA−Protein Cross-Links

In principle, DNA protein cross-links may be formed by the recombination of

a DNA radical with a protein radical. This trivial reaction requires that the two
radicals are formed independently and are formed in such a sufficient proxim-
ity that they react with one another rather than undergo other reactions such as
reactions with O 2 or GSH. Radicals in close proximity are formed in the spurs of
ionizing radiation, and under such conditions DNA-histone cross-linking may
occur in the cellular environment where only those • OH radicals that are formed
in the very neighborhood of DNA contribute to DNA damage. Yet, biologically-
active DNA is inactivated by the • OH-adduct of phenylalanine (by addition?; de
Jong et al. 1972), and DNA-protein cross-links are also observed when DNA pro-
tein mixtures are subjected to ionizing radiation in aqueous solution (Schüssler
and Jung 1989; Schuessler et al. 1997; Distel et al. 2002). This reaction can also be
triggered by secondary radicals like the hydroxyethyl radical (Schuessler et al.
1992). Altogether, this cross-linking must be due to a complex sequence of reac-
tions whose details are as yet not known.
Mechanistically, much more interesting are DNA

−

protein cross-links that are
generated by a single radical. As has been shown by the f flash-quench technique,
the G-derived radical is a good candidate (Nguyen et al. 2000). This technique
uses a ruthenium-based intercalator which after photoexcitation can be oxidized
by a quencher such as a Co(III) complex. The resulting intercalator in its higher
oxidation state can now oxidize G (G-free ODNs do not show this cross-linking
reaction). The deprotonated G • + ; that is, G • , has been suggested to undergo the
observed cross-linking reaction. As the nature of this cross-link is concerned,
the authors refer to studies that showed that basic amino acids such as serine,
lysine and threonine can covalently link to the radical formed by the reaction of
photoexcited benzophenone with dGuo and a dGuo derivative (Morin and Cadet
1994, 1995a, b). Proteins attached to DNA mediate hole transfer through DNA
(Wagenknecht et al. 2001), and this kind of process may lead to a DNA

−

− protein

cross-link, for example, via tyrosine (Wagenknecht et al. 2000).
Besides DNA strand breaks, DNA

protein cross-links are also formed upon
photodynamic action of chloroalbumine phthalocyanine (Ramakrishnan et al.
1988) and by a large variety of other photosensitizers (Villanueva et al. 1993).

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