Plasma lipoproteins

Triacylglycerol, cholesterol and cholesterol esters, as well as lipid-soluble vitamins, are transported in plasma complexed with proteins. There are four major classes of lipoproteins, classified by their density, which in turn reflects the relative content of lipid and protein, as shown in Table 5.6:

  • Chylomicrons, the least dense of the plasma lipoproteins, are formed in the intestinal mucosa and circulate as a source of triacylglycerol in the fed state (section 4.3.2.2).
  • Very low-density lipoproteins (VLDL) are assembled in the liver and circulate as a source of cholesterol and triacylglycerol for extrahepatic tissues.
  • Low-density lipoproteins (LDL) are formed in the circulation by removal of triacylglycerol from VLDL; intermediate-density lipoproteins (IDL) are an intermediate step in the conversion of VLDL to LDL.
  • High-density lipoproteins (HDL). The apoproteins are synthesized in the liver and small intestine and acquire lipids (and especially cholesterol) from tissues for transport back to the liver.
Table 5.6 Major classes of plasma lipoproteins

Chylomicrons

VLDL

IDL

LDL

HDL

Density (g/mL)

< 0.95

< 1.006

1.006—1.019

1.020-1.063

1.064—1.210

Diameter (nm)

75-1200

30-80

25-35

18-25

5-12

Mr (I03 kDa)

400

10-80

5-10

2.3

0.175-0.36

Per cent protein

1.5-2.5

5-10

15-20

20-25

40-55

Per cent

7-9

15-20

22

15-20

20-35

phospholipids

Per cent free

1-3

5-10

8

7-10

3-4

cholesterol

Per cent

84-89

50-65

22

7-10

3-5

triacylglycerol

Per cent

3-5

10-15

30

35-40

12

cholesteryl esters

Electrophoretic

At origin

Pre-beta

Between

Beta

Alpha

mobility

pre-beta and beta

Major apoproteins

A-I, A-II, B-48,

B-100, C-I,

B-100, C-III,

B-100

A-I, A-II, B-48,

C-I, C-II, C-III, E

C-II, C-III, E

C-I, C-II, C-III, E

D, E

Turnover in plasma

4-5 minutes

1-3 hours

1-3 hours

45%/day

4 days

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