1. Spring semester 2011
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2. Recommended literature
• R.K.Murray, D.K.Granner, V.W.Rodwell
Harper‘s Illustrated Chemistry 27th, 28th Ed.
• Harvey R.A., Ferrier D.R : Biochemistry 4th ed.
• M.Lieberman, A.Marks: Basic medical Biochemistry
(Clinical approach) 3rd edition
• Silbernagl, Stefan; Despopoulos, Agamemnon: Color atlas of
physiology, 6th ed. Published Stuttgart: Thieme 2009.
• Lectures: – materials on IS MUNI
2

3. Lipids.
Digestion and absorption, blood
plasma lipids, lipoproteins
 2011 Department of Biochemistry FM MU
(E.T.)
3

4. Digestion of lipids
Western diet contains 40 % of
lipids or more.
Triacylglycerols (TG)-90%
Phospholipids (PL)
Cholesterolesters (CE) Lingual and
Glycolipids (GL) gastric lipase
Lipophilic vitamins (LV)
pancreatic lipases
Bile acids + colipase
Primary products:
Absorption into mucosal free FA
2-monoacylglycerols
cells (enterocytes) in
lysophospholipids
form of micelles (particles cholesterol
< 20 nm) lipophilic vitamins 4

5. Cleavage of lipids by enzymes in the small
intestine
• Pancreatic lipase
Triacylglycerol  2-monoacylglycerol + 2 FA
O
O CH2 O C
C O CH
CH2 O C
O
< 1/4 TG
triacylglycerol  glycerol + FA
5
Orlistat - drug designed to treat obesity, it inhibits lipases

6. Orlistat
6

7. • Phospholipase A2
phospholipid  lysophospholipid + MK
O
O CH2 O C
C O CH O
CH2 O P O CH2 CH2 NH2
O
• Cholesterol esterase:
Cholesterol esters  cholesterol + FA
7

8. Emulsification of lipids
• is condition for effective digestion of hydrophobic lipids
• increase of effective surface oil-water, facilitation of contact
with enzymes
• is accomplished by action of detergents and mechanical
mixing due to peristalsis
Emulsificators in the small intestine
• salts of bile acid
• phospholipids
• salts of fatty acids
8

9. Colipase
• protein secreted from pancreas
• binds the lipase at ratio 1:1
• anchors lipase to bile acids on the surface of
emulsified lipid droplets
• facilitates the action of lipase
bile acids
triacylglycerols
colipase
lipase 9

10. Absorption of lipids by mucosal cells
(enterocytes)
Brush border Mucosal cells
Mixed micelles microvilli
diameter < 20 nm
10
Pasive difusion of fatty acid and monoacylglycerols

11. Absorption of lipids by mucosal cells– cont.
• long fatty acids and monoacylglycerol are resorbed by
diffusion
•short chain fatty acids (up to 10 C) do not enter micelles
• they are resorbed directly
• bile acids which remain in the intestine, are extensively
absorbed in ileum
• transport of cholesterol is mediated by NPC1L1 (Nieman-
Pick C1 like 1) (see the lecture in Biochemistry I)
11

12. Hormons affecting digestion of lipids
Secretin
• intestinal “S-cells“ produce secretin into the blood after the
stimulation by H+ entering the lumen
• secretin stimulates release of secrets containing HCO3- from
gallbladder and pancreas
Cholecystokinin (CCK)
• intestinal “I-cells“ produce cholecystokinin into the blood after the
stimulation of small peptides and lipids
• CCK stimulates secretion of amylase, lipase a proteases from
exocrine cells of pancreas
• potentiates effect of secretin on excretion of HCO3-
• stimulates the secretion of bile from gallbladder
12

13. Steatorrhoea (lipid malabsorption)
Loss of lipids by feces
(normaly is resorbed ~ 98% lipids of food)
Posible causes
lipids
Unsufficient supply of bile (damage of
liver, obstruction of bile duct
Disturbed function of pancreas
Disturbed function of intestinal mucose
Lipid Unsufficient intake of lipophilic
malabsorption vitamins 13

