Plasma lipids include triglycerides, cholesterol, phospholipids, and free fatty acids. They are transported in the form of lipoproteins which contain a core of triglycerides and cholesterol esters surrounded by a layer of phospholipids and proteins. The main classes of lipoproteins are chylomicrons, VLDL, LDL, and HDL, which differ in their size, lipid content, and apolipoprotein composition. Lipoproteins transport lipids through the blood and deliver them to tissues, playing important roles in lipid metabolism.

1. PLASMA LIPOPROTEINS
DR MUHAMMAD MUSTANSAR

2. Plasma Lipids
Plasma lipids are usually measured after 12 hours
fasting. The mean value of plasma lipids is 470 mg/dl.
The different types of Plasma lipids are :
Triglyceride
 normal range 50 – 150 (mean value 100 mg/dl)
Cholesterol
 normal range 100– 200(mean value 150 mg/dl)
Phospholipids
 normal range 150 –250 (mean value 200 mg/dl)
Free fatty acids
 normal range 10 – 30 (mean value 20 mg/dl)
2

4. Lipid transport in the circulation
Lipids are insoluble in plasma. In order to be transported they are combined
with specific proteins to form lipoproteins:
Proteins (apoproteins)
HO
O Non polar lipids in core
Cholesterol
R
(TAG and cholesterol esters)
O
R
HO
H
HO
Apoproteins are only weakly associated with a particular lipoprotein and are
easily transferred to another lipoprotein of a different class. Apoproteins have
various functions including:
• Structural role
• Binding sites for receptors
• Activators or co-enzymes for enzymes involved with lipid metabolism

5. Plasma Lipoproteins
All Lipids in plasma are
transported in the form
lipoproteins.
Any lipoprotein has:
central core: formed of non-
polar lipids (triglycerids &
cholesterol esters).
outer layer: formed of the
more polar lipids
(phospholipids & non-
esterified cholesterol), and
proteins (apoproteins)
5

6. Structure of lipoprotein
INTEGRAL APOPROTEINS
MONOLAYER OF
PHOSPHOLIPID
CHOLESTEROL AND CHOLESTERLOL
CORE
ESTERS
TRIGLYCERIDES
PERIPHERAL APOPROTEINS

7. Apoproteins
Are either peripheral (can be transferred) or integral
(cannot be transferred).
Function:
They act as activator for the enzymes that
act on lipoproteins
They are important for receptor recognition
to uptake the lipoproteins by certain
tissues.
7

8. General Functions of Lipoproteins
1. Keep lipids soluble as they transport them
in plasma
2. Provide an efficient mechanism for
delivering their lipid content to tissues
8

9. Plasma lipoproteins can be separated into different
fractions by two methods, electrophoresis and
ultracentrifugation.
Ultracentrifugation Electrophoresis
Non-mobile
Chylomicrons
fraction
VLDL β-lipoproteins
LDL Pre β- lipoproteins
HDL α - lipoproteins
Free FA –Albumin
(+)
9

10. 10

12. The five classes of lipoprotein
(all contain characteristic amounts TAG, cholesterol, cholesterol esters,
phospholipids and apoproteins)
Diameter Major apoliproteins
Class (nm) Source and function
Chylomicrons 500 Intestine. Transport of A, B48, C(I,II,III) E
(CM) dietary TAG
Very low density 43 Liver. Transport of B100, C(I,II,III) , E
Increasing density
lipoproteins endogenously synthesised
(VLDL) TAG
Low density lipoproteins 22 Formed in circulation by B100
(LDL) partial breakdown of IDL.
Delivers cholesterol to
peripheral tissues
High density lipoproteins 8 Liver. Removes “used” A, C(I,II,III), D, E
(HDL) cholesterol from tissues
and takes it to liver.
Donates apolipoproteins to
CM and VLDL

13. &%
Types of
Lipoproteins Lipid Content Origin
apoprotein
s
2% • TAG 90% (Main lipid)
Chylomicrons A, B48, C-II • Cholesterol (free & esters) 5% Intestine
&E • Phospholipids 3%
VLDL
• TAG 60% (Main lipid)
(very low % 10
B100, C, E
• Phospholipids 15% Liver
density • Cholesterol ( free & esters)15%
lipoproteins)
IDL • Cholesterol (free & esters)
(intermediate % 11 45% (Main lipid) Breakdown
density B100, E • Phospholipids 25 % of VLDL
lipoproteins) • TAG 30 %
• Cholesterol (free & esters) Breakdown
LDL
20 % 50% (Main lipid)
(low density B100 • Phospholipids 22 %
of VLDL via
lipoproteins) • TAG 8 % IDL
• Cholesterol (free & esters) 15%
HDL
50 % • Phospholipids 30% (Main
(high density Liver
A,C,D,E lipid) 13

14. Lipoprotein Functions Clinical
significance
•Transport dietary TAG Increased in
Chylomicron (mainly), cholesterol & deficiency of
cholesterol esters from the lipoprotein lipase
s
intestine to the peripheral
tissues
•Transport endogenous TAG Defect in their
VLDL from the liver to the peripheral synthesis leads to
tissues (Mobilize fat from fatty liver
liver)
LDL •Transport cholesterol from the Their increase
liver to the peripheral tissues leads to
atherosclerosis.
•Remove free cholesterol from Their decrease
peripheral tissues & esterifies leads to
14 HDL

15. 15

16. Uptake of LDL & its regulation
 The uptake of LDL occurs mainly in the liver
(75%), adrenals and adipose tissue.
 LDL are taken up by cells by LDL receptor-
mediated endocytosis.
 The interaction of LDL with LDL receptors
requires the presence of apoB100 which is found
only on LDL).
 The endocytosed membrane vesicles
(endosomes) fuse with lysosomes, in which the
apoproteins are degraded and the cholesterol
esters are hydrolyzed to yield free cholesterol.