1. Cholesterol Metabolism

2. Objectives Define cholesterol as the most important animal steroid.
Know The structure of cholesterol.
Discuss the Synthesis of cholesterol.
Identify the Regulation of cholesterol.
Describe the Excretion of cholesterol.
Define Hypercholesterolemia.

3. CHOLESTEROL METABOLISM
Overview:
Cholesterol is the most important animal steroid
Brain & egg yolk is very rich sources. The liver, kidney and red meat are rich sources.
Average diet supplies about g /day.

4. Cholesterol is an extremely important biological molecule that has roles in membrane structure as well as being a precursor for the synthesis of the steroid hormones and bile acids and vit D3
Both dietary cholesterol and that synthesized de novo are transported through the circulation in lipoprotein particles. The same is true of cholesteryl esters, the form in which cholesterol is stored in cells.

5. The synthesis and utilization of cholesterol must be tightly regulated in order to prevent over-accumulation and abnormal deposition within the body
Such deposition, eventually leading to atherosclerosis, is the leading contributory factor in diseases of the coronary arteries.

6. Most plasma cholesterol is in an esterified form , w is more hydropobic than free cholesterol.
Cholesteryl esters (CE) are not found in membranes
CE are normally present in low levels in most cells
Choesterol & CE must be transported in association with protein in LP or
solubilized by phospholipids and bile salts in the bile

8. Cholesterol: is a sterol (with 8 carbons at C17,= bet 5&6)
Sterols: are steroids with 8-10 carbon atoms in the side chain at C-17 & OH at C-3
Cholesterol is the major sterol in animal tissues
Plant sterols as B-sitosterol are poorly absorbed by humans, it blocks the absorption of dietary cholesterol
Dietary intake of plant steroid esters (trans fatty acid –free margarine ) helps in reduction of plasma cholesterol

9. Structure of cholesterol and its ester.
Plant sterols block the absorption of dietary cholesterol.

10. PLASMA CHOLESTEROL Plasma cholesterol level is 150 –
200 mg/dl (average 175 mg/dl)
Types: 30% of plasma cholesterol
are free and 70% are esterified
with polyunsaturated fatty acids
(PUFA)

11. Biosynthesis of Cholesterol

12. Cholesterol synthesis by all tissues esp
Cholesterol synthesis by all tissues esp.: liver, intestine ,adrenal cortex,& reproductive tissues
It occurs in the cytoplasm with enzymes in both the cytosol and the membrane of the endoplasmic reticulum
Synthesis begins with the transport of acetyl-CoA from the mitochondrion to the cytosol
It needs reducing equivalents in form NADPH & energy from hydrolysis of the high-energy thioester bond of acetyl CoA

13. The rate limiting step occurs at the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, HMGR catalyzed step.
The phosphorylation reactions are required to solubilize the intermediates in the pathway.
After dephosphorylation the intermediates require intracellular sterol carrier protein to keep them soluble
Release of pyrophosphate in the condensation steps make the reactions irreversible

14. Liver parenchymal cells contain two isoenzymes of HMGCoA synthhase The cytolsolic enzyme participates in cholesterol synthesis & the mitochondrial enzyme for ketone bodies synthesis HMGCoA synthase is an intrinsic membrane protein of the endoplasmic reticulum with the enzyme catalytic domain projecting into the cytosol

16. Squalene monooxygenase6C 5C
C30 to C27
Removal of CH3 at C4
= migration from C8 to C5
Reduction of= bet C24&C25
10C
15C
NADPH
NADP
Squalene synthase
O2+NADPH+H
Cyclization
CYCLE
1ST
27C
Squalene monooxygenase

17. Regulation of Cholesterol Synthesis

18. 1.Sterol-dependent regulation of gene expression:↑Cholesterol – transcription factor
2.Sterol-independent phosphorylation/dephosphorylation
3.Hormonal regulation
4.Inhibition by drugs: simvastatin, lovastatin &mevastatin

19. Cholesterol Degradation
Ring of sterol can’t be metabolized to CO2 & H2O in humans
Secretion as such in bile
Conversion to bile acids
Intest.
Egested as such
Modified by bacteria by reduction
Coprostanol & cholestanol
isomers )(
Neutral sterols

20. Bile acids & salts Bile consists of org. & inorg. watery mixture
Lecithin & bile salts are the most important org .quantitatively
Bile liver duodenum
stored in GB
Bile salts are more effective detergents than bile acids b/c of their enhanced amphipathic structure

22. HYPERCHOLESTEROLEMIA
It is the increase of plasma cholesterol above 220 mg/dl.
Hypercholesterolemia is associated with atherosclerosis, coronary heart disease (CHD), heart attacks, and stroke
Causes:
1) Overfeeding of diet rich in cholesterol, Fats specially
saturated FA ,or carbohydrates
2) Diabetes mellitus (D.M.)
3) Hypothyroidism: due to decreased conversion to bile acids
4) Obstructive jaundice: due to the obstruction no excretion of cholesterol or bile salts in the bile
5) Familial hypercholesterolemia

