Lipid METABOLISM Cyberjaya University College of Medical Sciences Prof Dr Noor Aini AH
LipidMETABOLISM
Cyberjaya University College of Medical Sciences Prof Dr Noor Aini AH
LIPID DIGESTION & ABSORPTION
• Dietary Lipids- 20-40% calorie intake- Mainly >90% - triacylglycerol (TG)- - 16-18 C, mixture- Others – phosphoglycerides, cholesterol
ester, cholesterol- Fat – solid at room temp - animal source - 40-60% saturated FA
Oil – liquid at room temp - plant source - 80-90% unsaturated FA - coconut oil – mainly saturated FA – 12-14C - olive oil – 79% oleic acid (monounsat) - safflower oil, corn oil – 76% linoleic acid (polyunsat) - fish oil (salmon)– 25-35% -3 (ecosapentaenoic acid)
-glycerol is the backbone to which 3 fatty acyl residues R1, R2, R3 are attached to C1,C2,C3 resp. -monoacylglycerols – 1- monoacylg, 2-monoacylg-diacylglycerol – 1,2- or 1,3-diacylg metabolic intermed.-1,2-diacylg – 2nd messenger
Digestion of Lipids
• Stomach - lingual & gastric lipases - hydrolyze short & medium chain FA - active in children - cow’s milk - short & medium chains - food fr stomach – to duodenum stim: - cholecystokinin contraction of gallbladder + release of
pancreatic juice - hepatocrinin bile secretion by liver
• Small intestines - Pancreatic juice – bicarb + pancreatic enzymes –
pancreatic lipase - cholesterol esterase - phospholipase A2
- Colipase - cofactor
- Bile from gall bladder
Bile – bile acid, phospholipids, cholesterol, salt, H2O, bilirubin etc
- peristalsis – help to break lipids
Emulsification of Lipids
-occurs in duodenum – bile
-Bile acid & phosphatidylcholine (found in bile)
micelles – aggregate formed in aq sol by subst composed by both polar & non polar groups – eg phosphatidylcholine – amphiphatic
mixed micelles – dispersed diet lipids for action of lipases
Micelles
Hydrolysis of TG
-pancreatic lipase
-Long chain FA –16-18C
-R3, R1
-Colipase – activates pancreatic lipase
- anchors bile salt to lipase
Pancreatic lipase
TG → 1,2-diacylglycerol + FA
1,2-diacylglycerol → 2-monoacylglycerol + FA
TG → 2-monoacylglycerol + 2FA
Phospholipid →lysoPL + FA
Phospholipase A2
Cholesterol Ester
→ Cholesterol + FA
cholesterol esterase
Absorption of Lipid
-FA + 2-monoacylglycerol + cholesterol + PL + other diet lipid eg fat soluble vitamins
packaged in micelles → microvilli lipids absorption – passive diffusion
Jejunum & ileum
-Bile salts reabsorbed to portal circ ileum
-Short & medium chain FA (C4-12) – absorbed directly into intes epit cell → intracell lipase → FA + glycerol →portal system
- cow’s milk, synthetic form - chronic pancreatitis
- In int cell – FA resynthesise to TG- 2-Monoacylglycerol + FACoA → Diacylglycerol +
CoA
Diacylglycerol + FACoA → TG + Co A
- Cholesterol + FA → Cholesteol ester- LysoPL + FA → PL- All lipids + apoprotein B48 → Chylomicron
- TG + CE + PL + C + other lipids + apoproteins chylomicrons
• Lipid malabsorption- Chronic pancreatitis - ↓ lipases- Bile duct obstruction – cholelithiasis, ca
head of pancreas - Chronic liver disease – liver cirrhosis
Steatorrhoea – bulky, fatty stool
• Apoproteins - proteins that form lipoproteins - in chylomicrons – ApoB-48 (most imp) - ApoAI, A2, A4 - ApoB-48 only present in chylomicrons - Apo CII & ApoE – transferred fr HDL - Apo CII – activates lipoprot lipase (LPL) - ApoE – recognise by liver cells recept
Lipid Transport
• Lipids are insoluble in aqueous materials such as blood plasma.– To transport these insoluble substances
within the lymph and blood, they are complexed with proteins to form lipoprotein.
