1. Lipoprotein metabolism
PSC 3110 Fall 2004
Lipoprotein metabolism By
Henry Wormser, Ph.D.
Professor of Medicinal Chemistry

2. Lipoproteins
particles found in plasma that transport lipids including cholesterol
lipoprotein classes
chylomicrons: take lipids from small intestine through lymph cells
very low density lipoproteins (VLDL)
intermediate density lipoproteins (IDL)
low density lipoproteins (LDL)
high density lipoproteins (HDL)

3. Composition and properties of human lipoproteins
Lipoprotein class
Density (g/mL)
Diameter (nm)
Protein % of dry wt
Phospholipid %
Triacylglycerol % of dry wt
HDL
5 – 15 33 29 8
LDL
1.019 – 1.063
18 – 28 25 21 4
IDL 18 22 31
VLDL
0.95 – 1.006 10 50
chylomicrons
< 0.95
1 - 2 7 84
Composition and properties of human lipoproteins
most proteins have densities of about 1.3 – 1.4 g/mL and lipid aggregates usually
have densities of about 0.8 g/mL

4. Lipoprotein structure

5. LDL molecule

6. The apolipoproteins major components of lipoproteins
often referred to as aproteins
classified by alphabetical designation (A thru E)
the use of roman numeral suffix describes the order in which the apolipoprotein emerge from a chromatographic column
responsible for recognition of particle by receptors

7. HELICAL WHEEL PROJECTION OF A PORTION OF APOLIPOPROTEIN A-I

8. LIPOPROTEINS
spherical particles with a hydrophobic core (TG and esterified cholesterol)
apolipoproteins on the surface
large: apoB (b-48 and B-100) atherogenic
smaller: apoA-I, apoC-II, apoE
classified on the basis of density and electrophoretic mobility (VLDL; LDL; IDL;HDL; Lp(a)

9. Apoproteins of human lipoproteins
A-I (28,300)- principal protein in HDL
90 –120 mg% in plasma; activates LCAT
A-II (8,700) – occurs as dimer mainly in HDL
30 – 50 mg %; enhances hepatic lipase activity
B-48 (240,000) – found only in chylomicron
<5 mg %; derived from apo-B-100 gene by RNA editing; lacks the LDL receptor-binding domain of apo-B-100
B-100 (500,000) – principal protein in LDL
80 –100 mg %; binds to LDL receptor

10. Apoproteins of human lipoproteins
C-I (7,000) – found in chylomicron, VLDL, HDL
4 – 7 mg %; may also activate LCAT
C-II (8,800) - found in chylomicron, VLDL, HDL
3 – 8 mg %; activates lipoprotein lipase
C-III (8,800) - found in chylomicron, VLDL, IDL, HDL
8 15 mg %; inhibits lipoprotein lipase
D (32,500) - found in HDL
8 – 10 mg %; also called cholesterol ester transfer protein (CETP)
E (34,100) - found in chylomicron, VLDL, IDL HDL
3 – 6 mg %; binds to LDL receptor
H (50,000) – found in chylomicron; also known as b-2-glycoprotein I (involved in TG metabolism)

11. Major lipoprotein classes
Chylomicrons (derived from diet)
density <<1.006
diameter nm
dietary triglycerides
apoB-48, apoA-I, apoA-II, apoA-IV, apoC-II/C-III, apoE
remains at origin in electrophoretic field

12. Chylomicron
formed through extrusion of resynthesized triglycerides from the mucosal cells into the intestinal lacteals
flow through the thoracic ducts into the suclavian veins
degraded to remnants by the action of lipoprotein lipase (LpL) which is located on capillary endothelial cell surface
remnants are taken up by liver parenchymal cells due to apoE-III and apoE-IV isoform recognition sites

13. Chylomicron metabolism

14. Major lipoprotein classes
VLDL
density >1.006
diameter nm
endogenous triglycerides
apoB-100, apoE, apoC-II/C-III
prebeta in electrophoresis
formed in the liver as nascent VLDL (contains only triglycerides, apoE and apoB)

15. VLDL
nascent VLDLs then interact with HDL to generate mature VLDLs (with added cholesterol, apoC-II and apoC-III)
mature VLDLs are acted upon by LpL to generate VLDL remnants (IDL)
IDL are further degraded by hepatic triglyceride lipase (HTGL) to generate LDLs

16. VLDL metabolism

17. Major lipoprotein classes
IDL (intermediate density lipoproteins)
density:
diameter: nm
cholesteryl esters and triglycerides
apoB-100, apoE, apoC-II/C-III
slow pre-beta

