ABSTRACT
Nitric oxide (NO) along with Carbon monoxide (CO) and Hydrogen Sulphide
(H2S) are biologically significant gaseous molecules
generally called as “gasotransmitters”. At a concentration higher or
lower than optimum value may result in toxicity or malfunctioning of
mammalian tissues. Soon after the acknowledgment of NO as
multifunctional bio-signalling molecule in 1987, many interesting
implications of this field emerged out. Meanwhile, several studies have
proven the NO-biosynthetic pathway responsible for normal functioning of
eye. High intraocular pressure (IOP) has been suggested as the main risk
factor in this context and collaborative approach with nitric oxide
releasers is said to control IOP and hence the relation with glaucoma.
Similar miracles were reflected from several other naturally produced
gaseous molecules,viz ., CO and H2S after
year 1990. The biological roles of both these molecules are now widely
accepted and in the current era investigations focused mainly with
development of efficient CO and H2S releasing compounds.
CO and H2S donors are also said to help in normalising
IOP like NO. Therefore the trio-gasotransmitters have collective
relation with the ophthalmic homeostasis in association with nervous
control. On one hand, the antimicrobial efficiency of these three
molecules is widely known and on the other hand, their collaborative
key-role in ocular nerve functioning makes it remarkable to state here
that their donors are supposed to act as a shield for both the
infectious as well as the non-infectious eye defects.
Keywords: Gasotransmitters; Ophthalmic diseases; NO; CO;
H2S, NORMs, CORMs, H2S-donors.
Introduction
The scientific recognition of carbon monoxide (CO) and hydrogen sulphide
(H2S) as bio-conjugated molecules sharing similar
functional role as nitric oxide (NO) resultedin coining the term
“gasotransmitters” for these molecules based on size, lipophilic
character,half-life and several other features [1, 2].Even though
these gases share a number of common features, they also possess
dissimilar characteristics and display noteworthy interactions, which
complicate the interpretation of their physiological activities.
In the late 1990’s, the scientific community saw a very unusual
phenomenon, the conversion of nitric oxide from harmful gases into an
important chemical messenger. The remarkable role of this molecule in
signal transduction and cytotoxicity is considered to be one of the
greatest marvels of biological chemistry in recent times. The biological
diversity of this molecule is well documented in neuroscience,
physiology and immunology [3,4]. Recommendations obtained from the
chemical stress of nitric oxide included the ”Molecule of the Year” vote
in 1992 by the journal ”Science”, published by the American Association
for the Advancement of Science (AAAS) [5]. At present, NO has been
accepted to be linked to many physiological mechanisms including
platelet aggregation and adhesion, neurotransmission, synaptic
plasticity, vascular permeability, hepatic metabolism, senescence, and
renal function [6-9]. In high concentration (μM), NO also plays a
strategic role in the immune system [10] and in suppression of
carcinogenic state [11, 12]. The prominence in the absence of
biology and medicine was emphasized in 1998 when the Nobel Prize for
Physiology and Medicine was awarded to Robert Furchgott, Louis Ignarro
and Ferid Murad for their role in transforming the role of NO in the
nervous, cardiovascular and physiological systems. In microbial world NO
plays mediator role in denitrification [13-15] (Figure 1 ).
In addition, the molecule is tailored in various ways due to its
important therapeutic potential [16,17].
NO2− NO N2O
N2
Figure 1 . Denitrification involving NO as intermediate
Carbon monoxide (CO) has long been known as a dangerous gas for mammals
and is called as a “silent killer” [18]. Carbon monoxide, when
inhaled enters the bloodstream, formscarboxyhaemoglobin (COHb) at a rate
240 times greater than oxygen [19]. This reduces the oxygen
transport ability and results in hypoxia [20]. Biologically, CO is
considered as a by-product of heme oxygenase (HO) metabolism [21],
and in the early stage of its biological exploration, CO was found as a
chronic neurotransmitting agent [22]. Therefore, the further studies
have altered the general perception of CO as a harmful molecule[23].
