The document summarizes lipid digestion and absorption. It discusses: 1. Lipid digestion begins in the mouth by lingual lipase and continues in the stomach. 2. In the small intestine, pancreatic lipase emulsified by bile salts completes most digestion. 3. Digested lipids are absorbed via micelles in the small intestine and transported through the body as chylomicrons before being stored or used for energy.

1. Digestion &Absorption of Lipids
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2. Digestion of lipids
- Digestion of fats & other lipids poses problems like,
1.Lipids are water insoluble
2.Lipolytic enzymes are water soluble (relieved by emulsification).
Dietary Sources of Lipids
1. Animal sources:Dairy products, meat, fish, pork, eggs etc.
2. Vegetable source:-sunflower oil, groundnut oil etc.
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3. Phases of lipid Digestion & Absorption
A. Preparatory phase: The digestion of lipids from oral cavity-intestine.
B. Transport phase: Transport of digested fats into enterocytes.
C. Transportation phase: Includes events occurring inside enterocytes & its
passage through lacteals.
NB: Fats role in stomach: satiety value by inducing release of enterogastrone
↓gastric motility  delay gastric emptying.
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4. A. Preparatory Phase
1.Digestion in mouth & stomach:-
a.Lingual Lipase: active in pH 2.0-7.5 (optimum 4-4.5).
- activity is continued in the stomach(in bolus) lasts for 2-3hrs.
- digests 30% of dietary TAG.
- more active on TAGs having short chain FAs at C3-postion.
 Milk fat is its best substrate b/c it is rich of Short & MCFAs.
- released FAs are directly absorbed from stomach wall to portal vein.
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5. Cont…
b. Gastric Lipase: minute amount in gastric secretion & its effect on fat
digestion is negligible b/c:
- No emulsification of Fat in stomach & its low quantity.
- Extremely low PH in stomach; b/c it works at pH ~7.0.
- Don’t act on fats with LCFAs but on milk & egg fats.
- It acts on the regurgitated intestinal content back to gastric lumen.
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6. Cont…
2. Digestion in SI:-main site.
- by pancreatic lipase(steapsin) & presence of bile salts(emulsifying agents).
- Pancreatic juice & bile enter the duodenum, via pancreatic & bile ducts respectively.
 Secretion of pancreatic juice(PJ) is stimulated by the passing acidic chyme to duodenum.
- acidic chyme stimulates secretin & CCK hormones production.
- secretin stimulates pancreas to secrete HCO3
- to increase the pH Pancreatic juice.
- CCK stimulates secretion of pancreatic enzymes & acts on gall bladder.
- Orlistat? Steatorrhea?
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7. Cont …
Contents of Pancreatic Juice:
a. Steapsin w/c act on most fats(optimum pH=6).
b. Phospholipase-2(lecithinase):-On Phospholipids
c.Cholesterol esterase:-On cholesterol esters.
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8. Role of bile salts
a. Have a detergent action on large lump of fat(emulsification).
b. help in emulsification of Fats:-Ca++ facilitates lipase action.
- Ca++ in intestine & FFA are immediately precipitated as soaps that
enhancing further lipase action by decreasing [FFA].
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9. Cont…
Mode of action of pancreatic lipase:
- Hydrolyze “primary ester linkage”/act first at C3a,B-DAG then at C1 β-MAG.
- It can’t readily hydrolyze the ester linkage of position-2(β).
- Then β-MAG a-MAG by isomerization where pancreatic lipase can act now.
NB: In rat sterol-ester hydrolase cleaves FA from β -position.
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10. B. Transport Phase
- FFA, α & β-MAGs mainly enter the microvilli by “simple diffusion”.
- Short,MCFA & unsaturated FA are easily absorbed
C. Transportation Phase:
- there are series of events in enterocytes
- 1st,α-monoglycerides(6%)FFA+ glycerol by Intestinal Lipase.
1. FA absorbed from intestinal lumen & FA from hydrolysis of α-MAG,are activated
to“Acyl-CoA” by ATP-dependent thiokinase.
