Lipoprotein metabolism for UG Students

1. LIPOPROTEIN METABOLISM
Amit Jha
Lecturer
Dept. of Biochemistry
UCMS, Bhairahawa

2. • Lipoproteins: Lipid complexed with proteins.
[Apoprotein]
• Lipids being water insoluble, requires a special transport
carrier (Lipoprotein) mechanism to reach site of requirement.

3. • Transport of hydrophobic lipid is achieved by:
– Association of more insoluble lipids with more “polar”
ones (eg: PL)
– then combining with free or esterified cholesterol
– & then combining with specific apoprotein to form
hydrophilic lipoprotein complex.
Structure of lipoproteins [3 layers]
Inner core Lipid layer made up of TG, CE
Covering surface made up of PL, Free Cholesterol
Outer surface made up of Apoprotein, specific for each lipoproteins

4. • Classification of Lipoproteins
– Are classified according to their:
• Hydrated density
• Electrophoretic mobility
• Based on apolipoprotein

5. • Classification as per hydrated density
• Goffman & colleagues (1954) separated lipoproteins
by ultracentrifugation into:-
1. Chylomicrons
2. VLDL
3. LDL [LDL1, IDL, LDL2]
4. HDL [HDL1, HDL2, HDL3]

6. Classification on the basis of Electrophoretic mobility
[Frederickson & colleagues 1967]
• Lipoproteins are (separated &) classified in relation to their
migration. [high protein content will move faster towards
anode]
– Origin: Chylomicrons
– β globulin region: LDL
– Pre- β globulin region: VLDL
– α globulin region: HDL

7. Classification based on apoprotein content
[Alauporic & colleagues (1972)]
Families Apoproteins Density class
LP-A Apo A (I, II) HDL
LP-B Apo-B (B-48, B-100) LDL & VLDL
LP-C Apo-C (I,II,III) VLDL, HDL, LDL
LP-D Apo-D HDL-3
LP-E Apo-E VLDL, HDL, LDL

9. • Apo-lipoproteins (apo-Lp) or apoprotein
– Protein part of lipoprotein.
– Apart from solubilising the lipid part, protein
components have specific functions.
Apoprotein Site of Synth. Component of Function
Apo A-I Intestine, Liver HDL-2 • Activation of LCAT
• Ligand for HDL receptor
• Antiatherogenic
Apo A-II Intestine, Liver HDL-3 • Inhibit LCAT
• Stimulate Lipase
Apo B100 Liver VLDL, LDL • Binds LDL receptor
Apo B48 Intestine CM
Apo C-I Liver CM, VLDL • Activates LCAT
Apo C-II Liver CM, VLDL • Activates lipoprotein lipase
Apo E Liver CM, VLDL, LDL • Ligand for hepatic uptake of
CM Remnant & IDL.

10. • Apo-E [Arginine-rich]
– Astrocytes also make apo-E.
– it is involved in cellular transport of lipids in CNS.
– Apo E-IV is implicated in development of senile dementia
& Alzheimer's disease.
– Apo-E is also associated with lipoprotein
glomerulopathy.

11. Lipid Profile Test
S. TG 50-150 mg/dL
S. Total Cholesterol 140-200 mg/dL
LDLC 80-130mg/dL
HDLC
Male
Female
30-60 mg/dL
35-75 mg/dL

12. • CHYLOMICRONS [Chylo: Milky, Microns: Small]
[Small milky globules]
– Largest & Least dense lipoproteins

13. SYNTHESIS OF APOPROTEIN
• Apo B48 synthesis (Intestine) begins on RER.
• As it moves through ER & golgi gets glycosylated.
ASSEMBLY OF CHYLOMICRONS
• Loading occurs during transition from ER to Golgi.
• “TG transfer protein” loads Apo-B-48 with lipid.
• In intestinal mucosal cells, TG & free cholesterol (dietary
origin) are coated with PL & Apo B48 to generate “Nascent
CM” [packed in a vesicle].
• Vesicle fuses with plasma memb. & releases CM into
lymphatics.