CO has now become an important molecule in the physical monitoring of
many organ systems. In the last few decades, investigations related to
CO have shown this gaseous molecule as a major chemical messenger.
In the seventeenth century, Carl Wilhelm Scheele recognized
H2S through chemical analysis. However, it has long been
speculated that the gas is derived from the sewage system, and is linked
to a series of special eye diseases that occur in sewage workers. The
disease is associated with severe inflammation, secondary bacterial
attacks and even blindness [24].Similar to NO and CO, internally
generated H2S is now considered a significant
gasotransmitter [25] and within a neuromodulator, which draws a lot
of attention in literature. Traditional neurotransmitters bind to and
activate membrane receptors, whereas gasotransmitters are able to freely
distribute to adjacent cells and directly bind to their target proteins
to supplement biological functions by contributing to their short-term
mutations [26].
Like NO which S-nitrosylates a variety of proteins, H2S
physically regulates the different protein functions by S-sulfhydration.
However, S-nitrosylation inhibits enzymes, while S-sulfhydration
stimulates them. Therefore, H2S is an important
physiologic gasotransmitter such as NO and CO [27-30].
The eye is one of the most sensitive parts of the brain. Any impairment
in eye function requires high quality care. Among eye health problems
intraocular pressure (IOP), cataract and retinal hypertension continue
to remain as potential risk factors in treatment. Due to our growing
interest in the synthesis of various chemicals tagged with NO, CO and
H2S [31-39] and the many biological actions of
H2S and the more precise delivery of this flexible gas
to target tissue in the form of H2S sponsors [40],
the current chapter focuses on in the applied interest of NO, CO and
H2S-compounds on eye-physiology. A historical view of
the emergence of the term ”gasotransmitter”, within the production of
NO, CO and H2Sin mammals, and to sought strong sponsors
of NORMS, CORMS and H2S-releasers (in the event of
chronic biosynthesis and digestion) applicable in the most common
eye-defects are the main objectives of thisliterature update.
A Meaningful Introduction Towards “Gasotransmitters”
In general, gasotransmitters refer to the distinctive class of molecules
like NO, CO and H2S, responsible for communication
amongstbody cells for a particular biological action. Albeit, these
molecules exist in solvated form while in biological medium, the
respectivedifferences in size, action, shape and bio-membrane
interactions stems their multitude biological roles reported so far. The
signal transduction pathway among such carriers may range from short to
long distances to transmit the required information[41]. The
properties and functional diversity found in these bioessential
signalling molecules, therefore gave rise to coin a new term in
reference to their biological relevanceas “gasotransmitters”.
Several parameters may be found differentiating neurotransmitters from
gasotransmitters. From the cellular biology it is clear that
neurotransmitters stored in vesicles get released by the intervention of
a suitable stimulus (Figure 2 , top). These responses are
receptor-specific in nature and depend on the molecular signalling to
bring forth a physiological move. Hence, synaptic vesicles behave as a
reservoir of information required at the time of safety or normal
physiological functioning. Whereas, gasotransmittersare endogenously
availedsmall molecules of signalling potentiality (“gaso” refers to
their gaseous nature under normal conditions) [41, 42].
Gasotransmitters have the main characteristic feature of diffusing
through cellular membrane without the aid of any receptor
(Figure 2 , bottom). No reservoir is required (like vesicles in
neurotransmitters), but are rapidly produced in response to a stimulus
when needed[1, 2]. Moreover, here in gasotransmitters cell
exocytosis fashion followed in neurotransmitters is not pronounced at
al.Therefore a separate term ‘gasotransmitter’ coined by Wang in 2002 is
suitable to distinguish them from neurotransmitters [25].The
vasorelaxant and gasotransmitter labelling of NO enhanced scientific
vigour to an extraordinary fashion and activated the seek for other
molecules of this class [43,44, 25]. [44]. Gases other than
these are also under interrogation to add further possible members to
this group.