14. Resynthesis of lipids within the mucosal cells:
1. Activation of FA
FA + CoA + ATP  Acyl-CoA + AMP + PPi
2. Resynthesis of triacylglycerols
Acyl-CoA + Monoacylglycerol  Diacylglycerol + CoA
Acyl-CoA + Diacylglycerol  Triacylglycerol + CoA
3. Resynthesis of phospholipids from lysophospholipids
4. Resynthesis of cholesterolesters
Processes are located in ER
FA with short chain and free glycerol do not také part in these processes and
14
are transported directly into the portal vein

15. Transport of lipids from enterocytes
AA    apoprotein B-48
(apoprotein A-I) CHE
PL
TG
chylomicron
FA with short
chain
glycerol
Lymphatic
Portal vein lacteals 15

16. Plasma lipids
Transport in form of lipoproteins
Lipid Plasma concentration
(mmol/l)
Triacylglycerol 0,9 - 2
Total cholesterol 3,8 - 5,8
Non-esterified cholesterol 1,3 - 1,9
Total phospholipids 1,8 - 5,8
Free fatty acids 0,4 - 0,8
16

17. Schematic structure of lipoproteins
Apoproteins
Phospholipids
Triacylglycerols
Apoproteins
Cholesterol
esters
Non-esterified
cholesterol
core superficial layer
17

18. Plasma lipoproteins TG
84% PL
54%
CH
Prot
Chylomicron CM VLDL
45%
50%
LDL HDL
18

19. Lipoproteins characteristics
Class Diameter Half-life Main lipid
(nm)
CM 100-1000 ~5-15 min TG
VLDL 30-90 ~2h TG
IDL 25-35 ~2h TG/CHE
LDL 20-75 ~2-4 d CHE
HDL 5-12 ~10 h PL/CHE
Separation of lipoproteins
• ultracentrifugation in gradient of salt
• elektrophoresis
(see seminars and practicals) 19

20. Apoproteins
• protein component of lipoproteins
• function of AP: enzyme activators and inhibitors
interaction with receptors
structural role
(transport)
• some of them are built in the lipoproteins, the other are
peripheral and exchange between particles
• synthesis in rough ER, attachment to lipid micelles
20

21. Chylomicrons (CM) – asembly and metabolism
• they are formed in intestinal mucosal cells
• they carry TG, CH and lipophilic vitamins admitted in food
• main apoprotein is apo-B 48, minor is apo-A (the other cannot
be synthesized in intestinal cells), synthesis of apoB-48 limits
formation of CM
• they are relased by exocytosis into the lacteals (lymphatic
vessels originating in the villi of the small intestine) – chyle
lymph
• they follow lymphatic veins and enter the blood in the thoracic
duct
21

22. Assembly of chylomicrons
AA    apoprotein B-48
(apoprotein A-I) CHE
PL
TG
chylomicron
SCFA glycerol
Lymphatic
Portal vein vessels 22

23. Metabolism of chylomicrons in blood
• they enter the blood 1-2 hours after the meal –
nascent chylomicrons
• modification of chylomicrons: in blood apo E and
apo C-II are transfered from circulating HDL to
chylomicrons
• in capilaries of most peripheral tissues are CM
degraded by lipoproteine lipase (LPL)
23

24. Chylomicron in blood (1st part)
blood
ApoCII
Ductus thoracicus ApoE
Adipose
lymph tissue,
muscles
LPL
Fatty acids
Mucosal
cells of the
intestine
glycerol
24

25. Lipoprotein lipase (LPL)
• negatively charged enzyme on the surface of endotelial cells in
capillaries (anchored by heparansulfate to the capillary walls)
• predominantly in adipose tissue and skeletal and cardiac muscle
• it is activated by apo-CII
• LPL can be released by heparin
• synthesis of isoenzyme in adipose tissue is stimulated by insulin
• deficit of LPL results in triacylglycerolemia
25

26. Action of LPL
• lipoprotein is attached to the enzyme bonded on endothelial
cells
• LPL catalyzes hydrolysis of TG contained in circulating
lipoproteins:
TG glycerol + 3 MK
O
O CH2 O C
C O CH
CH2 O C
O
26

