Every day we take a certain amount of cholesterol through food, which goes alongside that produced by the body (especially in the liver).
Often cholesterol is considered in negative terms, when in reality it is a fundamental molecule for the human organism. It is in fact:
- a structural component of plasma membranes, in particular, ensures a certain degree of fluidity
- the precursor of bile acids (in the liver), essential for the digestion of dietary fats; the precursor of steroid hormones (in the reproductive organs and in the adrenal cortex)
- the precursor of vitamin D (in the skin).
Cholesterol becomes harmful to the body only when its concentration in the plasma exceeds certain levels. In such situations it tends to form the so-called atherosclerotic plaques, which represent the starting point of numerous and serious cardiovascular diseases.
In free cholesterol, the carbon in position three is bonded to a hydroxyl (OH); for this reason it is not 100% hydrophobic. On the contrary, when this hydroxyl is esterified with a fatty acid, a totally insoluble cholesterol ester in water is obtained.
: lipoproteinsIn plasma, most of the cholesterol is in the esterified form, that is, in its less soluble form. For this reason it must be transported in a special way, through lipoproteins.
The main lipoproteins (LP) responsible for the transport of blood cholesterol are:
- chylomicrons
- VLDL (very low density lipoprotein)
- LDL (low density lipoprotein)
- HDL (high density lipoprotein)
These lipoproteins are classified according to their size (see figure), composition and density.
Each lipoprotein is composed of a hydrophobic lipid heart, rich in triglycerides and esterified cholesterol. This central part is surrounded by a peripheral portion consisting of proteins and phospholipids, which orient their polar head towards the outside. Between these two substances there is also a small amount of free cholesterol, which directs its hydroxyl towards the external portion. .
The proteins do not completely coat these molecules, but they tend to arrange themselves in patches.
The amount of lipids, as opposed to the size, tends to progressively decrease passing from chylomicrons, to VLDL, LDL and HDL. As the lipoprotein is depleted of fat, its protein content increases and together with it its density.
Higher than normal levels of LDL-cholesterol predispose to the development of atherosclerosis; for this reason the cholesterol transported by LDL is called "bad". On the contrary, HDLs are defined as "scavengers of the arteries", since the cholesterol associated with them, called "good", is transported to the liver after the "cleaning of the vessels".
The liver can thus direct the cholesterol transported by HDL to the synthesis of bile salts; if it is present in excess it can also eliminate it through the bile as free cholesterol.
The cardiovascular risk is lower the higher the HDL-cholesterol level and the lower the LDL-cholesterol level. In other words, within certain limits, it is better to have high total cholesterol levels associated with equally high HDL cholesterol values, rather than having low total cholesterol levels associated with low HDL cholesterol levels.
Summary Table of Lipoproteins
The lipoproteins maintain the lipids in solution in the plasma and carry them from one tissue to another. In particular:
chylomicrons are formed in enterocytes and convey to peripheral tissues the products of lipid digestion (triglycerides, cholesterol, cholesterol esters, fat-soluble vitamins) of food origin first in the lymph and then in the blood.
VLDLs are synthesized by hepatocytes. They transport triglycerides from the liver (where they have been synthesized, for example, from glucose) to other tissues (especially adipose and muscle).
LDL derive from VLDL, due to progressive depletion of their triglyceride content. They are loaded with cholesterol which they transport to the peripheral tissues, where they perform all the fundamental functions listed above.
HDL, secreted into the blood by the liver and intestines, transport cholesterol from peripheral tissues to the liver (they carry out the so-called reverse transport of cholesterol).