Lipoprotein
5 classes of plasma lipoproteins exist in humans:
General structure of a plasma lipoprotein
• range in size from 10 to 1000nm
• composed of a hydrophobic core containing cholesteryl esters, triglycerides, fatty acids and fat-soluble vitamins
• the surrounding hydrophilic layer is composed of various apolipoproteins, phospholipids and cholesterol.
Types of lipoproteins1. Chylomicrons2. VLDL (very low density lipoprotein)3. IDL (intermediate density lipoprotein)4. LDL (low density lipoprotein)5. HDL (high density lipoprotein)
Principal lipids carried by above lipoproteins areTriglyceridesCholesterol and cholesteryl estersPhospholipids
COMPOSITION OF LIPOPROTEINS
CHYLOMI VLDL LDL HDLTriglycerides 90% 60% 8% 5%Cholesterol & CE 5% 20% 50% 25%Phospholipids 3% 15% 22% 30%Apolipoprotein 2% 5% 20% 40%
Density increases with decreasing TG content or increasing protein content
Apoproteins - proteins in lipoproteins - in chylomicrons – ApoB-48 (most imp) -in VLDL / LDL - Apo B100- HDL Apo CII & ApoE transfer to Chylomicron, VLDL - Apo CII – activates lipoprot lipase (LPL) - ApoE – recognise by liver cells receptApo B100 – recognise by LDL receptor at endothelium
METABOLISM OF CHYLOMICRON
CHYLOMICRON
90%
5% 3%
2%
5%
TRIGLYCERI
CHOLESTER
PHOSPHOL
APOLIPOPR
Structure of chylomicron
48
Chylomicrons -synthesised in intestinal mucosal cells -carry dietary
lipids - contain apoB-48 - secreted into lymphatic system by exocytosis. - carried via lymphatic system- thoracic duct into systemic blood circulation.
Deficiency in apo B accumulation of lipids in intestinal epithelial cells & leads to hypolipoproteinemia
Lymphatic system to Thoracic duct and finally opens at left Subclavian vein(blood)
Main function-transport exogenous lipid (from diet) from intestine to liver and extrahepatic tissue: muscle, adipose tissue
In blood circulation
- + apo CII, apo E from HDL
- TG fatty acid + glycerol by LPL
- apo CII activate LPL
- FA adipose tissue – storage
muscle - oxidation
( FA + albumin )
Glycerol liver (+ glycerol kinase)
Chylomicron remnant TG liver
- receptor recognise apo E
-Endocytosis
-Hepatic lipase, lisosom – hydrolyses chylomicron
TG, CE, C, PL
- In liver - utilized
- reassembled with new apoproteins and
endogenous lipids to form VLDL.
Transfer of Apo E and Apo CII from HDL to chylomicron
In circulation chylomicron receives apoE and apoC-II from HDL
Deficiency in lipoprotein lipase or apoC-II accumulation of triglycerides in plasma (Type I hyperlipidemia) or (Familial hyperchylomicronemia)
Chylomicron
METABOLISM OF VLDL (VERY LOW DENSITY LIPOPROTEIN)
VLDL
60%
20%
15%
5%
20%
TRIGLYCERI
CHOLESTER
PHOSPHOL
APOLIPOPR
VLDL SynthesisIn liver - some of the lipid components brought by chylomicron remnants (exogenous)- lipid components synthesised by liver
(endogenous lipids) - + apoprotein B100 - secreted by exocytosis Function - carry both exogenous and endogenous lipids from
liver to extrahepatic tissues.
Synthesis of VLDL in Liver
In Blood circulation- VLDL receives apoE and apoC-II from HDL.- Cholesterol esters from HDL is also transferred
to VLDL by Cholesteryl ester transfer protein ( CETP).
• As the VLDL circulate through blood circulation, – they are acted by lipoprotein lipase and – triglycerides are degraded.
