Lipoproteins and Apolipoproteins

Major Lipoproteins and their Function

Name of Lipoprotein	Major lipid that is rich	Main transport function
Chylomicron	Triglyceride	Transports Triglyceride to tissue and liver from gut
VLDL	Triglyceride	Transports Triglyceride from liver to tissue
IDL	Cholesterol	Transports Cholesterol to liver and tissue: LDL precursor
LDL-c	Cholesterol	Transports Cholesterol to tissue
HDL-c	Cholesterol	Transports Cholesterol from tissue

Lipoprotein Fractions

1.  Chylomicron.

a. Transports dietary – derived triglyceride (TG) in the blood
b. Is synthesized in intestinal epithelium.

1. Requires apolipoprotein apolipoproteins B-48 for assembly and secretion.
2. Nascent chylomicrons in the circulation obtain apo CII and apo E from high density lipoprotein (HDL).

c. Is absent during fasting.
d. If increased, it forms a creamy superannuate/supernatant.

1. Test tube must be left upright in a refrigerator overnight.
2. Chylomicron floats on top of plasma because it has very little protein (low density).

e. Source of fatty acids and glycerol.

• Used to synthesize TG in the liver and adipose

f.   Its Hydrolysis by capillary lipoprotein lipase (CPL) leaves a chylomicron remnant.

• Which are removed by apo E receptors in the liver.

2. Very low density lipoprotein (VLDL).

a. Transports liver-synthesized TG in the blood.

• Requires apolipoprotein B-100 for assembly and secretion.

b. Is the source of fatty acids and glycerol.

1. Is Used to synthesized TG in the adipose tissue.
2. Hydrolysis by CPL produces intermediate – density lipoprotein (IDL) and low density lipoprotein (LDL).
3. Some of the IDL is removed from blood by apo E receptors in the liver.

c.   If increased, it from a creamy infranate in plasma.

1.     Protein is greater in VLDL than in chylomicrons, so it sinks rather floats in plasma.

2.      VLDL concentration can be calculated: VLDL (mmol/L)= TG÷2.2.

3. Low – density lipoprotein (LDL)

a. Transports cholesterol in the blood.
b. Derived from continued hydrolysis of IDL by CPL.
c. Removed from blood by LDL receptors in peripheral tissue.

d. Calculated LDL (mmol/L)= CH –HDL –TG ÷ 2.2 (VLDL)

1. Presence of chylomicrons falsely lower calculated LDL by increasing diet-derived triglyceride; hence, fasting is required for an accurate   calculated LDL.
2. To reduce the chance for a falsely low calculated LDL-c, LDL-c is directly measured if the serum TG>400 mg/dL.

e.   Functions of cholesterol.

1. Component of the cell membrane.
2. Synthesis of vitamin D, adrenal cortex hormones, bile salt and acids.

4. High – density lipoprotein (HDL)

a.”Good cholesterol”

• Increased by exercise, estrogen

b. It is synthesized by the liver and small intestine.

c. The functions of HDL-c.

1. The source of apolipoproteins for other lipoprotein fractions.

2. It removes cholesterol from atherosclerotic plaques.

i. It delivers cholesterol from peripheral tissue to the liver.

ii. Cholesterol is either excreted into bile or converted into bile acids/salts.

Apolipoproteins

• Apo A-I
• Apo A-II
• Apo A-IV
• Apo B-48
• Apo B-100
• Apo C-I
• Apo C-II
• Apo C-III
• Apo D
• Apo E