Chemistry Reference
In-Depth Information
plot of
X
total
/
X
intrinsic
vs
D
[expression (71)], where
X
is the quantity that is being
experimentally determined.
X
total
/
X
intrinsic
= 1 +
G
(OH)
D
/
t
incubation
v
(OH)
intrinsic
(71)
Irradiation would be carried out at the same O
2
status as in incubation, i.e.,
dose rates will have to be such that O
2
depletion by peroxyl radical formation
(Chap. 8) is minimal.
3.5.5
Metabolic OH-Radical Production
and Probing for OH Radicals in Vivo
At present, it seems that we do not yet have any reliable data on the rate of meta-
bolic production of
•
OH within the biological system. This will be proportional
to the rate of O
2
uptake. In a human organism, this has been estimated to be near
15 mol day
−1
, of which 1% is channeled into O
2
•
−
, i.e.,
10
−3
mol dm
−3
day
−1
(Halliwell and Gutteridge 1999). It is not very likely that there is a direct channel
into
•
OH, and it is reasonable to assume that its formation requires as precursors
O
2
•
−
and H
2
O
2
(in combination with low-valent transition metal ions). A major
source of H
2
O
2
may be O
2
•
−
(cf. the role of SOD). Considering the action of cata-
lase which keeps the cellular steady-state H
2
O
2
concentration at a low level, the
rate of
•
OH production must be significantly lower than that of O
2
•
−
. Hydroxyl-
radical production will not be uniform within the cell, and in a more complex
system such as a human it is even more likely that there are areas of low and high
•
OH production. This will make the probing very difficult, because a given probe
may be unevenly distributed within the body.
An approach to estimate the rate of
•
OH production
in the very neighborhood
of DNA
is on the basis of oxidized nucleobases excreted. From that, the total rate
of 'oxidative hits' at the DNA in man is estimated at 10
4
per cell per day times 6
×
∼
4
×
10
−6
mol body
−1
day
−1
(provided that part of the oxidized bases is not recycled in the cell). Putting
the ratio of DNA to the total of organic material in the body (without bones)
at 100 g/35 kg, we
extrapolate
to 7
10
13
cells per body (Ames and Shigenaga 1992), i.e., 1
×
10
−6
mol dm
−3
day
−1
. From an estimate of
the
•
OH steady-state concentration (in hepatocytes) (Boveris and Cadenas 1997),
one calculates a rate of
•
OH production of 1.5
×
10
−5
mol dm
−3
day
−1
.
The two latter values are much below the value of 2
×
10
−3
mol dm
−3
day
−1
calculated on the basis of salicylate data (Coudray et al. 1995) and the cellular
scavenging capacity discussed above. This raises the question whether the sa-
licylate assay can be taken as a reliable in vivo probe. It has been discussed above
that 2,5-dihydroxybenzoate is also formed enzymatically and that this product
cannot be used for monitoring
•
OH production. For this reason, reference has
always been made only to the other isomer, 2,3-dihydroxybenzoate. However,
the question has already been raised whether there are unknown cellular re-
actions (besides via
•
OH) which produce this isomer (Halliwell and Grootveld
1987). Evidence that this may indeed be the case comes from data compiled in
Table 3.8. Here, salicylate (or aspirin) has been given to either a rat or a human,
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