REVIEW
From O2 to H2S: a landscape view of gas biology
Misato Kashiba, Mayumi Kajimura, Nobuhito Goda, and Makoto Suematsu
Department of Biochemistry and Integrative Medical Biology, School of Medicine, Keio University, Tokyo, Japan(Received for publication on September 18, 2001)

Abstract. The majority of molecular oxygen (O2 ) consumed in the body is used as a substrate ofcytochrome c oxidase to maintain oxidative phospholylation for ATP synthesis. Rest of the O2 is used
for oxidative biosynthesis including synthesis of vasoactive substances such as prostaglandins and secondary gaseous mediators such as nitric oxide (NO) and carbon monoxide (CO). Thus, O2 is not onlyused for maintenance of energy supply but also for regulating blood supply into tissues. Nitrous oxide(N2 O), laughing gas for anesthesia, is generated endogenously through NO reductase in bacteria andfungi, and has recently been shown to modulate N-methyl-D-aspartic acid (NMDA) receptor function.
A number of other biologically active gases could participate in regulation of cell and tissue functions.
Carbon dioxide (CO2 ) is generated mainly through the Krebs cycle as a result of glucose oxidation andserves as a potent vasodilator, and hydrogen sulﬁde (H2 S) synthesized through degradation of cysteine
has recently been postulated to be a neuromodulater, although their receptor proteins for signaling
have not been veriﬁed as a discernible molecular entity. Easy penetration allow these gases to accessthe inner space of receptor proteins and to execute their biological actions. These gases are generated
and consumed in anaerobic bacteria through varied reactions distinct from those in mammals. This
review summarizes recent information on mechanisms for gas generation and reception in biologicalsystems. (Keio J Med 51 (1): 1–10, March 2002)
Key words: gas biology, nitric oxide, carbon monoxide, nitrous oxide, hydrogen sulﬁdeonists with each other. This property has well been
investigated in the ﬁeld of heme protein chemistry, since
all these gases have the ability to bind the prosthetic
heme of proteins but regulate their function in different
ways. However, a similar investigation has not fully been
carried out in other types of receptor proteins.
Among the gases utilized in the body, oxygen (O2 )has long been studied extensively regarding mechanisms for its transport and utilization and metabolism ina quantitative manner. Nitric oxide (NO) has also beenstudied well and considered to regulate a variety of biological events, since its generation system and biological signiﬁcance was ﬁrst recognized in 1980s.1–3 However, a whole picture of the interactions of NO withother gases and their pathophysiologic links still remain
largely unknown, except that this radical species reacts
with O2 4 or superoxide anions.5 Carbon monoxide(CO) has recently attracted great interest of researchers as a novel signaling molecule for regulation of neurovascular functions.6 Since anaerobic bacteria can generate and utilize this gas as a carbon source, there is a

Introduction
Gaseous substances constitute a unique class ofbiomaterials that are indispensable for maintaining thehomeostasis of biological systems. First, the gases arehighly membrane-permeable and thus serve as a substance that readily conveys the signal from one site toanother in autocrine, paracrine and/or juxtacrine fashions. Secondly, gases can exert their biological actions
through interactions with proteins in multiple ways.
These interactions involve covalent binding of gases toprosthetic metal complexes in receptor proteins, their
non-covalent binding to the critical region for regulation of the protein function, and their space occupancy
by them in and around the protein structure that leads
to reduced accessibility of other gases to the region. Suchfeatures of gases are distinct from other signaling molecules such as hormones. Thirdly, and most importantly,
different gases that share a similar chemical structure(e.g. O2 , NO, CO, CN) could not only exert comparable
biological actions but often compete with and are antag-

Reprint requests to: Dr. Misato Kashiba, Department of Biochemistry and Integrative Medical Biology, School of Medicine, Keio University,
35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan, e-mail: kashiba@sc.itc.keio.ac.jp1