• Fatty acids and glycerol are released.• Remnant VLDL is called IDL
• IDL will further lose some TG as well as ApoC and apoE to form Low density lipoprotein (LDL)
• So the conversion of VLDL to IDL and finally to Low density lipoprotein (LDL) occurs in circulation
Very low density lipoprotein (VLDL)
Very low density lipoprotein (VLDL)
METABOLISM OF LDL (LOW DENSITY LIPOPROTEIN)
LDL
8%
50% 22%
20%
42%
TRIGLYCERI
CHOLESTER
PHOSPHOL
APOLIPOPR
LDL - Contains high percentage of cholesterol and
cholesterol esters.
Function - LDL carry cholesterol to extrahepatic tissues. Receptors of LDL on cell surfaces recognise apoB-100 in
LDL and internalises by endocytosis.- Internalised LDL is acted by lysosomal enzymes Results in release of cholesterol into cell - Increases activity of ACAT (acyl CoA cholesterol
acyltransferase) Converts cholesterol to cholesterol ester – storage in
the cells
-When the plasma LDL is high - down regulation of cell surface receptors
- Up regulation occurs when cell increases receptors to internalise more LDL – can also by simvastatin or pravastatin drugs
- Cholesterol released inhibits HMG CoA reductase and decreases de novo synthesis by the cell
Defective LDL receptor results in increased plasma LDL called Type II hyperlipidemia (Familial hypercholesterolaemia)
Low density lipoprotein (LDL)
Elevated LDL – can undergo oxidation oxidised LDL taken by macrophages (scavenger receptor –SR-A)
that later forms the foam cells - SR-A is not down regulated. -These cells will be located below the endothelial layer
and cause injury to the site that ultimately causes formation of atherosclerotic plaque.
-This will narrow the blood vessel lumen -increases blood pressure -cardidovascular disease – MI, stroke
METABOLISM OF HDL (HIGH DENSITY LIPOPROTEIN)
HDL
5%
25%30%
40%
70%
TRIGLYCERI
CHOLESTER
PHOSPHOL
APOLIPOPR
Synthesis-liver and small intestine
-Released to circulation by exocytosis
-Disc shape – nascent HDL
-PL, C, apo E apo CII, apo A1
In blood
- + cholesterol fr extra hep tissue - free cholesterol received is esterified by the enzyme LCAT – Lecithin cholesterol acyltransferase. CE
-CE are carried to liver by HDL where it is degraded and the cholesterol can be converted to bile acids excretion.
- Some of the cholesterol esters are transferred to VLDL
Function - removing excess free cholesterol from extrahepatic
tissues.- act as reservoir of apoE and apoC-II.
High density lipoprotein (HDL)
• People with high HDL are resistant to the development of atherosclerosis
• Premenopausal women have higher HDL level than men or postmenopausal women
• Normal level > 1.4 mmol/L
Lipoprotein (a) Lp(a)- almost like LDL – but have apo(a) attached
apo B-100.- genetically determined - diet – trans fatty acids increase Lp(a)- oestrogen decreases LDL & Lp(a)- Lp(a) slows breakdown fibrin degradation
FATTY ACID SYNTHESIS
Fatty acids ( FA ) are synthesised actively by the liver. These are called endogenous fatty acids
Fatty acids are synthesised from acetyl CoA when there is caloric excess.
Dietary EXCESS carbohydrate (GLUCOSE) is the major source for the synthesis of FA
Other than glucose, amino acids and other sources of acetyl CoA can contribute towards FA synthesis.
FA synthesised can exist as free FA or as esters, mainly as triacylglycerol.
Others like cholesterol esters, phospholipids and sphingolipids also contain FA.
Triacyglycerol is also called triglycerides ( TG ) is the major storage of lipids.
TG is stored in adipose tissue.
FATTY ACID SYNTHESIS
Carbohydrates Amino acids Lipids(Glucose)
Acetyl CoA
Acetyl CoA carboxylase
Malonyl CoA
Fatty acid synthase complex
Palmitic acid
CO2
ATP
ADP
NADPH
NADP+
Glucose are converted to pyruvate (glycolysis) which then enter the mitochondria where it forms acetyl CoA.
