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Halliwell
B,et, al |
(FEBS
Letters, 486(1), 2000)
(Federation of European Biochemical Societies) |
Abstract
Because hydrogen peroxide (H2O2) is widely regarded as a cytotoxic
agent, levels must be minimized by the action of antioxidant defence
enzymes. In fact, H2O2 is poorly reactive in the absence of transition
metal ions. Exposure of certain human tissues to H2O2 may be greater
than is commonly supposed. Levels of H2O2 in the body may be controlled
not only by catabolism, but also by excretion, and H2O2 could play
a role in the regulation of renal function and as an antibacterial
agent in the urine. Urinary H2O2 levels are influenced by diet,
but under certain conditions might be a valuable biomarker of `oxidative
stress'.
1. Introduction
Hydrogen peroxide is freely miscible
with water and is apparently able to cross cell membranes readily,
high (usually 50+ micro-M) levels being cytotoxic. It is therefore
widely thought that H2O2 is very toxic in vivo and must be rapidly
eliminated, employing enzymes such as catalases, peroxidases (especially
glutathione peroxidases) and thioredoxin-linked systems. Paradoxically,
however, in chemical terms, H2O2 is poorly reactive: it can act
as a mild oxidizing or as a mild reducing agent, but it does not
oxidize most biological molecules readily, including lipids, DNA
and proteins (unless the latter have hyper-reactive thiol groups
or methionine residues). The danger of H2O2 largely comes from its
ready conversion to the indiscriminately reactive hydroxyl radical,
either by exposure to ultraviolet light or by interaction with a
range of transition metal ions, of which the most important is probably
iron.
Living organisms have evolved
mechanisms to sequester transition metal ions into protein-bound
forms that cannot catalyze hydroxyl radical formation and other
free radical reactions in vivo. These mechanisms are especially
important in such extracellular fluids as the blood plasma. Nevertheless,
H2O2 can contribute to Fenton chemistry not only by being one of
the substrates but also by providing the other, e.g. by liberating
iron from heme proteins. Addition of H2O2 to cells in culture can
lead to transition metal ion-dependent hydroxyl radical-mediated
oxidative DNA damage, although this damage appears to be rapidly
repaired provided that the cells are not rendered non-viable by
an excess of H2O2. (Spencer J, et
al, Biochem. Biophys. Res. Commun, 224, 1996)
However, levels of H2O2
at or below about 20-50 micro-M seem to have limited cytotoxicity
to many cell types. Indeed, there is a growing literature showing
that H2O2 can be used as an inter- and intra-cellular signalling
molecule. At sites of inflammation, H2O2 generated by activated
phagocytes appears to modulate the inflammatory process, e.g. by
up-regulating expression of adhesion molecules, controlling cell
proliferation or apoptosis and modulating platelet aggregation.
2. Exposure of human tissues to H2O2
Hydrogen peroxide is generated
in vivo by the dismutation of superoxide radical both non-enzymatically
and catalyzed by superoxide dismutase enzymes. Hydrogen peroxide
is also directly produced by a range of oxidase enzymes including
glycollate and monoamine oxidases as well as by the peroxisomal
pathway for beta-oxidation of fatty acids. With the apparent exception
of cardiac muscle, mitochondria in most tissues appear to have limited
capacity to remove H2O2, in that they readily generate substantial
amounts of H2O2 in vitro and probably in vivo. Although mitochondria
contain glutathione peroxidase and thioredoxin-linked peroxidase
activities, the efficiency of these enzymes in removing H2O2 is
uncertain given the ease with which mitochondria release H2O2. It
thus seems likely that most or all human cells are exposed to some
level of H2O2, with the mitochondria being an important source.
However, certain tissues may be exposed to higher H2O2 concentrations.
2.1. The oral cavity, oesophagus and stomach
Several beverages commonly drunk by
humans can contain H2O2 at concentrations above 100 micro-M, including
green and black tea and especially instant coffee. When such beverages
are ingested, the H2O2 they contain presumably rapidly diffuses
into the cells of the oral cavity and upper part of the gastrointestinal
tract. Oral bacteria also produce H2O2, although the resulting levels
of exposure of the oral tissues are uncertain. It is often suggested
that H2O2 released into saliva is used by salivary peroxidase to
oxidize thiocyanate into products toxic to certain bacterial strains.
