Lipoproteins Composition

Lipoproteins are composed of a neutral lipid core (containing TAG and cholesteryl esters) surrounded by a shell of amphipathic apolipoproteins,  phospholipid, and nonesterified (free) cholesterol (Fig. 1). These amphipathic compounds are oriented such that their polar portions are exposed on the surface of the lipoprotein, thereby rendering the particle soluble in aqueous solution.

 The TAG and cholesterol carried by the lipoproteins are obtained either from the diet (exogenous source) or from de novo synthesis (endogenous source). [Note: The cholesterol (C) content of plasma lipoproteins is now routinely measured in fasting blood.

Total C =LDL-C + HDL-C + VLDL-C, where VLDL-C is calculated by dividing TAG by 5 because the TAG/cholesterol ratio is 5/1 in VLDL. The goal value for total cholesterol is <200 mg/dl.]

Figure 1:  Structure of a typical lipoprotein particle.

1. Size and density: Chylomicrons are the lipoprotein particles lowest in density and largest in size and that contain the highest percentage of lipid  (as TAG) and the lowest percentage of protein. VLDL and LDL are successively denser, having higher ratios of protein to lipid. HDL particles are the smallest and densest. Plasma lipoproteins can be separated on the basis of their electrophoretic mobility, as shown in Figure 2, or on the basis of their density by ultracentrifugation.

Figure 2: Electrophoretic mobility of plasma lipoprotein particles. [Note: The order of low-density lipoprotein (LDL) and very-low-density lipoprotein (VLDL) is reversed if ultracentrifugation is used as the separation technique.]  HDL = high-density lipoprotein.

2. Apolipoproteins: The apolipoproteins associated with lipoprotein particles have a number of diverse functions, such as providing recognition sites for cell-surface receptors and serving as activators or coenzymes for enzymes involved in lipoprotein metabolism. Some of the apolipoproteins are required as essential structural components of the

particles and cannot be removed (in fact, the particles cannot be produced without them), whereas others are transferred freely between lipoproteins. Apolipoproteins are divided by structure and function into

several major classes, denoted by letters, with each class having subclasses (for example, apolipoprotein [apo] C-I, apo C-II, and apo CIII).

[Note: The functions of all the apolipoproteins are not yet known.]