DNA and Double-Stranded Oligonucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 424

DIAMIDE

While BSO interferes with the biosynthesis of GSH (Biaglow et al. 1983; Romero
and Sies 1984), DIAMIDE oxidizes GSH to the disulfide [reaction (87); Harris
1982]. It has also been considered to act as a sensitizer by preventing low O 2 con-
centrations from being depleted at high dose rates (Clark et al. 1983).

NEM and also diethylmaleate (DEM) reduce the GSH content of cells by binding
(Bump et al. 1982), but other reaction pathways have been considered as well
(Mullenger and Ormerod 1969). NEM, for example, is also effective when given
shortly after irradiation (Han et al. 1976) pointing to an interference with the
enzymatic repair rather than at the level of free-radical reactions. The reactions
with DNA radicals is too slow to be of any marked effect (Simic and Hayon 1971;
Hayon and Simic 1972).
Some radicals are sluggishly reduced by thiols but very fast by thiolates
(Chap. 7.4). The fraction of GSH present in the thiolate form (GS − ) [p K a (GSH)

≈

9.2; Wardman and von Sonntag 1995] is given by Eq. (88).

[GS − ]/(GSH] + [GS − ] = [1 + 10 (p K a - pH) ] −1 (88)

Thus at pH = 7.4, 1.6% of the total GSH is in the ionized form. This concentra-

tion is high enough for some radicals to be reduced by GS − in equilibrium rather
than by GSH.
Instead of reducing the cellular GSH level, it may also enhanced upon the ad-
dition of the drug OTZ which stimulates GSH synthesis thereby approximately
doubling the cellular GSH content. At low O 2 concentrations, this leads to an
increased protection (Russo et al. 1985).
Although there is such convincing evidence as to the involvement of thiols on
the oxygen effect, an intriguing problem remains. In their reactions with thiols,
the integrity of DNA is rarely restored, but, taking an • OH-base adduct as an ex-
ample, the resulting product formed by H-donation is a base hydrate (Chap. 11.2).
This is a DNA lesion that has to undergo repair as other DNA lesions. One may
argue that this type of damage is easier to repair than O 2 -mediated damage.

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