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Enzymatic cycle of myeloperoxidase
X
-
Products
HOX
(X = Cl, Br, SCN)
Myeloperoxidase
3+
+ H
2
O
2
Compound I + H
2
O
H
2
O
2
O
2
-
R
-
RH
RH
R
-
O
2
-
O
2
-
O
2
O
2
-
O
2
O
2
-
Compound III
Compound II
Figure 3.1.
The enzymatic cycles of myeloperoxidase (adapted from Davies [1] with
the permission of the Society of Free Radical Research Japan).
+
−
(3.2)
HOBr
H OBr
+
p
a
K
8 .
+
−
.
(3.3)
HOSCN
H OSCN p
a
+
K
5 .
Thus, the speciation of the oxidants is pH dependent (Fig. 3.2). At physio-
logical pH, the acidic forms of chlorine and bromine are present as the domi-
nating species. In the case of HOSCN, the anionic form, OSCN
−
, is the major
species.
The reactivity of HOCl and HOBr with amino acids, peptides, and proteins
is presented in this chapter. As a result of HOSCN as a significant product of
MPO-mediated reactions [14], its reactivity is also presented. The HOCl- and
HOBr-mediated oxidation of proteins generates moderately stable halamines
[15-18], which further oxidizes compounds (e.g., thiols); therefore, the reactiv-
ity of halamines is provided. Finally, chlorine dioxide (ClO
2
) has the oxidative
ability to modify proteins as well as the ability to perform antibacterial activity
against bacteria, viruses, and protozoa, and hence, the oxidations carried out
by ClO
2
are summarized.
3.1 HYPOHALOGENS
3.1.1 Hypochlorite
3.1.1.1 Kinetics of HOCl.
HOCl has been shown to be the major reactive
Cl
2
species in chlorination of a number of compounds [19]. The rate law for the
reactivity of Cl
2
with reported inorganic and organic compounds was first-order
with respect to [HOCl]
Total
and first-order with respect to the total concentration
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