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11. Cont.…
• 2. Glycerolα-Glycerol-P by glycerokinase for resynthesis of TG with active FA.
- β-MAG(72%) absorbed from intestinal lumen can combine directly with ‘Acyl-CoA’ TAG.
- TAGs formed in SER & merge with RER for the protein part(apo-B48)chylomicron.
- This re-esterification/formation of TAG from FFA,Glycerol,a/B-MAGs creates [ ]
gradient b/n enterocytes & intestinal lumen w/c let more diffusion of lipids.
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12. Chylomicrons
• Synthesized in the intestinal wall(0.5μm in diameter).
- Composed of TG(88%),cholesterol 3% (free/bound),PL(~8%)&<2% apo-B48.
2 types of chylomicrons
1. Nascent chylomicrons: have ‘apo-B48’ only.
2. Circulating chylomicron:contain additional apo-C,w/c from circulating HDL.
- responsible for postprandial lipaemia for 2-2 ½ hrs.
- degraded to FFA & glycerol by LPL w/c requires PL,heparin&apo-CII.
- Heparin helps the enzyme by removing the milky appearance(clearing).
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13. Micelles
- Are water soluble molecular aggregation of bile salts,HCO3
-,higher FAs,MAG & DAG,
lecithins,cholesterol.
- Are smaller than droplets of emulsified fats & absorbed from duodenum &jejunum.
- Bile salts of the “micelles” aren’t absorbed instead taken to liver via portal vein to
liver then released to gall bladder(aka enterohepatic circulation of bile salts).
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14. Digestion & Absorption of Cholesterol
Pancreatic juice contains cholesterol esterase.
- Hydrolysis of cholesterol esters.
- Cholesterol is absorbed from intestine almost in “free” form w/c is facilitated by
the presence of UFA & bile salts but inhibited by sitosterol & stigmasterol.
- 85-90% is in esterified form in lymph.
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15. Digestion & Absorption of Phospholipids
• Dietary PLs may be absorbed directly to portal vein and taken to liver.
Pancreatic juice contains phospholipase-A2 (lecithinase).
- In the presence of bile salts & Ca++ , it cleaves FA from C2lysophospholipid.
- Some lysophospholipid may be resynthesised to PL again in enterocytes while others
incorporated in ‘chylomicrons’ synthesis
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16. Clinical Significance
1.Steatorrheas(fat content in feces>5%) due to deficiency of pancreatic lipase due to
gene mutation, bile flow obstruction 2o to d/t diseases.
- Obstructive Jaundice & Steatorrhea? Vitamin-C deficiency and Steatorrhea?
- Hypocholesterolemia & Steatorrhea? Orlistat? Cystic fibrosis & digestion of lipids?
2.Chyluria:milky urine due to Chylous fistula(abnormal connection b/n urinary tract &
lymphatic drainage system of the intestine) etc.
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17. Metabolism of lipids
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18. Adipose tissue: A dynamic tissue(i.e. there is lipolysis & lipogenesis).
- Regulated by nutritional, metabolic & hormonal factors.
The lipids in the body physiologically exist in two forms:
a. “Element Constant” or structural lipids: content unchanged in any condition.
- Composed mainly of PLs & small amounts of Cholesterol of Cell membrane
b. “Element Variable”/stored lipids(fat depots):content varies.
- The amount fluctuates & composed mainly of TG/neutral fat.
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19. I.Lipogenesis
TG synthesis requires 2 substrates in adipose tissue: Acyl-CoA & α-Glycerol-P
1. Sources of Acyl-CoA/FFA in blood:Dietary & De-novo synthesis FAs.
a. FFA-pool No.1: Acyl-CoA from lipolysis in adipose tissue.
b. FFA-pool No.2:from lipolysis of TG of circulating chylomicrons & VLDL by LPL.
2.α-Glycerol-P:- 2 main sources.
a. Glycerolα-Glycerol-P by Glycerokinase though absent in adipose tissue.
