UNIT III:
Lipid Metabolism
Cholesterol and Steroid
Metabolism
5- Bile Acids and Bile Salts
 Bile consists of a watery mixture of organic and
inorganic compounds.
 Phosphatidylcholine (lecithin) and bile salts
(conjugated bile acids) are quantitatively the most
important organic components of bile.
 Bile can either pass directly from the liver where it is
synthesized into the duodenum through the common
bile duct, or be stored in the gallbladder when not
immediately needed for digestion.
2
A. Structure of the bile acids
 The bile acids contain 24 carbons, with
two or three hydroxyl groups and a side
chain that terminates in a carboxyl group.
 The carboxyl group has a pKa of about six
and, therefore, is not fully ionized at
physiologic pH—hence, the term “bile
acid.”
 The bile acids are amphipathic in that the
hydroxyl groups are α in orientation (they
lie “below” the plane of the rings) and
the methyl groups are β (they lie “above”
the plane of the rings).
 Therefore, the molecules have both a
polar and a nonpolar face, and can act as
emulsifying agents in the intestine,
helping prepare dietary triacylglycerol
and other complex lipids for degradation
by pancreatic digestive enzymes.
Figure 18.8 Bile acids.
3
B. Synthesis of bile acids
 Bile acids are synthesized in the liver by a multistep,
multiorganelle pathway in which hydroxyl groups are inserted
at specific positions on the steroid structure, the double bond
of the cholesterol B ring is reduced, and the hydrocarbon
chain is shortened by three carbons, introducing a carboxyl
group at the end of the chain.
 The most common resulting compounds, cholic acid (a triol)
and chenodeoxycholic acid (a diol), are called “primary” bile
acids.
Note:
 The rate-limiting step in bile acid synthesis is the introduction of a
hydroxyl group at carbon 7 of the steroid nucleus by cholesterol-7-αhydroxylase, an ER-associated cytochrome P450 (CYP) enzyme
found only in liver.
 The enzyme is down-regulated by cholic acid and up-regulated by
cholesterol
4
 Figure 18.9 Synthesis of cholic
acid, a bile acid.
5
C. Synthesis of bile salts
 Before the bile acids leave the liver, they are conjugated to a
molecule of either glycine or taurine (an end-product of
cysteine metabolism) by an amide bond between the carboxyl
group of the bile acid and the amino group of the added
compound.
 These new structures are called bile salts and include
glycocholic and glycochenodeoxycholic acids, and taurocholic
and taurochenodeoxycholic acids.
 The ratio of glycine to taurine forms in the bile is
approximately 3:1.
 Addition of glycine or taurine results in the presence of a
carboxyl group with a lower pKa (from glycine) or a sulfonate
group (from taurine), both of which are fully ionized
(negatively charged) at physiologic pH.
6
C. Synthesis of bile salts
 Bile salts are more effective detergents than bile acids because
of their enhanced amphipathic nature.
 Therefore, only the conjugated forms—that is, the bile salts—
are found in the bile.
 Individuals with genetic deficiencies in the conversion of
cholesterol to bile acids are treated with exogenously supplied
chenodeoxycholic acid.
Note:
 Bile salts provide the only significant mechanism for cholesterol
excretion, both as a metabolic product of cholesterol and as a
solubilizer of cholesterol in bile
7
 Figure 18.10 Bile salts. [Note “cholic” in the names.]
8
D. Action of intestinal flora on bile salts
 Bacteria in the intestine can remove glycine and
taurine from bile salts, regenerating bile acids.
 They can also convert some of the primary bile acids
into “secondary” bile acids by removing a hydroxyl
group, producing deoxycholic acid from cholic acid
and lithocholic acid from chenodeoxycholic acid
(Figure 18.11).
9
Figure 18.11 Enterohepatic circulation of bile salts and bile
acids.
10
E. Enterohepatic circulation
 Bile salts secreted into the intestine are efficiently
reabsorbed (greater than 95%) and reused.
 The mixture of primary and secondary bile acids and bile
salts is absorbed primarily in the ileum.
 They are actively transported from the intestinal mucosal
cells into the portal blood, and are efficiently removed by
the liver parenchymal cells.
