Lipoproteins

Introduction

Lipids are macromolecules that are insoluble in water and soluble in nonpolar solvents. These molecules are found in all tissues and cells of the body where they play important functions. 

Cholesterol and triglycerides are insoluble in water and therefore these lipids need to be transported in association with proteins in the circulation. Lipoproteins are complex molecules with a central core that contains cholesterol esters and triglycerides and surrounded by free cholesterol, phospholipids and apolipoproteins.

 Plasma lipoproteins can be divided into mainly six classes: chylomicrons, chylomicron remnants, VLDL, IDL, LDL and HDL.

Chylomicrons

Chylomicrons are produced exclusively in the intestine. They are involved with the transportation of dietary fats especially dietary triglycerides and cholesterol to peripheral tissues and liver. They contain apolipoproteins A-I, A-II, A-IV, A-V, B-48, C-II, C-III, and E and an outer shell of phospholipids, free cholesterol and proteins surrounds the particle. Chylomicrons have a density <0.930 g/ml.

These particles contain Apo B-48 is the core structural protein that each chylomicron particle contains. 

Chylomicron Remnants

The removal of triglyceride from the chylomicrons by peripheral tissues results in the formation of smaller particles called chylomicron remnants. Chylomicrons rapidly undergo lipolysis and form smaller particles called chylomicron remnants. Chylomicron Remnants have a density between 0.930- 1.006. These particles are enriched in cholesterol and are pro-atherogenic.

Very Low-Density Lipoproteins (VLDL)

VLDLs (very low-density lipoproteins) are synthesized in the liver and have a density between 0.930- 1.006.

These particles are produced by the liver and are triglyceride rich. They contain apolipoprotein B-100, C-I, C-II, C-III, and E. Apo B-100 is the core structural protein that each VLDL particle contains.

Intermediate-Density Lipoproteins (IDL)

The lipolysis of VLDL by muscle and adipose tissue results in the formation of IDL particles. They are enriched in cholesterol. These particles contain apolipoprotein B-100 and E. IDL have a density between 1.006- 1.019. These IDL particles are pro-atherogenic.

Low-Density Lipoproteins (LDL)

LDL particles are derived from VLDL and IDL particles and they are further enriched in cholesterol. LDL carries the majority of the cholesterol that is in the blood. The predominant apolipoprotein associated with LDL is B-100 and each LDL particle contains one Apo B-100 molecule. VLDL have a density between 1.019- 1.063. LDL is found in association with hypertriglyceridemia, obesity, type 2 diabetes, low HDL levels and infectious and inflammatory states. These LDL particles are considered to be more pro-atherogenic than large LDL particles.

High-Density Lipoproteins (HDL)

HDLs are smallest of the lipoproteins, HDLs are involved in the transport of cholesterol from the tissues back to the liver. High-density lipoproteins are produced by both the liver and the intestine. HDL have a density between 1.063- 1.210. The HDL particle contains around 50% protein and 50% lipids, mainly cholesterol. The predominant apolipoproteins found in HDL are A-I, A-II, some apo C and apo E. Apo A-I is the main structural protein and each HDL particle may contain multiple Apo A-I molecules.

These particles play an important role in reverse cholesterol transport from peripheral tissues to the liver which indicates that HDL may be anti-atherogenic.