- Glycerol from this tissue isn’t used for lipogenesis instead used for gluconeogenesis.
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20. Cont…
b. From Glycolysis: Dihydroxyacetone-Pα-Glycerol-P.
- Used for TG synthesis in adipose tissue.
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21. II. Lipolysis of TG
- By a hormone-sensitive TG lipaseFFA & glycerol.
Adipolytic lipases are three(lipases acting on tissue lipid):
1. Hormone sensitive TG lipase: Key regulating enzyme.
2. DAG lipase & 3. MAG lipase(are not Hormone sensitive) &LPL on VLDL & Chylomicron.
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22. Factors Affecting TG Metabolism
1. Glucose:- ↑glucose to adipose tissue↑α-Glycerol-PTAG formation.
- FFA outflow plasma FFA↓(not by decreasing lipolysis).
 In DM? & starvation?,availability of glucose in adipose tissue is grossly reduced?
- Lipolysis>re-esterification FFA & ↑plasma FFA.
2. Hormones: may favor lipolysis or esterification.
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23. Cont…
a. Hormones that ↑ the rate of Esterification:(Insulin):results in net Plasma FFA↓.
1. By enhancing phosphodiesterase activity ↓se cAMP.
- Low cAMP level  no activation of HSL↓se FFA & glycerol release.
2. By enhancing glucose uptake into adipose tissues via GLUT-4 more re-esterification.
3. ↑se FA synthesis as glucose is oxidized by HMP pathway & provides NADPH.
4. By enhancing the activity of PDH,ACC& Glycerol-P-acyl transferase.
5. Insulin also inhibits Hormone sensitive TG lipase via protein phosphatase.
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24. Cont…
Hormones that ↑se the rate of lipolysis: results net ↑se of FFA.
1.Catecholamines (E&NE):act rapidly by ↑sing cAMP level.
2.Others like:-Glucagon,Growth hormone,glucocorticoids.
- Act by activating adenyl cyclase↑cAMP in cells active PKAHSL-Po4(active).
In resting state,HSL is cytoplasmic & the fat droplet is covered by the perilipin.
- Active PKA phosphorylates both perilipin & HSL detachment of perilipin& lipolysis.
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25. Cont…
- Glucocorticoids:↑se plasma FFA w/c is achieved by:
1. depressing uptake of Glucose by peripheral tissueα-Glycero-P↓ ↓lipogenesis.
2. Stimulating synthesis of adenylate cyclase↑cAMP in cells.
- NE,E & glucagon released during physical exercise, stress, or fasting stimulate
lipolysis through the β-receptors,cAMP,PKA&HSL↑blood FA levels.
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26. Effect Of Drugs Like Methyl Xanthines
-Methylxanthines,theophylline&dyphylline to treat airway obstruction.
Drugs like caffeine & theophylline: inhibit PDE ↑cAMP ↑lipolysis.
NB:-Drinking of coffee, containing caffeine causes marked elevation of plasma FFA in
humans.
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27. Cont…
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Influence of hormones on adipose tissue metabolism
Dr.Tesaka W, Jimma University, Ethioipia

28. Assimilation of TG By Adipose Tissue:
- Chylomicrons & VLDL degraded by LPL to FFA & glycerol.
- Activity of LPL in adipose tissue is high in fed state & Low in starvation & DM.
- LPL is found attached in capillaries of adipose tissue, heart, skeletal muscle, lactating
mammary gland and, less amount in spleen,lungs,kidneys though not in liver & brain.
- LPL is found attached on walls of capillaries bound to heparan-sulfate.
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29. Brown Adipose Tissue
1.“White” fat (predominant fat depot) & 2. Brown fat(found in thoracic region).
Role in Thermogenesis
1.At times when a heat generation is necessary.
- During arousal from hibernation, when animals exposed to cold & in newborn animals.
2. Though not a prominent tissue in humans, it appears & responsible for diet-induced
thermogenesis w/c may account for how some persons can eat & don’t get fat.
