Lipid Transport and Lipoprotein Overview

Lipid Transport Overview

• Lipids are insoluble in water.
• Lipids are associated with proteins to form lipoproteins, which are water-soluble.
• The protein part of lipoproteins is called apolipoprotein.
• Lipoproteins transport lipids between various tissues.
• Total plasma lipid is approximately 400-600 mg/dL and includes triacylglycerol (20%), phospholipids (30%), cholesterol (40%), and unesterified free fatty acids.

Lipoprotein Structure

• Lipoproteins consist of a lipid core (primarily triacylglycerol and cholesterol esters) surrounded by a shell of phospholipids, cholesterol, and apolipoproteins.
• Apolipoproteins are proteins associated with lipoproteins.
• Apolipoproteins vary in amount, some are peripheral and some are integral.
• Apolipoproteins range from 1% in chylomicrons to 60% in some HDLs.

Lipoprotein Classification

• Lipoprotein density increases with protein content.
• Lipoproteins are separated by ultracentrifugation and electrophoresis.
• Types include chylomicrons, VLDL, IDL, LDL, and HDL.
• Lipoprotein (a) has a similar lipid composition to LDL but also contains a unique protein component.

Apolipoproteins

• Apolipoproteins are synthesized primarily in the liver and small intestine.
• Major classes are A to E are known.
• Apo B exists in two forms: Apo B-100 and Apo B-48. Apo B-100 is component of LDL, whereas Apo B-48 is part of chylomicrons.
• Apo (a) is present in Lp(a) with Apo B-100. It has a high carbohydrate content.
• Apolipoproteins promote lipid solubility and stability.
• Apolipoproteins act as ligands for lipoprotein receptors, regulating tissue uptake.
• Examples of apolipoproteins include Apo B-100/E for LDL receptor, Apo E for remnant receptor, and ApoA-I for HDL receptor.

Apolipoprotein Functions

• Some apolipoproteins act as enzyme activators (e.g., C-II for lipoprotein lipase, A-I for lecithin:cholesterol acyltransferase[LCAT]).
• Some apolipoproteins act as enzyme inhibitors (e.g., ApoA-II and apoC-III for lipoprotein lipase).

Metabolism of Chylomicrons

• Site of synthesis: intestinal mucosa.
• Function: Transport dietary lipids from the intestine to peripheral tissues.
• Structure: lipids (mostly triacylglycerol), proteins (including Apo B-48, E, C-II).
• Half-life: approximately 1 hour.

Metabolism of VLDL

• Site of synthesis: liver.
• Function: Transport endogenously synthesized lipids from the liver to peripheral tissues.
• Structure: lipids (primarily triacylglycerol), proteins (including Apo B-100, E, C-II).
• Half-life: 1-3 hours.

Formation of IDL

• The action of lipoprotein lipase (LpL) reduces TAG and Apo C content in VLDL.
• This forms VLDL remnants, also known as IDL.
• IDL contains less TAG and more cholesterol.
• IDL is either taken up by the liver via LDL receptors or converted to LDL.

This quiz covers the essentials of lipid transport and the structure of lipoproteins. Learn about the role of apolipoproteins and how different types of lipoproteins are classified based on their density and protein content. Test your knowledge on the various components of plasma lipids and their functions.