18. Major lipoprotein classes
LDL (low density lipoproteins)
density:
diameter: 18-25nm
cholesteryl esters
apoB-100
beta (electrophoresis)
< 130 LDL cholesterol is desirable, is borderline high and >160 is high

19. Major lipoprotein classes
HDL (high density lipoproteins)
density:
diameter: 5-12nm
cholesteryl esters and phospholipids
apoA-I, apoA-II, apoC-II/C-III and apoE
alpha (electrophoresis)

20. HDLs Several subfamilies exist Discoidal HDL :
contains cholesterol, phospholipid, apoA-I, apoA-II, apoE and is disc shaped;
it is formed in liver and intestine
It interacts with chylomicra remnants and lecithin-cholesterol acyl transferase (LCAT) to form HDL3

21. HDLs
HDL3
composed of cholesterol, cholesterol ester, phospholipid and apoA and apoE
interacts with the cell plasma membranes to remove free cholesterol
reaction with LCAT converts HDL3 to HDL2a (an HDL with a high apoE and cholesterol ester content)
cholesterol ester-rich HDL2a is then converted to triglyceride-rich HDL2b by concomitant transfer of HDL cholesterol esters to VLDL and VLDL triglycerides to HDL

22. HDL metabolism

23. Functions of HDL transfers proteins to other lipoproteins
picks up lipids from other lipoproteins
picks up cholesterol from cell membranes
converts cholesterol to cholesterol esters via the LCAT reaction
transfers cholesterol esters to other lipoproteins, which transport them to the liver (referred to as “reverse cholesterol transport)

24. Lipoproteins (a)- Lp(a)
another atherogenic family of lipoproteins(at least 19 different alleles)
they consist of LDL and a protein designated as (a)
the apoA is covalently linked to apoB-100 by a disulfide linkage
unusual in that it contains a kringle protein motif/domain (tri-looped structure with 3 intramolecular disulfide bonds – resembling a Danish pretzel)
high risk association with premature coronary artery disease and stroke

25. Cholesterol and lipid transport by lipoproteins

26. Cholesterol and lipid transport by lipoproteins

29. The LDL receptor
characterized by Michael Brown and Joseph Goldstein (Nobel prize winners in 1985)
based on work on familial hypercholesterolemia
receptor also called B/E receptor because of its ability to recognize particles containing both apos B and E
activity occurs mainly in the liver
receptor recognizes apo E more readily than apo B-100

30. Representation of the LDL receptor (839 aa)
extracellular domain is
responsible for apo-B-100/apo-E binding
intracellular domain is
responsible for clustering of LDL
receptors into coated
pit region of plasma
membrane

32. Cholesterol sources, biosynthesis and degradation
diet
only found in animal fat
biosynthesis
primarily synthesized in the liver from acetyl CoA
biosynthesis is inhibited by LDL uptake by the liver
degradation
only occurs in the liver
cholesterol is converted to bile acids

33. Biosynthesis of cholesterol
- synthesis of acetoacetyl CoA

34. Biosynthesis of cholesterol
- synthesis of mevalonate
rate-limiting step
and step subject to
inhibition by statins

35. Biosynthesis of cholesterol
synthesis of isopentenyl
pyrophosphate
A monoterpene

36. Synthesis of farnesyl
pyroposphate

37. Biosynthesis of cholesterol
- synthesis of squalene
a sesquiterpene
a triterpene

38. Synthesis of squalene

39. Biosynthesis of cholesterol
- synthesis of lanosterol
the allylamine antifungals
interfere with the epoxidation
step (naftidine, terfinabine)

40. Formation of the sterol ring system

41. Biosynthesis of cholesterol
The demethylation
of lanosterol is also
a useful step for drug
design – i.e. azole antifungals
ACAT inhibitors act
here

43. Biosynthesis summary

44. Bile acids from cholesterol
formed from cholesterol in the liver
stored in the gall bladder in bile as bile salts (sodium and potassium)
utilized during digestion of fats and other lipid substances (act as detergents)
rate limiting step is the conversion of cholesterol to 7-alpha cholesterol by 7-a-hydroxylase

46. Conversion of cholyl-CoA to glycocholic acid

47. Conversion of
cholyl CoA to
taurocholic acid

48. Taurine
Taurine is formed by the decarboxylation of cysteic acid, which in turn is made by oxidation of cysteine

50. Conversion of glycocholic acid to deoxycholic acid
Deoxycholic acid (secondary bile acid

51. Bile acids
cholic acid is the bile acid found in the largest amount in bile
cholic acid and chenodeoxycholic acid are referred to as primary bile acids
bile acids are converted to either glycine or taurine conjugates (in humans the ratio of glycine to taurine conjugates is 3:1)

