Phospholipids

​Chilomicrons​

• Chylomicrons are the largest (1000 nm) and Least dense (<0.95) of the lipoproteins.

• Chilomicrons contains contain only 1-2% protein, 85-88% triglycerides, ~8% phospholipids, ~3% cholesteryl esters and ~1% cholesterol and several types of apolipo proteins.

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​Apolipoprotein (Apo), are a proteins of plasma lipoprotein.

​Chylomicrons are transporting dietary triglycerides and cholesterol absorbed by intestinal epithelia.

​It can bind and transport blood lipids to various tissues of the body for metabolism and utilization.

Chylomicron originates in the intestinal mucosa and excreted into the plasma through the lymphatic system.

In the plasma, chylomicrons acquire apolipoproteins from HDL.

Once transported to tissues, triglycerides in chylomicrons are hydrolyzed by lipoproteinlipase of the endothelial cell walls.

The remnant chylomicron like residual cholesterol, is taken uinto the liver by endocytosis.​

​ Very Low Density Lipoproteins (VLDL)

VLDL are the next step down from chylomicrons in terms of size and lipid content.

VLDL are approximately 25-90 nm in size, Molecular Weight 6-27 million, with a density of ~0.98.

VLDL contains 5-12% protein, 50-55% triglycerides, 18-20% phospholipids, 12-15% cholesteryl esters and 8-10% cholesterol.

VLDL also contains several types of apolipoproteins from plasma HDL.

VLDL aoriginates in the liver involves association of lipids with apo-B100 mediated by microsomal triglyceride transfer protein during the translocation ofapo B100 to the lumen of endoplasmic reticulum .​

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Intermediate Density Lipoproteins (IDL)

Intermediate density lipoproteins are smaller than VLDL

Its size about 40 nm and more dense ~1.0.

They contain the same apolipoproteins as VLDL.

They are composed of 10-12% protein, 24-30% triglycerides, 25-27% phospholipids, 32-35% cholesteryl esters and 8-10% cholesterol.

IDLs are derived from triglyceride depletion of VLDL.

IDLs can be taken up by the liver for reprocessing or upon further triglyceride depletion,become LDL.

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Intermediate Density Lipoproteins (IDL)

Intermediate density lipoproteins are smaller than VLDL

Its size about 40 nm and more dense ~1.0.

They contain the same apolipoproteins as VLDL.

They are composed of 10-12% protein, 24-30% triglycerides, 25-27% phospholipids, 32-35% cholesteryl esters and 8-10% cholesterol.

IDLs are derived from triglyceride depletion of VLDL.

IDLs can be taken up by the liver for reprocessing or upon further triglyceride depletion,become LDL.

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​Low Density Lipoproteins (LDL)

Low density lipoproteins are smaller than IDL.

LDL size is about 26 nm and MW approximately 3.5 million and more dense ~1.04. They contain the apolipoprotein apo-B100.

LDL contains 20-22% protein, 10-15% triglycerides, 20-28% phospholipids, 37-48% cholesteryl esters and 8-10% cholesterol.

LDL and HDL transport both dietary and endogenous cholesterol in the plasma.

LDL is the main transporter of cholesterol and cholesteryl esters and makes up more than half of the total lipoprotein in plasma.

LDL is absorbed by the liver and other tissues by endocytosis.

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1. ​The cytoplasmic domain of the LDL receptor facilitates the formation of coated pits in the receptor-rich regions of the membrane. The ligand binding domain of the receptor recognizes apo-B100 on LDL, resulting in the formation of a clathrin-coated vesicle. The proton pumps lower the pH inside the vesicle resulting dissociation of LDL from its receptor. The clathrin coat vesicles now fused with lysozomes, resulting in enzymatic hydrolysis of peptide by cholesteryl ester. The LDL receptor can be recycled to the cell membrane. Insulin, tri-iodothyronine are also involved in the regulation of receptor mediated uptake of LDL.