Lipoproteins; structure, classification and Clinical disorders associated with lipoprotein metabolism
Definition
Lipoproteins are molecular complexes
that consist of lipids and proteins (conjugated proteins). They deliver the
lipid components (cholesterol, triacylglycerol ect.) to various tissues for
utilization.
Structure of lipoproteins
1. Chylomicrons
They are largest and least dense lipoproteins. They transport
exogenous lipids (Triglyceride, phospholipids, cholesterol esters, and
fat-soluble vitamins) from intestine to all cells.
2. VLDLs (Very Low-Density Lipoproteins)
VLDLs transport endogenous lipid (triglycerides
synthesized in hepatic cells) from the liver cells.
3. LDLs (low density lipoproteins)
LDLs are formed from VLDLs and carry cholesterol to
cells. They are rich in cholesterol and cholesterol esters (bad cholesterol).
4. HDLs (high density lipoproteins)
HDLs are the densest lipoproteins. They transport
cholesterol from cells to the liver (reverse cholesterol transport). They are
called good cholesterol. HDLs can be divided into HDL2 and HDL3 according to
density.
HDLs stimulate prostacyclin synthesis by the
endothelial cells. Prostacyclin inhibits platelets aggregation and prevent
thrombus formation.
HDLs help in removal of macrophages from the arterial
wall.
Clinical disorders associated with lipoprotein metabolism
Clinical disorders of lipids and lipoproteins can be
divided into: Hyperlipoproteinemia and Hypolipoproteinemia
A. Hyperlipoproteinemia
hyperlipoproteinemia can be divided into five types
according to changes of plasma lipoproteins:
1. Type I (familial lipoprotein
lipase deficiency) is due to deficiency of lipoprotein lipase and apo CII
It
is characterized by:
ü Hypertriglyceridemia
(high TG).
ü Hyperchylomicronemia
(high chylomicrons).
ü High
VLDLs.
ü Decrease
in HDL(α-lipoprotein).
ü Decrease
in LDL(β-lipoprotein).
2. Type II (familial
hypercholesterolemia, FHC) is due to metabolic defects including high apo B and
deficiency of LDL-receptor on fibroblasts. It is characterized by:
ü Hyper
β-lipoproteinemia (High LDL)
ü Increased
total cholesterol
ü VLDL
and triglyceride may be high.
3. Type III (familial Dys-beta
lipoproteinemia) is due to metabolic defect including high apo E and apo B. it
is characterized by:
ü Hyper
β-lipoproteinemia (High LDL)
ü High
VLDLs (pre β-lipoproteinemia)
4. Type IV (familial hypertriglyceridemia,
FHTG) is due to increased synthesis of triglyceride. It is characterized by:
ü High
VLDLs (pre β-lipoproteinemia).
ü Hypertriglyceridemia
(high TG).
ü Serum
cholesterol level may be normal or high.
ü Low
HDL and LDL.
5. Type V(combined hyperlipidemias).
ü High
chylomicrons and VLDLs.
ü High
triglyceride and serum cholesterol.
ü Decreased
HDL and LDL.
B. Hypolipoproteinemia
1. Abeta
lipoproteinemia is due to defect of apo B synthesis. It is characterized by:
ü Low
serum cholesterol.
ü Low
triglyceride.
ü Absence
of LDL.
ü Absence
of chylomicrons and/or VLDL.
2. Familial
α-lipoprotein deficiency (tangier's disease) is due to deficiency in apo A1 and
apo AII. It is characterized by low HDL.
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