52. Approximate composition of bile salts
Glycocholate – 24%
Glycochenodeoxycholate – 24%
Taurocholate – 12%
Taurochenodeoxycholate – 12%
Glycodeoxycholate- 16%
Taurodeoxycholate – 8%
Various lithocholate – 4%

53. Bile acids
fat digestion products are absorbed in the first 100 cm of small intestine
the primary and secondary bile acids are reabsorbed almost exclusively in the ileum returning to the liver by way of the portal circulation (98 to 99%)
this is known as the enterohepatic circulation
less than 500 mg a day escapes reabsorption and is excreted in the feces

54. Bile salts
detergent character of bile salts is due to the hydrophobic-hydrophilic nature of the molecules
the presence of hydroxyl (or sulfate) and the terminal carboxyl group on the tail gives the molecule its hydrophilic face
the steroid ring with its puckered plane provides the hydrophobic face

55. Function of bile salts
emulsification of fats due to detergent activity
aid in the absorption of fat-soluble vitamins (especially vitamin K)
accelerate the action of pancreatic lipase
have choleretic action –stimulate the liver to secrete bile
stimulate intestinal motility
keep cholesterol in solution (as micelles)

56. Mixed micelle formed by bile salts, triacylglycerols andf pancreatic
lipase

58. GALLSTONE THERAPEUTIC AGENTS
chenodeoxycholic acid (chenodiol; Chenix)
ursodeoxycholic acid (ursodiol; Actigall)
MAO:
reduce hepatic secretion of cholesterol into bile
inhibition of HMGCoA reductase: inhibit cholesterol biosynthesis
increase cholesterol solubility

59. Chenodiol and ursodiol
both are effective in dissolving cholesterol stones in some patients
ursodiol is the 7-beta epimer of chenodiol
most effective in dissolving small (<5 mm) floating stones in a functioning gallbladder
cannot dissolve stones that are more than 4% calcium by weight

60. Atherosclerosis
hardening of the arteries due to the deposition of atheromas
heart disease is the leading cause of death
caused by the deposition of cholesteryl esters on the walls of arteries
atherosclerosis is correlated with high LDL and low HDL

61. Photograph of an arterial plaque

62. Frederickson -WHO classification
Type I: incr. chylomicrons, reduced HDL, absence of lipoprotein lipase; deficiency of apo CII (hyperchylomironemia)
Type II-A: raised LDL; decreased catabolism of LDL (receptor deficiency or polygenic)
Type II-B: raised VLDL + LDL; often reduced HDL; increased production of VLDL + impaired LDL catabolism
Type III: raised IDL (dysbetalipoproteinemia); abnormal apolipoprotein E; impaired catabolism of IDL; elevated cholesterol and triglycerides (formerly known as broad beta disease)

63. Frederickson -WHO classification
Type IV: raised VLDL; often reduced HDL; impaired VLDL catabolism; dietary indiscretion ( formerly known as hyperprebetalipoproteinemia)
Type V: raised chylomicrons + VLDL; reduced HDL; reduced lipoprotein lipase + VLDL hypersecretion (formerly known as mixed lipemia)

64. Factors promoting elevated blood lipids
age
men >45 years of age; women > 55 years of age
family history of CAD
smoking
hypertension >140/90 mm Hg
low HDL cholesterol
obesity >30% overweight
diabetes mellitus
inactivity/ lack of exercise

65. Mechanisms of action of drugs
bind to bile acids/cholesterol
inhibit absorption/reabsorption
increase peroxisomal FA oxidation
stimulate lipoprotein lipase
inhibit triglyceride lipase
inhibit HMG CoA reductase
stimulates microsomal 7-alpha hydroxylase

66. Drug Classification systemic/non-sytemic cholesterol lowering agents
bile acid sequestrants
sitosterols*
probucol*
d-thyroxin*
HMG Co-A reductase inhibitors
* No longer available commercially in the U.S

67. Drug Classification mixed activity (nicotinic acid)
triglyceride lowering
clofibrate (Atromid-S)
gemfibrosil (Lopid)
fenofibrate (Tricor)

68. Bile sequestering resins

69. Bile sequestering resins

70. Bile acid sequestrants
po, safest, non systemic
bind to bile acids and inhibit reabsorption
increase 7-alpha hydroxylase activity leading to cholesterol degradation
decreases plasma LDL
problems:
abdominal discomfort, bloating, constipation
decreases drug absorption; wait 4 hrs after administration of BAS to give drugs

71. Colesevelam (WelChol)
polyalkylamine hydrochloride) cross linked with epichlorohydrin and alkylated with 1-bromodecane and (6-bromohexyl) trimethylammonium bromide
available as a 625 mg tablet
same mechanism of action as colestipol and cholestyramine

