Lipoproteins come in a range of sizes and contain triglycerides and cholesterol esters. They are classified according to their relative amounts of heavy protein and light lipid.
The left illustration shows that lipoproteins consist of a core containing triglycerides (lavender) and cholesterol esters (orange). This is surrounded by a single-layered coat of phospholipids (purple) and free cholesterol (orange) shown in the expanded segment. There are also proteins (red) attached to the coat; these are called apoproteins.
The illustration to the right shows lipoproteins of various sizes going from large, triglyceride-rich ones to small cholesterol ester-rich ones. The relative amount of triglyceride (lavender) and cholesterol ester (orange) is shown. The red particle on the surface of each represents an apoprotein
The largest lipoprotein is the chylomicron (1) from the intestine. The liver also secrets a relatively large one called a very low density lipoprotein (VLDL) represented by 4. Lipoproteins 6 and 7 represent high density lipoproteins (HDLs); they are cholesterol-ester rich as indicated by their mostly orange core.
VLDLs and chylomicrons lose their triglycerides (not their cholesterol esters) as they circulate though the body. Illustration 2 and 3 are representative of a chylomicron that is progressively losing triglyceride. It will continue to shrink until resembling 5 known as a chylomicron remnant. VLDLs (4) also lose triglycerides until they become similar to 5 but these are called intermediate density lipoproteins (IDLs) -- both have about equal amounts of triglyceride and cholesterol ester in their cores.
The two particles at the far right represent high density lipoproteins (HDLs). Their main component is cholesterol ester. Unlike the triglyceride-rich lipoproteins described above, they start out very small and progressively get larger. The ones shown here represent the larger ones -- small ones are too small to show.
Lipoproteins are classified according to their density or 'weight per unit volume'. The apoprotein molecules are much heavier than any other type of molecule involved. As the lipoprotein shrinks the heavy apoproteins occupy relatively more of the particle's volume. Scan the lipoproteins 1-7 and notice the constant size of the apoprotein (red). Large lipoproteins have relatively less apoprotein than small ones so they have low density; smaller ones have higher density.
When referring to a specific lipoprotein as having 'low density' it is important to specify 'relative to which other lipoprotein'. In the illustration it would be correct to say that particle 3 has low density -- compared to particle 5 -- but, it has high density compared to particle 1. Terms such as HDL (high density lipoprotein), IDL (intermediate density lipoprotein), LDL (low density lipoprotein) and VLDL (very low density lipoprotein) are general terms and don't refer to a particular particle. Rather, they refer to subgroups of lipoprotein particles of similar densities.
If the smaller sized particles are abundant the likelihood of developing coronary artery disease is increased. This is often referred to as pattern B; pattern A has relatively fewer small LDLs. The smaller particles are referred to as small dense LDL (sdLDL) while particles on the larger end of the scale are referred to as 'fluffy'. The clinical test that measures this distribution is called LDL-P ... 'p' stands for particles.
Continue to Lipoprotein Lipase.
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