Biomedical Engineering Reference
In-Depth Information
only recently received a serious consideration [5]. It is now accepted that the super-
oxide radical is an important agent of the toxicity of dioxygen and superoxide dis-
mutases (SODs) constitute the primary defense against this radical [6-9].
Univalently reduced oxygen is called the hydroperoxyl radical (HO
2
•
) in its pro-
tonated form and the O
2
•
radical in its ionized form. The HO
2
•
is a weak acid with
a p
Ka
of 4.8 [10]. The O
2
•
radical can be produced either by the univalent reduc-
tion of dioxygen or by the univalent oxidation of H
2
O
2
. For example, the O
2
•
can be
produced from the electrochemical reduction of dioxygen in aprotic solvents [11-15]
or in alkaline aqueous media [16, 17] or from the chemical reduction of dioxygen by
hydrated electrons or by hydrogen atoms generated during the photolysis [18-21],
radiolysis [10, 22-24], or ultrasonication of water [25, 26]. It could also be produced
from the reduction of dioxygen by carbanions [27, 28], reduced dyes or fl avins [29-
33], catecholamines [34], ferredoxins [35-37], or hemoproteins [38, 39]; and the oxi-
dation of H
2
O
2
by ceric ions [40]. The O
2
•
radical has been detected by a number
of physical methods including conductimetry [41], optical spectroscopy [42, 43],
electron-spin-resonance spectroscopy (ESR) [44-49], and mass spectrometry [50].
In biological systems, O
2
•
is generated as a reduced intermediate of molecular diox-
ygen in signifi cant quantities and is a primary species of the so-called reactive oxygen
species (ROS). As one part of the host defense systems and as cell/cell signaling mole-
cules, O
2
•
performs an essential function. Under normal physiological conditions, O
2
•
undergoes a disproportionation by non-catalytic or enzymatic reactions, leading to a
rather low and undetectable endogenous physiological concentration. An increase in the
activity of O
2
•
has been found to occur in response to traumatic brain injury ischemia
reperfusion and hypoxia [51] and O
2
•
may be involved in the etiology of aging, cancer,
and progressive neurodegenerative diseases, such as Parkinson's disease [52-56].
6.2 O
2
BIOASSAY: AN OVERVIEW
Due to its great roles in biological process, quantitative information on the O
2
•
level
in a variety of
in-vitro
and
in-vivo
models has become very essential in understanding
pathology and physiology of diseases relevant to ROS and free-radical biochemistry.
However, the short lifetime, low concentration, and high reactivity of O
2
•
substan-
tially make it relatively diffi cult to measure the concentration of O
2
•
. Actually, O
2
•
is
not so reactive itself, but it can be disproportionated rapidly in the presence of SODs or
react with other biomolecules to produce H
2
O
2
. H
2
O
2
can be easily turned to hydroxyl
radical, which is the most potent radical, by the Fenton reaction in the presence of
metal ions such as Fe
2
and Cu
2
[57-59].
Due to the rapidity of the spontaneous dismutation reactions, the steady-state con-
centrations of O
2
•
achieved by chemical or by enzymatic reactions are usually quite
low. The physical methods for detecting O
2
•
, although direct and unequivocal, are
restricted to measurements of steady-state concentrations and are thus often found to
lack of sensitivity. For distince, due to the reason mentioned above, when the EPR
method was employed for studying the O
2
•
production by xanthine oxidase, it was
necessary to use a high concentration of the reactants and to work at elevated pH so
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