23. Treatment of Hypercholesterolemia
1- Diet:
Decrease carbohydrate, saturated fatty acids and
cholesterol in diet.
Increase polyunsaturated fatty acids
2- Hypocholesterolemic drugs: Statin drugs e.g.
Atorvastatin (Lipitor) and simvastatin (Zocor) are used
to decrease plasma cholesterol levels in patients with
hypercholesterolemia
Statin drugs are competitive inhibitors of HMG CoA
reductase

24. Lipoproteins Metabolism

25. Introduction Lipid compounds: Relatively water insoluble
Therefore, they are transported in plasma (aqueous) as Lipoproteins

26. Lipoproteins and Related Clinical Problems
Atherosclerosis and hypertension
Coronary heart diseases
Lipoproteinemias (hypo- and hyper-)
Fatty liver

27. Lipoprotein Structure
Protein part: Apoproteins or apolipoproteins
These proteins may be structural or transferred
Lipid part:
According to the type of lipoproteins
Different lipid components in various combinations

28. Spherical molecules of lipids and proteins (apoproteins) = amphipathic molecules
Outer coat:
- Apoproteins
- Phospholipids
- Cholesterol (Unesterified)
Inner core:
- TG
- Cholesterol ester (CE)
Lipoprotein Structure

29. Apoproteins Five major classes (A-E) divided by structure & function
Each class has subclasses as Apo A1, Apo CII
Functions
Some are required as structural proteins
Some are activators,
Some are recognition sites.

30. Types of Lipoproteins There are various types of lipoproteins:
They differ in lipid and protein composition
and therefore, they differ in:
- Size and density
- Electrophoretic mobility

31. Chylomicrons Very low density Lipoprotein (VLDL) Low density
Lipoprotein (LDL)
High density
Lipoprotein (HDL)
Types and
Composition of
Lipoproteins

32. Composition of LDL and HDL
Low density lipoprotein (LDL)
Mostly free cholesterol
High density lipoprotein (HDL)
Mostly cholesterol ester
More % protein
More % phospholipids

33. Ultracentrifugation of
Lipoproteins

34. Lipoprotein Electrophoresis

35. Plasma Lipoproteins For triacylglycerol transport (TG-rich):
- Chylomicrons: TG of dietary origin
- VLDL: TG of endogenous (hepatic) synthesis
For cholesterol transport (cholesterol-rich):
LDL: Mainly free cholesterol
HDL: Mainly esterified cholesterol

36. Chylomicrons Assembled in intestinal mucosal cells Lowest density
Largest size
Highest % of lipids and lowest % proteins
Highest triacylglycerol (dietary origin)
Carry dietary lipids to peripheral tissues
Responsible for physiological milky appearance of plasma (up to 2 hours after meal)

37. Type I hyperlipoprotemia
Apo E
mediates uptake

38. Lipoprotein Lipase
Extracellular enzyme anchored by heparan sulphate to the capillary walls of most tissue esp those of adipose tissue, cardiac & skeletal muscles
Its synthesis & transfer to luminal surface of the capillary is stimulated by insulin
Activated by apoC-II

39. Isomers of lipoprotein lipase have different kms for TAG :high km in the adipose enzyme
Low km in the heart enzymes
is absent in adult liver which has hepatic lipase on the endothelial surface which assists in HDL metabolism mainly

40. Metabolism of VLDL

42. Choleteryl Ester Transfer Protein
VLDL CE
PL ,TAG
HDL
Choleteryl Ester Transfer Protein

43. Composition of LDL and HDL
Low density lipoprotein (LDL)
Mostly free cholesterol
High density lipoprotein (HDL)
Mostly cholesterol ester
More % protein
More % phospholipids

44. Low Density Lipoprotein
LDL carries about 70% of total plasma cholesterol
High LDL-C level is well established risk factor for development of coronary heart disease
The diagnosis of a primary defect is made after secondary defect causes have been ruled out

45. Low Density Lipoproteins (LDL)
Produced in the circulation as the end product of VLDLs
Compared to VLDLs:
It contains only apo B-100 Smaller size and more dense
Less TG
More cholesterol & cholesterol ester
Transport cholesterol from liver to peripheral tissues
Uptake of LDL at tissue level by
LDL receptor-mediated endocytosis Recognized by apo B-100

47. LDL: Receptor-Mediated Endocytosis

48. Receptor-Mediated Endocytosis
LDL receptor: Cell surface glycoprotein High-affinity, tightly regulated
LDL/Receptor binding and internalization of the complex by endocytosis
Release of cholesterol inside the cells for: Utilization Storage as cholesterol ester Excretion
Degradation of LDL: into amino acids, phospholipids and fatty acids
Degradation or recycling of receptor

