Biomedical Engineering Reference
In-Depth Information
and organics, releases several protons whereas biological H
2
S oxidation (2H
2
S
10H
), catalyzed by several sulfi de oxidase enzymes,
releases many more protons and electrons, useful in biological oxidation reactions
[4, 5]. H
2
S, which dissociates to HS
and H
with a pK near 6.8, exists as two reactive
sulfi de species under physiological conditions.
SSO
4
2
8e
3H
2
O
⇒
8.1.1.2 H
2
S biology
Because the potential difference between H
2
S and O
2
is high, many organisms have
stuck their metabolic foot between these two substrates. Prokaryotic H
2
S oxidation
has been known for years to result in benefi cial energetic gain [3], and H
2
S entered
the mainstream marine invertebrate physiology fi eld when the ocean fl oor hydrother-
mal vent communities were discovered. H
2
S was then recognized as the energy source
for animals containing intracellular chemoautotrophic sulfi de-oxidizing prokaryotic
symbionts that provide food for their gutless hosts [16]. In addition, non-symbiont-
containing animals from H
2
S-rich environments were also shown to gain energetic
benefi t from H
2
S oxidation [4]. For example, the ribbed mussel
Geukensia demissa
from H
2
S-rich sediment was shown to use H
2
S for mitochondrial oxidative phosphor-
ylation in a process called metazoan chemolithoheterotrophy [5].
When H
2
S was found to be a common constituent of mammalian tissues and cells,
researchers began to show that H
2
S could operate in manners similar to NO [17, 18].
For example, H
2
S modulates the function of heme proteins such as cytochrome
c
oxi-
dase, hemoglobin and myoglobin similar to NO, and H
2
S interacts with thiol groups
as a reductant [2]. Physiological effects of H
2
S include smooth muscle relaxation
and K
AT P
channel conductance [19-22], long-term neuronal potentiation via changes
in NMDA receptor [23, 24], and regulation of enzyme activity [2, 25-27] and meta-
bolic state [28]. Pathological effects of decreased H
2
S are implicated in cardiovascular
disease, Alzheimer's disease, and diabetes [29-33], and excess H
2
S production is
implicated in trisomy 21 [34]. Accordingly, cellular H
2
S concentration is most likely
tightly controlled, highlighting the important regulation of H
2
S production and con-
sumption pathways by cellular redox status, O
2
, and other factors [35, 36].
Although it is clear that H
2
S plays an important physiological role in mammalian
systems, it must be realized that many investigations have been carried out at O
2
and
H
2
S concentrations that do not represent physiological levels.
In-situ
H
2
S levels have
not been monitored partly because continuous physiological H
2
S measurements have
not been possible until recently. Without the benefi t of real time H
2
S measurements,
the effects of H
2
S under physiological conditions remain largely undefi ned.
8.1.2 H
2
S measurement in biological samples
8.1.2.1 Stability of sulfur
Like NO, the stability of H
2
S and HS
under physiological conditions is infl uenced
by a number of inorganic and organic components that catalyze oxidation reactions,
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