Chemistry Reference
In-Depth Information
12 . 8
Hydrogen Peroxide and Organic Hydroperoxides
The efficiency of H
2
O
2
to inactivate living cells is rather low compared to other
oxidizing agents such as ozone, and relatively high concentrations combined
with long exposure times are required (e.g., Prise et al. 1989; Toledo et al. 1998;
Thacker 1975; Swartling and Lindgren 1997; the fact that spores are quite resis-
tant against H
2
O
2
is a major concern in the food industry; Ito et al. 1973; Ste-
venson and Shafer 1983, where H
2
O
2
is an important disinfectant; for reviews
on the mechanisms of spore resistance, see Setlow 1995; Marquis et al. 1994). As
an uncharged and low-molecular-weight compound, H
2
O
2
can diffuse readily
through membranes. Thus, there is no major barrier to reach the DNA. It is now
generally agreed that DNA damage is due to a Fenton-type reaction (for details
of Fenton-type reactions see Chap. 2.5). Evidence for this has been obtained by
the addition of DMSO as
•
OH-scavenger which reduces SSB formation (Ward et
al. 1985) and chromosomal aberrations (Oya et al. 1986) in vivo as well as base
damage in vitro (Blakely et al. 1990). Moreover, when the DNA was freed from
transition metal ions by exhaustive dialysis with a transition metal ion chelator
an effect of H
2
O
2
was no longer observed. Although this experiment shows that
transition metal ions must be involved, it does not discriminate between iron or
copper as the relevant agent, and there is an ongoing discussion as to what extent
these two transition metal ions contribute to DNA damage in cells (Barbouti
et al. 2001; Bar-Or and Winkler 2002; Galaris et al. 2002). Mitochondrial DNA
shows a 10-15 times higher steady-state level of damaged bases, and this has, in
part, been attributed to the higher iron levels in mitochondrial as compared to
nuclear DNA (Eaton and Qian 2002).
In mammalian cells, the number of SSBs induced at equitoxic doses is much
greater for H
2
O
2
than for X-rays (Bradley and Erickson 1981). HeLa cells exposed
to 1
10
−4
mol dm
−3
H
2
O
2
at 0
1 °C results in the same number of SSBs as a dose
of 300 Gy (Szmigiero and Studzian 1988). Based on a value of 1000 SSBs per cell
per Gy (Elkind 1979), a somewhat higher value of 6000 Gy equivalent was esti-
mated from extrapolated data (Ward et al. 1985). In Chinese hamster V79 cells,
such lesions were already detected upon incubation with 1
×
−
10
−5
mol dm
−3
H
2
O
2
at 4 °C for 20 min (Prise et al. 1989). In addition, damaged bases are formed as
has been shown in in vitro (Demple and Linn 1982; Blakely et al. 1990) and in in
vivo studies (Dizdaroglu et al. 1991).
The ratios of two typical A and G products are compiled in Table 12.13 for
×
γ
-radiolysis in N
2
O/O
2
-saturated solutions which produces 90%
•
OH and 10%
O
2
•
−
and for H
2
O
2
. If in the reaction of DNA-bound transition metal ions with
H
2
O
2
•
OH would be the only agent that is responsible for the observed dam-
age one would expect that these product ratios should be identical, but they are
markedly different (in order to avoid potential uncertainties, data from the same
group where chosen).
Since in both cases the primary damage is set by
•
OH, these differences can
be explained if the transition metal ion which must be still near the damaged
site modifies the damage prior to a reaction of oxygen. Alternatively, H
2
O
2
which
was present at rather high concentrations in these experiments (0.1 to 0.4 mol
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