Note:
 Bile acids are hydrophobic and require a carrier in the portal
blood. Albumin carries them in a noncovalent complex, just
as it transports fatty acids in blood.
 The liver converts both primary and secondary bile acids
into bile salts by conjugation with glycine or taurine, and
secretes them into the bile.
11
E. Enterohepatic circulation
 The continuous process of secretion of bile salts into the bile,
their passage through the duodenum where some are
converted to bile acids, and their subsequent return to the
liver as a mixture of bile acids and salts is termed the
enterohepatic circulation.
 Between 15 and 30 g of bile salts are secreted from the liver
into the duodenum each day, yet only about 0.5 g is lost daily
in the feces.
 Approximately 0.5 g/day is synthesized from cholesterol in the
liver to replace the lost bile acids. Bile acid sequestrants, such
as cholestyramine, bind bile acids in the gut, prevent their
reabsorption, and so promote their excretion.
 They are used in the treatment of hypercholesterolemia
because the removal of bile acids relieves the inhibition on bile
acid synthesis in the liver, thereby diverting additional
cholesterol into that pathway.
12
Figure 18.11 Enterohepatic circulation of bile salts and bile acids.
Note:
• Dietary fiber also binds bile acids and increases their excretion.
13
F. Bile salt deficiency: cholelithiasis
 The movement of cholesterol from the liver into the bile
must be accompanied by the simultaneous secretion of
phospholipid and bile salts.
 If this dual process is disrupted and more cholesterol
enters the bile than can be solubilized by the bile salts and
lecithin present, the cholesterol may precipitate in the
gallbladder, initiating the occurrence of cholesterol
gallstone disease—cholelithiasis.
14
F. Bile salt deficiency: cholelithiasis
 This disorder is typically caused by a decrease of bile acids in the
bile, which may result from:
1.
Gross malabsorption of bile acids from the intestine, as seen in
patients with severe ileal disease;
2. Obstruction of the biliary tract, interrupting the enterohepatic
circulation
3. Severe hepatic dysfunction, leading to decreased synthesis of bile
salts, or other abnormalities in bile production; or
4. Excessive feedback suppression of bile acid synthesis as a result of
an accelerated rate of recycling of bile acids
 Cholelithiasis also may result from increased biliary cholesterol
excretion, as seen with the use of fibrates.
Note:
 Fibrates, such as gemfibrozil, are derivatives of fibric acid. They are
used to reduce triacylglycerol levels in blood through up-regulation of
fatty acid β-oxidation.
15
F. Bile salt deficiency: cholelithiasis
 Laparoscopic cholecystectomy
(surgical removal of the gallbladder
through a small incision) is currently
the treatment of choice.
 However, for patients who are
unable to undergo surgery,
administration of chenodeoxycholic
acid to supplement the body's supply
of bile acids results in a gradual
(months to years) dissolution of the
gallstones.
 Figure 18.12 Gallbladder with
gallstones
16
6- Plasma Lipoproteins
 The plasma lipoproteins are spherical macromolecular
complexes of lipids and specific proteins
(apolipoproteins or apoproteins).
 The lipoprotein particles include:




chylomicrons,
very-low-density lipoproteins (VLDL),
low-density lipoproteins (LDL),
and high-density lipoproteins (HDL).
 They differ in lipid and protein composition, size,
density, and site of origin.
17
6- Plasma Lipoproteins
 Lipoproteins function both to keep their component
lipids soluble as they transport them in the plasma and to
provide an efficient mechanism for transporting their lipid
contents to (and from) the tissues.
 In humans, the transport system is less perfect than in
other animals and, as a result, humans experience a
gradual deposition of lipid—especially cholesterol—in
tissues.
 This is a potentially life-threatening occurrence when the
lipid deposition contributes to plaque formation, causing
the narrowing of blood vessels (atherosclerosis).
18
19
Figure 18.13 Approximate size and density of
serum lipoproteins. Each family of lipoproteins
exhibits a range of sizes and densities; this figure
shows typical values.
The width of the rings approximates the amount of
each component.
Note:
Although cholesterol and its esters are shown as
one component in the center of each particle,
physically cholesterol is a surface component
whereas cholesteryl esters are located in the
interior of the lipoproteins.