NB:Brown adipose tissue is reduced or absent in obese persons & lifelong time in rats.
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30. Characteristics of Brown Adipose Tissue
- A high content of mitochondria & brown due to high content of cytochromes.
- Has a well-developed blood supply & rich in carnitine.
- Unlike white adipose tissue, it has the enzyme Glycerokinase.
- Low ATP-synthase activity & high O2 consumption.
- White adipose tissue stores fat; but brown burn it off as heat.
- Newborns have lots of brown fat to help them keep warm.
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31. Mechanism of Heat Production
Oxidation & phosphorylation aren’t coupled in mitochondria.
- Thus,oxidation produces much heat.
Chemiosmotic theory: there should be H+-gradient in inter-mitochondrial space to
generate ATP on inner membrane.
- No H+-gradient in this tissue due to thermogenic protein, called Thermogenin.
- During cold stress in rats, brown adipose tissue accounts for 82% of heat.
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32. Oxidation of Fatty Acids
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33. Sources of Plasma FFA
1. Lipolysis in adipose tissue(Pool No-1) & from circulating chylomicrons & VLDL
2. A small portion from dietary source(SC & MCFA) & FFA synthesized by liver.
Site of Oxidation:-Liver,heart,Kidney,Muscle,Brain,Lungs,testes&adipose tissue.
- FA is a source of energy for heart of neonates but glucose & lactate for fetal heart.
- After meal,plasma contains 10-30% FFA w/c form loose “albumin-FFA complex” that
dissociate at plasma membrane.
- SCFAs are H2O soluble & exist as unionized or FA anion form.
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34. Types of Fat oxidation
- 1. β-oxidation(major)
- 2.Others(α-oxidation,ω-oxidation&Peroxisomal oxidations).
1. β-oxidation
Site: Mitochondrial matrix by β-oxidase system& by cleaving 2Cs as acetyl-CoA at a
time from β-position.
- Has 4 repeating steps(per cycle) in mitochondria.
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35. Activation of FAs
- It is a Cytosolic process for LCFAs & mitochondrial for SCFAs.
- by Acyl-CoA Synthetase/Ligase(Thiokinases)thioester bond
- Activates all forms of FAs(Saturated,UFA,OH-FAs)
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36. Carnitine
1. Aka β–OH–trimethyl ammonium butyrate(from Lys & SAM mainly in liver& kidney).
- FAs activated in the cytosol but oxidized in mitochondrial matrix.
- Helps LCFAs to cross inner mitochondrial membrane & not for SCFAs.
- Works with 3 Enzymes:CAT-I, CACT, CAT-II.
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37. Steps of β-oxidation
1.Dehydrogenation:Removal of 2H atoms from C2 & C3 by acyl-CoA dehydrogenase Δ2-
transenoyl-CoA(also α,β-unsaturated acyl-CoA)&FAD as a co-enzyme.
2.Hydration:1mole of H2O is added to z double bond to form 3/β-OH acyl-CoA by Δ2-
enoyl-CoA hydratase.
3.Dehydrogenation:removal of 2H atoms by 3–OH–acyl-CoA dehydrogenase3/β-ketoacyl-
CoA) & NAD+ is a coenzyme.
4.Thiolytic cleavage:b/n C2 & C3 by thiolase(3–keto acyl thiolase)+CoA.
- SIDS(sudden infant death syndrome)?
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38. 18/03/2024 38
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39. Exercise
• Net ATP from β-oxidation of palmitic acid?(Palmitic acid is C15H31COOH).
- In β-oxidation, it will undergoes 7cycles7Acetyl-CoA
- acetyl-CoA/cycle(last cycle-one extra)=8 acetyl-CoA.
Bioenergetics of β-Oxidation & Its efficiency.
- 8 acetyl-CoA = 8x12ATP=96ATP& 7 cycles= 5 ATP/cycle=5x7=35
Total=35+96=131 ATP, Net=131-2=129ATP from 1palmitate.
