The SILENT killer (CO)!!!
As signalling molecules
Discovered in 1772 by Joseph Priestly
He referred to it “nitrous air”.
A colorless and a toxic gas
Since then, it has received the label
of being a toxic gas and an air
pollutant until over two hundred
He had also discovered “Oxygen”
THE NOBEL ASSEMBLY AT KAROLINSKA INSTITUTE
Prize in Physiology & Medicine in October 12,1998 jointly to
Robert F. Furchgott, Louis J.
Ignarro and Ferid Murad for their discoveries concerning
“nitric oxide as a signalling molecule in the cardiovascular
What is Nitric Oxide?
First described in 1979 as a potent relaxant of
peripheral vascular smooth muscle.
Used by the body as a signaling molecule.
Serves different functions depending on body
system. i.e. neurotransmitter, vasodilator,
First gas known to act as a biological messenger
The structure and nature of
Nitric oxide is a diatomic free radical consisting of one atom
of nitrogen and one atom of oxygen
Lipid soluble and very small for easy passage between cell
Short lived, usually degraded or reacted within a few seconds
The natural form is gas
The nitrogen atom in NO is derived from the terminal guanidino group of L-
Synthesis of Nitric Oxide
Nitric oxide is synthesized from L-arginine
This reaction is catalyzed by nitric oxide
synthase, a 1,294 aa enzyme
Types of NOS
NOS I / nNOS
Central and peripheral neuronal cells
Ca+2 dependent, used for neuronal communication
NOS II / iNOS
Most nucleated cells, particularly macrophages
Independent of intracellular Ca+2
Inducible in presence of inflammatory cytokines
NOS III / eNOS
Vascular endothelial cells
Properties of NOS
NOS l & NOS lll are called as constitutive forms of NOS
They produce less NO in the body
On the other hand NOS ll / iNOS produces more NO in the body
1. Its high concentration & Activity in the body
2. Pathological conditions are associated with the cytokines
Properties of NOS
All three NOS are isoenzymes and are dimers
Similar/homologous with cytochrome P450
Each isoform contains iron protoporphyrin ix (heam) , flavin adenin
dinucleotide (FAD) , flavin mononucleotide (FMN) &
thetrahydrobiopterin (H4B) as bound prosthetic group
These prosthetic group and the ligand which is going to bind to the
enzyme in the presence of the reduced NADPH control the assembly
of the enzyme into “The Active Dimer form”
Properties of NOS
NOS enzymes are functionally BIMODAL in nature which is
associated with distinct structural domains
The oxygease domain binds to Heam while reductase bind to calcium
calmodulin , FMN , FAD , NADPH
NOS enzymes are the only Flavo Heam enzymes that use H4B as a
The crystal nature of the NOS heam (oxygenase) domain in iNOS &
eNOS has revealed how L arginine heam & H4B bind in the active
L-Arginine is usually present in excess in endothelial cell cytoplasm, so the rate of
production of NO is determined by the activity of the enzyme rather than by
Nevertheless, very high doses of L-arginine can restore endothelial NO biosynthesis
in some pathological states (e.g. hypercholesterolaemia) in which endothelial
function is impaired. Possible explanations for this paradox include:
compartmentation: i.e. existence of a distinct pool of substrate in a cell compartment
with access to the synthase enzyme, which can become depleted despite apparently
plentiful total cytoplasmic arginine concentrations
competition with endogenous inhibitors of NOS such as asymmetric dimethylarginine
(ADMA), which is elevated in plasma from patients with hypercholesterolaemia
reassembly/reactivation of enzyme in which transfer of electrons has become
uncoupled from L-arginine
relative depletion of arginine, which can inhibit NOS activity by inhibiting translation
of iNOS mRNA
Activation of NOS
Glutamate neurotransmitter binds to NMDA receptors
Ca++ channels open causing Ca influx into cell
Activation of calmodulin, which activates NOS
Mechanism for start of synthesis dependent on body
NO synthesis takes place in endothelial cells, lung cells,
and neuronal cells
What is the role of Nitric Oxide
in the human body?
Nitric Oxide in the human body has many uses which are
best summarized under five categories.