14. MODIFICATION OF NASCENT CM
– Nascent CM [functionally incomplete] via thoracic duct
enters blood & receives apo C-II & apo E (from HDL) to
form mature CM.
FATE OF CM
• Mature CM are removed from circulation by extra-hepatic
tissues [Adipose tissue, Skeletal & Cardiac muscle].
• T1/2: <1hr (in blood).
• Apo C-II activates Lipoprotein lipase on endothelium of extra
hepatic tissue.
• Lipoprotein lipase hydrolyses chylomicron.
[TGs → Glycerol + FFA]

15. • FFAs released are taken up by peripheral tissues.
• Adipose tissue: Stored
• Muscle: Used as energy source.
• Glycerols released are taken up by liver for:-
• Lipid synthesis
• gluconeogenesis
• Due to loss of TG content, size of mature CM is reduced
to half & % amount of Free & esterified cholesterol gets
doubled [remaining particle is k/a CM remnant].

16. LIVER TAKE UP CM REMNANTS
Hepatocytic receptors [apo E dependent] binds with Apo-E of
CM remnants.
↓
CM remnants gets internalized.
↓
Endocytosed vesicle then fuses with a lysosome
↓
Enzymatic hydrolysis of Apo B48, Apo-E, TG, Cholesterol esters
releases Amino acids, free cholesterol, FFA & Glycerol.
↓
Resecreted from liver as PL-rich lipoprotein known as Remnant
remnant. [Metabolic fate not known.]
Receptor mediated endocytosis

18. • Function of CM: Transport dietary TG from intestine to
Adipose tissue for storage & Muscle for energy
• Clinical significance
– Patient with
• Lipoprotein lipase deficiency
• Apo C-II deficiency (Type-I hyperlipoproteinemia)
• Familial lipoprotein lipase deficiency
– Shows a dramatic accumulation of CM in plasma

19. • Lipoprotein lipase [LpL] [S/N]
– Found in skeletal muscle, adipose tissue, heart, spleen,
lung, renal medulla, aorta, diaphragm and lactating
mammary gland. [absent in liver]
– Present in walls of blood capillaries of extra hepatic tissue
anchored to capillary wall by -vely charged proteoglycan
chains of heparin sulphate.
– Attacks TGs of mature CM and VLDL.
– LpL activity is inhibited by Apo A-II & Apo C-III.
– Requires Apo C-II & PLs as cofactors for its activity.
– Apo C-II promotes binding of CM & VLDL to enzyme.

20. • LpL of adipose tissue has large Km
– Allows removal of FA from circulating CM & their storage
as TG when CM level in plasma is high.
• LpL of cardiac muscle has low Km
– Allows heart continuing assess to circulating fuel even
when CM level is low.
• Following injection of heparin, LpL is released from tissues
& lipemia is thus cleared. [post-heparin lipolytic activity]
• Lack of C-II leads to ↓ed activity of LpL & consequent
accumulation of CM & VLDL in blood.

21. • Synthesis of VLDL [Site: Liver ]
– VLDL synthesis is more const. [occurs even in fasting
state]
– Apo-B-100 (major apoprotein) +nt when it is secreted.
VLDL Metabolism
• Release of VLDL
VLDL is released from Hepatic parenchymal cells
↓
Space of Disse
Hepatic sinusoids
↓
Released into blood as “Nascent VLDL”

22. • Modification of circulating VLDL
Exchange rxn between VLDL & HDL by CETP
– Cholesteryl ester transfer protein transfers TG from
VLDL to HDL & CE from HDL to VLDL.
VLDL pass through circulation
↓
Lipoprotein lipase: degrades TG
↓
VLDL ↓ in size & become denser
↓
Apo-C & Apo-E returned to HDL [retain apo B-100]

23. • Metabolic fate of VLDL
– T1/2 of VLDL in plasma: 1-3 hrs
Apo-C-II of VLDL
↓(+)
Lipoprotein Lipase
TG---------------------------------- IDL + FA
• FA taken up by Adipose tissue & muscle.
• VLDL remnant is known as IDL [Intermediate density
lipoprotein].