2.2. The respiratory system
The cells lining the respiratory system,
in common with the oral and oesophageal epithelium, are exposed
to high O2 concentrations (21%) as compared with most other body
tissues. Hydrogen peroxide is present in exhaled air of humans from
phagocytes (e.g. alveolar macrophages, neutrophils in the oral cavity,
or neutrophils recruited to the lungs in inflammatory lung diseases)
or other lung cells. Amounts of exhaled H2O2 appear greater in subjects
with inflammatory lung diseases.
2.3. The kidney, urinary tract and bladder
Substantial quantities of H2O2, at concentrations sometimes exceeding
100 micro-M can be detected in freshly voided human urine, even
in babies. The H2O2 detected in human urine appears to arise, at
least in part, by superoxide radical-dependent auto-oxidation of
urinary molecules, some of which originate from diet. Traces of
superoxide dismutase are present in urine: this enzyme, as well
as the acidic pH of urine, should facilitate both enzymic and non-enzymic
dismutation of superoxide radicals to H2O2. The high levels of H2O2
that can be detected in some urine samples, strongly suggest that
at least some H2O2 generation occurs within the bladder. Hydrogen
peroxide has an antibacterial effect and it may be that its presence
at high levels in urine could be advantageous in diminishing infections
of the bladder and urinary tract. Indeed, there are suggestions
that H2O2 is involved in modulation of renal function. Excretion
of H2O2 may also represent a metabolic mechanism for controlling
its levels in the human body, a valuable tool for assessment of
`oxidative stress'.
2.4. Vascular endothelial and circulating blood cells
Some studies have claimed substantial
levels of H2O2 (up to ~35 micro-M) in human blood plasma, but others
have claimed levels to be very low. In part, it is degraded by the
traces of catalase present, but H2O2 can also react with heme proteins,
ascorbate, and protein-SH groups. In vivo, H2O2 generated in plasma
could also diffuse into erythrocytes, white cells, endothelial cells
and platelets for metabolism. Human plasma may be continuously generating
H2O2, at least under pathological conditions.
2.5. Ocular tissues
The presence of H2O2, at widely varying
levels (in some cases, 100 micro-M or more), has been reported in
human and other animal aqueous and vitreous humors. Any impairment
in the capacity of the lens epithelium, retina or other ocular tissues
to dispose of H2O2 would then result in its accumulation. The origin
of this H2O2 is uncertain, but oxidation of glutathione or ascorbate
is one possibility.
3. Conclusion
Hydrogen peroxide appears to be a ubiquitous
molecule. We exhale it, excrete it and take it in from diet. It
can be detected in drinking water, rain-water and sea water (Willey
J et al, Eco. Prog Ser. 178, 1999; Fujiwara K, Nippon Bunseki Kagakkai
9, 1999.) These data emphasize
the importance of metal ion sequestration in preventing the toxicity
of H2O2 in vivo by decreasing the occurrence of Fenton chemistry.
CAUTION:
Hydrogen peroxide is an irritant of the eyes, mucous membranes,
and skin. Inhalation of high concentrations of the vapor or mist
may cause extreme irritation of the nose and throat. Severe systemic
poisoning may cause headache, dizziness, vomiting, diarrhea, tremors,
numbness, convulsions, pulmonary edema, unconsciousness, and shock.
Exposure for a short period of time to the mist or spray may cause
stinging and tearing of the eyes. Skin contact with liquid hydrogen
peroxide causes a temporary whitening or bleaching of the skin;
if the skin is not washed promptly, redness and blisters may develop.
Ingestion of hydrogen peroxide may cause irritation of the upper
gastrointestinal tract and severe damage to the esophagus and stomach.
(Hathaway G et al. Proctor
and Hughes' chemical hazards of the workplace. Van Nostrand Reinhold.
1991) Splashes of high
concentrations of hydrogen peroxide in the eyes may cause severe
corneal damage. At very low concentrations (1 to 3 percent), instillation
of hydrogen peroxide into the eye causes severe pain that later
subsides. (Grant W, Toxicology
of the eye. Charles C Thomas, 1986)
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