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Kashiba M, et al: Biological actions of gases

possibility that CO serves as a potential cross-talk molecule for regulation of host-parasite relationship. However, such a hypothesis has not yet been elucidated.
Hydrogen sulﬁde (H2 S) is a gas generated throughdegradation of cysteine, and is also used to synthesize
the amino acid; the cysteine-generating pathways inmammals are distinct from those in bacteria, as described later. Despite the well-known toxicity of H2 S, apossible physiologic signiﬁcance of this gas has recently
been reported. This article aims to review the present
status of information on generation, reception and utilization of gas molecules, and challenges in raise possible
working hypotheses as to how these gases and their reception systems could interact with each other in vivo.
Oxygen Sensing Through Biological Systems
Every organism has the ability to sense reductionof oxygen concentrations. Hypoxia causes both acuteand chronic responses. Acute responses involve posttranslational modiﬁcation of proteins through redox
and/or phosphorylation-dephosphorylation mechanismsthat occur in seconds to minutes, while chronic responses involve alterations in gene expression over minutes or hours.7 In mammals, approximately 95% of molecular oxygen (O2 ) consumed in the body is utilized
as a substrate for cytochrome c oxidase. This reaction
contributes to generation of the mitochondrial inner
membrane potential, and thus to oxidative phosphorylation.
Several different strategies seem to be in operation
for sensing oxygen concentrations with acute response
in biological systems. First, the mitochondrial electrontransfer system serves not only as a major energy source
for ATP synthesis but also as an oxygen-sensing apparatus. Upon hypoxia, the activity of cytochrome oxidase
is suppressed as a result of the shortage of the substrate,
leading to leakage of electrons from the transfer systemand subsequent generation of reactive oxygen species(ROS). Considering that ROS could either directly interact with effector proteins such as potassium channels
and/or transcriptional factor such as hypoxia-inducible
factor 1 (HIF-1), this reaction serves as an O2 -sensing
mechanism. Second, ROS is also generated through thereaction of NAD(P)H oxidase in phagocytes and vascular smooth muscle cells.8,9 In response to the reduced
O2 concentrations, amounts of ROS could be decreased,thus serving as a possible mechanism for O2 sensing.
However, considering its extremely low Km for O2 , thisenzyme system is unlikely to account for a major O2 sensing system; for NADPH oxidase in phagocytes, as
small as 1 mmHg O2 is enough to generate sufﬁcientamounts of superoxide anion.10 The third mechanism
involves generation of biologically active gases from O2 .
As discussed later in this article, small amounts of O2

consumed in the body are also used as a substrate forgeneration of NO and CO through the reactions ofNO synthase and heme oxygenase, respectively. Since
amounts of these signaling gases are altered in proportion to local O2 concentrations, these reaction pathways
also serve as an alternative mechanism for O2 sensing.
Finally, in the simplest model, the sensor protein
receives O2 and simultaneously executes biologicalactions directly. Hemoglobin (Hb) is such a well-known
example of an O2 sensor: upon hypoxia, Hb reduces the
afﬁnity to O2 and releases it, while increasing the O2
afﬁnity to limit its delivery under normoxic conditions.
Recent investigation revealed that such Hb-dependent
O2 sensing by erythrocytes not only contributes to regulation of tissue O2 supply but also directly regulates
local vascular tone through the release of ATP from thecells.11 Although the detailed mechanisms remain unknown, circulating erythrocytes have the ability to open
a channel to release ATP upon hypoxia through mechanisms involving the cystic ﬁbrosis transmembrane
conductance regulator (CFTR), a member of the ATPbinding cassette. The released ATP then binds to purinergic receptor on vascular endothelial cells and thereby stimulates endothelial cell-dependent NO release.
In the bacterial kingdom, FixL, a heme protein thatmodulates gene expression responsible for nitrogen ﬁxation in rhizobia, is such an example of the direct oxygen sensor (DOS).12,13 FixL consists of three functionally separated domains: transmembrane, O2 sensor andkinase domain. Binding of O2 at the ferrous heme in
FixL is responsible for regulating the kinase activity. A
similar DOS in Escherichia coli has been reported although its physiological role remains undeﬁned.14
Besides the sensing mechanisms with acute responses,
several transcriptional factors have been known toparticipate in chronic adaptation against hypoxia andto stimulate expression of speciﬁc gene products foradaptation to hypoxic or oxidative stress. In mammals,
the transcriptional regulator, HIF-1, is an essential
mediator of O2 homeostasis.15,16 Under normoxic
conditions, levels of HIF-1 are regulated by removalof the HIF-1a subunit through ubiquination and proteasomal destruction. Hypoxia suppresses ubiquinationof HIF-1a, and prevents its destruction, facilitating
its translocation into the nucleus for binding to hypoxia-responsive elements in promoter regions of variedgenes such as erythropoietin,17 vascular endothelial
growth factor18 and heme oxygenase-1.19 Other important transcriptional factors sensing alterations in
O2 concentrations are nuclear factor kB (NF-kB) and
nrf2. NF-kB is bound to one of several inhibitory proteins such as IkB which prevent its nuclear translocation under normoxic conditions. Hyperoxia or elevations of ROS cause the release of NF-kB from theinhibitory proteins, allowing it to bind to target gene

