1. Cholesterol exogenous (dietary) cholesterol delivered to
liver via chylomicron remnants.
endogenous cholesterol synthesized primarily
in the liver from AcCoA (extrahepatic tissues
also synthesize cholesterol)
in the liver, cholesterol is either converted to
bile salts or to cholesterol esters and packaged
into VLDLs.
Peripheral tissues obtain most of their exogenous
cholesterol from LDLs and convert it back to
cholesterol for use in membranes or store in
cholesterol ester droplets.
LDLs deliver cholsterol to the tissues - HDLs
circulate it back to the liver.
Excess cholesterol is disposed of by the liver as
bile salts.

2. Lipoproteins-1 Function Types of lipoproteins % TAGS 8% 85%
hydrophobic core (TAGS, cholesterol esters)
hydrophilic surface (P-lipids, cholesterol, and
apolipoproteins)
Function
transport of lipids in blood
Types of lipoproteins
(classified according to density)
chylomicron
very low density (VLDL)
intermediate density (IDL)
low density (LDL)
high density (HDL)
Protein content increase, lipid decreases as density
increases.
% TAGS 8%
85%
Chylomicron VLDL IDL LDL HDL
% Protein 2%
33%

3. Lipoproteins-2 Chylomicron: 85% TAG, 4% chol., 8% protein 80 -500nm
formed in intestinal epithelial cells
deliver exogenous TAGS to tissue
ApoCII activates lipases in capillary cell
walls releasing FFA to tissue
chylomicron remnants return to liver where
they bind to ApoE receptor and are taken up
1/2 life in blood minutes
VLDL:
50% TAGs, 22% choles., 10% protein nm
formed in liver
deliver endogenous lipids to other tissues
(mainly muscle and fat cells)
ApoCII activates lipases in capillary cell
walls releasing FFA to tissue
converted to IDLs and LDL as lipids are
released

4. Lipoproteins-3 IDL: (31% TAGs, 29% choles., 18% protein)
formed from VLDLs as lipids removed
some IDLs return to liver
rest converted to LDLs by further removal
of lipids
LDL: “bad” cholesterol
10% TAGs, 45% choles., 25% protein nm
formed as lipids removed from VLDLs
and IDLs.
all Apolipoproteins lost except ApoB100
bind to LDL receptor via ApoB100 and
taken up by endocytosis by hepatic and other
tissues (50-75% taken up by liver). Primary
mode of cholesterol delivery to tissues.
Synthesis of LDL receptor is inhibited by
high levels of intracellular cholesterol and
stimulated by low levels of cholesterol.
Therefore, cholesterol uptake is closly
matched to intracellular cholesterol levels.

5. Lipoproteins-4 HDL: “good” cholesterol
8% TAGs, 30% choles., 33% protein nm
formed in liver
scanvenge cholesterol from cell surfaces
and other lipoproteins and deliver it to liver.
Convert cholesterol to cholesterol ester
bind to “scanvenger receptor” on liver cell
surface - cholesterol esters taken up and
HDLs released and reenter circulation.