Chemistry Reference
In-Depth Information
strong oxidant and its redox potential is similar to that of
•
OH [222]. Both
SO
•−
and
•
OH can possibly react with phosphate ions to produce phosphate
radicals (Chapter 5). Reactions of phosphate radicals with aromatic amino
acids and their peptides are discussed in Chapter 5.
1.3.3 Nitrogen Species
Nitric oxide (
•
NO) has been studied extensively due to its importance in bio-
logical chemistry. A low level of
•
NO production is involved in several
processes such as immune system control, blood pressure modulation, signal
transduction, smooth muscle relaxation, memory, and inhibition of platelet
aggregation [21, 223]. However, high levels of
•
NO can result in injury to
tissues [224]. Recent studies hypothesized that dietary nitrite may form
•
NO
in the human stomach [225, 226]. One of the reasons of
•
NO toxicity may be
its reaction with
O
•−
to form peroxynitrite. Peroxynitrite can oxidize as well
as nitrate a number of biological molecules including proteins. Peroxynitrite
can either directly promote one- and two-electron oxidations in molecules or
decompose to secondary radicals such as
•
OH and
•
NO
2
(Chapter 5).
•
NO
2
is
a mild and selective oxidant (Chapter 5). The secondary radicals thus result
in nitration and lipid peroxidation [227]. In the occurrence of high levels of
bicarbonate in interstitial (30 mM) and intracellular (12 mM) fluids, the forma-
tion of a short-lived adduct between peroxynitrite and CO
2
occurs. This adduct
decomposes into oxidizing intermediates,
CO
•−
and
•
NO
2
. The kinetics of nitro-
gen species and nitration products of free amino acids and proteins are pre-
sented in Chapter 5.
1.3.4 Sulfur Species
Thiol proteins play key roles in diverse physiological processes [228, 229]. Thiol
proteins are also important molecules by which reactive species involve them-
selves in cellular transduction pathways [230]. During oxidative stress, oxida-
tion of protein thiols takes place. The reactions of protein thiols (Pr-SH) can
occur through one-electron and two-electron pathways (Fig. 1.8) [230]. In
two-electron pathways, sulfenic acid (Pr-SOH) is initially formed, which can
undergo several secondary reactions. Sulfenic acid can also form mixed disul-
fides (Pr-SS-G) by reacting with GSH. The formation of sulfenic acid is a
prominent feature in several acute and chronic diseases. Significantly, more
than 200 different cellular proteins have been shown to undergo cysteine
oxidation [231].
One-electron oxidants such as radicals and transition metal ions produce
the corresponding thiyl radical (Pr-S
•
). This radical may react with either other
biomolecules or may participate in further reactions with thiol molecules
and oxygen. The reaction with oxygen ultimately forms superoxide and hydro-
gen peroxide. A recent study on the oxidation of methionine-lysine peptide
by
•
OH in the presence of CAT resulted in the formation of methionine
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