NO in the nervous system
NO in the circulatory system
NO in the muscular system
NO in the immune system
NO in the digestive system
Nitric Oxide in the Nervous
Nitric oxide as a neurotransmitter
NO is a signaling molecule, but not necessarily a neurotransmitter
NO signals inhibition of smooth muscle contraction, adaptive
relaxation, and localized vasodilation
Nitric oxide believed to play a role in long term memory
Memory mechanism proposed is a retrograde messenger that
facilitates long term potentiation of neurons (memory)
Synthesis mechanism involving Ca/Calmodulin activates NOS-I
NO travels from postsynaptic neuron back to presynaptic neuron
which activates guanylyl cyclase, the enzyme that catalyzes cGMP
This starts a cycle of nerve action potentials driven by NO
Is Nitric Oxide a
NO serves in the body as a neurotransmitter, but there
are definite differences between other neurotransmitters
used commonly in the body
NO is synthesized on demand vs. constant synthesis
NO diffuses out of the cells making it vs. storage in vesicles and release
NO does not bind to surface receptors, but instead exits in cytoplasm,
enters the target cell, and binds with intracellular guanylyl cyclase
Similarities to normal NTs
Present in presynaptic terminal
Natural removal from synaptic junction
Nitric Oxide in the Circulatory
NO serves as a vasodilator
Released in response to high blood flow rate and signaling
molecules (Ach and bradykinin)
Highly localized and effects are brief
If NO synthesis is inhibited, blood pressure shoots
NO aids in gas exchange between hemoglobin and cells
Hemoglobin is a vasoconstrictor, Fe scavenges NO
NO is protected by cysteine group when O2 binds to hemoglobin
During O2 delivery, NO locally dilates blood vessels to aid in gas
Excess NO is picked up by HGB with CO2
Nitric Oxide in the Muscular
NO was orginally called EDRF (endothelium derived
NO signals inhibition of smooth muscle contraction
Ca2+ is released from the vascular lumen activating NOS
NO is synthesized from NOS III in vascular endothelial cells
This causes guanylyl cyclase to produce cGMP
A rise in cGMP causes Ca+2 pumps to be activated, thus
reducing Ca2+ concentration in the cell
This causes muscle relaxation
Endothelium-derived NO acts locally on underlying vascular smooth
muscle or on adherent monocytes or platelets.
The potential for action at a distance is neatly demonstrated by Rhodnius
prolixus, a blood-sucking insect that produces a salivary
vasodilator/platelet inhibitor with the properties of a nitrovasodilator. This
consists of a mixture of nitrosylated haemoproteins, which bind NO in the
salivary glands of the insect but release it in the tissues of its prey.
The consequent vasodilatation and inhibition of platelet activation
presumably facilitates extraction of the bug's meal in liquid form.
A strong, but still controversial, case has been made that NO can also act
at a distance in the mammalian circulation via reversible interactions with
• Haem has an affinity for NO > 10 000 times greater than for oxygen. In the
absence of oxygen, NO bound to haem is relatively stable, but in the
presence of oxygen NO is converted to nitrate and the haem iron oxidised
• Distinct from this inactivation reaction, a specific cysteine residue in globin
combines reversibly with NO under physiological conditions.
• The resulting S-nitrosylated haemoglobin is believed to be involved in
various NO-related activities, including the control of vascular resistance,
blood pressure and respiration.
• Key features of the proposed mechanism include the following.
• Nitrosylation of haemoglobin is reversible.
• It depends on the state of oxygenation of the haemoglobin, which
consequently takes up NO in the lungs and releases it in tissues, in
concert with release of oxygen.
• Haemoglobin acts as an O2 sensor and could regulate vascular tone
(and hence tissue perfusion) in response to the local partial pressure of
O2 by releasing NO in this way. This mechanism is impaired in sickle cell
disease (a common inherited disorder caused by a molecular variant of
• NO is not released into the cytoplasm of erythrocytes (where it would
promptly be inactivated by haem), but is transported out of the red cells
via cysteine residues in the haemoglobin-binding cytoplasmic domain
of an anion exchanger called AE1.
• S-nitrosylated albumin also constitutes a source of circulating NO
bioactivity. An alternative view is that nitrite anion, rather than
nitrosylated protein, is the main intravascular NO storage molecule
Nitric Oxide in the Immune
NOS II catalyzes synthesis of NO used in host defense
Activation of NOS II is independent of Ca+2 in the cell
Synthesis of NO happens in most nucleated cells,
NO is a potent inhibitor of viral replication
NO is a bactericidal agent
NO is created from the nitrates extracted from food near
This kills bacteria in the mouth that may be harmful to the
Nitric Oxide in the Digestive
NO is used in adaptive relaxation
NO promotes the stretching of the stomach in response to
When the stomach gets full, stretch receptors trigger
smooth muscle relaxation through NO releasing neurons
Nitric Oxide Metabolism
NO may also be involved in the regulation of protein activity
through S-nitrosylation. In the extracellular milieu, NO reacts
with oxygen and water to form nitrates and nitrites.
NO toxicity is linked to its ability to combine with superoxide
anions (O2–) to form peroxynitrite (ONOO–), an oxidizing free
radical that can cause DNA fragmentation and lipid oxidation.
In the mitochondria, ONOO– acts on the respiratory chain (I-
IV) complex and manganese superoxide dismutase
(MnSOD), to generate superoxide anions and hydrogen
peroxide (H2O2), respectively.
