Chylomicrons, responsible for the transport of lipid molecules absorbed in the intestine, are not the only lipoproteins present in our body. In the article dedicated to the absorption of fats, we defined lipoproteins as particles characterized by a heart of a lipidic nature, wrapped in a sort of protein shell. These proteins, being water-soluble, give these particles the ability to circulate without too many problems in the aqueous environment.
In addition to chylomicrons we must remember three other very important lipoproteins, called respectively: VLDL, LDL and HDL.
These acronyms are acronyms referring to their density:
VLDL: very low density lipoproteins
LDL: low density lipoproteins
HDL: high density lipoproteins
The density referred to is related to their lipid content. In particular, the density is lower the greater are the triglycerides enclosed within the particle. It follows that:
VLDLs are lipoproteins with a high triglyceride content
LDL are lipoproteins with a low triglyceride content *
HDL are lipoproteins that are extremely low in triglycerides *
* On the other hand, LDL and HDL are characterized by a high cholesterol content.
Each of these lipoproteins plays different roles:
VLDL: they have the task of transferring triglycerides from the liver to the tissues; in particular, after being synthesized in the liver, they are poured into the bloodstream and transferred above all to muscle and adipose tissue.
LDL and HDL: carry cholesterol in the bloodstream. While LDLs have the purpose of transferring it to tissues, HDLs are responsible for removing excess cholesterol in the plasma.
Difference between chylomicrons and VLDL: while the former originate in the intestine and convey triglycerides from the diet to the tissues, VLDLs are assembled above all in liver cells (hepatocytes) and mainly transport triglycerides of endogenous origin.
The liver synthesizes VLDL by enclosing a large amount of triglycerides within them. Unlike chylomicrons, these lipids do not come directly from the diet but are synthesized in the liver (endogenous origin). For example, if there is an excess of glucose in the blood, the liver is able to convert these sugars into triglycerides. The same thing happens in the case of a diet that is high in calories and too rich in protein.
Within the VLDL we therefore find triglycerides in large quantities, but also a modest content of fat-soluble vitamins, phospholipids and cholesterol. All these substances are enclosed in a protein shell.
VLDLs exocytosis from the liver cell and from there they pass into the bloodstream. Once here, the very low density lipoproteins can carry out their main action, which we have said is to transfer triglycerides to the tissues, especially to muscles and fat reserves.
When the VLDLs reach the capillaries that supply these tissues, they are able to bind to the vascular wall and release triglycerides which can: deposit in the adipose tissue increasing its size or be oxidized to produce the energy necessary for cellular metabolism.
VLDL, losing a good part of their triglyceride load, increase their density and the cholesterol content becomes more relevant in percentage terms. The VLDL, after having transferred a good part of the triglycerides to the tissues, are transformed first into IDL (Intermediate Density Lipoproteins) and then, losing some more of their lipid load, into LDL.
Inside the LDL the most relevant substance is cholesterol. The low density lipoproteins have in fact the purpose of traveling in the bloodstream and releasing cholesterol to the various cells of the organism.
All cells need cholesterol, as this lipid enters the composition of plasma membranes. There are also cells that metabolize greater quantities of cholesterol, as they use it for further purposes. Endocrine cells, for example, use cholesterol as a starting molecule to produce steroid hormones; Examples are the cells of the adrenal cortex, which produce cortisol and aldosterone, the testes, which produce male sex hormones, and the ovaries which obviously produce female sex hormones.
HDL, similar to other lipoproteins, are synthesized by the liver. They are characterized by a high content of phospholipids, a modest content of triglycerides and the usual protein mantle that surrounds them. HDLs perform the opposite function to LDLs. These particles are in fact able to bind to cell walls and absorb excess cholesterol. At this point, the cholesterol-laden HDLs return to the liver, where they penetrate inside the liver cell releasing their lipid load. The liver can thus recover excess cholesterol or eliminate it through the bile.
