HDL cholesterol and LDL cholesterol

HDL and LDL cholesterol refer to the lipoproteins that carry cholesterol in the blood. HDL is high density lipoprotein (HDL stands for high density lipoprotein ), while LDL is low – density lipoprotein (LDL stands for low density lipoprotein ).

This classification of lipoproteins is due to the technique used for their separation: ultracentrifugation. In this technique, the blood plasma is centrifuged at high speed in a tube and the lipoproteins are separated by their density, leaving the densest (HDL) at the bottom of the tube and the least dense (LDL and other lipoproteins) floating at different levels. of the tube.

Cholesterol is a lipid molecule that is not soluble in water. Lipoproteins are spherical particles made up of proteins and lipids that carry cholesterol and other lipids through the blood. The main lipoproteins in the transport of cholesterol are HDL and LDL.

HDLLDL
DefinitionHigh-density lipoprotein.Low-density lipoprotein.
Density (gr / ml)1,063-1,2101,019-1,063
Composition
  • Total lipids: 40-48%
  • Proteins: 52-60%
  • Total lipids: 75-80%
  • Proteins: 20-25%
Cholesterol24-45% of total lipids.47-51% of total lipids.
Diameter120-70 angstrom250 angstrom

What is HDL cholesterol?

HDL or HDL cholesterol (HDL-C) are the lipoproteins that carry cholesterol from cell membranes to the liver. They are also known as good cholesterol, although this statement is wrong, because HDL does not only contain cholesterol.

Origin of HDL

HDL originate from the Apo A1 protein that is synthesized in the extracellular space. This protein circulates through the tissues capturing cholesterol and phospholipids from the membranes, and once it enters the circulation, it exchanges proteins and cholesterol esters with other lipoproteins.

HDL characteristics

  • It has a higher percentage of proteins with respect to lipids.
  • Density between 1,063 and 1,210 gr / ml.
  • Diameter from 70 to 120 angstrom, it is the smallest of the lipoproteins.

Structure of HDL

HDL, like the other lipoproteins, are spherical particles, made up of:

  • An outer layer of phospholipids, proteins, and cholesterol.
  • A hydrophobic liquid center with triglycerides and cholesterol ester.

HDL proteins can be recognition proteins such as Apo A1 and Apo A2 and enzymes such as LCAT.

Function of HDL

The main function of all lipoproteins is to solubilize fats in the blood. HDL are the lipoproteins responsible for transporting cholesterol from extrahepatic tissues to the liver. This is what is known as the reverse transport of cholesterol .

In the liver, cholesterol is transformed into bile acids for elimination through the intestine, or recycled.

HDL values ​​in human blood

High levels of HDL cholesterol are associated with protection against cardiovascular disease (CVD). Normal HDL values ​​for an adult should be between 40 and 60 mg / dl (1.04-1.55 mmol / L in SI units). Values ​​greater than 60 mg / dl are recommended to prevent CVD. HDL values ​​below 40 mg / dl are high risk for CVD.

What is LDL cholesterol?

LDL or LDL cholesterol (LDL-C) are the lipoproteins that transport cholesterol from the liver to extrahepatic tissues. They are also known as bad cholesterol, as high levels of this lipoprotein are deposited in the arteries, producing atherosclerotic plaque.

Origin of LDL

LDL is derived from VLDL (larger, triglyceride-rich, very low-density lipoproteins) that are formed in the liver. These VLDLs travel through the bloodstream, delivering triglycerides to the tissues. In this way, they are reduced in size and gaining in cholesterol content, until they become LDL.

LDL characteristics

  • It has a higher percentage of lipids in relation to proteins.
  • Density between 1,019 and 1,063 gr / ml.
  • Diameter about 250 angstrom.
  • The characteristic protein of LDL is apolipoprotein B100.

Structure of LDL

LDL is also spheroidal particles, larger than HDL. They present:

  • an outer layer of phospholipids, cholesterol, and the protein Apo B100.
  • A fatty center full of cholesterol and triglycerides.

The main LDL protein is apo B100, which is a protein that is recognized by cell receptors.

LDL function

LDL is responsible for carrying cholesterol to the tissues that need it. The liver leaves assembled the VLDL loaded with triglycerides and cholesterol. Triglycerides are released as they pass through the circulation. LDLs are the result of the reduction in size of these VLDLs.

The apo B100 protein of LDL binds to cell receptors and enters the cell by endocytosis. The cell can thus use cholesterol to form membranes and other compounds.

LDL values ​​in human blood

Elevated LDL cholesterol levels are associated with cardiovascular disease (CVD). Optimal LDL values ​​for an adult should be below 100 mg / dl (less than 2.6 mmol / L, SI). Values ​​between 130-159 mg / dl (3.36–4.11 mmol / L, SI) are borderline risk for CVD, while above 159 mg / dl. the person is at high risk for CVD.

People with high LDL levels should consult with their doctor to implement measures that lead to the reduction of cardiovascular risk.

Good cholesterol and bad cholesterol

Cholesterol is an important molecule for the functioning of cells and animal organisms. This compound is the precursor to steroid hormones, such as sex hormones and corticosteroids. In addition, it is part of the cell membranes, sandwiched between the phospholipids of the lipid bilayer, regulating the fluidity of the membranes.

To say that there is a good cholesterol or a bad one is an oversimplification of the role of transporter lipoproteins. The cholesterol that accumulates in the arteries promotes the formation of fatty plaques or atheromas, leading to atherosclerosis.

The lipoproteins that transport cholesterol to the arteries (LDL) are badly called bad cholesterol, while the lipoproteins that take cholesterol from the membranes (HDL) are nicknamed good cholesterol.

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