Chapter 5 Metabolism of Lipids

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Chapter 5 Metabolism of Lipids
The biochemistry and molecular biology department
of CMU
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Concept

• Lipids are substances that are insoluble or
  immiscible in water, but soluble in organic
  solvents.

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Fats

(Triglyceride or triacylglycerole)
To store and supply energy
Lipids
Phospholipids
To be important membrane components
Glycolipids
Lipoids
Cholesterol
Cholesterol ester
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Contents

• Section 1 Fatty acids
• Section 2 Metabolism of Triglycerids
• Section 3 Metabolism of Phospholipids
• Section 4 Metabolism of Cholesterols
• Section 5 Metabolism of Plasma Lipoproteins

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Section 1 Fatty acids
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1.1 Classification of fatty acids
Numerical Symbol Common Name Comments
140 Myristic acid Saturated
160 Palmitic acid Saturated
180 Stearic acid  Saturated
161 ? 9 Palmitoleic acid Unsaturated
181 ? 9 Oleic acid Unsaturated
182 ? 9,12 Linoleic acid EFA
183 ? 9,12,15 Linolenic acid EFA
204 ? 5,8,11,14 Arachidonic acid EFA

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Essential Fatty Acids (EFA)

• Linoleic, linolenic and arachidonic acids are
  called essential fatty acids, because they cannot
  be synthesized by the body and must be obtained
  through diet.

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1.2 Important Derivatives of Arachidonic acids

• Arachidonic acids (AA) in turn gives rise to
  biologically important substances known as the
  eicosanoids.
• Prostaglandins (PGs)
• Thromboxanes (TXs)
• Leukotrienes (LTs)

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Section 2 Metabolism of Triglycerides
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Triglyceride (TG) or triacylglycerol (TAG)
Glycerol
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Overview of triglycerides metabolism
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2.1 Degradation of TG

• 2.1.1 Fat catabolism (lipolysis)
• 2.1.2 ß-Oxidation of Fatty acids
• 2.1.3 Other Oxidations of Fatty acids
• 2.1.4 Ketone Bodies Formation and Utilization

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2.1.1 Fat catabolism (lipolysis)

• Fat mobilization
• The triacylglycerol stored in the
  adipocytes are hydrolyzed by lipases, to produce
  free fatty acids (FFA) and glycerol, which are
  released to the blood, this process is called
  fat mobilization.

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The fatty acids thus released diffusively from
the adipocyte into the blood, where they bind to
the serum albumin.
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Hormone sensitive lipase (HSL)

• TG lipase is the rate-limiting enzyme in the TG
  degradation in adipose tissue. It is also named
  HSL because it is regulated by some hormones.

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Effect of hormones on lipolysis

• Lipolytic Hormones
• epinephrine
• norepinephrine
• adrenocorticotropic hormone (ACTH)
• thyroid stimulating hormone (TSH)
• Glucagon etc.
• Antilipolytic Hormones insulin

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glycerol metabolism
Place liver, kidney, intestine
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Note

• In muscle cells and adipocytes, the activity of
  glycerol kinase is low, so these tissues cannot
  use glycerol as fuel.

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2.1.2 ß-Oxidation of Fatty acids

• Fatty acids are one of the main energy materials
  of human and other mammalian.
• Fatty acid catabolism can be subdivided into 3
  stages.

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Stage 1 Activation of FAs

• Acyl-CoA Synthetase (Thiokinase), which locates
  on the cytoplasm, catalyzes the activation of
  long chain fatty acids.

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Key points of FA activation

• 1. Irreversible
• 2. Consume 2 P
• 3. Site cytosol

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Stage 2Transport of acyl CoA into the
mitochondria ( rate-limiting step)

• Carrier carnitine

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Rate-limiting enzyme

• carnitine acyltransferase ?

