REACTIVE INORGANIC OXY-SPECIES OF C, N, S, AND P - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 233

The type of substrate determines the rate-limiting step and the yield of
radicals diffusing out of the solvent cage [248]. The reactivity of peroxynitrite
with transition metal centers are complex and several different mechanisms
are involved [228, 236, 248-255]. Significantly, the ferryl intermediate has been
detected in the reaction of peroxynitrite with chloroperoxidase [256]. There-
fore, reactions of peroxynitrite can generate a secondary oxidizing species in
addition to • NO 2 . However, the oxidizing species may be reversed by available
reductants such as gSH, ascorbic acid, and even excess peroxynitrite.
Recently, the kinetics and spectral measurements on the Fe(III)EdTA/
peroxynitrite system as a function of pH (10.4-12.3) at 25°C in aqueous solu-
tion were made [257]. The reaction between Fe(III)EdTA and peroxynitrite
immediately formed a purple-colored species with an absorption maximum at
520 nm, similar to species obtained in the Fe(III)EdTA/H 2 O 2 system in an
alkaline medium [257-260]. However, the molar extinction coefficient
ε 520 nm = 13/M/cm was much lower than ε 520 nm = 520/M/cm for the Fe III (EdTA)
O 2 ] 3− , a purple high-spin Fe III side-on-bound peroxo complex, observed in the
Fe(III)EdTA/H 2 O 2 system. The rate of the reaction between Fe(III)EdTA
and peroxynitrite increased rapidly and slowed down with increasing concen-
tration of peroxynitrite. This indicates peroxynitrite first binds to Fe(III), the
initial complex, which then formed the colored species. This species may be
an end-on peroxynitrite complex because of much lower absorptivity com-
pared to the peroxo-complex species [257].
5.2.3.6 Reactivity with Amino Acids, Peptides, and Proteins. The kinetics
of the reactions of peroxynitrite with essential amino acids have been deter-
mined [261]. Met, Cys, and Trp were the only amino acids to undergo a direct
reaction with peroxynitrite. The second-order rate constants of these amino
acids are given in Table 5.6 [205, 206, 210, 216, 248, 261, 262, 263, 264, 265, 266,
267, 268, 269, 273, 274]. Other amino acids were modified indirectly by oxidiz-
ing species (e.g., • NO 2 , CO −• , ferryl), suggesting their reactions with peroxyni-
trite in the absence or presence of transition metals [261]. direct and indirect
reactions lead to the formation of oxidized, nitrated, and nitrosated products,
discussed below.
Methionine reacted with peroxynitrite with a second-order rate constant
on the order of 10 2 /M/s (Table 5.6). The nucleophilic sulfur atom in the side
chain of Met reacted with peroxynitrite to form methionine sulfoxide and
nitrite (Table 5.7) [248]. Similar oxidations of Met in proteins, such as α1-
antitrypsin inhibitor and glutamine synthetase, in vitro by peroxynitrite, have
also been observed. Other sulfur-containing amino acids, such as Cys, reacted
the fastest with peroxynitrite (Table 5.6). The second-order rate constants
of the reaction of peroxynitrite with homocysteine, gSH, and the thiol of
albumin were on the order of ∼10 3 /M/s (Table 5.6). Comparatively, thiols in
glyceraldehyde 3-phosphate dehydrogenase, creatine kinase, tyrosine phos-
phatase, and perooxiredoxin reacted faster with second-order rate constant
values in the range of ∼10 5 -10 7 /M/s (Table 5.6). Plots of apparent second-order