Lipoproteins

Lipoproteins are particles that carry lipids such as triglycerides, phospholipids, cholesterol and proteins called apolipoproteins through the blood, to and from the liver and the cells in the body.
These lipids being non-water soluble need to be transported through the blood since they cannot float in the blood stream. Lipoproteins differ in their density and the types of proteins that they contain. The more lipids in relation to proteins in the lipoprotein, the less dense lipoproteins are. (1)Ridker PM, Rifai N, Rose L, Buring JE, Cook NR. Comparison of C-Reactive Protein and Low-Density Lipoprotein Cholesterol Levels in the Prediction of First Cardiovascular Events. N Engl J Med 2002; 347:1557-1565November 14, 2002. Available here. (2)Toth PP. The “Good Cholesterol”. Circulation.2005; 111: e89-e91. Available here. (3)Krauss RM. Heterogeneity of plasma low-density lipoproteins and atherosclerosis risk. Curr Opin Lipidol. 1994 Oct;5(5):339-49. Available here. (4)Campos H, Genest JJ, Blijlevens E, McNamara JR, Jenner JL, Ordovas JM, et al. Low density lipoprotein particle size and coronary artery disease. Arteriosclerosis, Thrombosis, and Vascular Biology. 1992; 12: 187-195 Available here.

Lipoproteins

There are various types of lipoproteins:

  • Chylomicrons – made in the intestine, they mainly transport triglycerides (but also some phospholipids and cholesterol) to the lymph vessels. The function of chylomicrons is to deliver triglycerides to the body to be used as fuel.
  • Very low density lipoproteins (VLDL) – made in the liver, their function is to carry triglycerides and, in a lower proportion, cholesterol to the body cells. They contain a protein called “apo B100” and protein consists of between 5% to 10% of their overall composition. When VLDL loses most of the triglycerides it turns into LDL, since it becomes
    “smaller” and, therefore, the proportion of protein (and of cholesterol) in their composition increases.
  • Intermediate-density lipoproteins (IDL) – lipoproteins formed by the degradation of VLDL. They mainly carry triglycerides and cholesterol through the blood. They are either moved to the liver for degradation or form LDL particles.
  • Low-density lipoproteins (LDL) – made in the liver as VLDL, become LDL after losing a lot of triglycerides. LDLs carry most of the cholesterol trough the blood to deliver it to all body cells. This is the most misunderstood lipoprotein particle. Here is why:

Small dense LDL particlesLDLs come in different sizes. Till recently all LDL particles were considered as “Bad” however it has been shown that the small, dense LDLs and not the large LDLs cause all the trouble. As the following illustration shows the small, dense LDLs are susceptible to oxidation and are able to easily penetrate the artery wall, depositing the contents (mainly cholesterol particles) and therefore contributing to plaque formation.

  • High-density lipoproteins (HDL) – made in the liver and in the small intestine. This particle picks up and absorbs cholesterol molecules from the body cells, carrying them to the liver for breakdown and excretion. It is known as “good cholesterol” although it is not really cholesterol but a particle carrying cholesterol. High HDL is related to a lower risk of heart disease and stroke. Low HDL in the body increases the risks.

References   [ + ]

1. Ridker PM, Rifai N, Rose L, Buring JE, Cook NR. Comparison of C-Reactive Protein and Low-Density Lipoprotein Cholesterol Levels in the Prediction of First Cardiovascular Events. N Engl J Med 2002; 347:1557-1565November 14, 2002. Available here.
2. Toth PP. The “Good Cholesterol”. Circulation.2005; 111: e89-e91. Available here.
3. Krauss RM. Heterogeneity of plasma low-density lipoproteins and atherosclerosis risk. Curr Opin Lipidol. 1994 Oct;5(5):339-49. Available here.
4. Campos H, Genest JJ, Blijlevens E, McNamara JR, Jenner JL, Ordovas JM, et al. Low density lipoprotein particle size and coronary artery disease. Arteriosclerosis, Thrombosis, and Vascular Biology. 1992; 12: 187-195 Available here.