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Stage 3 ß-oxidation of FAs

• ß-oxidation means ß-C reaction.
• Four steps in one round
• step 1 Dehydrogenate
• step 2 Hydration
• step 3 Dehydrogenate
• step 4 Thiolytic cleavage

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• Step 1. Dehydrogenate

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• Step 2. Hydration

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Step 3. Dehydrogenate
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Step 4. Thiolytic cleavage

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ß- oxidation of fatty acids
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The ß-oxidation pathway is cyclic
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Summary

• one cycle of the ß-oxidation
• fatty acyl-CoA FAD NAD HS-CoA ?fatty
  acyl-CoA (2 C less) FADH2 NADH H
  acetyl-CoA

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The product of the ß-oxidation is in the form of
FADH2, NADH, acetyl CoA, only after Krebs cycle
and oxidative phosphorylation, can ATP be
produced.
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Energy yield from one molecule of palmitic acid
The net ATP production 131-2 129
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2.1.3 Other Oxidations of Fatty acids

• 1. Oxidation of unsaturated fatty acids
• 2. Peroxisomal fatty acid oxidation
• 3. Oxidation of propionyl-CoA

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1. Oxidation of unsaturated fatty acid

• Mitochondria
• Isomerase cis ? trans
• Epimerase D (-) ? L ()

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2. Peroxisomal fatty acid oxidation
Very long chain fatty acids
Acyl-CoA oxidase
FAD
shorter chain fatty acids
ß-oxidation
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3. Oxidation of propionyl-CoA
propionyl-CoA
Carboxylase (biotin) Epimerase Mutase (VB12)
succinyl-CoA
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2.1.4 Ketone Bodies Formation and Utilization

• Ketone bodies are water-soluble fuels normally
  exported by the liver but overproduced during
  fasting or in untreated diabetes mellitus,
  including acetoacetate, ß-hydroxybutyrate, and
  acetone.

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The formation of ketone bodies (Ketogenesis) Loca
tion hepatic mitochondria Material acetyl
CoA Rate-limiting enzyme HMG-CoA synthase
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Utilization of ketone bodies (ketolysis) at
extrahepatic tissues
Succinyl-CoA transsulfurase

•  

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HSCoA ATP
AMP PPi
Acetoacetate thiokinase
-
Lack of succinyl-CoA transsulfurase and
Acetoacetate thiokinase in the liver.
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Biological Significance

• Ketone bodies replace glucose as the major source
  of energy for many tissues especially the brain,
  heart and muscles during times of prolonged
  starvation.

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Normal physiological responses to
carbohydrate shortages cause the liver to
increase the production of ketone bodies from the
acetyl-CoA generated from fatty acid oxidation.

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Plasma concentrations of metabolic fuels (mmol/L)
in the fed and starving states
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Ketosis consists of ketonemia, ketonuria and
smell of acetone in breath
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Causes for ketosis

• Severe diabetes mellitus
• Starvation
• Hyperemesis (vomiting) in early pregnancy

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2.2 Lipogenesis
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2.2.1 Synthesis of fatty acid
palmitic acid (C160)
palmitoylCoA
stearic acid (C180)
stearoylCoA
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oleic acid (C181 D9)
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oleoylCoA
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H3C
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1. Palmitic Acid Synthesis

• Location cytosol of liver, adipose tissue,
  kidney, brain and breast.
• Precursor acetyl CoA
• Other materials ATP, NADPH, CO2

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Citrate-pyruvate cycle

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The sources of NADPH are as follows

• Pentose phosphate pathway

• Malic enzyme

• Cytoplasmic isocitrate dehydrogenase

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Process of synthesis

• (1) Carboxylation of Acetyl CoA
• (2) Repetitive steps catalyzed by fatty acid
  synthase

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(1) Carboxylation of Acetyl CoA

• Malonyl-CoA serves as the donor of two-carbon
  unit.

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• Acetyl-CoA Carboxylase is the rate limiting
  enzyme of the fatty acid synthesis pathway.
• The mammalian enzyme is regulated, by
• phosphorylation
• allosteric regulation by local metabolites.

