Cholesterol Metabolism

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.

Objectives

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 0.5 - 1 g /day.

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.

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.

• MostMost plasma cholesterolplasma cholesterol is in an esterified form , w is more hydropobichydropobic than free cholesterol.

• Cholesteryl esters (CE) are not found not found in membranes in membranes

• CE CE are normally present inare normally present in low levels low levels in in most cellsmost cells

• Choesterol & CE must be Choesterol & CE must be transportedtransported in association with protein in LP in association with protein in LP or or

solubilizedsolubilized by phospholipids and bile by phospholipids and bile salts in the bilesalts in the bile

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

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

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)

Biosynthesis of Cholesterol

• Cholesterol synthesis by all tissues esp.: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 NADPHNADPH & energy

from hydrolysishydrolysis of the high-energy thioesterthioester bond of

acetyl CoA

• The rate limiting step occurs at the 3-hydroxy-3-3-hydroxy-3-

methylglutaryl-CoA (HMG-CoA) reductasemethylglutaryl-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

Liver parenchymal cells contain two Liver parenchymal cells contain two isoenzymes of HMGCoA synthhaseisoenzymes of HMGCoA synthhase

The The cytolsolic enzyme cytolsolic enzyme participates in participates in cholesterol synthesis & the cholesterol synthesis & the mitochondrial enzymemitochondrial enzyme for ketone for ketone bodies synthesisbodies synthesis

HMGCoA synthaseHMGCoA synthase is an intrinsic membrane protein of the is an intrinsic membrane protein of the endoplasmic reticulum with the enzyme endoplasmic reticulum with the enzyme catalytic domain projecting into the catalytic domain projecting into the cytosolcytosol

6C

5C

10C

15C

Squalene synthaseNADPH NADP

O2+NADPH+HCyclizationCYCLE

1ST

27C

C30 to C27Removal of CH3 at C4

=migration from C8 to C5Reduction of= bet C24&C25

Squalene monooxygenase

Regulation of Cholesterol Synthesis

• 1.Sterol-dependent regulation of 1.Sterol-dependent regulation of

gene expressiongene expression::↑Cholesterol – ↑Cholesterol –

transcription factortranscription factor

• 2.Sterol-independent 2.Sterol-independent

phosphorylation/dephosphorylationphosphorylation/dephosphorylation

• 3.Hormonal regulation3.Hormonal regulation

• 4.Inhibition by drugs: simvastatin, 4.Inhibition by drugs: simvastatin,

lovastatin &mevastatinlovastatin &mevastatin

Cholesterol Degradation

• Ring of sterol can’t be metabolized to CO2 & H2O in humans

Conversion to bile acids Secretion as such in bile

Modified by bacteria by reductionCoprostanolCoprostanol & cholestanolcholestanol

isomersisomers)( )( Egested as such

Neutral sterolsNeutral sterols

Intest.

Bile acids &Bile acids & saltssalts

• Bile consists of org. & inorg. watery mixture

• Lecithin & bile salts are the most important org .quantitatively

• Bile liver duodenum duodenum • stored in GBstored in GB • Bile salts are more effective detergents Bile salts are more effective detergents

than bile acids b/c of their enhanced than bile acids b/c of their enhanced amphipathic structureamphipathic structure

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

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

LipoproteinsMetabolism

Lipid compounds: Relatively water insoluble

Therefore, they are transported in plasma (aqueous) as Lipoproteins

Introduction

Lipoproteins and Related Clinical Problems

• Atherosclerosis and hypertension

• Coronary heart diseases

• Lipoproteinemias (hypo- and hyper-)

• Fatty liver

Lipoprotein Structure

Protein part: Apoproteins or apolipoproteinsThese proteins may be structural or transferred

Lipid part:• According to the type of lipoproteins• Different lipid components in various combinations

Lipoprotein Structure

Spherical molecules of lipids and proteins (apoproteins) = amphipathic molecules

Outer coat: - Apoproteins - Phospholipids - Cholesterol (Unesterified)

Inner core: - TG - Cholesterol ester (CE)

ApoproteinsApoproteins

• Five major classes (A-E) divided by structure & function

• Each class has subclasses as Apo A1, Apo CII

FunctionsFunctions

• Some are required as structural proteins

• Some are activators,

• Some are recognition sites.

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

Types andComposition

ofLipoproteins

Chylomicrons

Very low densityLipoprotein (VLDL)

Low densityLipoprotein (LDL)

High density Lipoprotein (HDL)

Composition of LDL and HDL

High density lipoprotein (HDL)Mostly cholesterol esterMore % proteinMore % phospholipids

Low density lipoprotein (LDL) Mostly free cholesterol

Ultracentrifugationof

Lipoproteins

Lipoprotein Electrophoresis

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

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)

Apo E mediates uptake

Type I hyperlipoprotemia

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

• 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

Metabolism of VLDL

VLDL

HDL

PL ,TAGCE

Choleteryl Ester Transfer Protein

Composition of LDL and HDL

High density lipoprotein (HDL)Mostly cholesterol esterMore % proteinMore % phospholipids

Low density lipoprotein (LDL) Mostly free cholesterol

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

Low Density Lipoproteins (LDL)

Produced in the circulation as the end product of VLDLsCompared to VLDLs:

It contains only apo B-100Smaller size and more denseLess TGMore cholesterol & cholesterol ester

Transport cholesterol from liver to peripheral tissuesUptake of LDL at tissue level by

LDL receptor-mediated endocytosis Recognized by apo B-100

LDL: Receptor-Mediated Endocytosis

Receptor-Mediated Endocytosis

• LDL receptor: Cell surface glycoproteinHigh-affinity, tightly regulated

• LDL/Receptor binding and internalization of the complex by endocytosis

• Release of cholesterol inside the cells for:UtilizationStorage as cholesterol esterExcretion

• Degradation of LDL: into amino acids, phospholipids and fatty acids

• Degradation or recycling of receptor

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 LDLDecrease 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 LDLIncrease de novo synthesis of cholesterol

• 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

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 factorsNegative Risk Factors: HDL-C concentration ≥ 60 mg/ dL (≥ 1.6 mmol/L) ; its

presence removes one risk factor from the total count

Categories of Risk for LDL Cholesterol Goals

Risk Risk CategoryCategory

CHD & CHD risk

equivalents

Multiple (2+) risk factors

0-1 risk factor

LDL goal mg/dLLDL goal mg/dL

< 100

<130

<160

For cholesterol the molar concenteration can be changed to mass concentration by the formula:mg/dL =mmol/L x 38.7

HDL Metabolism

PC = Phosphatidylcholine/Lecithin

High Density Lipoproteins (HDL)

• Produced by intestine and liver

• Nascent HDL: Disk-shapedContains apo A-I, C-II and EContains 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

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/LCATActivator: Apo A-I Substrate: Cholesterol, Co-substrate: PCProduct: Cholesterol ester (& Lyso-PC)

• Reverse cholesterol transport

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

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

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 atherosclerosisThere ie premature atherosclerosis

Type III hyperlipoteinemia = F. dysbetalipoproteinemia or broad B diseaseDeficiency of apo E-> accumulation of chylomicron remnants in plasma .

There’s hyperchloesterolemia with premature atherosclerosisThere’s hyperchloesterolemia with premature atherosclerosis

Abnormalities in lipoprotein metabolism

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