-B-oxidation of Odd chain Fatty acid? Net ATP of 17C fatty acid?
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40. Cont….
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a. Methylmalonate Pathway
- Found only in animals & occurs in the mitochondria of liver,cardiac and skeletal muscles, kidney&
other tissues.
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41. Regulation of β-Oxidation
- Main site at carnitine-shuttle:
A. Inhibition: CPT-I by Malonyl-CoA when there is plenty of ATP from glucose.
B. Activation:-starvation, fasting & exercise ATP↓se AMP↑se.
- AMP activates AMP dependent protein kinase(AMPK)AMPK phosphorylates ACC
Inactivate ACC FA synthesis will quit.
• Transcription factor:-PPARα(a TF regulates gene expression in muscle,liver & adipose
tissue essential for fatty acid oxidation.
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42. Peroxisomal β-Oxidation
- Oxidation of VLCFAs like ≥C22
- VLCFAs,branched FAs, Some Prostaglandins, LTs are degraded.
- The enzymes in peroxisomes don’t attack SCFAs.
- Acetyl-CoA,MCFAs, shortened cholesterol side chain/bile acid(C24)& H2O2.
- Not linked to phosphorylations  No ATP formation.
- Enzymes are induced by high fat diet or hypolipidemic drugs(Clofibrate).
- End-products:-octanoyl-CoA & acetyl-CoA:removed from peroxisomes to mitochondria with help
of carnitine for further oxidation.
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43. Difference b/n peroxisomal & mitochondrial β-oxidation
1. The 1stoxidation step in peroxisome is catalyzed by Acyl-CoA oxidase.
Acyl-CoA+O2= trans-2,3-dehydroacyl-CoA+H2O2
2.In peroxisome:peroxisomal CAT instead of CAT to transport activated acyls to mitochondria.
3.The NADH formed in the 3rd oxidative step can’t be reoxidized in the peroxisome,so reducing
equivalent is exported to the cytosol.
4. The β-ketothiolase in peroxisomal β-oxidation is less specific.
- Malate/OAA shuttle(peroxisomal redox shuttle)?Peroxisomal malateDH & Cytosolic Malate DH?
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44. 1.Zellweger’s(Cerebrohepatorenal) Syndrome(Named After Hans Zellweger)
- Congenital disorder chxzed by the reduction or absence of functional peroxisomes.
- Fail to Oxidise VLCFA accumulation of VLCFA in brain,liver& kidney.
2.Brown–Schilder’s Disease(Named after Paul Schilder)
- Aka Adreno leukodystrophy(ALD) autosomal recessive disorder.
- Insufficient oxidation of VLCFAs due to deficiency of peroxisomal matrix proteins.
- VLCFAS are accumulated & myelin sheaths are destroyed.
- Leukodystrophy is chxzd by degeneration of the white matter in the brain.
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45. 3. CPT Deficiency
a. In liver  Hypoglycaemia & low plasma ketone bodies. How?
b. In Muscle:-recurrent muscle weakness & myoglobinuria.
4. Jamaican Vomiting Sickness:
- Caused by eating the unripe fruit of the Akee tree,w/c contains a toxin,hypoglycin
that inactivates medium & short-chain acyl-CoA dehydrogenase  blocks β-oxidation
 hypoglycemia & excretion of the respective FAs.
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46. II.Oxidation Of Unsaturated Fatty Acids
- Most FAs in TAG & PLs of animals and plants are unsaturated.
- bonds are in cis-configuration & can’t be acted upon by enoyl-CoA hydratase.
- Needs 2-auxiliary enzymes Isomerase & Reductase
a. Oxidation of MUFAs: Requires only enoyl-CoA Isomerase additionally.
- Oleate,an abundant C18 MUFA with a cis-double bond b/n C9 & C10(cis-∆9).
- Oleoyl-CoA then undergoes 3 passes in β-oxidation cycle 3 acetyl-CoA & 12-
carbon UFA,cis-∆3-dodecenoylCoA.