72. Bile sequestering resins
drug interactions (decreased serum level)
aspirin
clindamycin
clofibrate
furosemide
glipzide
tolbutamide
phenytoin
imipramine
methyldopa
nicotinic acid
penicillin G
propranolol
tetracycline
thiazide diuretics
digoxin
hydrocortisone
phosphate supplements

73. PLANT STEROLS

74. More plant sterols

76. HMG CoA reductase 3 different regulatory mechanisms are involved:
covalent modification: phosphorylation by cAMP-dependent protein kinases inactivate the reductase. This inactivation can be reversed by 2 specific phosphatases
degradation of the enzyme – half life of 3 hours and the half-life depends on cholesterol levels
gene expression: cholesterol levels control the amount of mRNA

81. Synthetic statins

82. HMG CoA reductase inhibitors
Precaution:
mild elevation of serum aminotransferase (should be measured at 2 to 4 month intervals)
minor increases in creatine kinase (myopathy, muscle pain and tenderness)
do not give during pregnancy

83. Selected hypolipidemic products

84. FIBRIC ACID DERIVATIVES

85. Clofibrate (Atromid-S)
Precautions
enhances coumarin activity
renal/hepatic injury contraindication
pregnancy/nursing
cholelithiasis
most commonly reported ADR are GI related
liver malignancies (not very common; but has led to scant usage)

86. CLOFIBRATE Primary activity on triglycerides MOA: ancillary:
increases lipoprotein lipase
lowers VLDL
increases peroxisomal FFA oxidation
inhibits cholesterol biosynthesis
increases biliary secretion of cholesterol
ancillary:
decreases platelet adhesiveness/fibrinogen

87. Gemfibrosil (Lopid) MOA precautions half life: 1.1 hours
stimulates lipoprotein lipase
interact with PPARa (peroxisome proliferator-activated receptors)
inhibits triglyceride lipolysis in adipose tissue
decreases FFA uptake by the liver
decreases hepatic VLDL/TG synthesis
slight cholesterol lowering effect
precautions
similar to clofibrate
myositis (voluntary muscle inflammation)
GI (indigestion, abdominal pain, diarrhea)
cholelithiasis (increased cholesterol biliary secretion)
half life: 1.1 hours

88. Fenofibrate (Tricor)
a relatively new fibric acid derivative (micronized form of the drug)
lowers plasma TG
inhibits TG synthesis
stimulates catabolism of VLDL
indicated primarily for hypertriglyceridemia
same side effects and precaution as in other fibric acid compounds
half-life: 20 hours
Dose: mg/day with meals

89. Now also available as a 200 mg tablet

90. NICOTINIC ACID (Niacin)
A water soluble vitamin of the B family;
nicotinamide is not active
Once converted to the amide, it is
incorporated into NAD
In order to be effective, it has to be dosed at the rate of 1.5 to 3.5 gm daily.
A sustained release dosage form is available
adverse effects: GI disturbances (erosion and ulceration)
red flush especially in the face and neck area
caused by vasodilation of capillaries

91. Nicotinic acid (Niacin)
MOA
dual plasma triglyceride and cholesterol lowering
decreases VLDL and LDL
decreases TG lipase in adipose tissue
increases lipoprotein lipase in adipose tissue
precaution
transient cutaneous flush
histamine release
potentiates BP effect of antihypertensives

92. Advicor® niacin-extended-release and lovastatin tablets
reduces LDL-C, TC, TG and increases HDL-C
available as 500/20, 750/20 and 100/20 mg tablets

93. Rosuvastatin (Crestor)
New statins: rosuvastatin (ZD4522)
nicknamed” superstastin/ gorilla statin” because of its powerful
effect on LDL cholesterol

94. Ezetimibe (Zetia)
This drug blocks the intestinal absorption of cholesterol. A dose of 10 mg qd leads to
a 19% reduction of LDL; shows real promise in combo product with statins (Schering-
Plough and Merck)

95. Investigational drugs
acylCoA: cholesterol acyltransferase inhibitors
Orphan nuclear receptors:
LXR – “oxycholesterol receptor” --- enhanced cholesterol efflux
FXR – “bile acid receptor” ---- decreased cholesterol conversion to bile salts

96. ACAT Inhibitors

97. ACAT Inhibitors

98. Squalene synthase inhibitors
squalestin 1, a fermentation product derived from Phloma species (Coelomycetes)
a potent inhibitor of squalene synthase
produces a marked decrease in serum cholesterol and apoB levels
may represent an alternative clinical therapy to hypercholesterolemia

99. The End