Keio J Med 2002; 51 (1): 1–10

3

promoters.20–22 Similarly, under normoxic conditions,nrf2 is retained in the cytoplasm by a repressor protein
keap1. Elevation of ROS leads to the activation of
modiﬁcation of nrf2 and/or keap1, although the molecular mechanism(s) for this event remains unknown. The
modiﬁcation follows dissociation of nrf2 and keap1.
Free nrf2 translocates into the nucleus and binds toantioxidant response element (ARE) of the gene promoters, resulting in the induction of ARE-regulated
gene products that constitute the phase-2 drug metabolizing system.23–25 Glutathione S-transferase and heme
oxygenase-1 are involved in the nrf2-responsible cluster. Thus, this pathway accounts for an important protective mechanism against oxidative stress associatedwith xenobiotic metabolism.
Nitric Oxide
NO is a radical species synthesized from oxygen andL-arginine by NO synthase (NOS, EC1.14.13.39).26,27
This gas has a short half-life, is highly reactive, and is
involved in diverse biological actions such as vascularrelaxation and neurotransmission. It also exerts theability to kill microorganisms with its excess amounts
generated through the inducible enzyme, serving as ahost defense mechanism against infection. Biologicalsigniﬁcance of this gas molecule has been described
extensively in other review articles.28–33
NO is a free radical and reacts with various biomolecules (Fig. 1). NO forms dinitrosyl complexes withiron and binds easily with proteins that contain theheme group. Soluble guanylyl cyclase (GC) also contains a heme group, and its combination with NO
explains the mechanism of this mediator’s action on
vascular smooth muscle. When NO binds to the prosthetic heme of the enzyme, the proximal histidine
bond in the axial position is broken, leading to conformational changes critical for activation of the cyclase
domain to produce guanosine-3 0 ,5 0 -monophosphate(cGMP).34 cGMP acts as a second messenger, activatingcGMP-dependent protein kinases, and facilitates thephosphorylation of various proteins.35,36 With excess
amounts, NO also reacts with cytochrome P450, another heme containing enzyme, and inactivates it irreversibly.37 As mentioned later, this is a distinct property compared with CO, which can bind to the enzyme
and inhibit it reversibly.
Although depending primarily on its synthesis by the
enzyme, bioavailability of NO in vivo is also inﬂuenced
by its scavenging and/or sink-buffering systems suchas heme proteins, superoxide5 and thiols38–41 including glutathione (GSH) and cysteine residues of proteins.39,40 NO rapidly loses biological activity particularly under ambient conditions, because NO reacts withmolecular oxygen.4 On the other hand, its biologicalFig. 1 Mechanisms for reception of NO in biological systems. Dark
backgrounds indicate the enzyme systems in bacterial kingdom.

