Lipids are insoluble in water and so natural compounds such as cholesterol, phospholipids and triglyceride need to be associated with proteins, forming water soluble lipoproteins, in order to be transported around the body. These lipoprotein complexes vary with respect to size and density - the smaller the complex, the greater the density. Also, the denser the complex, the higher the proportion of cholesterol to triglyceride.
The complexes, in descending order of molecular mass, are:
- very low density lipoprotein (VLDL)
- low density lipoprotein (LDL)
- high density lipoprotein (HDL)
- chylomicron remnants
The protein components of lipoproteins are apolipoproteins or enzymes. Apolipoproteins are structural proteins, which also may contain:
- receptor binding sites, or
- proteins that modify the binding of other apolipoproteins to receptors.
Lipoproteins may also contain other lipid-soluble substances e.g. lipid-soluble vitamins. These are thus able to be distributed around the body via the lipoproteins.
The major sources of lipoproteins are the gut and the liver.