Lipoprotein in general

Because lipids, such as cholesterol and triglycerides, are insoluble in water these lipids must be transported in association with proteins (lipoproteins) in the circulation. Large quantities of fatty acids from meals must be transported as triglycerides to avoid toxicity.

The plasma lipoproteins are complexes of lipid and protein which appear as spherical particles when viewed in the electron microscope with negative staining.

These lipoproteins have, in their basic structure, a lipid core to be transported (triacylglycerols (TAG), phospholipids, and cholesterol esters). A hydrophilic layer in which apolipoproteins are embedded thus provides structural stability as well as identity for each type of the lipoprotein.

Plasma lipoproteins are separated by hydrated density; electrophretic mobility; size; and their relative content of cholesterol, triglycerides, and protein into five major classes: from ultra-low-density lipoprotein (ULDL = chylomicrons) to very-low-density lipoprotein (VLDL), intermediate-density lipoproteins (IDL), and high-density lipoproteins (HDL).

Chylomicrons (CM) are the largest in diameter with the lowest density and the highest TAG content. The size of chylomicrons varies depending on the amount of fat ingested.

Lipoprotein lipase (LPL) is an extracellular enzyme that is essential in lipoprotein metabolism.
Lipoprotein in general

Very-low-density lipoprotein

The lipoproteins of human serum transport hydrophobic compounds, including cholesterol, triglyceride, and phospholipid, through the aqueous plasma compartment to sites of utilization or catabolism.

The major lipoprotein types: very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL).

In normal man, the plasma low density lipoproteins (LDL) are the major carriers of plasma cholesterol, whereas the very low-density lipoproteins (VLDL) transport endogenous triglycerides.

VLDL functions is to transport triglycerides from the liver to peripheral tissues, where they are hydrolyzed to provide fatty acids that can be oxidized to produce adenosine triphosphate (ATP) for energy production.

VLDL, a component of non-high-density lipoprotein cholesterol, is identified as a risk factor for atherosclerotic cardiovascular disease.

They are triglyceride-rich, but their triglyceride content is lower and cholesterol content higher than that of chylomicrons.

Through the activity of extrahepatic lipoprotein lipase, VLDL lose most of their content of triglyceride to form particles poor in triglyceride and rich in cholesterol termed intermediate density lipoproteins (IDL).
Very-low-density lipoprotein

High Density Lipoprotein cholesterol

Cholesterol is found in all cells. It is needed for many body functions. Two types of cholesterol are HDL and LDL. HDL is a “good” cholesterol that removes LDL (bad) cholesterol from the bloodstream.

High-density lipoprotein (HDL) is one of the five major fat and protein particles (lipoproteins) whose role it is to enable blood fats (lipids), such as cholesterol and triglycerides, to be transported within the water-based bloodstream.

HDL is a class of heterogeneous lipoproteins containing approximately equal amounts of lipid and protein. HDL particles are characterized by high density (more 1.063 g/mL) and small size (Stoke’s diameter 5 to 17 nm).

The various HDL subclasses vary in quantitative and qualitative content of lipids, apolipoproteins, enzymes, and lipid transfer proteins, resulting in differences in shape, density, size, charge, and antigenicity.

In a normal healthy individual, HDL carries about a quarter of the total amount of cholesterol in the blood, whereas most of the remainder is carried in LDL (low density lipoprotein) “bad cholesterol” particles.

HDL is important for the synthesis of steroid hormones but it is better known for its protective role against cardiovascular disease. A low HDL is thought to speed up the rate at which arteries fur up.

HDL (good) cholesterol protects against heart disease, so for HDL, higher numbers are better. A level less than 40 mg/dL is low and is considered a major risk factor because it increases the risk for developing heart disease. HDL levels of 60 mg/dL or more help to lower the risk for heart disease.

