Lipoproteins

There are 4 major lipids in plasma

          Fatty acids

            Phospholipids

            Triglycerides

            Cholesterol

 

Lipoproteins – lipids are transported in the blood stream, but lipids are not soluble in plasma water

Lipoproteins are complexes composed of hydrophilic lipids like phospholipids, free cholesterol and proteins (apolipoproteins) on the outside and hydrophobic lipid like triglycerides and cholesteryl esters on inside.

Lipoproteins are differed according to their size, density and composition of lipid proteins

There are 4 main types of lipoproteins

1.    Chylomicrons

2.    VLDL (very low-density lipoprotein)

3.    LDL (low density lipoprotein)

4.    HDL (high density lipoprotein)

 

Lipoproteins involve to transport lipids in plasma by the protein portion and transport their lipid content to and from tissues

Chylomicrons transport triglycerides of dilatory origin.

VLDL transport triglycerides endogenous syntheses (from liver to tissues)

LDL transport mainly free cholesterols (from liver to tissues)

HDL transport mainly esterified cholesterols (from tissues to liver)

 

Apolipoproteins

Integral – Apo B100, Apo B48

Peripheral – Apo AI, Apo AII, Apo CI, Apo CII, Apo E

Functions of apolipoproteins

Activates many enzymes in the lipoprotein metabolic pathways

Maintain the structural integrity of the lipoprotein complex

Facilitate the uptake of lipoprotein in cells through their recognition by specific cell surface receptors

 Can act as lipid transfer protein

 

There are two types of lipid transport

1.    Exogenous lipid transport – involves chylomicron metabolism aided by HDL

2.    Endogenous lipid transport – involves metabolism of VLDL and HDL

 

Chylomicrons

Interstitial mucosal cells produce nascent (functionally incomplete) chylomicrons. They are lowest density and largest in size. It contains higher triacylglycerols (TAG). Carry dietary lipids (dietary cholesterol and TAG) to peripheral tissues. Responsible for physiological milky appearance of plasma. The apolipoproteins available in nascent chylomicrons before entering to blood circulation (Apo B 48, Apo AI, Apo AII). Chylomicrons leave the intestine via the lymphatic system into blood circulation. In the blood stream chylomicrons acquire Apo CII and Apo E from HDL to become mature chylomicrons. Chylomicrons move adipose tissue and muscles. In the capillary wall of adipose tissue and muscles, there is an extracellular enzyme called lipoprotein lipase (LPL). Apo CII in the chylomicrons activates Lipoprotein lipase. TAG in chylomicrons is hydrolyzed by LPL to fatty acid and glycerol. Fatty acid either stored by adipose tissue or generate energy by muscle. Apo CII and Apo E are transported back to HDL.

VLDL

They are assembled in liver and contains high triacylglycerols. Carry endogenously produced triacylglycerol from liver to peripheral tissues. Nascent VLDL contains Apo B 100. Nascent VLDL acquire Apo CII and Apo E from circulating HDL become mature VLDL. VLDL move adipose tissue and muscles. In the capillary wall of adipose tissue and muscles, there is an extracellular enzyme called lipoprotein lipase (LPL). Apo CII in the VLDL activates Lipoprotein lipase. TAG in VLDL is hydrolyzed by LPL to fatty acid and glycerol. Fatty acid either stored by adipose tissue or generate energy by muscle. Apo CII and Apo E are transported back to HDL, converting VLDL to IDL.

LDL

Composition 50% of cholesterol and CE, Apo B 100. It provides cholesterol to peripheral tissues or returns it to the liver. By cell surface membrane LDL receptors recognize Apo B100 in LDL or apo E in VLDL. These receptors are called Apo B/E receptors. After binding the LDL receptor complex is internalized by endocytosis. LDL are primary plasma carriers of cholesterol for delivery to all tissues. Uptake occur predominantly in liver (75%), adrenal and adipose tissue.

HDL

HDL is a fraction of plasma lipoproteins. They made in liver and small intestine. HDL pick up cholesterol from body cells and take it back to the liver. Potential to reverse heart disease. HDL is a reservoir of apolipoproteins. HDL transfers cholesterol esters to other lipoproteins in exchange for various lipids. HDL and other lipoproteins carry the cholesterol esters back to the liver