20
A. Composition of plasma lipoproteins
 Lipoproteins are composed of a neutral lipid core (containing
triacylglycerol, and cholesteryl esters) surrounded by a shell of
amphipathic apolipoproteins, phospholipid, and nonesterified
cholesterol.
 These amphipathic compounds are oriented so that their polar
portions are exposed on the surface of the lipoprotein, thus
making the particle soluble in aqueous solution.
 The triacylglycerol and cholesterol carried by the lipoproteins
are obtained either from the diet (exogenous source) or from
de novo synthesis (endogenous source).
Note:
 Lipoprotein particles constantly interchange lipids and
apolipoproteins with each other; therefore, the actual
apolipoprotein and lipid content of each class of particles can
be somewhat variable.
21
A. Composition of plasma lipoproteins
1. Size and density of lipoprotein
particles:
22
 Chylomicrons are the lipoprotein
particles lowest in density and largest
in size, and contain the highest
percentage of lipid and the lowest
percentage of protein.
 VLDLs and LDLs are successively
denser, having higher ratios of protein
to lipid.
 HDL particles are the densest.
 Plasma lipoproteins can be separated
on the basis of their electrophoretic
mobility, as shown in Figure 18.15, or
on the basis of their density by
ultracentrifugation.
Figure 18.14 Structure of a
typical lipoprotein particle.
Figure 18.15 Electrophoretic mobility of
plasma lipoproteins. The order of LDL
and VLDL is reversed if
ultracentrifugation is used as the
separation technique.
Gradient density centrifugation
schematic showing the typical subclasses present
23
A. Composition of plasma lipoproteins
2. Apolipoproteins:
 The apolipoproteins associated with lipoprotein particles have a
number of diverse functions, such as providing recognition sites for
cell-surface receptors, and serving as activators or coenzymes for
enzymes involved in lipoprotein metabolism.
 Some of the apolipoproteins are required as essential structural
components of the particles and cannot be removed (in fact, the
particles cannot be produced without them), whereas others are
transfered freely between lipoproteins.
 Apolipoproteins are divided by structure and function into five
major classes, A through E, with most classes having subclasses, for
example, apolipoprotein (or apo) A-I and apo C-II.
Note: Functions of all of the apolipoproteins are not yet known.
24
B. Metabolism of chylomicrons
 Chylomicrons are assembled in intestinal mucosal cells and
carry dietary triacylglycerol, cholesterol, fat-soluble vitamins,
and cholesteryl esters (plus additional lipids made in these
cells) to the peripheral tissues.
Note:
 TAGs account for close to 90% of the lipids in a chylomicron.
25
Figure 18.16 Metabolism of chylomicrons. CM = chylomicron; TAG = triacylglycerol; C =
cholesterol; CE = cholesteryl esters. Apo B-48, apo C-II, and apo E are apolipoproteins
found as specific components of plasma lipoproteins. The lipoproteins are not drawn
to scale (see Figure 18.13 for details of the size and density of lipoproteins).
26
B. Metabolism of chylomicrons
1.
Synthesis of apolipoproteins:


Apolipoprotein B-48 is unique to chylomicrons.
Its synthesis begins on the rough ER; it is glycosylated as it
moves through the RER and Golgi.
Note:
 Apo B-48 is so named because it constitutes the N-terminal,
48% of the protein coded for by the apo B gene.
 Apo B-100, which is synthesized by the liver and found in VLDL
and LDL, represents the entire protein coded for by the apo B
gene.
 Posttranscriptional editing of a cytosine to a uracil in intestinal
apo B-100 mRNA creates a nonsense codon, allowing
translation of only 48% of the mRNA.
27
B. Metabolism of chylomicrons
2. Assembly of chylomicrons:
 The enzymes involved in triacylglycerol, cholesterol, and
phospholipid synthesis are located in the smooth ER.
 Assembly of the apolipoproteins and lipid into chylomicrons
requires microsomal triacylglycerol transfer protein, which
loads apo B-48 with lipid.
 This occurs before transition from the ER to the Golgi, where
the particles are packaged in secretory vesicles.
 These fuse with the plasma membrane releasing the
lipoproteins, which then enter the lymphatic system and,
ultimately, the blood.
