Chemistry Reference
In-Depth Information
Fig. 12.16.
OER for the survival of V 79-135B cells as a function of the O
2
concentration in the me-
dium. Data taken from Millar et al. (1979)
OER = (
m
[O
2
] +
K
) / ([O
2
] +
K
)
(81)
The concept is based on a very simplified scheme [reactions (82)-(86)].
DNA + ionizing radiation
→
damaged DNA
(82)
DNA + ionizing radiation
→
DNA
•
(various DNA radicals)
(83)
DNA
•
→
damaged DNA
(84)
DNA
•
+ RSH
→
DNA (chemical repair)
(85)
DNA
•
+ O
2
→
DNA
−
O
2
•
(oxygen fixation)
(86)
The importance of reaction (84) in this scheme has been pointed out (Schulte-
Frohlinde and Bothe 1990), and on the model level, transforming DNA (Held et al.
1981) or SSB formation (Prakash Rao et al. 1992), a chemical repair by RSH beyond
•
OH-scavenging [reaction (85)] has been clearly shown.
However, this simplified scheme does not account for the fact that in many cas-
es the oxygen effect is biphasic or even more complex (e.g., Ewing and Powers1976,
1980; Millar et al. 1979; Tallentire et al. 1972), and the mathematics of such a com-
plex situation has been discussed (Millar and Scott 1981; Alper 1983, 1984; Koch
1985; Scott 1986). An example of such a multiphasic OER is shown in Fig. 12.16.
The time scale of the oxygen effect is of particular interest as it yields infor-
mation as to the lifetime of the free-radical lesions in a cellular environment.
Several approaches to measure such lifetimes have been followed (Adams et al.
1976b; Michael 1984; Adams 1985).
In the double-pulse method, the first pulse directed on wet cells consumes, by
free-radical reactions, all the O
2
which is dissolved in the cells. The second pulse
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