REACTIVE INORGANIC OXY-SPECIES OF C, N, S, AND P - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 238

proteins [289]. An enhancement of 3-nitrotyrosine has been reported in
numerous diseases [289]. A study has been performed to demonstrate that
imidazole-based thiourea and selenourea derivatives inhibited protein tyro-
sine nitration, mediated by peroxynitrite and peroxidase [290].
Recently, acetylation of amino acids L-His and L-Lys by acetyl radicals,
formed from the reaction of diacetyl and methylglyoxal, has been studied [291,
292]. Peroxynitrite reacted with diacetyl and methylglyoxal with second-order
rate constants of 1.0 × 10 4 /M/s and 1.0 × 10 3 /M/s, respectively, at pH 7.2 and
25°C. The mechanism of acylation of amino acids mediated by acetyl radicals
was confirmed by EPR measurements [292].
In recent years, studies have started appearing on the reactivity of peroxyni-
trite with peroxiredoxins (Prx's) [263, 293-304]. generally, Prx's have shown
high reactivity with peroxynitrite ( k ∼ 10 6 -10 7 /M/s at pH 7.4 and 25°C) [293].
For example, tryparedoxin peroxidases from Trypanosoma cruzi had high
efficiency in the catalytic elimination of peroxynitrite and hydrogen peroxide.
Studied tryparedoxin peroxidases were cytosolic (c-TxNPx) and mitochon-
drial (m-TxNPx). Two cysteine residues in both TxNPxs (Cys52 and Cys173
in c-TxNPx and Cys81 and Cys204 in m-TxNPx) were identified that played
the catalytic role in reducing peroxynitrite and hydrogen peroxide [293]. The
HRP compound I also showed efficient reaction of its protein-cysteine residues
with peroxynitrite ( k = 3.7 × 10 5 /M/s) and hydrogen peroxide ( k = 3.4 × 10 7 /M/s)
at pH 7.4 and 25°C [305]. Significantly, alkyl hydroperoxide reductase (AhpE)
showed ∼10 3 times faster reactivity with peroxynitrite than with hydrogen
peroxide [306, 307]. AhpE is a novel subgroup of the Prx family and contains
Mycobacterium tuberculosis AhpE ( Mt AhpE) and AhpE-like proteins. In the
mechanism study of the reactions, peroxidative thiol oxidation and sulfenic
acid overoxidation through sulfenate anion were proposed. Besides peroxyni-
trite reaction, oxidation ( k ∼ 10 8 /M/s) and overoxidation ( k ∼ 10 8 /M/s) of
Mt AhpE by fatty acid-derived hydroperoxides were also suggested.
The biological chemistry of peroxynitrite was recently evaluated [133]. The
concentrations of target molecules (R) and rate constants of their reactions
with peroxynitrite ( k ) ultimately determine the relevance of peroxynitrite
targets in the biological system. Based on these two parameters, only a few
targets such as CO 2 , Prx's, other thiol proteins (e.g., gSH peroxidase), and some
heme proteins need to be considered (Fig. 5.17) [133]. A range of [R]× k = 60-
100/second has been estimated for the reaction of peroxynitrite with CO 2 ,
considering the concentration of CO 2 is high (≥1.3 mM) and the rate constant
is pH dependent with a value of 5.8 × 10 4 /M/s at 37°C. This range is the estab-
lished benchmark to assess the relative importance of other biological targets.
For example, the rate constants of peroxynitrite with Prx5 is 7.0 × 10 7 /M/s at
pH 7.4 and 37°C and the concentration is >1 µM, giving [R] × k >70/second,
which is higher than that of CO 2 . Similarly, the estimated concentration of
2 µM of the selenium-containing protein, gSH peroxidase, in hepatocytes and
the rapid reaction with peroxynitrite ( k = 8 × 10 6 /M/s; pH 7.4, 25°C) gives
[R] k > 16/M/s. This value is below that of CO 2 but is still significant. displayed