28
B. Metabolism of chylomicrons
3. Modification of nascent chylomicron particles:
 The particle released by the intestinal mucosal cell is called a
“nascent” chylomicron because it is functionally incomplete.
 When it reaches the plasma, the particle is rapidly modified,
receiving apo E (which is recognized by hepatic receptors) and C
apolipoproteins.
 The latter include apo C-II, which is necessary for the activation of
lipoprotein lipase, the enzyme that degrades the triacylglycerol
contained in the chylomicron.
 The source of these apolipoproteins is circulating HDL.
29
Figure 18.16 Metabolism of chylomicrons. CM = chylomicron; TAG = triacylglycerol; C =
cholesterol; CE = cholesteryl esters. Apo B-48, apo C-II, and apo E are apolipoproteins
found as specific components of plasma lipoproteins. The lipoproteins are not drawn
to scale (see Figure 18.13 for details of the size and density of lipoproteins).
30
B. Metabolism of chylomicrons
4. Degradation of triacylglycerol by lipoprotein lipase:
 Lipoprotein lipase is an extracellular enzyme that is anchored by
heparan sulfate to the capillary walls of most tissues, but
predominantly those of adipose tissue and cardiac and skeletal
muscle.
 Adult liver does not have this enzyme.
Note:
 A hepatic lipase is found on the surface of endothelial cells of the
liver. However, it does not significantly attack chylomicrons or
VLDL triacylglycerol, but rather assists with HDL metabolism
 Lipoprotein lipase, activated by apo C-II on circulating lipoprotein
particles, hydrolyzes the triacylglycerol contained in these
particles to yield fatty acids and glycerol.
31
B. Metabolism of chylomicrons
 The fatty acids are stored (by the adipose) or used for energy (by
the muscle). If they are not immediately taken up by a cell, the
long-chain fatty acids are transported by serum albumin until their
uptake does occur.
 Glycerol is used by the liver, for example, in lipid synthesis,
glycolysis, or gluconeogenesis.
Note:
 Patients with a deficiency of lipoprotein lipase or apo C-II (Type
1 hyperlipoproteinemia, or familial lipoprotein lipase deficiency)
show a dramatic accumulation (1000 mg/dl or greater) of
chylomicrons in the plasma (hypertriacylglycerolemia), even in
the fasted state.
32
B. Metabolism of chylomicrons
5. Regulation of lipoprotein lipase activity:
 Lipoprotein lipase synthesis and transfer to the luminal surface of
the capillary is stimulated by insulin (signifying a fed state).
 Furthermore, isomers of lipoprotein lipase have different Km
values for triacylglycerol.
 For example, the adipose enzyme has a large Km, allowing the
removal of fatty acids from circulating lipoprotein particles and
their storage as triacylglycerols only when plasma lipoprotein
concentrations are elevated.
 Conversely, heart muscle lipoprotein lipase has a small Km,
allowing the heart continuing access to the circulating fuel, even
when plasma lipoprotein concentrations are low.
Note: The highest concentration of lipoprotein lipase is in cardiac
muscle, reflecting the use of fatty acids to provide much of the
energy needed for cardiac function.
33
B. Metabolism of chylomicrons
6. 6. Formation of chylomicron remnants:
 As the chylomicron circulates and more than 90% of the
triacylglycerol in its core is degraded by lipoprotein lipase, the
particle decreases in size and increases in density.
 In addition, the C apoproteins (but not apo E) are returned to HDL.
 The remaining particle, called a “remnant,” is rapidly removed
from the circulation by the liver, whose cell membranes contain
lipoprotein receptors that recognize apo E.
 Chylomicron remnants bind to these receptors and are taken into
the hepatocytes by endocytosis.
 The endocytosed vesicle then fuses with a lysosome, and the
apolipoproteins, cholesteryl esters, and other components of the
remnant are hydrolytically degraded, releasing amino acids, free
cholesterol, and fatty acids.
 The receptor is recycled.
34
Figure 18.16 Metabolism of chylomicrons. CM = chylomicron; TAG = triacylglycerol; C =
cholesterol; CE = cholesteryl esters. Apo B-48, apo C-II, and apo E are apolipoproteins
found as specific components of plasma lipoproteins. The lipoproteins are not drawn
to scale (see Figure 18.13 for details of the size and density of lipoproteins).