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(2) Repetitive steps catalyzed by fatty acid
synthase

• Fatty acid synthesis from acetyl-CoA
  malonyl-CoA occurs by a series of reactions that
  are
• in bacteria catalyzed by seven separate enzymes.
• in mammals catalyzed by individual domains of a
  single large polypeptide.

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Fatty acid synthase complex(multifunctional
enzyme)

• Acyl carrier protein (ACP)
• Acetyl-CoA-ACP transacetylase (AT)
• ß-Ketoacyl-ACP synthase (KS)
• Malonyl-CoA-ACP transferase (MT)
• ß-Ketoacyl-ACP reductase (KR)
• ß-Hydroacyl-ACP dehydratase (HD)
• Enoyl-ACP reductase (ER)
• Thioesterase (TE)

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ACP contains 4-phosphopantotheine.
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The overall reaction of synthesis

• acetyl-CoA 7 malonyl-CoA 14 NADPH 14H
• palmitate 7 CO2 14 NADP 8 HSCoA 6H2O

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Routes of synthesis of other fatty acids
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2. Elongation of palmitate

• Elongation beyond the 16-C length of the
  palmitate occurs in mitochondria and endoplasmic
  reticulum (ER).

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• Fatty acid elongation within mitochondria uses
  the acetyl-CoA as donor of 2-carbon units and
  NADPH serves as electron donor for the final
  reduction step.
• Fatty acids esterified to coenzyme A are
  substrates for the ER elongation machinery, which
  uses malonyl-CoA as donor of 2-carbon units.

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3. The synthesis of unsaturated fatty acid

• Formation of a double bond in a fatty acid
  involves several endoplasmic reticulum membrane
  proteins in mammalian cells

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• Desaturases introduce double bonds at specific
  positions in a fatty acid chain.

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2.2.2 Synthesis of Triacylglycerol

• Monoacylglycerol pathway (small intestine)
• Diacylglycerol pathway (liver, adipose tissue)

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1. Monoacylglycerol pathway
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2. Diacylglycerol pathway
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Summary

• Places small intestine, liver, adipose tissue
• Materials
• Endogenous glucose?amino acid?glycerol
• Exogenous free fatty acid and monoacylglycerol

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Adipose tissue generate fat mainly from glucose

• In adipose tissue, the acetyl CoA for the
  synthesis of fatty acid is mainly from glucose.
• The lack of glycerol kinase make the only source
  of glycerol 3-phosphate in adipose tissue is
  glucose.

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Obesity results from an imbalance between energy
input and output
adipose tissue
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Section 3 Metabolism of Phospholipids
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• Phospholipid refers to phosphorous-containing
  lipids.

Glycerophospholipids
Phospholipids
Sphingolipids
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3.1 Classification and Structure of
Glycerophospholipids

• Glycerophospholipids are lipids with a glycerol,
  fatty acids, a phosphate group and a nitrogenous
  base.

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glycerol
fatty acids
nitrogenous base
Phosphatidylcholine
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glycerol
fatty acyl group
Nitrogenous base
fatty acyl group
The basic structure of glycerophospholipid
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In general, glycerophospholipids contain a
saturated fatty acid at C-1 and an unsaturated
fatty acid (usually arachidonic acid) at C-2.
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The major function of phospholipids is to form
biomembrane.
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• Hydrophobic tail fatty acids
• Polar head nitrogenous base

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Some common glycerophospholipid
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Some common glycerophospholipid
(continue)
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3.2 Synthesis of Glycerophospholipid

• Location
• All tissue of body, especially liver kidney
• Endoplasmic reticulum
• Pathways
• CDP-diacylglycerol pathway
• Diacylglycerol pathway

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The system of synthesis

• a. FA
• Glycerol
• b. poly unsaturated fatty acid from plant oil
• c. choline
• ethanolamine
• serine
• inositol
• d.  ATP, CTP
• e. Enzymes and cofactors

from carbohydrate
from food or synthesis in body
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Diacylglycerol pathway
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CDP-Diacylglycerol pathway
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Phosphatidylcholine (Lecithin)
Phosphatidylethanolamine (Cephalin)
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CDP-diacylglycerol
Phosphatidylserine
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Phosphatidylglycerol
Diphosphatidyl glycerol (Cardiolipin)
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Phosphatidylinositol
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3.3 Degradation of glycerophospholipids by
phospholipase
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Actions of phospholipases on lecithin