By analogy with cytochrome P450, it is believed that the flavins accept
electrons from NADPH and transfer them to the haem iron, which
binds oxygen and catalyses the stepwise oxidation of L-arginine, via a
hydroxyl-arginine intermediate, to NO and citrulline.
In pathological states, the enzyme can undergo structural change
leading to electron transfer between substrates, enzyme cofactors and
products becoming 'uncoupled', so that electrons are transferred to
molecular oxygen, leading to the synthesis of superoxide anion rather
This is important, as superoxide anion is a reactive oxygen species and
reacts with NO to form a toxic product (peroxynitrite anion)
New research ideas involving
The role NO might play in neuronal
The mechanism of NO inhibiting the different
forms of NOS
Diazeniumdiolates as NO releasing drugs
Excessive NO release as the cause of most brain
damage after stroke
Carbon monoxide (CO) is a colorless, odorless, and tasteless gas
that is slightly less dense than air. It is toxic to humans when
encountered in concentrations above about 35 ppm
Carbon monoxide consists of one carbon atom and
one oxygen atom, connected by a triple bond that consists of
two covalent bonds as well as one dative covalent bond. It is the
In biology, carbon monoxide is naturally produced by the action
of heme oxygenase 1 and 2 on the heme from hemoglobin breakdown.
This process produces a certain amount of carboxyhemoglobin in
normal persons, even if they do not breathe any carbon monoxide.
Following the first report that carbon monoxide is a
normal neurotransmitter in 1993, as well as one of three gases that
naturally modulate inflammatory responses in the body
Carbon monoxide is produced naturally by the human body
as a signaling molecule.
Thus, carbon monoxide have a physiological role in the body,
such as a neurotransmitter or a blood vessel relaxant.
Because of carbon monoxide's role in the body, abnormalities
in its metabolism have been linked to a variety of diseases,
including neurodegenerations, hypertension, heart failure,
The most common symptoms of carbon monoxide poisoning may
resemble other types of poisonings and infections, including
symptoms such as headache, nausea, vomiting, dizziness, fatigue,
and a feeling of weakness.
Affected families often believe they are victims of food poisoning.
Infants may be irritable and feed poorly. Neurological signs
include confusion, disorientation, visual disturbance, syncope and
Some descriptions of carbon monoxide poisoning
include retinal hemorrhages, and an abnormal cherry-red blood
carbon monoxide has received a great deal of clinical attention as a
biological regulator. In many tissues, all three gases are known to act
as anti-inflammatories, vasodilators, and encouragers
of neovascular growth.
However, the issues are complex, as neovascular growth is not always
beneficial, since it plays a role in tumor growth, and also the damage
from wet macular degeneration, a disease for which smoking (a major
source of carbon monoxide in the blood, several times more than
natural production) increases the risk from 4 to 6 times.
› Normal heme
reaction in the body
known to produce CO.
› Levels increased in:
Mechanism of action of CO
CO combines reversibly with the oxygen binding sites
of hemoglobin and has an affinity Hemoglobin about
220times that of oxygen. The product formed
carboxyhemoglobin, cannot transport oxygen.
Thus brain and heart are most
The free carboxy group present combines
with hemoglobin, thus forming carboxy
hemoglobin. Therefore Reduces the
oxygen supply to the tissues.
1. CO binds to platelet hemoproteins and
increases NO efflux.
2. Platelet-derived NO reacts with neutrophil-
derived superoxide which activates
platelets and causes platelet-neutrophil
3. Reactive products and adhesion molecules
promote firm aggregation and stimulate
degranulation of neutrophils.
4. Endothelial cells acitaved by
myeloperoxidase facilitating firm neutrophil
adhesion and further degranulation.
5. Reactive oxygen species (ROS) initiate lipid
peroxidation and adducts interact with
brain myelin basic protein. The altered
myelin basic protein triggers an adaptive
immunologic response that causes
CO regulates blood
flow and blood fluidity
Vascular tone SMC proliferation Platelet aggregation
Cross talk between CO AND NO
CO AND NO are two endogenously produced gases that can act as second
Heme oxygenase and nitric oxide synthase are the enzyme systems
responsible for generating CO and NO share similar properties, such as ability
to activate soluble guanylate cyclase to increase cyclic GMP.
it is becoming increasingly clear that these 2 gases do not always work
independently, but rather can modulate each others activity.
Although much is known about the heme oxygenase/CO and nitric oxide
synthase/nitric oxide pathways, how these two imp systems interact is Less well
The current known relationship between CO and NO it relates to their
production and physiological function.
• RANG and DALE’S Pharmacology. Sixth edition (2007). 265-
• The sources on the world wide web.