Figure 2. Diagram showing the mechanism of neurotransmitter (2A,
top) and
gasotransmitter (2B, bottom)action.
Biosynthesisand target of NO, CO and H2S
Biological synthesis and target of NO
NOis biologically synthesized by the catalytic action (oxidation of the
nitrogen coloured red in Figure 3 ) ofnitric oxide synthase
(NOS) over L-arginine as substrate and resulting in the formation of
L-citrulline [45]as shown in Figure 3 . Since, the NO
production is involved in every system of a human body, there are three
distinctive gene products and isoforms of NOS, producing NO in the
presence of oxygen, flavins and NADPH [46]. Figure 4(i) andFigure 4(ii) represent these three members as NOS-I, NOS-II and
NOS-II. These are also called as eNOS (endothelial), iNOS (inducible)
and nNOS (neural), respectively, because of the specific
target/production locus during their biosynthesis.NOS-I is central and
peripheral nervous system linked [47], NOS-III is
vaso-relaxationconnected [46] and NOS-II is immunological directed.
From the literature it is found that NOS-II functions independent of the
presence of calcium, and is a source of considerable amount of NO
produced for a longer duration as compared to NOS-I and NOS-III
[48].Several examples L-arginine structural analogues have been
found to prevent NOS from producing
NO,viz ., NG-monomethyl-L-arginine
(L-NMMA), NG-nitro-L-arginine (LNA),
NG-nitro-L-arginine methyl ester (L-NAME), etc.
[49]. However, by making instant and sufficient arginine
availability, the action could get reversed initially.
Figure3. Reaction showing thebiosynthesis of nitric oxide
Figure4. Interaction of three NOS isoforms
Biological Production and Target of COAs per the metabolic pathways concerned with the CO-biosynthesis,
almost 14% of 500 μmol/day is obtained from lipid peroxidation and
from photooxidation plus self-activation of cytochrome p-450. Bacteria
and Xenobiotics also contribute the same minor percentage [67,68].
Major contribution(almost 86%) is generated by the
erythrocyte-breakdown, wherein, the haem-oxygenase (HO) catalyzes this
oxidation. Like NOS, HO also exists in two isoforms,viz, HO-1
and HO-2. These are alsocalled as inducible and
constitutive,respectively. Both the isoforms show same rate-limiting
step while catabolizing heme, the difference lies with the regulation,
amino acid sequence, and distribution in the tissues. Another HO has
been recently identified and named as HO-3. This form of HO was
detected in the several organs of rats. Till date no haem-degradation
study has been reported for this newly detected HO-member [69].The
metabolic pathwayof HO-catalyzed haem oxidation involves several
important stages as has been illustrated in Figure 5 . In
addition to CO other intermediatory products like of
α-meso-hydroxyheme, verdoheme, biliverdin (converts to bilirubin as
excretory product conjugated by glucoronic acid shown inFigure 6 ) are also involved. [70,71].The bioaction of
HO-1 under stressful situation gets enhanced and the CO-production
gets increased than the optimal value [72]. Therefore, suchan
elevation in the concentration can be used as a sign convention
medically to read the associated behaviour. The similar correlation
has been found in several diseases wherein a patient is expected to
suffer from stress and strain conditions.For instance in
bronchiectasis,asthma, cystic fibrosis, hyperglycemia and other
diseases CO level appears higher than the normal [73]. Hence, the
detection level of CO because of inducible HO-1 can help in diagnosis
of pathophysiological state.
Figure 5. Oxidation of heme by heme oxygenase (HO)forming CO as
a by-product.
