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EICOSANOIDS
• C20 polyunsaturated fatty acids e.g. Arachidonic acid → Eicosanoids
→physiologically, pathologically and pharmacologically active compounds
• PG – Prostaglandins
• TX - Thromboxanes
• LT - Leukotrienes
• LX - Lipoxin
FUNCTIONS
• Physiologic:-
→ act as local hormones
through G protein linked receptors
→ biochemical effects
inflammatory response (calor, rubor, tumor, dolor)
gastric integrity
renal function
smooth muscle contraction (intestine / uterus / bronchus)
blood vessel diameter
platelet homeostasis
• Pathologic:-
→ hypersensitivity ( allergic reaction – may be fatal)
DIFFERENCE FROM HORMONES
• Produced in small amounts
• Produced by all tissues( not in glands)
• Act locally
• Not stored
• Short half life
SYNTHESIS- SUBSTRATE 1. The dietary Essential fatty acids Linoleic (ш 6 ,
18 C, dienoic fatty acid)
and Linolenic acids (ш 6 eicosa trienoic fatty acid)
→ Arachidonic acid
2. Arachidonic acid is found in the plasma membrane phospholipids at C2/ diet
3. Phospholipase A2 causes the hydrolytic release of Arachidonic acid
ARACHIDONIC ACID has 20 C and 4 double bonds 20:4 (5,8,11,14)
EICOSANOID SYNTHESIS
• Dietary precursor is the Essential Fatty acid, linoleic acid (ω6 fatty acid)
→desaturated →elongated
→ Arachidonic acid ( ω6 FA )
→ membrane bound phospholipids
→ released by Phospholipase A2
2 PATHWAYS FOR ARACHIDONIC ACID 2 pathways compete for Arachidonic substrate:-
• Cyclo oxygenase → PG2 , TX2 (Prostanoids)
• Lipooxygenase → LT4, LX4
CYCLOOXYGENASE PATHWAY • Prostaglandin H synthase enzyme (PGH synthase)
• Enzyme is bound to Endoplasmic reticulum
• Enzyme has 2 catalytic activities:-
(1) Fatty acid cyclooxygenase (COX)
requires 2 molecules of Oxygen
(2) Peroxidase – dependent on reduced
Glutathione
→oxidative cyclization of Arachidonic acid
→PGH2
→ variety of PG and TX(PGD, PGE2, PGF2, PGG2, TXA2, PGI2)
• Each cell type produces only 1 type of prostanoid
• Cyclooxygenase is a suicide enzyme (self catalyzed destruction→↓PG activity)
PGH SYNTHASE ISOZYMES • 2 isozymes :- COX 1 and COX 2
1. COX 1 – present in most tissues
required for healthy gastric tissue,
renal homeostasis
platelet aggregation
2. COX 2 - limited no. of tissues
induced by products of activated
immune cells
and inflammatory cells
→ Calor, Rubor, Tumor, Dolor of
Inflammation
INHIBITION OF PG SYNTHESIS • Steroidal anti inflammatory agent ( Cortisol):-
→↓Phospholipase A2 →↓Arachidonic acid
• Non steroidal anti inflammatory agents(NSAIDS)
(Aspirin, Indomethacin, Phenylbutazone, Ibuprofen):-
→↓Cyclooxygenase by competing with Arachidonate
→↓COX 1 and COX2 →↓PGH2
{ Aspirin toxicity →systemic inhibition of COX 1 and COX2
→damage to stomach and kidneys + ↓blood clotting }
{ Celecoxib (COX 2 inhibitor)→↓inflammatory process but COX 1 physiologic functions maintained→↑risk of heart attacks ( due to ↓PGI2) }
FUNCTIONS OF PROSTAGLANDINS • PGE2 (most tissues)→ vasodilation
relaxation of smooth muscle
↓acid secretion from stomach
↑release of Renin from JG cells
uterine contractions→ labor
FUNCTIONS OF PROSTAGLANDINS • PGF2α(most tissues)→
Vasoconstriction
Bronchoconstriction
Contraction of smooth muscle
↓Progesterone secretion + regression of Corpus Luteum →interrupts early pregnancy + onset of labor →termination of pregnancy / Labor
CLINICAL USES OF EICOSANOIDS • Induction of labor
• Prevention of conception
• Termination of pregnancy
• Relief of Asthma and nasal congestion
• Control BP in Hypertension
• ↓HCl formation in peptic ulcer
• PGD2 is sleep promoting substance
THROMBOXANES
• TXA2 is produced by COX-1 in activated platelets →
↑adherence+ aggregation of circulating platelets
contraction of vascular smooth muscle (vasoconstriction)
→formation of blood clots(thrombi)
• PGI2 (Prostacyclin) produced by COX2 in vascular endothelial cells →↓platelet aggregation
+ ↑vasodilation→↓ thrombogenesis
• Opposing effects of TXA2 and PGI2 →↓thrombi
(at site of injury only)
• Aspirin →↓COX1→↓TXA2 in platelets( inhibition cant be overcome because of lack of nuclei)
• →↓COX2→↓PGI2 in endothelial cells (inhibition can be overcome because of nuclei → can generate more enzyme)
• →↓thrombi
• →↓risk of stroke / heart attack (by low dose Aspirin)
•
LIPOOXYGENASE PATHWAY • Lipooxygenases enzymes (LOXs)
→linear hydroperoxy acids (- OOH)
• 5 Lipooxygenase converts Arachidonic acid →
5-hydroperoxy-6,8,11,14 eicosa tetra enoic acid
(5-HPETE)→ Leucotrienes (4 double bonds)
(in leucocytes, platelets and macrophages)
→mediators of allergic response and inflammation
• NSAIDS do not affect their synthesis
• Inhibitors of 5- lipooxygenase and leucotriene receptor antagonists →treatment of Asthma
PHYSIOLOGIC ACTIONS OF LEUKOTRIENES AND LIPOXINS
• Leukotrienes :-
Slow reaction g substance of anaphylaxis (SRS-A)
is mixture of Leukotrienes→
Bronchoconstriction,
Inflammatory/ hypersensitivity reaction
e.g. Asthma
• Lipoxins :-
Anti inflammatory role i.e. counter regulatory
compounds (Chalones) of immune response