Since acetyl CoA cannot cross mitochondrial membrane it condenses with oxaloacetate to form citrate – Citrate shuttle
Acetyl CoA + Oxaloacetate Citrate
Citrate is transported out into cytoplasm
Citrate Acetyl CoA + oxalocaetate Citrate lyase
Citrate in cytosol is converted to Acetyl CoA and Oxaloacetate
TG synthesis
Fatty Acid synthesis
1. Acetyl CoA + oxaloacetate
2. Citrate Shuttle
3. Conversion of Acetyl CoA to Malonyl CoA
4. Elongation reactions at FA synthase complex
Acetyl CoA Malonyl CoA
Malonyl CoA serves as the immediate donor of the 2-carbon unit which is added sequentially to lenghthen the fatty acid chain by the fatty acid synthase.
Acetyl CoA carboxylase is the rate limiting enzyme of FA synthesis.
Acetyl CoA carboxylase
CO2
Fatty acid synthase complex
Fatty acid synthase catalyses the synthesis of fatty acids in elongation of fatty acid chain until forming palmitate (16 C ).
Initially acetyl CoA provides the -methyl carbon of palmitate.
Malonyl CoA provides the 2-carbon units that are added to the growing fatty acyl chain.
Fatty acid synthase complex
Palmitate and other fatty acids can also be desaturated to form unsaturated fatty acids
Finally these fatty acids are activated to fatty acyl CoA which interact with glycerol 3-phosphate to form triacylglycerol and packed into VLDL and secreted into circulation.
REGULATION OF FATTY ACID SYNTHESIS
Allosteric modification
Acetyl CoA carboxylase is activated by citrate which causes it to polymerise.Inhibited by palmitoyl CoA.
Regulation of Acetyl CoA carboxylase
1.Stimulated by citrate
2.Inhibited by Palmitoyl CoA
3.Inhibited glucagon & stimulated by insulin
However, humans cannot synthesise the following polyunsaturated fatty acids due to lack of specific enzymes:
Linoleic acid ( 18: 9,12)Linolenic acid ( 18:9,12,15 )
The above fatty acids are called ESSENTIAL fatty acids
Triacylglycerol formed in adipose tissue will be stored.
Triacyglycerol synthesis is activated by insulin.
Triacylglycerol formed in liver is packed into VLDL and secreted into circulation.
VLDL as it passes through capillary, it is acted by lipoprotein lipase which hydrolyses the triacylglycerol to fatty acids and glycerol.
Fatty acids are taken up by peripheral tissue to generate energy through -oxidation and electron transport chain.
Fatty acids taken up by adipose tissue forms triacyglycerol and are stored.
Excessive storage can cause obesityObesity can be determined from BMI (Body Mass Index) = Wt / (Ht)2 . Normal ~ 20 - 25BMI Value > 30 indicates obesity.
abnormal lipid metabolism leads to obesity – risk factor DM type II, cardiovascular diseases, osteoarthritis, gall stone formation and others.
High fat with saturated fatty acids
•
High fiber, high fish protein and PUFA
LIPOLYSIS & β OXIDATION
Lypolysis- Breakdown of TG FA + glycerol- Adipose tissue- Hormone sensitive lipase- Provide FA – fasting, strenuous
exercise,stress- Hormones – glucagon, adrenaline
- FA released + albumin tissue (muscle) oxidation- Glycerol liver – gluconeogenesis
Regulation- Fasting blood glucose glucagon c-AMP lypolysis- Stress/strenuous exercise adrenaline c-AMP lypolysis- Fed state blood glucose Insulin/ glucagon
lypolysis
Fatty Acid Oxidation ( - oxidation)- Aerobic- FA catabolism CO2, H2O + energy (ATP + heat)- Sources : - diet – carried by lipoprotein (LPL) - lipolysis- Most tissues except – brain, erythrocytes- Main energy source
- oxidation- In mitochondria- From carboxyl end ie C3/
2 C fragment ‘cut off’ from FA chain each cycle to produce acetyl-CoA consecutively
Products:Acetyl-CoA Kreb cycle
NADH, FADH2 Resp chain ATP
3 important steps1. FA activation Fatty acyl-CoA - in cytosol2. Transport Fatty acyl-CoA into mitochondria - carnitine shuttle3. Reactions that takes place in -oxidation - oxidation - hydration - oxidation - cleavage
FA activationLong chain FA are activated by thiokinase present at the
outer membrane of mitochondriaLong chain fatty acid
Thiokinase ATP
CoASH (coenzyme A)
Fatty acyl CoAFatty acyl CoA then carried into mitochondria by
carnitine shuttle
AMP
-Oxidation of fatty acids
4 steps are repeated until all carbons of an even chain fatty acids are oxidised to acetyl CoA.
