HALOGENATED SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 96

HOCl than for gly-Cl. Chloramines oxidized Prx2 to a disulfide-linked dimer.
An implication of these results in relation to the oxidation of Prx2 by H 2 O 2
has been studied to understand the erythrocyte's ability to resist oxidative
damage [112].
Bromamines are likely the major products of HOBr-mediated oxidation of
biological molecules. A few studies showed NH 2 Br had a much higher reactiv-
ity than the corresponding NH 2 Cl (Table 3.4) [29, 121]. Melatonin was oxidized
efficiently by Tau-Br, while limited oxidation activity was observed using
Tau-Cl [122]. A study was also conducted to oxidize Trp as free or as a residue
in albumin by Tau-Br [123]. Fluorescence analysis on the oxidation of free Trp
by Tau-Br and other oxidants are shown in Figure 3.11 [123]. Similar reactivity
of HOCl and HOBr with Trp was observed. Significantly, Tau-Br was a much
more efficient oxidant than Tau-Cl (Fig. 3.11a). This was also supported by
results of dose-dependent effects of Tau-Br on the consumption of Trp in
comparison with Tau-Cl (Fig. 3.11b). The increased oxidation efficiency of Trp
residues in albumin by Tau-Br in comparison to any other oxidants was also
obtained [123]. The formation of formylkynurenine, an oxidized product of
Trp, was more pronounced in the oxidation of albumin by Tau-Br than other
oxidants. Depletion of Cys and the formation of a carbonyl group were not
significantly different among the tested oxidants. These results clearly demon-
strated that Tau-Br had a higher reactivity for Trp residues in proteins.
3.2.3 Reduction of Halamines
N -Halogenated species can be rapidly reduced by a one-electron process. The
rates of reduction of a series of halamines and haloamides by e a − and O •− have
been determined (Table 3.5) [124, 125]. The halogenated species were reduced
by e a − with k > 10 9 /M/s, while only N -halogenated imides were reduced by O •−
( k > 10 6 /M/s). The reaction scheme is presented in Figure 3.12 [124, 125]. Del-
eterious N -centered radicals and Cl − /Br − were produced in the reactions of
chloramines/bromamines with e a − and O •− (Fig. 3.12). Comparatively, in the
case of N -bromoimides, the heterolytic cleavage of the N-Br bond produced
bromine atoms (Br • ), which may initiate chain reactions. A chain length of
10-20 was suggested. These results indicate that at the elevated O •− concentra-
tion, there may be tissue damage in vivo due to the production of destructive
Br • and organic radicals. This would be pronounced due to an efficient chain
reaction [125].
3.3 CHLORINE DIOXIDE
Chlorine dioxide (ClO 2 ) is produced in the chlorite-mediated peroxidase cycle.
This is demonstrated in Figure 3.13a [126]. The two-electron oxidation of ferric
horseradish peroxidase (HRP) by chlorite produces compound I at a rate
constant k 1 . Compounds I and II are reduced by chlorite via a one-electron