• PLA1 fatty acid lysolecithin
• PLA2 fatty acid acyl
  glycerophosphoryl choline
• PLC 1,2 diacylglycerol phosphoryl choline
• PLD phosphatidic acid choline

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• Lysophospholipids, the products of Phospholipase
  A hydrolysis, are powerful detergents.

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Section 4 Metabolism of Cholesterol
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4.1 Structure and function of cholesterol

• 1. Function of cholesterol
• (1) It is a constituent of all cell membranes.
• (2) It is necessary for the synthesis of all
  steroid hormones, bile salts and vitamin D.

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2. Structure of cholesterol
All steroids have cyclopentano penhydro
phenanthrene ring system.
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Cholesterol ester
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4.2 Synthesis of cholesterol

• Location
• All tissue except brain and mature red blood
  cells.
• The major organ is liver (80).
• Enzymes locate in cytosol and endoplasmic
  reticulum.
• Materials
• Acetyl CoA, NADPH(H), ATP

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Acetyl-CoA is the direct and the only carbon
source.
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Acetyl-CoA
HMG-CoA
Acetoacetyl-CoA
HMG CoA reductase is the rate-limiting enzyme
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The total process of cholesterol de novo synthesis
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Regulation of cholesterol synthesis

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4.3 Transformation and excretion of cholesterol
Bile acids
Steroid hormones
Vitamin D
Cholesterol
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1. Conversion of Cholesterol into bile acid

• (1) Classification of bile acids
• The primary bile acids are synthesized in the
  liver from cholesterol. The 7?-hydroxylase is
  rate-limiting enzyme in the pathway for synthesis
  of the bile acids.

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• The secondary bile acids are products that the
  primary bile acids in the intestine are subjected
  to some further changes by the activity of the
  intestinal bacteria.

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Classification of bile acids
Classification Free bile acids Conjugated bile acids Conjugated bile acids
Primary bile acids Cholic acid Glycocholic acid Taurocholic acid
Primary bile acids Chenodeoxy-cholic acid Glycocheno-deoxycholic acid Taurocheno-deoxycholic acid
Secondary bile acids Deoxycholic acid Glycodeoxy-cholic acid Taurodeoxy-cholic acid
Secondary bile acids Lithocholic acid Glycolitho-cholic acid Taurolitho-cholic acid
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(2) Strcture of bile acids

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(3) Enterohepatic Cycle of bile acids

• Conversion to bile salts, that are secreted into
  the intestine, is the only mechanism by which
  cholesterol is excreted.
• Most bile acids are reabsorbed in the ileum ,
  returned to the liver by the portal vein, and
  re-secreted into the intestine. This is the
  enterohepatic cycle.

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(4) Function of bile acids

• Bile acids are amphipathic, with detergent
  properties.
• Emulsify fat and aid digestion of fats
  fat-soluble vitamins in the intestine.
• Increase solubility of cholesterol in bile.

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2. Conversion of cholesterol into steroid hormones

• Tissues adrenal cortex, gonads
• Steroid hormones cortisol (glucocorti-coid),
  corticosterone and aldosterone (mineralocorticoid)
  , progesterone, testosterone, and estradiol

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Steroids derived from cholesterol
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3. Conversion into 7-dehydrocholesterol

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4.4 Esterification of cholesterol

• in cells

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in plasma
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Section 5 Plasma lipoprotein
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5.1 blood lipid

• Concept All the lipids contained in plasma,
  including fat, phosphalipids, cholesterol,
  cholesterol ester and fatty acid.
• Blood lipid exist and transport in the form of
  lipoprotein.