24. • IDL
– Cholesterol > TG
– ↓ in size & more dense than VLDL
– Contain Apo-B100 & lack Apo C-II [returned to HDL].
– IDL further looses TG & gets converted to LDL.
VLDL--------------IDL----------------- LDL
[Lipoprotein cascade pathway]
– Little fractions of IDLs taken up by hepatocytes through
receptor-mediated endocytosis using Apo-E as ligand.

25. • Functions of VLDL
– Carries TG from Liver to peripheral tissue for energy.
• Clinical Significance
– Fatty Liver
• Occurs in condition in there is an imbalance between
hepatic TG synthesis & VLDL secretion.
• Eg: Obesity, Uncontrolled DM, Chronic alcoholism
– Abetalipoproteinemia
Defect in Triacylglycerol transfer protein.
↓
Leads to inability to load Apo-B with lipid
↓
No chylomicrons & VLDL formation
↓
TG accumulates in Liver & intestine

27. Dietary FA more than req. for immediate energy source
↓
converted back to TG in liver
↓
& packed with specific apo-protein
↓
To form VLDL
• Excess dietary carbohydrate also converted to TG in liver &
exported as VLDL.

28. • LDL
– Isn’t synthesized or secreted by Liver & Intestine.
– Formed principally by degradation of circulating VLDL.
VLDL----------------IDL----------------------LDL
TG TG
• In comparision to VLDL & IDL, LDL are:
– Richer in FC & CE
– Poorer in TG & Total Lipid
– Smaller in Diameter
– Higher in Density
• Principal CE +nt in LDL: Cholesterol Linoleate

29. LDL Metabolism
1. Receptor mediated Endocytosis
2. Effects Of endocytosed Cholesterol on cellular
cholesterol homeostasis
3. Uptake of chemically modified LDL by macrophase
Scavenger

30. • Receptor-mediated endocytosis
– Primary function of LDL: Provide cholesterol to peripheral
tissues.
– LDL receptors (apo B100) receptors
• are -vely charged glycoproteins clustered in pits on cell
memb.
• Intracellular side of pit is coated with protein clathrin
[stabilizes shape of pit]
– After binding LDL, LDL-receptor complex is internalized
by endocytosis.
– LDL vesicle rapidly loses its clathrin coat & fuses with
other similar vesicles, forming larger endosomes.

31. • CURL—Compartment for Uncoupling of Receptor & Ligand
Endosomal ATPase pumps H+
↓
fall in endosomal pH
↓
LDL separates from its receptor
↓
Receptors migrate to one side of endosome
& LDLs stay free within vesicle lumen
[This str. is called CURL]
↓
Receptors can be recycled
Lipoprotein remnants transferred to lysosomes
↓
Degraded by Lysosomal enzymes
↓
Releases FC, AAs, FAs, & PLs [Utilized by cell]

33. • Effect of endocytosed cholesterol on cellular cholesterol
homeostasis:
– High cholesterol
i. Inhibits HMG CoA reductase  ↓es cholesterol synthesis.
ii. Down regulates LDL receptor  ↓es synt. of new LDL receptor
 ↓es further entry of LDL-C into cells.
iii. ↑es activity of ACAT.
Cholesterol utilised for structural or synthetic purpose.
• Excess cholesterol is esterified by Acyl CoA:Cholesterol
acyltransferase (ACAT).
• ACAT transfers FA from a Fatty acyl CoA derivative to
cholesterol, producing CE (stored in cell).

34. Uptake of chemically modified LDL by
macrophage scavenger receptors:
– Macrophages possess high levels of scavenger receptor
class A (SR-A),
• mediate endocytosis of chemically modified LDL in
which lipid components or apo B have been oxidized.
– SR-A isn’t down-regulated in response to ↑ed intracellular
cholesterol.
– CE accumulate in macrophages & cause their
transformation into “foam” cells, which participate in
formation of atherosclerotic plaque.