35
C. Metabolism of VLDL
 VLDLs are produced in the liver.
 They are composed predominantly of triacylglycerol
(approximately 60%), and their function is to carry this lipid
from the liver to the peripheral tissues.
 There, the triacylglycerol is degraded by lipoprotein lipase, as
discussed for chylomicrons.
Note:
 “Fatty liver” (hepatic steatosis) occurs in conditions in which
there is an imbalance between hepatic triacylglycerol
synthesis and the secretion of VLDL.
 Such conditions include obesity, uncontrolled diabetes
mellitus, and chronic ethanol ingestion.
36
Figure 18.17 Metabolism of VLDL and LDL. TAG = triacylglycerol; VLDL = very-low-density lipoprotein;
LDL = low-density-lipoprotein; IDL = intermediate-density lipoprotein; C = cholesterol; CE =
cholesteryl esters. Apo B-100, apo C-II, and apo E are apolipoproteins found as specific components
of plasma lipoproteins. Lipoproteins are not drawn to scale (see Figure 18.13 for details of the size
and density of lipoproteins).
37
C. Metabolism of VLDL
1. Release of VLDL:
 VLDL are secreted directly into the blood by the liver as
nascent VLDL particles containing apo B-100.
 They must obtain apo C-II and apo E from circulating HDL
(see Figure 18.17). As with chylomicrons, apo C-II is required
for activation of lipoprotein lipase.
Note:
 Abetalipoproteinemia is a rare hypolipoproteinemia caused by a
defect in microsomal triacylglycerol transfer protein, leading to
an inability to load apo B with lipid. As a consequence, no VLDL
or chylomicrons are formed, and triacylglycerols accumulate in
the liver and intestine.
38
 Figure 18.18 Transfer of
cholesteryl esters (CE) from
HDL to VLDL in exchange for
triacylglycerol (TAG) or
phospholipids (PL).
39
C. Metabolism of VLDL
2. Modification of circulating VLDL:
 As VLDL pass through the circulation, triacylglycerol is degraded
by lipoprotein lipase, causing the VLDL to decrease in size and
become denser.
 Surface components, including the C and E apoproteins, are
returned to HDL, but the particles retain apo B-100.
 Finally, some triacylglycerols are transferred from VLDL to HDL in
an exchange reaction that concomitantly transfers some
cholesteryl esters from HDL to VLDL.
 This exchange is accomplished by cholesteryl ester transfer
protein (Figure 18.18).
40
C. Metabolism of VLDL
3. Production of LDL from VLDL in the plasma:
 With these modifications, the VLDL is converted in the plasma to
LDL.
 Intermediate-sized particles, the intermediate-density lipoproteins
(IDL) or VLDL remnants, are observed during this transition.
 IDLs can also be taken up by cells through receptor-mediated
endocytosis that uses apo E as the ligand.
41
C. Metabolism of VLDL
Note:
 Apo E is normally present in three isoforms, E-2, E-3, and E-4.
 Apo E2 binds poorly to receptors, and patients who are
homozygotic for apo E2 are deficient in the clearance of
chylomicron remnants and IDL.
 The individuals have familial Type III hyperlipoproteinemia
(familial dysbetalipoproteinemia, or broad beta disease), with
hypercholesterolemia and premature atherosclerosis.
 Not yet understood is the fact that the E-4 isoform confers
increased susceptibility to and decreased age of onset of lateonset Alzheimer disease, doubling the lifetime risk.
42
D. Metabolism of LDL
 LDL particles contain much less triacylglycerol than their VLDL
predecessors, and have a high concentration of cholesterol
and cholesteryl esters (Figure 18.19).
1. Receptor-mediated endocytosis:
 The primary function of LDL particles is to provide cholesterol to
the peripheral tissues (or return it to the liver).
 They do so by binding to cell surface membrane LDL receptors
that recognize apo B-100 (but not apo B-48).
 Because these LDL receptors can also bind apo E, they are
known as apo B-100/apo E receptors.
 A summary of the uptake and degradation of LDL particles is
presented in Figure 18.20.