Figure 6. Chemical structure ofGlucuronic acid
CO in mammals has been found to show target specific action in two
ways,viz. ,soluble guanylyl cyclase (sGC) pathway and Non-cGMP
pathways. It is well established fact that NO binds with heme by
replacing one histidine unit to result in the activation of soluble
guanylyl cyclase (sGC) [74]. Generally NO binds to heme b to
formaunstable short lived six-coordinate systemto finally cleave the
heme-His105 bond, and forms a five coordinated heme complex with NO
(Figure 7 ). Therefore, it is conformational change in other
words that leadsto sGC activation [75].The resulting sGC catalyzes
the conversion of GTP to cGMP in the fashion as given
inScheme-1. The cGMP formation is related to a number of
sequential pathways entailed with several clinical implications
asillustrated inScheme-2 (red coloured).
Figure.7 . Diagram showing the activation of guanylyl cyclase by
NO and CO/YC-1.[Taken from Ref. E. Martin, K. Czarnecki, V. Jayaraman,
F. Murad and
J. Kincaid, J. Am. Chem. Soc., 127(2005) 4625-4631]
Scheme-1: sGC catalyzing the Conversion of GTP tocGMP
Scheme-2: Cyclic guanosine monophosphate (cGMP)routes
ameliorating medical conditions shown as red.cGKs = cGMP-dependent
protein kinases, Iα and Iβ; IRAG =Inositol
1,4,5-triphosphate(IP3 )receptor-associated cGKIβ
substrate; VASP = vasodilator-stimulated phosphoprotein; PDEs
=phosphodiesterases; cAMP = cyclic adenosine monophosphate [Adopted
from Ref. Brian E. Mann and Roberto Motterlini, Chem. Commun., (2007)
4197-4208]
Therefore, in an analogous way like NO, this molecule (CO) alsoyields
cGMP by activating guanylyl cyclase. However, it may be mentioned here
that this activation is only 1/80theffective as NO.
Some trials have been found to use synthetic compounds like YC-1
(Figure 8 ), when used in combination with CO can increase this
effectiveness up to the level of NO. This indicates that there should be
some naturally existing signalling molecule like YC-1 that shows the
guanyl cyclase activation similar to NO.There are contrary observations
reported for CO-activity in the same activation study. CO while
coordinating withguanylyl cyclase results in six-coordinated iron
complex and central metal ion continues to remain as
FeII. The retention of five-coordinated inability of
CO with iron is due to the increased radii of the ion anddecrease in the
electronegativity. This shows CO-based vasorelaxation goes through a no
similar pathway as in NO. On the other hand, considering YC-1
interaction with the displacement of His-105 produces a 5-coordinate
complex with retaining CO as co-ligand [79](Figure8 ),there
is another interpretation for the CO-dependent stimulation of sGC
[80].
Figure 8. Structure of
YC-1:1-Benzyl-3-(5′-hydroxymethyl-2furyl)indazole
Non-cGMP CO-pathway is remarkably in other targets to lead
vasorelaxation, incorporating the involvement of potassium channels
(BKCa). The conductance BKCachannels are
found distributed almost in everytissue, and get affected byseveral
modulators. Among contributing factors towards such channels, protein
kinases, endogenous NO and CO along with heme are mentionable. Many
physiological processes including neuronal excitability, contractility
of muscles, and vascular tone maintenance are because of
BKCa activity by involving the suitable yield of action
potential [81].
Biosynthesis and target sites of H2S
H2S is considered as the recently recognized third
member among gasotransmitters. This gas is also produced endogenously
through enzymatic reactions (Figure 9 ) and is responsible for
maintaining physiological balance.Recently, non-enzymatic pathways have
also been reported to be responsible for biosynthesis of hydrogen
sulphide. The optimal concentration of H2S under normal
conditions is said to be present in micromoles (µM) [82].L-cysteine
is the important bioessential source of this gas involving the role of
cystathionine β-synthase (CBS), cytosolic/pyridoxal-5′-phosphate
(P5P)-dependent enzymes, and cystathionineγ-lyase (CSE)[83,84] or
the tandem enzymes, cysteine aminotransferase (CAT) and
3-mercaptopyruvate sulphur transferase (3-MST), mainly confining the
activity in mitochondria. Absolute H2S is produced from
CSE and CBS, whereas 3-mercaptopyruvate transfers sulphur to cysteine
through the catalytic application of 3-MST resulting in the formation of
persulfide [85, 86]. Thereafter, persulfide acts as a
H2S-releaser endogenouslycausing the reduction of
disulfides,e.g, reduction of dihydrolipoic acid (DHLA) or
thioredoxin(Trx) under physiological terms [87]. Strictly, therefore
it is thioredoxin reductase (TR) acting sequence wise in combination
with glutathione (GSH) and sulfur oxidase (SO) in this phenomenon. The
significance of H2S (oxidized form) in respiratory chain
is well documented acting as electron donor in Q,III and IV steps of the
chain that results in the generation of energy currency (ATP) and
cellular oxygenconsumption. Hence, biosynthesis and
H2S-mediated sensing of oxygen represent essential role
of H2S-biochemistry.