35. Functional LDL receptors deficiency
• causes a significant ↑ in plasma LDL &, thus plasma
cholesterol.
• Patients with such def. have Type II hyperlipidemia (familial
hypercholesterolemia) & premature atherosclerosis.]
Wolman disease & Niemann-Pick disease, type C
– Rare
– autosomal recessive
– Defect: ↓ ability to
– Hydrolyze lysosomal CE (Wolman disease)
– Transport unesterified cholesterol out of lysosome
(Niemann-Pick disease, type C)

36. HDL Metabolism
– Originate in Liver & S. intestine as small protein rich
particle [contain little FC & CE]
• Apo-proteins of HDL [Apo-A-I, II & IV, C-I, II & III, D, E]
• HDL is a reservoir of apolipoproteins:
– HDL particles serve as a circulating reservoir of:
– Apo C-II: transferred to VLDL & CM [Activator of LpL].
– Apo E: Req. for receptor-mediated endocytosis of IDL
& CM remnants.

37. • Synthesis of HDL
– Hepatic HDL
• Apo-A, Apo-C, Apo E are synthesised by
polysomes on RER
• Assembled with lipids to form nascent HDL.
• Released into circulation.
– Intestinal HDL
• Apo-A is synthesised by polysomes on RER
• Assembled with lipid to form nascent HDL.
• Released into circulation where it receives apo C
& apo E from nascent HDL released from Liver.

38. • HDL uptake of unesterified cholesterol:
– Nascent HDL (disk-shaped) containing primarily:
• PL (largely lecithin)
• Apoproteins A, C, & E.
• HDL are excellent acceptors of free cholesterol (from other
lipoproteins & cell memb.) due to their high PL content.
• Nascent HDL accumulate cholesterol, rapidly converted to
spherical particles.

39. • Esterification of cholesterol:
– When cholesterol is taken up by HDL, it is immediately
esterified by plasma enzyme “Lecithin:cholesterol
acyltransferase” (LCAT).
– LCAT [Lecithin:cholesterol acyltransferase]
• Synthesized by liver.
• Activated by Apo A-I.
• LCAT binds to nascent HDL, transfers FA from C2 of
lecithin to cholesterol to produce:-
– a hydrophobic CE [sequestered in core of HDL]
– & lysolecithin [transferred to plasma albumin ]

40. • As nascent HDL accumulates CE,
– It 1st becomes a relatively cholesteryl ester–poor HDL3.
– & eventually, a CE–rich HDL2 particle [carries these esters
to liver]
– CETP moves some CE from HDL to VLDL in exchange for
TG.

41. Dual role of SRB1 receptor in HDL metabolism
• In liver and steroidogenic tissues, SR-B1 receptor binds
HDL via aop A-I, allows selective transfer of CE to these
cells.
• In other extra hepatic tissues, SRB1 receptor mediates
acceptance of free cholesterol from cells by HDL.

42. HDL Cycle

43. • Reverse cholesterol transport: [S/N]
– Key component of cholesterol homeostasis.
– Selective transfer of cholesterol from peripheral cells
to HDL, & from HDL to:
• Liver for bile acid synthesis
• Steroidogenic cell for hormone synthesis,
– This is basis for inverse relationship seen between:
• Plasma HDL level & atherosclerosis,
• & HDL's designation as “good” cholesterol carrier.

44. • Reverse cholesterol transport involves:
 Efflux of cholesterol from peripheral cells to HDL
 Esterification of cholesterol by LCAT
 Binding of cholesteryl ester–rich HDL (HDL2) to liver &
steroidogenic cells
 Selective transfer of cholesteryl esters into these cells,
 & release of lipid-depleted HDL (HDL3).

45. Non HDL cholesterol
• = LDL + VLDL + IDL + Lp(a)
• Also known as atherogenic cholesterol.
• Estimation is helpful in evaluating the risk of CVS disease.
Non HDLc Level
100-130mg/dl Very low risk
130-160 mg/dl Borderline risk
160-190 mg/dl High risk
> 190 mg/dl Very high risk

46. Lipoprotein ‘a’ [Lp(a)]
• Inhibits fibrinolysis.
• Present only in some people.
• Binds apo B100 via disulfide bond.
• Lp(a) level > 30mg/dL is associated with high risk (3 times
more) for heart attacks.
• High Lp(a) level along with high LDL increases the risk of
heart attacks by 6 times.
• Nicotinic acid reduces serum Lp(a) level.