43
D. Metabolism of LDL
Note:
The numbers in brackets below refer to corresponding numbers
on that figure.
 A similar mechanism of receptor-mediated endocytosis is used for
the cellular uptake and degradation of chylomicron remnants and
IDLs by the liver.
44
Figure 18.19 Composition of the plasma lipoproteins.
Note the high concentration of cholesterol and
cholesteryl esters in LDL.
45
D. Metabolism of LDL
[1] LDL receptors are negatively charged glycoproteins that are
clustered in pits on cell membranes. The intracellular side of
the pit is coated with the protein clathrin, which stabilizes the
shape of the pit.
[2] After binding, the LDL-receptor complex is internalized by
endocytosis.
Note:
 A deficiency of functional LDL receptors causes a significant
elevation in plasma LDL and, therefore, of plasma cholesterol.
Patients with such deficiencies have Type II hyperlipidemia
(familial hypercholesterolemia) and premature atherosclerosis.
[3] The vesicle containing the LDL rapidly loses its clathrin coat
and fuses with other similar vesicles, forming larger vesicles
called endosomes.
46
D. Metabolism of LDL
[4] The pH of the endosome falls (due to the proton-pumping activity
of endosomal ATPase), which allows separation of the LDL from its
receptor.
- The receptors then migrate to one side of the endosome, whereas
the LDLs stay free within the lumen of the vesicle.
Note:
 This structure is called CURL—the Compartment for Uncoupling of
Receptor and Ligand.
[5] The receptors can be recycled, whereas the lipoprotein remnants in
the vesicle are transferred to lysosomes and degraded by lysosomal
(hydrolytic) enzymes, releasing free cholesterol, amino acids, fatty
acids, and phospholipids.
- These compounds can be reutilized by the cell.
47
Note:
• Rare autosomal
recessive deficiencies
in the ability to
hydrolyze lysosomal
cholesteryl esters
(Wolman disease), or
to transport
unesterified
cholesterol out of the
lysosome (NiemannPick disease, type C)
have been identified.
48
Figure 18.20 Cellular
uptake and degradation
of LDL. ACAT = acyl
CoA:cholesterol
acyltransferase.
D. Metabolism of LDL
2. Effect of endocytosed cholesterol on cellular
cholesterol homeostasis:
 The chylomicron remnant-, IDL-, and LDL-derived cholesterol
affects cellular cholesterol content in several ways (see Figure
18.20).
 First, HMG CoA reductase is inhibited by high cholesterol, as a
result of which, de novo cholesterol synthesis decreases.
 Second, synthesis of new LDL receptor protein is reduced by
decreasing the expression of the LDL receptor gene, thus limiting
further entry of LDL cholesterol into cells.
Note:
 Regulation of the LDL receptor gene involves a SRE and a SREBP, as
was seen in the regulation of the gene for HMG CoA reductase
49
D. Metabolism of LDL
 Third, if the cholesterol is not required immediately for
some structural or synthetic purpose, it is esterified by acyl
CoA:cholesterol acyltransferase (ACAT).
 ACAT transfers a fatty acid from a fatty acyl CoA derivative
to cholesterol, producing a cholesteryl ester that can be
stored in the cell (Figure 18.21).
 The activity of ACAT is enhanced in the presence of
increased intracellular cholesterol.
50
 Figure 18.21 Synthesis of
intracellular cholesteryl ester by
ACAT.
51
D. Metabolism of LDL
3. Uptake of chemically modified LDL by macrophage scavenger
receptors:
 In addition to the highly specific and regulated receptor-mediated
pathway for LDL uptake described above, macrophages possess
high levels of scavenger receptor activity.
 These receptors, known as scavenger receptor class A (SR-A), can
bind a broad range of ligands, and mediate the endocytosis of
chemically modified LDL in which the lipid components or apo B
have been oxidized.
 Unlike the LDL receptor, the scavenger receptor is not downregulated in response to increased intracellular cholesterol.
 Cholesteryl esters accumulate in macrophages and cause their
transformation into “foam” cells, which participate in the
formation of atherosclerotic plaque (Figure 18.22).
52
 Figure 18.22 Role of oxidized lipoproteins in plaque
formation in arterial wall.
53