Production of H2S in tissues via cystosolic enzymes
CBS and CSE
(ii) (a) Generation of H2S via tandem enzymes, CAT and
(3-MST) and(b)
H2S oxidation in the mitochondria
Figure 9. Biosynthesis of H2S and its oxidation
[Adopted from Ref. K. R. Olsona, J. A. Donald, R. A. Dombkowski and S.
F. Perry, Respiratory Physiology & Neurobiology ,184 (2012) 117–
129]
The biological synthesis of H2S as in the case of NO and
CO, must be followed by the consumption or target phenomenon. This gas
has no colour, flammable and smellslike that of rotten eggs.Itsacid
strengthis weak (pKa value of 6.98 at 25 °C and 6.76 at 37 °C). The
dissociation of this gas in water may be represented as shown in
Scheme-3 (Ka1=1.3×10−7 M,
Ka2 =1×10-19) [39]. The
H2S that remains undissociated is volatile, while the
dissociated form HS− is not volatile. In physiological
medium these dissociation patterns are pH-dependant.At the 7.4 pH,
one-third of H2S remains udissociated. Physiological pH
does not support the substantial presence of S2−(because high pH is required for it). As per another scientific
observation H2Smainly persists in HS-form(82%) because of having weak acid strength (pKa1:
6.76; pKa2: 19.6)[97].
Scheme-3. Reactions showing the dissociation of
H2S in aqueous solution
Therefore, both the H2S as well as
SH−are contributoryunder biological activity of
hydrogen sulfide, despite the fact that SH−,
represents more nucleophilic potential than cysteine (Cys) or reduced
form of glutathione (GSH), thatswiftly coordinates with bio-metallic
centres or interacts with othercompounds[98].In mitochondrial chain
reactions, H2Sdisplays sequential oxidation trend.
Initially, it gets oxidized to thiosulfate, followedby conversion to
sulfite and subsequently to sulfate. The first oxidation stage
isnon-enzymatic in nature, while the rest steps are carried out
enzymatically using thiosulfate cyanide sulfurtransferase (TST). Despite
the observation showing sulfate as major end product of the metabolic
pathways followed by H2S, urinary thiosulfate represents
a non-specific indicator to sense thequantity of H2S
production within a body [99]. Overall, the target of
H2S is highly influenced by several factors especially
the rapid oxidation disfavouring the long-distance transport. Therefore
the development of efficient storage system endogenously applicable is
suggested.For instance,in NO-association,H2S gets stored
as nitrosothiols(RS-NO) implying the dual gas-combinatory fashion. This
opens several areas of interest to seek answer for the unexplored
queries regarding the isolated and combined gasotransmitters
research.Meanwhile, the solvated fashion of H2S reveals
different solubility trend based on the nature of the solvent. Due to
lipophilic feature of this gas, H2Seasily crosses cell
membrane. The tested solubility experiments have shown itto be five
times moresoluble in lipophilic solvent as compared to water. The data
furnished from its solubility behaviour in various solvents other than
water have been keenly recorded [100, 101].The solution chemistry on
further exploration depicts that concentration H2S under
varied physiological abnormalities within mammalian blood. Harmful
implications of H2S at the level of <100 ppm
gets expressed in the formsore throat, eye irritation, dizziness, etc
[102–104].The exposure at greater than 1000 ppm affects CNS,
respiratory chain and may more even cause death[104].