27. Action of LPL on chylomicrons
CM
• free fatty acids are
Apo CII taken up by tissues,
small part returns back
into the plasma and is
LPL
LPL transported by serum
albumin
• LPL hydrolyses TG to fatty acids and CM-remnat
glycerol
Is rapidly removed
• more than 90% of TG in CM is from circulation by
degraded by LPL, the particle decrases in the liver játry
size and increases density  remnant (Apo-E receptor)
27

28. Chylomicron in blood (2nd part)
ApoCII
ApoE
Adipose
tissue,
muscles
LPL
ApoCII
Fatty acids
liver Glycerol
apoE
apoE, apoB/E,
remnant
28

29. • LPL degrades about 90% TG in chylomicrons
• chylomicrons in blood are removed during aprox. 30
min
The fate of fatty acids released by the action of LPL:
 - oxidation in tissues (muscle, myocard) – yield of energy
• deposition in form of TG in adipous tissue
Removal of remnats from blood
•They bind to the receptors in liver and are taken up
• receptors recognize apo-E
• cholesterol taken in food is transported into the liver in 29
this way

30. VLDL – asembly and metabolism
• they are formed in hepatocytes
• they are composed of 60% of TG that are synthesized in
the liver
• contains cholesterol
• content of apoproteins:apo-B 100, small amount of Apo-A and
Apo-C
• they are secreted into the blood as nascent particles
30

31. Apo-B 100
• apoprotein in LDL and VLDL
•It is integral protein
• very long chain (4 536 AA)
• apo-B 48 and apo-B 100 have the same mRNA, but apo-
B 48 has only 48% of chain leghth of apo-B 100)
• synthesis of apo-B 100 is inhibited by insulin, VLDL are
formed in post-resorption phase
31

32. What is the origin of TG in liver?
• fatty acids are synthesized in liver from acetyl-CoA
• acetyl CoA originates mainly from metabolism of
saccharides (after the meal)
• free fatty acids can be also taken up from blood (during
starvation)
• TG are synthesized from fatty acids
32

33. Metabolism of VLDL-
overview VLDL
Liver
apoB/E
Apo-C,E
HDL Tissues
LPA
Apo C FA
G
IDL HDL
LDL CHE
33

34. Degradation of VLDL by lipoprotein lipase
Apo CII
free fatty acids enter tissues
LPL
IDL
particle become
smaller in size
LDL
Taken up by liver (apo-E
receptor) 34

35. Metabolism of VLDL in steps
•Nascent VLDL enter the blood
• apo E and apo CII are transfered from HDL
• triacylglycerols are degraded by lipoprotein lipase to fatty
acid and glycerol (similarly like chylomicrons)
•VLDL changes to IDL
• IDL are either taken up by liver or are converted to LDL
35

36. Hepatic lipase
• enzym on luminal wall of liver sinusoids
• it acts similarly like LPL
• it degrades TG in IDL, VLDL and HDL when they pass
through the liver
• it can be released by heparin
36

37. Heart muscle and adipose LPL
KM of heart muscle LPL is aprox. 10x lower than
adipose LPL
Synthesis of adipose LPL is activated by insulin
What follows from it ?
37

38. Metabolism of IDL and LDL
• IDL and LDL may be enriched by cholesterol esters from
HDL (role of cholesterolester transfer protein CETP)
• IDL are taken up by liver Apo-B/E receptors
• LDL are taken up by periferal tissues (1/3) and liver (2/3) by
the process of receptor mediated endocytoses (Apo-B/E)
• at physiological conditions 30-40% of new formed LDL is
catabolized during 24 h
38

39. LDL receptors
LDL receptor (apo B/E Non specific (scavenger)
receptor) receptors (SR-A)
Is regulated by intracelular membrane receptor with broad
content of cholesterol specifity
Located on the surface of
macrophages and Kupfer-cells
in liver
Are not down regulated
Uptake of modified and
redundant LDL
39

40. Uptake of LDL by specific receptors (apo
B/apoE)
•LDL receptor is negativelly charged membrane
glycoprotein lokalized on the surface of clathrin coated
pits,
• it recognizes apo E and apo B 100
40

41. • after binding, the complex LDL-receptor is internalized by
endocytoses
41

42. •vesicles containing LDL loses clatrin coat and fuses with
lysosomes forming endosomes
•LDL disociates from its receptor, the receptor migrates to one
side of endosome, separates a recycles back to the membrane
membrány
recyclation of a
receptoru
Lysosome
42
endolysosom

