DNA and Double-Stranded Oligonucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 421

Table 12 .19. Chemical repair rates for O 2 -dependent free-radical precursors of lesions in
V70 cells measured using the gas explosion technique. (Prise et al. 1992)

Rate/s −1 t ½ /ms

Cell survival 1. 3

530 ± 210

DSB (pH 9.6) 380 ± 180 1.8

DSB (pH 7.6) 670 ± 360 1.0

SSB 3.4

210 ± 85

Table 12 . 20. Second-order rate constants (unit: dm 3 mol −1 s −1 ) for fixation by O 2 and
chemical repair by GSH of SSB and DSB free-radical precursors. (Prise et al. 1999)

SSBs DSBs

(2.3 ± 0.02) × 10 8 (8.9 ± 1.2) × 10 7 O 2

(1.4 ± 0.3) × 10 5 (2.9 ± 0.4) × 10 5 GSH

strains contain less of GSH. Since fixation and chemical repair are competing
processes, a faster repair would also result in a shorter lifetime of the O 2 -depen-
dent damage. This being observed, the data support the above concept.
With pBR 322 plasmid DNA in aqueous solution containing 0.01 mol dm −3
GSH and using the gas explosion technique, the rate of repair of SSB precursors
was determined at 1370 s −1 as compared to 2900 s −1 for DSB precursors (Prise et
al. 1993). The OER for SSB was 3.0 and that for DSB 7.5. This is strong evidence
that in this model system DSBs are formed by two close-by SSBs.
The chemical repair rates for the survival of V70 cells and two typical le-
sions, DSBs and SSBs are shown in Table 12.19. There is no difference between
cell survival and DSB formation when the large error bars are taken into ac-
count. In contrast, the repair rate for SSBs is much slower. This difference may
be accounted for on the basis of the clustered lesion model (Sect. 12.5) consider-
ing that if one of two precursors of nearby SSBs has been successfully repaired, a
DSB can no longer develop. Note that the probability of one or the other of these
radicals being chemically repaired is twice that of a that of an SSB free-radical
precursor.
Such studies have been extended to plasmid pBR 322 DNA by modifying the
gas explosion technique and using H 2 S as a repair agent (Prise et al. 1998). With
this assay, it was shown that the post-irradiation protection of DNA by H 2 S is
time-dependent, having first-order rate constants of 21 s −1 for SSB and 10 s −1 for