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47. Cont…
- This product can’t be acted upon by enoyl-CoA hydratase.
- Then cis-∆3-enoyl-CoA  trans-∆2-enoyl-CoA by enoyl-CoA isomerase.
Elaidinization?
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48. b.Oxidation of PUFAs
- Requires 2-auxiliary enzymes,enoyl-CoA isomerase & 2,4-dienoyl-CoA reductase.
e.g., a C18 PUFA with 2-cis bonds denoted Cis-∆9,Cis-∆12.
- Linoleoyl-CoA undergoes 3 passes through the typical β-oxidation 3 acetyl-CoA+
coenzyme A ester of a C-12 UFA with a cis-∆3,cis-∆6 configuration.
- Enoyl-CoA isomerase & 2,4-dienoyl-CoA reductase allows re-entry of this intermediate
into β-oxidation6 cetyl CoA.
- Total of 9 moles acetyl CoA from linoleate.
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49. Cont…
- The introduction of 2 additional enzymes(enoyl-CoA Isomerase &3-hydroxyacyl-CoA
racemase)makes it possible to handle any combination of double bonds in UFAs.
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Key:A= enoyl-CoA isomerase;
B=enoyl-CoA hydratase;&
C=3-hydroxyacyl-CoA epimerase.
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50. Cont…
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Oxidation of PUFAs
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51. Ketogenesis
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52. Cont.…
- The acetyl-CoA is oxidized in TCA cycle if fat & CHO are balanced.
- Under certain metabolic conditions liver produce ketone bodies acetoacetate & β-OH
butyrate w/c pass by diffusion to blood.
- Acetoacetate  acetone by spontaneous decarboxylation.
- These 3 molecules are collectively known as Ketone bodies/acetone bodies.
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53. Concentration of Ketone Bodies:
- In normal well fed individual not exceeds 1mg/dl &<1mg/day is lost in urine.
- Ketoacidosis:Acetoacetate&β-OH-butyrate(moderatelystrongacids).
 Ketonemia,Ketosis & Ketonuria?
buffered by alkalis (cations)in blood & tissues  loss of buffer cations,w/c progressively
depletes the alkali reserve↓ ketoacidosis(may be fatal in uncontrolled DM).???
 Causes of Ketoacidosis? Starvation, DM, Alcoholism?
- Others like high fat diet & after vigorous exercise can cause ketoacidosis.
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54. Site of Formation & Fate
- Produced only in Liver & pass to the blood.
- Extrahepatic tissues can pick & use ketone bodies as respiratory substrates.
- Liver can’t utilize them b/c it lacks thiophorase(β-ketoacyl:CoAtransferase).
- Ketone bodies are utilized by extrahepatic tissues as “fuel” by skeletal & cardiac
muscles, brain & kidney.
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55. Ketogenesis in Liver
1. Acetoacetyl-CoA:the starting material for ketogenesis.
a. Directly during the course of β-oxidation of FA,or
b.from condensation of 2Acetyl-CoA.
- β-OH-butyrate is quantitatively the predominant one
in blood & urine in Ketosis.
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56. Cont…
1. Activation of Acetoacetate to acetoacetyl-CoA in 2 ways.
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Or
minor
2. Fate of β-OH-Butyrate
a.β-OH-butyrate can be converted back to “acetoacetate” by β-OH-butyrate
dehydrogenase & NAD+(reversible rxn)(major route).

57. Cont…
- Acetoacetyl-CoA formed is splitted to “acetyl-CoA” by thiolase&oxidised in TCA cycle.
3. Fate of Acetone:-Acetone is difficult to be oxidised in vivo.
- Excess of acetone can be exhaled & excreted in urine.
- Imparts fruity smell of breath & urine.
Ketosis Susceptibility
- Dogs are least susceptible,female,infants and young children are more susceptible.
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58. Factors Determining Magnitude of Ketogenesis
1. Ketogenic Substances
- ↑plasma FFA & Ketogenic-AAs. DM & starvation?
