What is Apolipoprotein B?
Apolipoprotein B is the main protein component of low density lipoprotein (LDL or "bad" cholesterol), responsible for transporting cholesterol to the tissues.
Apolipoprotein B, or more simply APOB, also constitutes about 40 percent of the protein fraction of very low density lipoproteins (VLDL) and chylomicrons (which carry triglycerides of endogenous and exogenous origin.
Functions in the body
APOB is essential for the assembly, secretion and metabolism of these lipoproteins.
Although the function of apolipoprotein B is still far from being fully understood, we know that it acts as a ligand for the LDL receptors located in many cells of the body. In practice, it represents the "key" which, once inserted in the lock of the respective "window" located on the external surface of the cell, allows cholesterol to enter.
The high affinity interaction between LDL and the LDL receptor (located on the surface of most of the cells of our organism) occurs through APOB, which is the physiological ligand and therefore largely responsible for regulating LDL cholesterol levels in the body. plasma.
Why is it measured in blood?
The dosages of apolipoprotein B, together with those of other blood indicators, allow to quantify the risk of suffering from arterio-coronary diseases. In fact, there is a clear relationship between high levels of APOB and atherosclerosis, so evident that the dosage of APOB in the blood represents a better indicator of cardiovascular risk than LDL cholesterol (which in turn is more indicative of total cholesterol). Similarly to the latter, apolipoprotein B levels can be elevated due to genetic and / or environmental factors (high-calorie diet rich in saturated fats and simple sugars, overweight, sedentary lifestyle, etc.).
In the blood we find two isoforms, APOB48, synthesized exclusively by the small intestine, and APOB100, produced in the liver.
Normal values Apolipoprotein B nel plasma: 35-100 mg/100 ml
Cause di APOB Elevate
Increase: pregnancy; hypercholesterolemia; defects in LDL receptors; obstruction of the biliary tract; nephrotic syndrome.
Decrease: liver disease; sepsis; administration of estrogen; familial apolipoprotein B deficiency.
There is a clear relationship between the concentration of APOB100 in the blood and the number of lipoprotein particles of hepatic origin (VLDL, IDL, LDL), since each of these has one, and only one, APOB100 protein.
High levels of APOB100 in the blood are therefore synonymous with a high number of LDL lipoproteins, but they do not provide any information on their content in lipids and in particular in cholesterol (the various particles can have a more or less important load of triglycerides and cholesterol). So how to explain the association between elevated APOB100 levels and increased cardiovascular risk?
It is believed that the presence of a high number of lipoproteins in the bloodstream gives rise to competition mechanisms for cellular APOB100 receptors (LDL-R). Since the number of these "locks" and their respective "windows" is limited, the surplus of "keys" (lipoproteins) in the blood makes them more susceptible to those phenomena of oxidative and non-oxidative origin, which underlie the formation of atheromas. These considerations would explain the relationship between elevated levels of apolipoprotein B and increased risk of coronary heart disease, as well as the greater reliability of this parameter compared to LDL cholesterol.
Levels of apolipoprotein B are often related to those of apoliporotein A1 (which characterizes HDL or "good" cholesterol). The lower this is report (APO-A1 / APO-B) and the greater the cardiovascular risk.