activity increases signiﬁcantly under hypoxia.42,43 Bioavailability of NO also depends on the rate of NO release from its adducts with biomolecules. Although thelife time of NO is believed to be short, its adducts withthiol (S-nitrosothiols, RS-NO) are fairly stable underphysiologic conditions. Thus, the rate of NO releasefrom these complexes is an important factor to control
local NO bioavailability. Analysis in vitro revealed
that various reducing agents, such as thiols and vitamin
C have the potency to release NO from S-nitrosothiols.44–46 Pathophysiologic implications for roles ofvitamin C in regulation of local concentrations of NO
have not fully been investigated.
Nitrous Oxide
Nitrous oxide (N2 O) is a colorless gas with a slightly
sweet taste and odor. The density of the gas is 1.5 times
that of air. This gas molecule is stable and rather inert
chemically at 37  C. N2 O is formed by both enzymaticand non-enzymatic reduction of NO (Fig. 2). In vitro
analysis revealed that N2 O is formed by the reaction
between NO and thiol.47 Hyun et al.,48 reported that
NO is reduced to N2 O by the cytosolic fraction of hepatocytes, suggesting the possible formation of this gas inmammalian cells. In bacteria, N2 O is produced during

4Kashiba M, et al: Biological actions of gases

Fig. 2 Origins of nitrous oxide (N2 O) and its reception and consumption in biological systems. Dark backgrounds indicate the enzyme systems for bacteria.

denitriﬁcation. NO is reduced to N2 O by nitric oxide
reductase.
N2 O has been used clinically as a general anesthetic
for more than a century. Low potency, low solubility,
and rapid induction as well as rapid recovery account for
the widespread acceptance of N2 O as one of the safest
and least toxic of the inhaled anesthetics.49,50 JevtovicTodorovic, et al.,51 reported that N2 O inhibits bothionic currents and excitotoxic neurodegeneration mediated through the N-methyl-D-aspartic acid (NMDA)
receptor. N2 O inhibits methionine synthase by slowing
the conversion of homocysteine to methionine and byincreasing homocysteine concentrations in lymphocyte
cell cultures52 and in human liver biopsy samples.53
Although N2 O neither serves as a ligand to heme
iron nor reacts with thiols, it is detectable at the inner
structure of heme proteins such as Hb, myoglobin, andcytochrome c oxidase.54–56 The ability of N2 O to alter
the structure and function of the heme proteins wasshown by shifts in infrared spectra of cysteine thiols of
Hb55 and by partial and reversible inhibition of cytochrome c oxidase.56 Precise mechanisms for this noncovalent binding between N2 O and the proteins and its
link to biological events need further investigation.
Carbon Monoxide
CO is produced by oxidative degradation of protoheme IX through the action of heme oxygenase (HO;Fig. 3 Generation and reception of carbon monoxide (CO) in mammals and bacteria. Dark backgrounds indicate the enzyme systems forbacteria.

EC 1.14.99.3).57 The enzyme decomposes protoheme
IX by oxidative cleavage of its alpha-methene bridgeand generates biliverdin-IXa and divalent iron together
with this gas (Fig. 3). Biliverdin-IXa is then converted
to bilirubin-IXa through the reaction of biliverdin reductase. In mammals, two forms of the HO isoenzymes
are responsible for oxidative degradation of heme:
HO-1 and HO-2.58,59 HO-1 is inducible in response tostressors, while HO-2 is constitutive. HO-3 has recently
been recognized as a novel isozyme, but the role inendogenous heme degradation in vivo is not fully addressed yet because of its greater Michaelis constantto protoheme IX than that of the other two isozymes. A
variety of stressors such as cytokines, hypoxia, ROS,
and exposure to heme and heavy metals serve as inducers of HO-1. Detail mechanisms for transcriptional
regulation of HO-1 expression are summarized in previous review articles.60
Three products of the HO reaction are all biologically active. Biliverdin and bilirubin are potent scavengers of ROS.61,62 Divalent iron enhances the toxicity

