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In-Depth Information
S
SO
k
1
O
O
OH
-
O
OH
-
O
O
K
a
(H
2
CO
4
)
K
a
(H
2
CO
3
)
HO
2
-
OH
-
O
O
OH
HO
OH
HO
O
CO
2
HO
2
-
+ H
+
H
2
O
2
OH
-
+ H
+
H
2
O
Figure 5.9.
Scheme for the activation of H
2
O
2
by
HCO /CO
3
−
in the oxidation of cys-
teine and methionine. Hydration (right side) and the analogous perhydration (left side)
pathways are shown. At physiological pH values, the pathway involving direct reaction
of
HO
−
with CO
2
is likely to be significant (center) (adapted from Richardson et al.
[74] with the permission of Elsevier Inc.).
2
significant oxidation of a third methionine (Met226) was also observed. An
additional example is the catalytic decomposition of
HCO
−
by catalase [73].
5.1.3 Carboxyl Radical
The production of the carboxyl radical (
CO
•−
) during hydralazine and
carbon tetrachloride metabolism has been reported [75, 76]. The α-keto acid
decarboxylation promoted by hydrogen peroxide/transition metal ions or
peroxynitrite is the likely source of
CO
•−
at neutral pH [77]. The carboxyl
radical is a powerful one-electron reductant with a redox potential of −1.85V
(CO /CO pH
2
•−
, . )
[78, 79]. The reduction of the disulfide bond in aponeo-
carzinostatin, the aporiboflavin-binding protein, and bovine immunoglobin
has been investigated in detail [80]. These proteins do not contain free Cys in
the native form. The reactions yielded protein-bound cysteine-free thiols
under γ-ray irradiation, which were pH and protein concentration dependent.
The efficiency of the chain increased upon acidification of the solution and
decreased sharply below pH 3.6. The major protein radical species formed was
protonated disulfide radical anion under acidic conditions. This radical decayed
to thiyl radical, which could react with formate to propagate the chain reaction
with the generation of
CO
•−
. The decay of the disulfide radical anion at pH 8
7 0
2
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