43. •pH in endolysosome falls, lipoprotein is degraded, cholesterol,
amino acids, fatty acids, phospholipids releases
• cholesterol is esterified by ACAT (acylCoA-cholesterol-
acyltransferase) and is stored in the cell
ACAT
CH CHE
AK
43
MK PL

44. The level of cholesterol in the cell is strictly
regulated
• down- regulation of cholesterol intake in form of LDL
(increased level of cholesterol in the cell decreases the number
of receptors on the surface – expression of LDL-receptor gene
is decreased)
• regulation of intracellular synthesis of cholesterol
 regulation of transcription of HMG-CoA synthase gene
by SREBP
Inhibition of HMG-CoA – reductase by cholesterol
44

45. Role of sterol regulatory element-binding protein
(SREBP) in regulation of intake and synyhesis of
cholesterol
• precursor of SREBP is an integral protein of ER membrane
• when sterol level in the cell is low, N-terminal peptide is
released from precursor molecule and migrates to the nucleus,
where it binds to sterol-regulatory element in promotor area of
genes regulating cholesterol
• SREBP regulates synthesis of LDL receptors and HMGCoA
reductase
See also: http://www.biocarta.com/pathfiles/m_s1pPathway.asp
45

46. Familiar hypercholesterolemia
(type II hyperlipidemia)
Defeciency in production of
LDL-receptors
High level of LDL in blood
Synthesis of cholesterol in the
cell is not inhibited cell
produces excess of cholesterol
Increased risk of myocardial
Characteristic tendon
infarction xanthomas
46

47. Uptake of lipoproteins by scavenger
receptor SRA
• SRA –receptors on surface of phagycytosing cells
(macrophages in the cell wall, in lung alveoles and
peritoneum, Kupfer cells)
• receptor does not have down-regulation
• it preferably take up modified LDL (oxidized, glycated)
• it can take up also undamaged LDL, if the capacity of down-
regulated receptor is exceeded
47

48. Formation of foam cells and plaque
•Makrophages filled by lipids become foam cells
• they canoothcumulate in subendotellial area of the cell wall
• growth factors and cytokines stimulate the migration of
smooth muscle cells and their proliferation
• these processes can result in formation of atherosclerotic
plaque
48

49. Effect of hormones on LDL uptake in liver
Insulin and trijodthyronin increase uptake of LDL by the
liver,
Glucocortikoids have opposite effect
(mechanism is not known)
Why non-controlled diabetes and hypothyroidism are risk
factors for development of atherosklerosis and are very often
connected with hypercholesterolemia?
49

50. HDL and their role in metabolism of lipids
reverse transport of cholesterol (RTC)
- HDL take up cholesterol from peripheral tissues and
transports it to the liver
HDL exist in several modifications
They differ in size, shape, by content of lipids and
apoproteins
• they have different functions
• main subfraction according to the density: HDL2, HDL3
• remodelation of HDL – changes of HDL in circulation as 50
a consequence of exchange of lipids

51. HDL – assembly and metabolism
• base of HDL structure are apo A I and apo A II, they
contain also apoC and apo E
• for transport of cholesterol from tissues are important
small particles so-called „lipid free“ a „lipid poor“ apo
A
• thet are secreted by liver and enterocytes, can be also
formed in circulation from larger HDL particles
51

52. SR-B1 receptor
HDL-receptor with dual function in metabolism of HDL
• It binds HDL in liver and steroidogenic cells through
apo A-I and mediates transport of cholesterol inside the
cells
• it mediates the transport of cholesterol from the cells
into the HDL in periferal tissues
52

53. How is cholesterol deposited in the cells?
•Cholesterol esters are present in form of lipid droplets
in cytoplasma
•Revers transport of cholesterol is started by their
hydrolysis by cholesterylester hydrolase
•Free cholesterol is transported to the cell membrane
53