Gasotransmitters in the Mission of Vision (Eye-Health
Contribution)Eye is one of the most important sense organs performing the function
of vision through interacting with light, involving several
physicochemical phenomena to memorize the surroundings, and therefore
acts as a natural perception mediator to translate the observations to
the brain. So, ultimately light-phenomenon to nerve actions, so many
tissues collaborate to let such a complex process to happen. The
physiology of eye is not restricted to a simple conduction process
only, but is subjected to the role of the gasotransmitters introducedvide supra . This section details the role of NO, CO and
H2S in maintaining a healthy eye, so is the title
established as “the aim of gasotransmitters in the mission of
vision”.
NO News is Good News for Eyes: NO Donors for the Treatment
of Eye Diseases
Since the dawn of NO-recognition as a key signalling molecule, the
diversified biological role of this free radical met with its extended
role in so many areas of physiological investigations. Keeping in view
the combinatory functional status of NO and cGMP are entailed with a
range of biological actions, there are numerous evidencessupporting the
fact that the NO-metabolic pathways are also involved in the normal
functioning of an eye.The respective neurological role served as a
motivational move for the researchers to find the possible
responsiveness while studyingeye functioning. These responsible roles
include dynamics of aqueous humour (AqH) dynamics, retinal
neurotransmission and other light induced pathways. Any malfunctioning
that results in the respective NO-generation can cause eye abnormality
[48]. As of now, the normal tissue functioning of eye in concern
with eNOS and nNOSbasedrolehas got wide acceptance [105]. This is
because neural and immunologic expressive forms of NOS have been found
in retina. Several reports are evidential in supporting nNOSresponsible
in photoreception via NO-generation, and similar effect in bipolar
cells. This in turn leads to stimulus for guanylate cyclase
photosensitive rod cells and thereby increasing the calcium channel
activation. By stopping NOS action in the retina of cats, results
indicate impairment in photo-transduction [106]. Moreover, iNOS has
also been found responsible for keeping normal phagocytosis in the outer
section of retina. Also, the role that NO plays in maintaining
circulation of retina, links the molecule with ophthalmologic role.
The optimal concentration as discussed in the biosynthesis is always
mainly eyed to confirm the normal functioning of any tissue. At the
concentrations other than optimum value results in so many eye diseases
[105]. In case of low NO-concentration (eNOS or nNOS abnormal
functioning) substances that could act as NO-donors could be
administered. On the other side, iNOS as pointed in the above sections
isonly intervening in pathological conditions expressive in terms of
several cytokines (interleukin-1, interleukin-6, etc), inflammation and
endotoxins. Once initiated, iNOS continues to produce sufficient NO
followed by conversion phenomenon free radicals, nitrogen dioxide or
nitrites as defensive way against pathogens.In hyperactivity of iNOS in
several disorders like cataracts, age-related macular degeneration
(AMD), myopia and uveitis, iNOS inhibitioncould be suggested.
As is widely known that blood pressure plays important role in keeping
normal eye-sight, and on the other hand NO is also considered as blood
pressure regulator. Under such a stemming fact, some recent studies have
foundeffectivemutual relationof NO with hypertension based cataracts.
The increase in lens nitrite (as NO metabolite) is suggested as one of
the key regulator for toincrease oxidative stress of lenticular part and
hypertensive cataract formation [107]. Thus, NOinvolvement in
retinal action continues to be of significant interest [108]. Let us
specify this role by illustrating the role of NO in eye defects as
elaborated below figures (Figure 10−figure 13 ): [Adopted from
L. K. Wareham1, E. S. Buys and R. M. Sappington, The Nitric
Oxide-Guanylate Cyclase Pathway and Glaucoma, Nitric Oxide ,
77(2018) 75–87, Ref. 111].