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5.2 Classification of plasma lipoproteins

• 1. electrophoresis method
• ?- Lipoprotein fast
• pre ?-Lipoprotein
• ?-Lipoprotein
• CM (chylomicron) slow

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2. Ultra centrifugation method

• high density lipoprotein (HDL) high
• low density lipoprotein ( LDL)
• very low density lipoprotein ( VLDL)
• CM (chylomicron )
  low

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electron microscope
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Separation of plasma lipoproteins by
electrophoresis on agarose gel
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5.3 Structure
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5.4 Composition of lipoprotein
CM VLDL LDL HDL
Density(g/ml) Density(g/ml) lt1.006 0.95-1.006 1.006-1.063 1.063-1.210
Protein 2 10 23 55
Phospholipids 9 18 20 24
Cholesterol 1 7 8 2
Cholesteryl esters 3 12 37 15
TG 85 50 10 4
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5.5 Apolipoproteins
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Functions of apolipoproteins

• a . To combine and transport lipids.
• b .  To regulate lipoprotein metabolism.
• apo A II activates hepatic lipase(HL)
• apo A I activates LCAT
• apo C II activates lipoprotein lipase(LPL)
• c. To recognize the lipoprotein receptors.

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5.6 Metabolism of plasma lipoprotein

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1. CM

• Chylomicrons are formed in the intestinal mucosal
  cells and secreted into the lacteals of lymphatic
  system.

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structure of CM
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Metabolic fate of CM

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summary of CM

• Site of formation intestinal mucosal cells
• Function transport exogenous TG
• key E LPL in blood
• HL in liver
• apoC? is the activator of LPL
• apo E and apo B-48 will be recognized by the LRP
  receptor

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2. VLDL

• Very low density lipoproteins (VLDL) are
  synthesized in the liver and produce a turbidity
  in plasma.

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Nascent VLDL
Metabolic fate of VLDL and production of LDL
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Summary of VLDL

• Formation site liver
• Function VLDL carries endogenous triglycerides
  from liver to peripheral tissues for energy
  needs.
• key E LPL in blood
• HL in liver

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3. LDL

• Most of the LDL particles are derived from VLDL,
  but a small part is directly released from liver.
  They are cholesterol rich lipoprotein molecules
  containing only apo B-100.

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LDL receptors
Cholesterol ester
protein
Cholesterol
LDL
Cholesteryl oleate
Amino acids
Internalization
Lysosomal hydrolysis
LDL binding
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Michael Brown and Joseph Goldstein were awarded
Nobel prize in 1985 for their work on LDL
receptors.
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Summary of LDL

• Formation site from VLDL in blood
• Function transport cholesterol from liver to the
  peripheral tissues. LDL concentration in blood
  has positive correlation with incidence of
  cardiovascular diseases.

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Fates of cholesterol in the cells

• 1. Incorporated into cell membranes.
• 2. Metabolized to steroid hormones.
• 3. Re-esterified and stored. The
  re-esterification is catalyzed by ACAT.
• 4. Expulsion of cholesterol from the cell,
  esterified by LCAT and transported by HDL and
  finally excreted through liver.

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4. HDL

• LDL variety is called bad cholesterol whereas
  HDL is known as good cholesterol .

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Forward and reverse cholesterol transport
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Reverse cholesterol transport

• Cholesterol from tissues reach liver, and is
  later excreted. This is called reverse
  cholesterol transport by HDL.

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Metabolism of HDL in reverse cholesterol transport
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CETP

• Cholesterol ester transfer protein (CETP)
  transfer cholesterol ester in HDL to VLDL and
  LDL.

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Summary of HDL

• Formation site liver and intestine
• Function transport cholesterol from peripheral
  tissues to liver

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summary of lipoprotein metabolism

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5.7 Hyperlipidemias
classification Lipoprotein Blood lipids
? CM TAG? ? ? CH?
?a LDL CH? ?
?b LDL, VLDL CH? ? TAG? ?
? IDL CH? ? TAG? ?
? VLDL TAG? ?
? VLDL, CM TAG? ? ? CH?