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and thiyl radicals, which are formed from the one-electron oxidation of thiol
by NO
2
. Pathway 3 involves the direct combination of
•
NO and thiol to form
a putative intermediate radical (RSNOH). The following conclusions were
drawn using both experiment and kinetic simulation: (1) the simultaneous
occurrence of radical and nonradical pathways for S-nitrosation; (2) the direct
addition of
•
NO to thiol for S-nitrosation is not significant; and (3) S-nitrosation
of gSH is not catalyzed or enhanced by the hydrophobic environment of
protein [107]. Therefore, any possible reaction that scavenges the thiyl radical
would favor N
2
O
3
-mediated nitrosation over gS
•
-mediated nitrosation. Fur-
thermore, significant levels of SOd, gSH, other thiols, oxygen, and any other
targets for gS
•
in cells may facilitate the nitrosation of gSH by N
2
O
3
. Also,
any mechanism associated with enhancing the formation of the thiyl radical,
such as action of peroxidases and increased formation of superoxide, may
support nitrosation through thiyl radicals. Conclusions (2) and (3) may not be
valid for some individual proteins containing thiol residues in hydrophobic
protein environments due to the possible reactivity with nitrogen oxides to
cause S-nitrosation.
A recent study on the reactivity of thiyl radicals derived from cysteine,
gSH, and penicillamine with
•
NO showed a near diffusion-controlled rate
constant (
k
= (2−3) × 10
9
/M/s) [114]. The high rate constants suggest the for-
mation of
S
-nitrosothiols in biological systems. Reactions of thiyl radicals with
cyclic nitroxides have also been studied [115]. The rate constants of their reac-
tions were determined to be (5−7) × 10
8
/M/s at pH 5-7 and were independent
of the structure of the thiyl radical and the nitroxide. The main products of
the reactions were identified as the corresponding amines [115]. The proposed
mechanism could account for the protective effect of nitroxides against thiyl
radicals.
In recent years, the formation of
•
NO in natural waters has been explored
[116, 117]. The
NO
−
ion, a trace compound in natural waters, is photochemi-
cally unstable because it can absorb in the sea-level UV region to yield
•
NO
[109, 117-122]:
NO H O
−
+
+ →
h
ν
•
NO OH OH
+
•
+
−
.
(5.25)
2
2
In natural waters, the reactions of alkyl peroxy radicals and
•
NO may be
one of the sources of alkyl nitrates [123]. The photoformation rates of
•
NO in
river water and seawater have been measured as (9.4−300) × 10
−12
/M/s and
(5.3−39) × 10
−12
/M/s, respectively [116, 117]. The scavenging rate constants for
•
NO in the surface seawater samples were measured as 0.0-0.33/second [116].
The concentrations of
•
NO were determined as (2.4 − 32) × 10
−11
M [116].
Nitrification and denitrification processes in natural waters can also produce
•
NO [124]. However, a recent study demonstrated there is a negligible contri-
bution of photobiological processes to produce
•
NO in natural waters [116].
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