Each set of reaction produces 1 NADH, 1 FADH2 1molecule of acetyl CoA and an acyl CoA that is 2 carbons less than the original.
Thus, palmitoyl CoA with 16 carbons will undergo 7 sets of - oxidation to complete its conversion to acetyl CoA.
-Oxidation of even chain fatty acid
Palmitoyl CoACH3 – CH2 – CH2 – CH2 – CH2 – CH2 – CH2 – CH2
CH2CH2 CH2CH2
CoAS~C– CH2 – CH2 – CH2
Acetyl CoA + Acyl CoA + NADH + FADH2
O
Regulation of fatty acid oxidation:-- oxidation is regulated by the levels of ATP and NADH- glucose - Malonyl CoA an intermediate in fatty acid synthesis inhibits carnitine acyltransferase I. This prevents oxidation during synthesis.-Fasting lipolysis FA oxidation acetyl Co A & NADH inhibit pyruvate dehydrogenase save glucose
HUNGER state
FA glucose
FA-CoA pyruvate
Acylcarnitine
(-) (-) PDH
Acetyl CoA Acetyl CoA
FED state
Glucose FA
Pyruvate FA CoA PDH (-) Acylcarnitine Trans I Acetyl CoA Acylcarnitine
Malonyl CoA (-) -oxidation
FA
Oxidation of odd chain fatty acidsFatty acids with odd number of carbons also oxidised by
- oxidation.
However, in the last set of reaction it will yield an acetyl CoA and a 3carbon propionyl CoA.
Propionyl CoA is converted to succinyl CoA
Energy Comparison• Glucose ( MW = 180 g/mol)• Lauric acid (12:0) (MW= 200 g/mol)• Glucose– Consider 1 mol– 32 mol ATP/180 g = 0.18 mol ATP/g glucose• Lauric Acid– Consider 1 mol– 78 mol ATP/200 g = 0.39 mol ATP/g lauric acid
Dietary Values– 4 Cal/g for carbohydrates– 9 Cal/g for fats and oils
METABOLISM OF KETONE BODIESExcess breakdown of fatty acids lead to the
formation of ketone bodies;-acetoacetate, - hydroxybutyrate and acetone.During;High fat and low carbohydrate diet Fasting / StarvationUncontrolled diabetes mellitus
Ketogenesis or ketone formation occurs mainly in the liver
- in mitochondriaAs increased fatty acid oxidation produce high
concentration of acetyl CoA.Less oxaloacetate to form citrate – FA synthesisThis causes accumulation of acetyl CoA which
reacts as follows:
Acetyl CoA + Acetyl CoA
Thiolase CoA
Acetoacetyl CoA
HMG CoA synthetase Acetyl CoA
Hydroxymethylglutaryl CoA ( HMG CoA )
HMG CoA lyaseAcetoacetate
Synthesis of
Acetoacetate
HMG-CoA synthetase stimulated during fasting
Formation of - hydroxybutyrate and acetone.
These ketone bodies are released into circulation where they are utilized by extrahepatic tissues to generate energy. - Acetone excreted via lungs
Utilization of ketone bodies to generate energyHydroxybutyrate
Hydroxybutyrate dehydrogenase
Acetoacetate
Succinyl CoA transferase
Acetoacetyl CoAThiolase
Acetyl CoA + Acetyl CoA
NAD+
NADH
Succinyl CoA
Succinate
CoA
During starvation BRAIN adapts to use ketone bodies to produce energy.
During fasting HMG CoA synthetase is induced by high glucagon: insulin ratio.