49. LDL Receptor-Mediated Endocytosis: Regulation
Down-regulation: High intracellular cholesterol content Degradation of LDL receptors Inhibition of recepotor synthesis at gene level Decrease No. of receptor at cell surface Decrease further uptake of LDL Decrease de novo synthesis of cholesterol
Up-regulation: Low intracellular cholesterol content Recycling of LDL receptors Stimulation of recepotor synthesis at gene level Increase No. of receptor at cell surface Increase further uptake of LDL Increase de novo synthesis of cholesterol

50. Plasma LDL can be measured by ultracentrifugation, but this is not a practical technique
Calculated LDL:
LDL-C= total cholesterol –[HDL-C]+ TAG/2.2) in case of mmol/L or 5 in case of mg/dL
LDL exits in a range of sizes & densities which can be detected by electrophoresis.
There is evidence that small dense LDL is atherogenic

51. Coronary Heart Disease Risk Factors Determined by (Exclusive of LDL –C) NCEP ATP III
Positive Risk Factors:
Age ≥ 45 y for men , ≥55 y for females or premature menopause
Family history of premature CHD
Current cigarette smoking
Hypertension ( BP≥ 140/90 or taking antihypertensive drugs
HDL-C concentration < 40 mg/dL ( < 1.0 mmol/L)
Diabetes mellitus = CHD risk equivalent
Metabolic syndrome (multiple metabolic risk factors
Negative Risk Factors:
HDL-C concentration ≥ 60 mg/ dL (≥ 1.6 mmol/L) ; its presence removes one risk factor from the total count

52. Categories of Risk for LDL Cholesterol Goals
LDL goal mg/dL
< 100
<130
<160
Risk Category
CHD & CHD risk equivalents
Multiple (2+) risk factors
0-1 risk factor
For cholesterol the molar concenteration can be changed to mass concentration by the formula:
mg/dL =mmol/L x 38.7

53. HDL Metabolism
PC = Phosphatidylcholine/Lecithin

54. High Density Lipoproteins (HDL)
Produced by intestine and liver
Nascent HDL: Disk-shaped Contains apo A-I, C-II and E Contains primarily phospholipid (PC)
Mature HDL (HDL2): First, the HDL3 collects cholesterol (C) Then, C is converted to CE (C- ester) The HDL2 is the spherical mature particle

55. Functions of HDL Reservoir of apoproteins e.g., Apo C-II and E to VLDL
Uptake of cholesterol: From other lipoproteins & cell membranes
(HDL is suitable for uptake of cholesterol because of high content of PC that can both solublizes cholesterol and acts as a source of fatty acid for cholesterol esterification)
Esterification of cholesterol: Enzyme:PCAT/LCAT Activator: Apo A-I Substrate: Cholesterol, Co-substrate: PC Product: Cholesterol ester (& Lyso-PC)
Reverse cholesterol transport

56. Why Is HDL a Good Cholesterol carrier?
Inverse relation between plasma HDL levels and atherosclerosis …. How?
Reverse cholesterol transport involves: Efflux of cholesterol from peripheral tissues and other lipoproteins to HDL3
Esterification of cholesterol & binding of HDL2 to liver and stroidogenic cells by scavenger receptor class B (SR-B1)
Selective transfer of cholesterol ester into these cells
Release of lipid-depleted HDL3

57. Lp(a) Simulates LDL but apo(a) covalently linked to apo B-100
Competes with plasminogen to plasminogen activator
Genetical element
Estrogen decreases it while trans fats increases it

58. Abnormalities in lipoprotein metabolism
Type I hyperlipoproteinemia = F.lipoprotein lipase deficiency
Due deficiency of lipoprotein lipase or apo C-II
-> accumulate of triaryglycerol –rich lipoptn in plasma
Type II hyperlipidemia = F.hypercholesterolemia
-familial hyperbetalipoproteinalmia
deficiency of functional LDL receptors
-> elevation of plasma cholesterol but plasma TG remains normal
There ie premature atherosclerosis
Type III hyperlipoteinemia
= F. dysbetalipoproteinemia or broad B disease
Deficiency of apo E
-> accumulation of chylomicron remnants in plasma .
There’s hyperchloesterolemia with premature atherosclerosis

59. Abetalipoproteinemia defective triacyglycerol transfer protein
No chylomicrons  ↓TG& ↑ TG in small intestine & liver
No VLDL
No VLDL  no LDL  ↓ cholesterol
Hypobetalipoproteinanemia
There ‘s ↓ apo B-100 synthesis
↓ VLDL  ↓ TG & ↑ TG in liver
↓ LDL  ↓ cholesterol
Fatty liver : there’s imbalance between hepatic TG synthesis & secretion of VLDL
Causes . hepatitis ,DM, chronic ethanol ingestin