2. EICOSANOIDS
Eicosanoids are metabolites of
arachidonic acid or other
unsaturated fatty acids derived
from the action of the
cyclooxygenase or lypoxygenase
enzymatic pathway
Cyclooxygenase – prostanoids
Lypoxygenase - leukotrienes
Eicosanoids are classified into 2
main groups;
3. Cont…
1. Leukotrienes and lipoxins
2. Prostanoids
a) Prostaglandins (PGs)
b) Prostacyclins (PGI2)
c) Thromboxanes (TXs)
4. Cont…
The key precursor fatty acid is arachidonic acid,
which is an essential fatty acid as it cannot be
synthesized de novo in animals
The primary precursor is obtained from the diet in
the form of linoleic acid (polyunsaturated omega-6
fatty acid). It typically occurs in nature as a
triglyceride ester
5. Prostaglandins
PGs are extremely potent, biologically active lipid
mediators that are synthesized throughout the body
PGs have diverse hormone-like effects in animals
They are derived enzymatically from the fatty acid
arachidonic acid
6. Biosynthesis of PGs
Involves the action of multiple enzymes, some of
which are rate limiting
1. The first step is production of free arachidonic acid
from membrane phospholipids upon stimulation of
the enzyme phospholipase A2
2. The COX pathway, accomplished by catalytic
activity of two distinct cyclooxygenase (COX-1
and COX-2) isozymes encoded by separate genes
-produces PGG2 and then PGH2
7. Cont…
3. PGH2 is then converted into
-PGI2 by prostacyclin synthase
-TXA2 by thromboxane synthase, and
-PGE2, PGD2 and PGF2α by their respective synthase
enzymes
PGs are produced in the endoplasmic reticulum, exit
the cell and signal through G protein-linked
receptors at the cell surface
8.
9. COX-1 COX-2
Constitutive
Present in most tissues
Synthesizes PGs that
regulate physiologic
processes
Especially important in
-gastric mucosa
-kidneys
-platelets
-vascular endothelium
Inducible
Induced mainly at sites of
inflammation by cytokines
Synthesizes PGs that
mediate inflammation,
pain, and fever
Constitutive expression
primarily in
-brain
-kidneys
Differences between COX-1 and COX-2
10. Cont…
Both COX iso-enzymes are inhibited by NSAIDs,
such as acetylsalicylic acid and ibuprofen
This prevents synthesis of endogenous PGs
Because of differences in the structures of the
binding sites, COX-1 is completely inhibited by
aspirin, whereas COX-2 is only partially inhibited
Synthesis of COX-2 is inhibited by steroidal anti-
inflammatory drugs at the level of transcription
11. PG Receptors
PGs function close to the site of synthesis and are
deactivated to inactive metabolites before moving
into the circulation
They act locally in very low concentrations to
produce profound physiological changes through
receptor-mediated G-protein linked signaling
pathways
The immediate effect of the appearance of PGs is a
change in the rate of production of second
messengers such as cAMP or Ca2+ and a change in
the activation of a specific protein kinase
12. Biological Functions of PGs
Maintenance of IOP (Intraocular Pressure)
Vasodilation/vasoconstriction
Cause aggregation or disaggregation of platelets
Regulate inflammation
Induce labor
Acts on parietal cells in the stomach wall to inhibit
acid secretion
Increase glomerular filtration rate
Sensitize spinal neurons to pain
13.
14. Inhibitors of PGs
NSAIDs (inhibit cyclooxygenase)
Corticosteroids (inhibit phospholipase
A2 production); used in allergic reaction and to
relive inflammation
COX-2 selective inhibitors or coxibs eg; Celecoxib,
used to treat arthritis pain and RA
15. Clinical use of synthetic PG analogs
To induce labour or abortion
PGE1 analogue - misoprostol
PGE2 analogue - dinoprostol
PGF2 analogue – dinoprost
To prevent closure of patent ductus arteriosus in
newborns with particular CHD (PGE1)
As a vasodilator in severe Raynaud's
phenomenon or ischemia of a limb
16. Cont…
In treatment of IOP and glaucoma, PGF2α analogs eg;
travoprost, latanoprost and bimatoprost
In pulmonary hypertension
To prevent and treat peptic ulcers (PGE)
To treat erectile dysfunction or in penile
rehabilitation following surgery (PGE1
as alprostadil)
17. How PGs are different from true
hormones
Are formed in almost all tissues rather than in
specialized glands
Act locally rather than to distance sites
Act on their parent cells
Are not transported via blood
Are synthesized as per needed
Are not stored