Keio J Med 2002; 51 (1): 1–10

of H2 O2 through the Fenton reaction. On the other
hand, it simultaneously binds to iron-responsive proteinthat causes stabilization of ferritin mRNA and results ina reduction of free iron to be stored in cells. Because
not only CO but also other products of the reaction
have such important physiologic roles, the consequencesof the induction of HO-1 in response to stress stimuli
serve as a protective mechanism as a whole against
oxidative tissue injury.
There are four major organs which abundantly express HO activity under normal conditions: brain, liver,spleen and testes. The role of CO as a neural signaling
molecule was ﬁrst reported in the brain.63 However,previous studies using HO-1 or HO-2 knockout mice
did not fully prove the hypothesis that CO serves as aneurotransmitter.64 Roles of endogenous CO for regulation of vascular systems were ﬁrst reported in the
rat liver by our laboratory, providing quantitative evidence for CO generation and its roles in regulation ofsinusoidal tone.6,65 In the liver, HO-2 is constitutively
expressed in hepatocytes, while HO-1 occurs in Kupffer
cells.66 Since then, an increasing body of evidence suggests that CO generated in the liver contributes not
only to regulation of microvascular tone but also tomodulation of bile output and xenobiotic metabolism.67–69 Mechanisms by which CO regulates drug
metabolism involve inhibition of cytochromes P450 and
an increase in paracellular permeability across thejunction between hepatocytes that allows diffusion ofxenobiotics into bile canaliculi. Since excess loading of
xenobiotics to hepatocytes is a cause of degradationof cytochrome P450-associated heme, the HO reaction
could serve as a fail-safe system that limits unnecessary
degradation of the heme in the liver.
Recent studies in our laboratory have suggested that,
under stress conditions, an increase in CO generation
through the HO-1 induction alters bile output and biliary constituents through multiple mechanisms. Distinct
from its role in the unstimulated liver, CO generated
through the inducible HO appears to restore biliary insufﬁciency in endotoxemic livers.70,71 Such restorationof bile excretion by CO results in part from improvement of lobular perfusion through its vasodilating
action. In this case, CO appears to exert its vasodilatory
actions through inhibition of cytochrome P450 monooxygenases rather than by activation of sGC (soluble
guanylate cyclase); the HO-1-dependent overproduction of CO does not induce any notable elevation of thetissue cGMP contents despite causing marked vasorelaxation.68 CO-elicited choleresis appears to occur
not only through a mechanism involving the increased
sinusoidal perfusion but also through a direct effect on
hepatocytes. CO at micromolar concentrations appears
to stimulate transcellular transport of bilirubin-IXa andglutathione into bile. We have recently found that5

