Biomedical Engineering Reference
In-Depth Information
Methionine
reducing
conditions
SAMe
SAC
MS
Homocysteine
vasorelaxation
oxidizing
conditions
CBS
cGMP
Cystathionine
sGC
K AT P channel
inhibition of
mitochondrial
respiration,
ROS production
NO
CGL
RSS?
Cysteine
x
RSNO
CBS
overproduction
H 2 S
CGL
O 2
oxidation to
sulfite, thiosulfate, sulfate
Glutathione
FIGURE 8.1 H 2 S metabolic pathways. Under oxidative stress, homocysteine is metabolized toward H 2 S
production by the enzymes cystathionine
lyase (CGL). H 2 S affects vas-
cular smooth muscle cells directly via K AT P channels as well as indirectly by reacting with S-nitrosated thiols
(RSNO) to release NO and produce persulfi des (RSS?), with NO causing vasodilation by soluble guanylate
cyclase (sGC) activation of the cyclic guanidine monophosphate (cGMP) pathway. Under reducing conditions,
homocysteine is metabolized to methionine by methionine synthase (MS) and can be subsequently converted
back to homocysteine through intermediates S-adenosyl methionine (SAMe) and S-adenosyl cysteine (SAC).
An overproduction of H 2 S not adequately removed by oxidation pathways may result in mitochondrial respira-
tion inhibition and reactive oxygen species (ROS) generation (after [41]).
β
synthase (CBS) and cystathionine
γ
sensor (PHSS) for real-time physiological measurement of H 2 S in vivo and in vitro and
discuss its usefulness to better understand the role of H 2 S in cell signaling.
8.1.1 Signifi cance of H 2 S in the life sciences
8.1.1.1 H 2 S chemistry
H 2 S was present in the reducing atmosphere of early earth, along with CO 2 , NH 3 ,
CH 4 , CO, and H 2 . Early organisms made energy using limited oxidative reactions,
and in some reactions H 2 S served as a source of reducing equivalents, for example
in anoxygenic photosynthesis (2H 2 S
H 2 O). The low energy
wavelengths of light (870 nm and 840 nm) necessary for this reaction carried out by
purple/green bacteria indicate the ease of H 2 S oxidation.
Environmental H 2 S is formed mainly by geothermal and biological activity.
Solubility is high, 2 mM total sulfi de kPa 1 , and the potential difference between H 2 S
and O 2 is high,
CO 2
2S
CH 2 O
800 mV, making H 2 S oxidation reactions energetically
favorable. Abiotic H 2 S oxidation (H 2 S
270 mV to
SO 4 2
2H ), catalyzed by metals
2O 2
Search WWH ::




Custom Search