Lipoprotein

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Lipoproteins are complex particles composed of a water-repelling core of mainly lipids as non-polar cholesterol esters (CEs) and triglycerides (TGs) surrounded by an amphipathic phospholipid monolayer that includes unesterified free cholesterol (FC) and proteins known as apolipoproteins or aproteins. Apolipoproteins provide a framework for lipoprotein assembly; determine the metabolic fate of the lipoprotein by activating or inhibiting key enzymes; and finally act as ligands for receptor molecules. Lipoproteins are classified into five classes, defined by their respective density on electrophoresis as Chylomicrons, Very Low Density Lipoproteins (VLDL), Intermediate Density Lipoproteins (IDL), Low Density Lipoprotein (LDL) and High Density Lipoproteins (HDL).

Classification

Lipoproteins can be classified according to their density, size and constituents. It is also possible to classify lipoproteins as "alpha" and "beta", akin to the classification of proteins in serum protein electrophoresis. The lipoproteins that have a low density are the triglyceride rich proteins which include chylomicrons, chylomicron remnants and VLDL. LDL, HDL and lipoprotein(a) have a less triglyceride content and a higher density.

Shown below is a table summarizing the structure of each lipoprotein.

Class	Origin	Size (nm)	Density (g/mL)	Electrophoretic Mobility	Triglyceride (%)	Cholesteryl Ester (%)	Free Cholesterol (%)	Phospholipid (%)	Protein (%)	Apolipoprotein
Chylomicron	Intestine	80–500	<0.93	α2	86	3	2	7	2	B-48, E, A-I, A-II, A-IV, C
VLDL	Liver	30–80	0.95–1.006	Pre-β	55	12	7	18	8	B-100, A-I, C-I, C-II, C-III, E
IDL	VLDL	25–35	1.006–1.019	Slow pre-β	23	29	9	19	19	B-100, E
LDL	IDL	21.6	1.019–1.063	β	6	42	8	22	22	B-100
HDL2	Liver, intestine	10	1.063–1.125	α	5	17	5	33	40	A-I, A-II, A-IV
HDL3	Liver, intestine	7.5	1.125–1.210	α	3	13	4	25	55	A-I, A-II
Lp(a)	Liver	30	1.055–1.085	Slow pre-β	3	33	9	22	33	B-100, apo(a)

Function

The lipids are often an essential part of the complex, even if they seem to have no catalytic activity themselves. To isolate transmembrane lipoproteins from their associated membranes, detergents are often needed.

All cells use and rely on fats and, for all animal cells, cholesterol as building blocks to create the multiple membranes which cells use to both control internal water content, internal water soluble elements and to organize their internal structure and protein enzymatic systems.

Lipoproteins in the blood, a water medium, carry fats around the body. The protein particles have charged groups aimed outward so as to attract water molecules; this makes them soluble in the salt water based blood pool. Triglyceride-fats and cholesterol are carried internally, shielded by the protein particle from the water.

The interaction of the proteins forming the surface of the particles with (a) enzymes in the blood, (b) with each other and (c) with specific proteins on the surfaces of cells determine whether triglycerides and cholesterol will be added to or removed from the lipoprotein transport particles.

Regarding atheroma development and progression vs. regression, the key issue has always been cholesterol transport patterns, not cholesterol concentration itself.

Shown below is a table summarizing the function of each lipoprotein.

Lipoprotein	Function
Chylomicron	Carries triacylglycerol (fat) from the intestines to the liver and to adipose tissue
Chylomicron remnant
Very low density lipoprotein (VLDL)	Carries newly synthesised triacylglycerol from the liver to adipose tissue
Intermediate density lipoprotein (IDL)	Intermediate between VLDL and LDL, not usually detectable in the blood
Low density lipoprotein (LDL)	Carries cholesterol from the liver to cells of the body, sometimes referred to as the "bad cholesterol" lipoprotein
High density lipoprotein (HDL)	Collects cholesterol from the body's tissues and brings it back to the liver, sometimes referred to as the "good cholesterol" lipoprotein
Lipoprotein(a)

References

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