addition of CO to the perfusate showed biphasic
alterations as a function of the concentrations. At
low concentrations CO caused choleresis, while athigh concentrations, it led to a decrease in bile. The
CO-induced increase in bile ﬂow paralleled with theincreased ﬂux of bilirubin-IXa in bile. On the other
hand, the addition of CO did not affect amounts ofphospholipids or bile acids in the bile. Althoughdetailed mechanisms by which CO facilitates excretionof the organic anions are largely unknown, these results
suggest that CO serves as a shadow player that guarantees biliary bilirubin excretion during the heme detoxiﬁcation under stress conditions.
Because of its high and reversible binding properties
to ferroheme proteins, CO can utilize soluble GC andcytochromes P450 as receptor proteins for signal transduction. Since NO can utilize these receptor proteins to
execute its signaling,31,34,37 CO has long been thought
to be merely a substitute gaseous mediator for NO.
However, it has recently been shown that, because thepotency of CO to alter the protein function differs fromthat of NO, CO serves as a partial antagonist for NOmediated activation of soluble GC in vivo. sGC is aheme protein and a heterodimeric enzyme converting
guanosine 5 0 -triphosphate to cGMP. It has been demonstrated that iron (Fe) protoporphyrin is involoved in
the enzyme activity.72,73 There is cogent evidence thatNO binding to Fe of the prosthetic heme causes a breakin the proximal His-Fe bond, forming a ﬁve-coordinated nitrosyl heme complex that is thought to result
in conformational changes and a hundred-fold increasein cGMP generation.74 CO also shares a high afﬁnity
for the heme iron of the sGC; however, it forms a
six-coordinated heme complex with the His-Fe bondremaining intact, presumably inducing smaller conformational changes than observed in the NO-binding
to the enzyme (Fig. 3).74 Thus, the potency of CO toactivate sGC is far less than that of NO. Based on thesedata in vitro, Imai, et al. developed new transgenic micein which HO-1 gene was preferentially expressed in
vascular smooth muscle cells. Of interest is that the
animals exhibited systemic hypertension and reduced
vasodilatory responses to exogenously applied nitrovasodilators.75 Since these animals maintain functional
integrity of soluble GC, mechanisms appear to involvea competition between NO and CO at the prosthetic
heme; CO serves as an agonist for GC when local NO isnegligible, while acting as an antagonist for GC when
NO is sufﬁciently generated in situ. These results suggest that different gases can interact with the same receptor protein and modulate its function in vivo. At thesame time, considering that excess administration of
NO could induce HO-1 in varied experimental models,
the results shed light on mechanisms for resistance tonitrovasodilators in patients with coronary disease.

6Kashiba M, et al: Biological actions of gases

CO is an important carbon source for microorganisms. They utilize CO to be converted to methane
and acetyl CoA through the reaction of CO dehydrogenase.76 Interestingly, this enzyme is a non-heme enzyme that possesses molybdenium or nickel to constitute the catalytic center for CO reception. It was also
shown that anaerobic bacteria with hemolytic toxins
express HO.77,78 This 28 KDa protein catalyzes thesame reaction as that of mammalian HO. These bacteria can catalyze heme derived from food and erythrocytes extravasated from hosts, and thereby recycle
and store free iron for themselves. Aono, et al.79,80
recently revealed a CO-sensing heme protein CooA.
The CooA-CO complex serves as a transcriptional factor that stimulates bacterial replication. It should be
noted that HO-2, a constitutive CO generator, occurs in
the autonomic nervous plexus in intestinal tracts.81
These lines of evidence led us to raise the possibility
that intestinal bacterial ﬂora could stand in a position toutilize the gas from their host to stimulate their replication in the gut. Further investigation is necessary todemonstrate such a hypothesis in vivo.
Hydrogen Sulﬁde
H2 S is a colorless and ﬂammable gas with the characteristic smell of rotten eggs at a low concentration. It
dissolves well in water. Water solution of H2 S is notstable and forms elemental sulfur as a result of the reaction with O2 . H2 S is known as a toxic gas. This gas is
produced in large quantities when sulfur-containing
proteinaceous materials undergo putrefaction. Industrial sources include pulp and paper operations, tanneries, mining, and petroleum reﬁneries produce H2 S.
Exposure to 250 ppm H2 S causes pulmonary edema andinhalation of 1,000 ppm can produce coma and may befatal. Levels in excess of 50 ppm for one hour may leadto keratoconjunctivitis.82
Interestingly, this toxic H2 S is synthesized endogenously in mammalian tissues (Fig. 4). It is produced mainly by two pyridoxal-5 0 phosphate-dependent
enzymes responsible for metabolism of L-cysteine,
cystathionine b-synthase (CBS, EC 4.2.1.22) and cystathionine g-lyase (CSE, EC 4.4.1.1). It should be notedthat H2 S generation is closely associated with thecatabolism of cysteine and methionine as well as withgluthathione metabolism. Both CBS and CSE are responsible for metabolism of methionine into cysteine,
which is in turn used for generation of H2 S. CBS is aheme protein.83 CBS deﬁciency, an autosomal recessively inherited disorder, is the leading cause of homocystinuria in humans.84 Untreated patients develop a
number of phenotypes which include skeletal abnormalities, dislocated optic lenses, mild to profound mental retardation, and vascular disorders. Some patients

