Lipoproteins

Introduction

Lipoproteins contain both fats and proteins. They are found in both soluble complexes (egg yolk, mammalian blood plasma, etc.) and insoluble ones like cell membranes. It carries cholesterol and triglycerides to different cells of the body. Apart from the genetic condition, lipoproteins levels may be increased due to the intake of excess fats, and some other medical conditions. In this tutorial, we will learn about lipoproteins in depth.

Different Forms of Lipoproteins

Lipoproteins are round in shape and constitute fats and proteins. It transports cholesterol, a type of lipid in the bloodstream. Cholesterol and triglycerides are two types of lipids found in lipoproteins. Our body makes various types of lipoproteins. Some of the lipoproteins are harmful and some are beneficial. There are four types of lipoproteins.

The scientists categorized lipoproteins based on their biological role, contents, and biological functions which can be described below −

Chylomicrons

• Chylomicron is rich in triglycerides and larger in shape. They are synthesized by the intestinal cells.

• They carry dietary triglycerides to adipose tissue and skeletal tissue and dietary cholesterol to the cells of the liver.

• On the surface target tissue of the cell, lipase hydrolyzes the triglycerides. The fatty acids are produced in this process which is either used in metabolism or for storage.

• The cholesterol-rich chylomicrons gets transported to the liver cell through blood.

Very low-density lipoprotein (VDEL)

• The size of very low-density lipoprotein is not constant but smaller than chylomicrons.

• VDEL is generated by the liver.

Intermediate density lipoprotein (IDL)

• These transport triglycerides and cholesterol.

Low-density lipoprotein (LDL)

• These are the principal carriers of cholesterol in the blood.

• Cholesterol is transported by LDL to the tissues that need cholesterol like the adrenal glands and the gonads.

• It is considered bad cholesterol.

High-density lipoprotein (HDL)

• Transports cholesterol from different parts of the body tissues to the liver.

• HDL possesses anti-inflammatory and antioxidant properties.

• They help prevent the build-up of plaques in the blood vessels that lead to atherosclerosis.

• It is considered good cholesterol.

Structure of Lipoproteins

The shape of the lipoproteins is spherical, micelle-like particles. It constitutes triglycerides and cholesterol esters as core elements. Further, it is encircled by a phospholipid, cholesterol, and amphipathic coat of proteins. On the surface of lipoprotein particles, apolipoproteins are present. These apolipoproteins assist to stabilize the lipids.

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Features of Lipoproteins

Lipoproteins	Density(g/ml)	Mean diameter (nm)	Electrophoretic mobility	Source	Function
Chylomicrons	<0.95	500	Remains at origin	intestine	Transports exogenous glycerides
VLDL	0.96-1.006	43	Pre-$\mathrm{\beta}$	liver	Transports endogenous triglycerides
IDL	1.007-1.019	27	Broad -$\mathrm{\beta}$	Catabolism of VLDL	Precursor of LDL
LDL	1.02-1.063	22	$\mathrm{\beta}$	Catabolism of VLDL through IDL	Transports cholesterol
HDL	1.064-1.21	8	$\mathrm{\alpha}$	Liver, intestine, catabolism of Chylomicrons and VLDL	Reverse cholesterol transport

Functions of Lipoproteins

Lipoprotein plays an essential role in the body. The main functions of lipoproteins are listed below −

• Transportation and absorption of lipids i.e., triglycerides and cholesterol in the intestines.

• It carries lipids to different tissues of the body from the liver.

• It transports lipids from different tissues of the body to the liver. That is known as reverse cholesterol transport.

Clinical Significance

The clinical significance of lipoproteins is listed below −

• The clinical significance of lipids is mainly connected with heart and liver diseases.

• The high level of lipoproteins causes coronary heart diseases and different types of lipoproteins disorders.

• There should be always an appropriate balance of lipoprotein levels in the body. As the level of LDL and VLDL increases, there is an increased risk of stroke, heart attack, and other heart-related diseases.

• Higher LDL and triglyceride levels and lower HDL cholesterol increase the risk of cardiovascular diseases. Deposition of dense small LDL particles is more likely to develop atherosclerosis. The reason behind it is the small dense LDL particles are circulated for a longer period of time and it is easy for them to enter the arteries and stick to them. As a result, the arteries' passage is blocked

• The LDL particles are very vulnerable to oxidation.

• High concentration of lipoprotein in the plasma depends on genetic factors, environment, and lifestyle.

How to Improve Lipoprotein Level?

The changes in our lifestyle, changes in dietary and proper medication improves lipoprotein levels. Some of these are listed below −

• Be physically active. We should do 30 minutes of exercise per day.

• We should eat food that is rich in fibers and low in fats.

• If the individual is with hypertension and blood sugar it should always be at the control level.

• Maintain a healthy weight.

• Reduce stress levels.

• Quit smoking and reduce the intake of alcohol.

• Eat a balanced diet and take a supplement of omega 3 fatty acids.

Conclusion

Lipoproteins are round in shape and constitute fats and proteins. It transports cholesterol, a type of lipid in the bloodstream. Our body makes various types of lipoproteins. Some of the lipoproteins are harmful and some are beneficial. As the level of LDL and VLDL increases, there is an increased risk of stroke, heart attack, and other heart-related diseases. Higher LDL and triglyceride levels and lower HDL cholesterol increases the risk of cardiovascular diseases. The changes in our lifestyle, changes in diet, and proper medication improves lipoprotein levels.

FAQs

Q1. What will happen in reverse cholesterol transport?

Ans. In reverse cholesterol, the body draws out surplus cholesterol from the tissues of the body to the liver. After that, either gallbladder may remove it from the body or the body distributes it.

Q2. What is a triglyceride?

Ans. Out of the three forms of lipids, triglycerides are the main form of lipids. Normally in the diet, approximately 95% of lipids are present in the form of triglycerides. They can be solid and liquid at room temperature.

Q3. What is the treatment for low-density lipoprotein?

Ans. To attain a normal range of low-density lipoprotein health professionals prescribe drugs that reduce the cholesterol level. Ezetimibe, statins, and PCSK9 inhibitors are some examples of cholesterol-reducing drugs. Diet and lifestyle changes also reduce the LDL level.

Q4. What are the risk factors of bad cholesterol?

Ans. Adults above the age of 40 and women who have undergone menopause are at risk of high LDL levels. In addition to this diet rich in fats, a family history of high cholesterol, smoking and excessive intake of alcohol, obesity, and health conditions like hypertension, diabetes, kidney diseases, and HIV contribute to increasing bad cholesterol levels.

Q5. Why are high levels of triglycerides harmful to health?

Ans. As the level of triglycerides increase fat deposition in the artery wall will also increase. As a consequence, the passage of arteries becomes narrow and it affects the blood distribution to the heart. High-level triglycerides increase the level of stroke, heart attack, or any other heart-related disorders.