Liver cannot generate energy from ketone bodies since Succinyl CoA tranferase are not sufficiently present .
In uncontrolled diabetes, excessive ketone bodies are produced causes ketonemia leading to ketonuria.
Starvation and ketone bodies
CHOLESTEROL METABOLISM
Cholesterol Ester
Cholesterol Ester-FA chain replaces OH – C3
-Usually unsaturated FA
-oleic acid – cholesteryl oleate
-linoleic – cholesteryl linoleate
Ester Formation
1. In tissue – liver, intestine, adrenal cortex, arterial wall
ACAT - acyl Co A + cholesterol cholesterol ester + Co Ash
2. In plasma LCAT - phosphatidylcholine + cholesterol cholesterol + lysophsphatidylcholine
Sources of cholesterol-diet – 40%
-De novo synthesis – 60%
-Diet 200-300mg/day
-Digestion & absorption – small intestine
chylomicron
Synthesis of Cholesterol-liver, intestine, adrenal cortex, ovary, testis
-In cytosol
-From acetyl CoA – CHO, lipid, amino acid
-Acetyl Co A from mitochondria cytosol via citrate shuttle
5 major steps
1. Acetyl CoA 3-hydoxy-3-methyl glutaryl CoA (HMGCoA)
2. HMGCoA mevalonate
3. Mevalonate isoprene based molecule isopentenyl pyrophosphate (IPP), with loss of CO2
4. IPP squalene
5. squalene lanosterol cholesterol
Regulation of Cholesterol synthesis - Mainly by regulation of HMG CoA reductase
1. Feedback inhibition
- cholesterol cellular level - activity, synth
- mevalonate
2. Covalent modification
- active in dephosphorylated state
- inactive in phosphorylated state
3. Hormonal
- insulin – dephospho of HMGCoA reductase
- glucagon – phoshorylation
- insulin - synthesis of HMG CoA reductase – control of gene expression
4. Enzyme degradation - cholesterol in cell degradation
5. Supression of LDL receptor – down regulation by cholesterol itself6. Circadian rhythm - midnight (max 6 hrs after dark) - at noon ( min 6 hrs after light)
Maintaining normal Blood Cholesterol Level1. Diet - cholesterol intake - 10-15% - no intake - 40% - sat FA - synthesis cholesterol - unsat FA - synth cholesterol Mech – Unsat FA – upregulates LDL receptor - palmitate inhibits bile acid synthesis - Low cal diet - CHO, Lipid - acetyl CoA2. Exercise – Use of FA for energy - HDL3. Stop smoking – smoking HDL
Drug therapy – if above fails
1. Mevinolin, Mevastatin, Lovastatin:
- HMG-CoA reductase inhibitors.
- increased cellular uptake of LDLs, since the intracellular synthesis of cholesterol is inhibited and cells are therefore dependent on extracellular sources of cholesterol.
2. Cholestyramine or colestipol (resins): - nonabsorbable resins that bind bile acids - not reabsorbed by the liver but excreted. - Reduce feedback inhibiton bile acid synthesis - cholesterol is converted to bile acids to maintain a steady level in circulation.
Familial Hypercholesterolaemia- Defect in LDL receptor cellular cholesterol – no inhibition to HMGCoA
reductase - syntheis cholesterol – further blood cholesterol
- Atherosclerosis at young age
Synthesis of Bile Acids-In liver
-From cholesterol
-Cholesterol 7-hydroxylase
-1° bile acid synthesized in liver – cholic acid
- chenodeoxycholic
-2° - deoxycholic acid, lithocholic acid – formed in intestine
-95% reabsobed – enterohepatic circulation
Conjugation
-in liver
-With glycine or taurine
-cholic acid, → glycocholic or taurocholic
-chenodeoxycholic →glycochenodeoxydholic or taurochenodeoxycholic
→secreted to the intestine via common bile duct or stored in gallbladder
In intestine
-Deconjugation – remove glycine & taurine
-Then cholic acid → deoxycholic acid
-chenodeoxycholic → lithocholic acid
→ 95%reabsorbed via intrahepatic ciculation.