Fig. 4 Generation of hydrogen sulﬁde (H2 S) by cystathionine b-synthase (CBS) and cystathionine g-lyase (CSE). Note that H2 S generation is closely associated with catabolism of cysteine and methionine
as well as with gluthathione metabolism.

respond to vitamin B6 administration while others areunresponsive to this therapeutic intervention.85 A
growing body of evidence suggests that vascular disorders found in one-third of the patients with premature arterial disease or cerebrovascular disease are theresult of mild hyperhomocysteinemia, some of which
may result from heterozygous CBS deﬁciency86,87
Overexpression of the synthase in trisomy 21 (Down
syndrome) could be involved in generation of the syndrome phenotype. The CBS gene maps to 21q22.388
and plays a central role in sulfur amino acid and S-adenosylmethionine metabolism.89 It is thus conceivablethat the synthase overexpression in trisomy 21 contributes to the pathophysiology of patients with Down syndrome. The activity of CBS is regulated presumably
at transcriptional level by glucocorticoids and cyclic
AMP.90 S-adenosylmethionine enhances the afﬁnity to
the enzyme by allosteric activation.91,92 Post-translational proteolysis similarly affects the afﬁnity of thesynthase.93 CSE is linked to homocystinuria.94 Malignant lymphoma cells show a marked reduction of CSE
and do not grow in media devoid of L-cystine.94–96

Keio J Med 2002; 51 (1): 1–10

CSE is absent in tissues of mammalian fetuses and its
activity occurs after delivery and increases graduallyduring lactation.97–99 H2 S is also formed in bacteria asa major gaseous substance. Since the gas molecule isable to go through cell membranes without speciﬁctransporters, H2 S formed in bacteria might also affectcellular function in host organs. However, such a possibility has not been investigated yet.
Kimura et al.100,101 reported that physiologically
relevant concentrations of H2 S increase the productionof cAMP and selectively enhance NMDA receptormediated responses and facilitate the induction of hippocampal long-term potentiation. However, its receptors as a discernible entity have not yet been identiﬁed.
Since H2 S can bind to heme proteins as an axial ligand
for the prosthetic group, this gas has the potency tomodulate the activities of heme-containing enzymes invitro, though the physiologic signiﬁcance of this event isquite unknown. Since H2 S serves as a thiol with strong
reducing activities, this compound might also be an
important redox controlling molecule like other small
thiol groups such as cysteine and glutathione. However,such a possibility should further be studied.102
Future Directions
Despites the facts that toxic actions of individual
gases have well been described over the years, their
physiologic roles are poorly understood. Besides thegas substances mentioned in this article, biologicalactions of other gases such as CO2 , and SO
2 have notfully been investigated. Furthermore, other small molecules such as acrolein that are generated upon lipid
peroxidation could also exert their biological actions
under pathologic conditions.103 Since several different
gases share the same receptor proteins and react with awide range of biomaterials through multiple mechanisms, the whole picture of mechanisms by which gases
regulate homeostasis of biological systems is largely
unknown and too complicated to understand at present.
Because of the biological properties of these gases
such as covalent binding to prosthetic metal complexes,
non-covalent binding to the critical site of proteins,and space occupancy in and around the critical protein
structure, details of the atomic basis of the gas-protein
interactions should be examined further through biophysical analyses using advanced spectrophotometry
combined with structural biology. At the same time, inorder to understand roles of gaseous substances in regulation of the host-parasite relationship, in vivo experiments using germ-free and monocontaminated animals
combined with gene-targeting manipulation at the hostside would be essential. Development of a concept ofgas biology greatly depends on utilization and combination of these lines of technology.

7

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