A variety of things can affect cholesterol levels: Diet, Weight, Physical Activity. Low HDL among others due to: Insulin resistance, Familial Combined Hyperlipidaemia (FCH), Tangier disease, Medicines including beta blockers, thiazide diuretics, androgens, progestogens and anabolic steroids.
High Density Lipoprotein cholesterol 

Blood lipid

Lipid is a broad term for fats and fatty substances in the bloodstream including fatty acids, phospholipids, lipoproteins, triglycerides and cholesterol and other sterols.

Lipids do not dissolve in water, thus move through the bloodstream suspended in protein carriers- lipoproteins. All of these lipoprotein contain phospholipids, triglycerides, and cholesterol but in varying amounts.

Lipids are considered structural, rather than chemical, fats and are important for many body functions, including cell maintenance and repair, dissolving fat-soluble vitamins and other nutrients and storing energy.

High levels of lipid may result from genetic makeup or lifestyle choices or both. Lifestyles factors such as high-fat diet, obesity, smoking, and physical inactivity can cause or contribute to high cholesterol levels, increasing an individual’s risk for atherosclerosis. Cholesterol is an important lipid in the blood.

When the level of lipids in the bloodstream become too high (hyperlipidaemia), lipid droplets can settled out of the blood flow and accumulate along the inner walls of the arteries, forming blockages (occlusions) and eventually compacting into arterial plaque, a brittle layer incorporating other debris such as blood cells that causes the arteries to become stiffened and inflamed.

Abnormalities of blood lipid level are best determined by measurement of the lipoproteins rather than the analysis of the blood lipid fractions alone. Thus disorders on blood lipid level are classified according to the concentration of the lipoproteins.
Blood lipid

What is cholesterol?

Coronary heart disease is leading cause of death of Americans and preventive measures have emphasized reducing risk factors such as high blood cholesterol levels.

What is cholesterol? Cholesterol is one of the most vital and important biochemical compounds in nature. It is a combination of two acetate molecules that join to form a waxy substance.

Cholesterol naturally present in everywhere in the body, including the brain nerves, muscles, skin, liver, intestines and heart.

Humans manufacture 80% to 90% of their cholesterol within their own cells. Cholesterol is produced in the liver, which can make all the cholesterol that the body needs.

All cell membranes are composed of cholesterol and cholesterol-derived compounds. Brain and nerve tissue contain the highest proportion of cholesterol on the body.

Cholesterol is used in the production of certain vitamins and hormones, such as vitamin D, cortisol, estrogen and testosterone. It takes only small amount of cholesterol to meet these needs.

Cholesterol and other fats cannot dissolve in blood. They have to be carried to and from the cells through a special medium called lipoproteins. Cholesterol must be attached to transport molecules lipoproteins to remain in circulation.

When the body have high levels of bad cholesterol (LDL), fatty deposits may form in the artery walls, which the process known as atherosclerosis. It could lead to heart attack.
What is cholesterol?

High density lipoprotein

Lipoproteins are globular particles of varying size and composition, configured so that their outer surface is hydrophilic and their inner core, which contains immiscible lipids, hydrophobic.

High density lipoproteins (HDL) are the smallest and the densest of all plasma lipoproteins.

Protein forms the major building blocks of HDL. HDL carries a particular large number of proteins as compared to other lipoprotein species.

HDLs are composed of lipids (– including phospholipid, cholesterol and triglyceride) and apolipoproteins. The core consists of cholesterol ester and triglycerides.

The major structural apolipoproteins of HDL is apolipoproteins A-1 (apoA-I) and the majority of HDLs contain apoA-I.

HDL plays an important role in transporting cholesterol from peripheral tissues directly back to the liver or indirectly via intermediate density lipoprotein (IDL) and low density lipoprotein, in a process popularly referred to as reverse cholesterol transport pathway.

HDL display antithrombotic and anti-inflammatory properties and have been shown to inhibit the activation and binding of monocytes to the endothelium.
High density lipoprotein