2. Antiketogenic Substances
- CHO,glycerol of fat,Malonyl-CoA & glucogenic amino acids.
 If the ratio of Ketogenic/Antiketogenicfactors is>2,ketone bodies appear in urine.
End!
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59. CHOLESTEROL
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60. Cholesterol
• Normal adult has 35g of cholesterol most of it in the cell membranes.
RDI:300mg/day according to FDA.
• Importance of cholesterol
- Membrane packing & fluidity
- a precursor for bile acids & steroid hormones,vitamin-D3 etc.
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61. Metabolism of Cholesterol
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 Site of Synthesis: mainly in liver(75-80%),Intestine, Gonads & Skin.
- Low amount in Adipose tissue,muscle,aorta & neural tissues.
- Brain of newborns can synthesize cholesterol but not adults’.
- Acetyl-CoA is the starting material(all 27C) of cholesterol after 37 steps.
- ~1g/day is synthesized (endogenous cholesterol) from acetyl CoA.
- Acetyl CoA from pyruvate oxidation & FA oxidation exported by citrate shuttle.
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62. 62
Transport of Acetyl-CoA by Citrate Shuttle
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63. Steps of biosynthesis:
5-Groups of Reactions
1.Synthesis of mevalonate:a 6C compound from acetyl-CoA by by 2 steps reaction.
a. HMG-CoA is formed in Cytosol.
b. Next the rate-limiting step,
- HMG-CoA  Mevalonate.
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Statins?
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64. 2. Formation of Isoprenoid Units/Isopentenyl pyrophosphate:
- Mevalonate is phosphorylated sequentially(3) & loss of Co2.
- 5C Isoprenoid units are regarded as building blocks of steroid nucleus.
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Diphosphomevalonate decarboxylase
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65. 3.Formation of Squalene
- Squalene a 30C aliphatic chain is synthesised from condensation of 6 isoprenoid units.
- Squalene synthetase is microsomal liver enzyme.
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66. 4. Cyclisation of Squalene
-Takes place in 2 steps by oxidosqualene cyclase.
5. Conversion of lanosterol to cholesterol:in 19 steps.
- Removal of 3 angular–CH3 groups by demethylation with unknown mechanism.
- CH3-group at C14 is first eliminated.
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67. Regulation of Cholesterol Biosynthesis
1. SREBP(Sterol Regulatory Element-Binding Protein) Pathway
- The steps up to HMG-CoA synthesis are reversible.
- The formation of mevalonate is irreversible.
- only hepatic synthesis is inhibted by dietary cholesterol.
- Liver HMG-CoA reductase:inhibited by mevalonate & cholesterol.
- Fasting & starvation activates HMG-CoA lyase not HMGR.
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68. Cholesterol Homoeostasis
1. By regulating HMGR activity & level(major).
-HMGR has 2-domains:cytoslolic(catalytic domain)&membrane sterol sensing domain (SSD).
- At high[Cholesterol],the SSD faces to the cytosolic side and let the HMGR be tagged by
Ub& degraded by Proteosome.
- At low ATP high AMPactivation of AMPK pathwayphosphorylate HMGR(inactive).
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69. 2. Regulating excess of free cholesterol in cells via SREBP(as TF).
a. At high[Choles],SREBP binds to SCAP & INSIG-1 proteins no expression of HMGR.
b. At low[Choles],INSIG-1 dissociates from SREBP-SCAP complex &SREBP-SCAP taken to
GA cleaved by S1P(site 1protease)& S2Pfree SREBP  binds to its Receptor in
nucleus to sterol regulatory element(SRE) expression of HMGR.
3.Regulation of plasma chole via LDL-mediated uptake & HDL-mediated reverse transport.
- LDL is the main transporter of Cholesterol(1500 cholesterol ester).
- LDL-R expression also mediated by SREBP at low [Choles] & blocked at high[Choles].