54. How is cholesterol taken up from tissues ?
Two mechanism are expected
difussion formation of new HDL
54

55. Formation of new HDL
Lipid poor Periferal
particles tissue játra
SR-B1
ABCA1 is an ATP-
ABCA1 binding cassette steroidogenní
tkáně
Tangier disease – extremly
rare genetic disease –
LCAT CETP functional ABCA1
transporter is missing.
It results in intracellular
spherical HDL accumulation of lipids
Disk-shaped HDL LDL/VLDL
* Gene expression for pro ABCA1 is regulated by the amount of cholesterol 55
inside the cell

56. Mechanism of cholesterol difusion into HDL
•HDL attaches to cell surface
after the interaction with SR-
Periferal B1 receptor
tissue
•ABCG4 (ATP-binding casette
protein G4) transports
SRB-I
cholesterol into the HDL
ABCG4 •Gradient of cholesterol
concentration is mediated by
LCAT on the HDL surface
56

57. For further conversions of HDL are
important:
Lecitin cholesterol acyltransferase (LCAT)
Cholesterol ester transfer protein (CETP)
LCAT
• transfers a fatty acid from lecitine (phosphatidylcholine)
na cholesterol
• plasmatic enzyme, it acts on the surface of HDL,
activated by apo A-I
57

58. Function of LCAT
CH2 O CO
CO O CH
O CH3
+
CH2 O P O CH2 CH2 N CH3
O- CH3
• LCAT transfers acyl of fatty acid to
OH group of cholesterol
• non-esterified cholesterol is HO
converted to esterified it is less
polar and more voluminous – it is
sequestered into the core of HDL
58

59. CETP
Cholesterolester –transfer-protein
It transfers esterified cholesterol from HDL to other
lipoproteins
CE VLDL
CETP
CE HDL
59

60. Further metabolism of HDL
• esterified cholesterol formed by the action of LCAT
accumulates in the core of HDL
• particles become spheric
• spheric HDL interacts with the other lipoproteins
• uptake in liver is mediated by SR-B I receptor
60

61. Cholesterol uptake by SR-B1 receptor in the liver
•HDL binds to receptor on hepatocytes
• the complex is not endocytosed
• only cholesterol is transported into the cells, transport
is mediated by ABCG transporter
•HDL dissociates from receptor and re-enters again
circulation
61

62. Conversions of HDL - summarization
Lipid poor
particles Peripheral játra SR-B1
tissue SR-B1
ABCA1 steroidogenic
cells
SRB-I
LCAT CETP
Disk-shaped spherical HDL LDL/VLDL
(HDL2 a HDL3)
HDL
Remodelation by interaction with the other 62
lipid poor particles lipoproteins

63. Cholesterol balance
Food Biosynthesis
80-500 mg 800 – 1000 mg
Cholesterol pool
Steroidal hormons,
Cholesterol Bile acids sebum, intestinal
(bile) epitelium
800 mg 500 mg 200 mg
1000-1500 mg/day is secreted 63

64. “Good” and “bad”
cholesterol
HO HO
Cholesterol in HDL is considered to be
the good cholesterol, because it accepts
free cholesterol from peripheral tissue
64

65. Increased intake of cholesterol or defects of LDL receptors
Increased plasmatic LDL,
The long half-life of LDL  possibility of oxidation
Damaged and redundant LDL are taken up by SRA receptors of
makrophages, formation of foam cells
oxidized LDL are strongly aterogenic
LDL cholesterol – bad cholesterol
65

66. High level of HDL-cholesterol
• HDL act as cholesterol scavenger, picking up excess
cholesterol in blood and taking it back to the liver for
disposal. The higher HDL cholesterol level, the better
prognosis of coronary heart diseases risk.
HDL-cholesterol = good cholesterol
When the higher level of cholesterol in blood is found,
the distribution between LDL and HDL fractions is
determined – examination of HDL- and LDL-
cholesterol is performed – see practicals
66

67. Lipoprotein (a)
• Lp(a) has very similar structure as LDL
• it contains additional apoprotein molecule (a) [apo(a)], that is
covalently attached to apo B-100
• apo(a) has homologous structure with plazminogen
• large quantities Lp(a) in plasma are associated with the increased
risk of coronary heart disease
• hypothesis: Lp(a) slows down the breakdown of blood clots that
trigger heart attack because it competes with plasminogen for
binding to fibrin
67