Regulation
-Cholesterol 7-hydroxylase
-Product inhibition, palmitate- excretion bind to resin (cholestyramine) → inhibition to 7-hydroxylase
Eicosanoids
-20 C polyunsaturated fatty acids
-stored in membrane PL
Omega-6 class of essential FA
-Eicosatrienoic acid (20:3-6), eicosatetraenoic acid (20:4 -6) – from linoleic acid (18:2 -6)
Omega-3 class of essential FA
-eicosapentanoic acid (20:5-3)- derived from linolenic acid (18:3 -3)
20:4 -6 – arachidonic acid
-most important
-From membrane phospholipid by phospholipase A2 - from phosphatidylcholine and phospholipase C from phosphatidylinositol
PL A2
-Phosphatidylcholine arachidonic acid
PLC DAG lipase
-Phosphatidylinositol 1,2 DAG MAG + arachidonic acid
-PLA2 is activated by calcium ion
Ecosapentaenoic acid (EPA) 20:5 3
-Synthesized from linolenic acid (18:3 3)
-High in fish oil
-Precursor for series-3 prostaglandin, series-5 leukotrienes
-Reduce the extent of inflammatory response induced by the series-2 PG, series-4 leukotrienes
-Reduce cholesterol, TG level in hyperTG patients
Eicosanoid metabolism
-From arachidonic acid
1.Cyclooxygenase pathway
-endothelial cells
Produces prostaglandin and thromboxanes
2.Lipoxygenase pathway
-neutrophil leukocytes
produces leukotrienes, hydroxyeicosatetraenoic acids (HETEs), lipoxins
-Platelets – both cyclooxygenase and lipoxygenase paths
3.Cytochrome P450 forms epoxides
Prostaglandin
-Ecosanoid – 20C
-Synthesized from arachidonic acid (20:4 -6) – cyclooxygenase pathway
-Contain cyclopentane ring, C8,9,10,11,12
-7types of rings – A,B,D,E,F,G,H,I series – differ in the substituent on the cyclopentane ring
-PGE1,PGE2,PGE3 – 1,2,3 means no of double bond in the HC chain
-2-series – primary PGs – PGD2,PGE2,PGF2,PGI2 & thromboxane A2 - widely dist in body
Prostanoic acid – parent structure for PG
Pathways for eicosanoid formation
ARACHIDONIC ACID
Cyclooxygenase
PGH 2
Lipoxy-genase
HPETE
Cytochrome P450
Epoxides
Prostaglandins Thromboxanes
Leukotrienes HETE Lipoxins
PGG2
Biosynthesis of prostaglandin
Formation arachidonic acid
-Stimulation by inflammatory stimuli such cytokines, bradykinin, adrenaline, thrombin – release Ca2+ - activates PL A2 – hydrolyse arachidonic from membrane PL
-Inhibited by corticosteroids
-Activation of PL C
Cyclooxygenase pathway
1. Arachidonic acid prostaglandin endoperoxide (PGH2) by Cyclooxygenase
-Stimulated by inflammatory stimuli
-Self inactivation – act 15-30s – prevents overproduction of PG
-Can be inhibited by NSAID – aspirin, indomethacine etc
2. PGH2 PGE2, PGI2 by PG synthase
3. PGH2 tromboxane A2 by TXA2 synthase
Functions:
-Mediates physiolgical response
- inflammatory response
- production of pain and fever
- regulation of blood pressure
- regulation of blood coagulation
- induction of labour
- reduce gastric acid secretion
- sleep promoting substance – PGD2
Lipoxygenase Pathway
-3 types of Lipoxygenase – 5, lipoxygenase, 12 & 15
1. A.A hydroxyperoxy-eicosatetaenoic acid (HPETEs)
- short lived
2. HPETEs leukotrienes, lipoxins, hydroxyeicosatetraenoic acids (HETEs)
LEUKOTRIENES
-Lipooxygenase pathway
-Contains 3 conjugated double bonds
-LTA4, LTB4, LTC4, LTD4, LTE4
-Brochoconstriction
-Increase vascular permeability
-Attraction & activation of leucocytes – inflammatory response
-SRS-A (LTC4, LTD4, LTE4)
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