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70. Transport of Cholesterol
- Absorbed from intestine, incorporated into chylomicrons.
- 80-90% in the lymph is esterified with LCFA.
- 70% is esterified’ & transported as “lipoproteins” in plasma.
- Largest proportion of cholesterol is found in LDL(B-lipoprotein) & also in HDL.
- Free cholesterol exchange b/n tissues & LPs whereas cholesterol esters don’t.
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71. Cholesterol balance in Tissues
a. Increase of cholesterol in cells: due to
- ↑sed synthesis of Cholesterol & hydrolysis of CE by cholesterol ester hydrolase.
- Uptake & delivery of cholesterol in cells by circulating LDL.
b. Decrease of cholesterol in cells:-due to
- Efflux of cholesterol from cells to HDL & esterification by AcylCoA-cholesterol acyltransferase.
- Utilization of cholesterol for synthesis of steroid hormones.
- formation of cholic acids & formation of vit-D3 in hepatocytes.
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72. Other Factors
1. Dietary fats:-↑sed intake of fats(mainly Saturated FA)↑es Choles in blood.
2. Dietary cholesterol:↑se cholesterol in diet ↓es endogenous synthesis.
3. Dietary CHOs(increase calorie intake)↑se cholesterol level.
4.Heredity & in A&AB blood groups slight increase blood cholesterol.
5. Physical exercise:-lowers serum cholesterol & HDL↑.
6.↑Plasma FFA:↑se cholesterol in circulation.
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73. Fate of Cholesterol
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74. A.Bile Acids
a.1o Bile Acids:- Synthesized in the liver from cholesterol by oxidation.
1.Cholic acid(main acid in bile) & 2.Chenodeoxy cholic acid.
b. 2o bile acids:- produced in intestine from 1o bile acids by the action of intestinal
bacteria,by deconjugation &7α-dehydroxylation.
1.Deoxycholic acid from cholic acid & 2.Lithocholic acid from chenodeoxycholic acid.
- Bile salts: taurine or glycine,Na and K salts of these conjugated bile acids.
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75. Biosynthesis of Bile Acids
1.1st step is the 7α-hydroxylation of the cholesterol to form 7α-OH cholesterol, by
7α-hydroxylase,amicrosomal enzyme(rate limiting step).
- Vitamin-C deficiency leads to cholesterol accumulation & atherosclerosis in scorbutic animals.
2. Cholic acid &Chenodeoxy cholic acid formation from 7-a-OH cholesterol by 12α-
hydroxylase & involves several steps.
Vitamin-C deficiency & Gall Stone?
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76. Formation of bile acids
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77. Cont…
- Chenodeoxycholic acid(aka chenocholic acid named b/c it was 1st isolated from domestic
goose(Greek,Cheno= goose).
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78. Regulation of Bile Acid Synthesis
- Each day the amount of bile acid lost in feces= synthesised in liver
7-α-hydroxylase regulation
1.nuclear bile acid binding receptor Farnesoid-X receptor (FXR).
- When bile acid pool↑ses in enterohepatic circulation,FXR is activated & the transcription of the
7-α-hydroxylase gene is suppressed.
2. Enhanced by endogenous & dietary cholesterol(cholesterol feeding).
3. Bile acids,exert"feedback”inhibition on 7-α-hydroxylase.
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HSD3B=3β-hydroxy steroid
dehydrogenase

79. Bile
- A viscous fluid produced by the liver cells & doesn’t contain digestive enzymes.
-Secreted by liver  canaliculi & bile duct gallbladder(~ 500-1000ml/day).
Functions of Bile Salts
- Decrease Surface tension(as emulsifier) & for Micelles formation
- Absorption of fat soluble Vitamins & carotene.
- Have Choleretic action;stimulate liver to secrete bile as long as bile acids are absorbed.
- keep cholesterol in solution(Cholesterol remains soluble in gallbladder bile by bile salts).
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80. 80
18/03/2024 Dr.Tesaka W, Jimma University, Ethioipia