LIPOPROTEIN TRANSPORT

CHYLOMICRON a very large lipoprotein that transports triglycerides and other dietary lipids away from the small intestine, first in the lymph and then in the blood

LIPOPROTEINS particles formed by the assembly of proteins and phospholipids that transport lipids in lymph and in blood

In the mucosal cells, triglycerides and other dietary lipids are incorporated into a lipid transport particle called a chylomicron, which is one type of lipoprotein. Lipoproteins are protein-containing spherical particles that act as the primary carriers of lipids in blood. Lipids are not typically soluble in water and require carriers for transport, and so cannot travel freely in the bloodstream. As a result, clinical laboratory tests that monitor blood lipid levels typically measure amounts and types of lipids present in lipoproteins in blood. Unlike other lipoproteins, however, chylomicrons are so large that they cannot enter blood immediately after their formation, so they first enter the lymphatic system, which then delivers them into the bloodstream.

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VERY LOW-DENSITY LIPOPROTEIN (VLDL) a lipoprotein responsible for transporting primarily triglycerides to adipose tissue, cardiac muscle, and skeletal muscles

LOW-DENSITY LIPOPROTEIN (LDL) a lipoprotein responsible for transporting primarily cholesterol from the liver through the bloodstream to the tissues

Lipoproteins are classified by their density, which is determined by the relative ratios of triglycerides, cholesterol, and proteins present. This classification scheme also separates them by their functions. Chylomicrons (the largest and least-dense particles) and very low-density lipoproteins (VLDLs) are similar in that they both primarily transport triglycerides to adipose tissue, and cardiac and skeletal muscles. They differ, however, in their site of origin; VLDLs originate from the liver, while chylomicrons originate in the small intestine, and carry essentially all lipids present in our diet. Low-density lipoproteins (LDLs) transport cholesterol from the liver to all cells and are often referred to as “bad” cholesterol, as elevated levels are associated with an increase in the risk of heart disease. (INFOGRAPHIC 6.6)

INFOGRAPHIC 6.6 Lipoprotein Structure and Function Lipoproteins consist of a lipid core of triglycerides and cholesterol surrounded by a single layer of phospholipids containing both cholesterol and proteins.

Question 6.6

Which lipoprotein transports fat and cholesterol from dietary sources?

The chylomicron transports dietary fats and cholesterol to muscle and adipose tissue.

HIGH-DENSITY LIPOPROTEIN (HDL) a lipoprotein responsible for transporting cholesterol from the bloodstream and tissues back to the liver

In contrast, the Inuit had high levels of the so-called “good” cholesterol, or high-density lipoproteins (HDLs). HDL contains a lot of protein, so rather than delivering cholesterol around the body, HDL picks up as much excess cholesterol as it can from the cells and tissues and takes it back to the liver, which then either uses the cholesterol to make bile acids, excretes it directly into bile, or recycles it. This action is thought to explain why high levels of HDL are associated with low risk for heart disease. The commonly held view of cholesterol, as being either “good” or “bad,” however, is potentially misleading. Cholesterol is necessary for numerous essential functions throughout the body, and since most cells in the body receive cholesterol transported by LDLs, even LDLs are necessary. However, when the LDL level rises too high and HDL level drops too low, it is this ratio of lipoproteins (LDL to HDL) that is “bad” because high LDL increases the risk of cardiovascular disease. Chapter 7 will further explore lipoprotein levels in the blood and their relationship to the risk of heart disease. (INFOGRAPHIC 6.7)

INFOGRAPHIC 6.7 Lipoproteins Circulate Lipids Throughout the Body Each lipoprotein serves as a vehicle that carries lipids through the watery environment of the bloodstream.

Question 6.7

Which lipoprotein picks up excess cholesterol from cells and delivers it to the liver for use or excretion?

High-density lipoproteins (HDLs) pick up cholesterol from the cells of the body and deliver them to the liver for use or excretion.

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Dyerberg and Bang knew they needed extra help to identify the mysterious, long-chain fatty acid in the Greenlanders’ blood samples. So Dyerberg bought a plane ticket for Minnesota to visit the lab of Ralph Holman, PhD, an international expert on lipids.

EICOSAPENTAENOIC ACID (EPA) a 20-carbon omega-3 fatty acid that can be produced in the body by the metabolism of the essential fatty acid alpha-linolenic acid, or provided in the diet by oily fish

DOCOSAHEXAENOIC ACID (DHA) a 22-carbon omega-3 fatty acid that can be produced in the body from the essential fatty acid alpha-linopenic acid, also found in oily fish

As soon as he arrived, Dyerberg held the results up to the chemist. Immediately, Holman identified the mystery fatty acid: That’s eicosapentaenoic acid, or EPA, he told the young scientist. And the other spike at the end of the graph, representing another long-chain fatty acid present in higher levels in the Inuit than in Danish adults is docosahexaenoic acid, or DHA. You get both from eating fatty fish.

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As Dyerberg left the lab, his head was swimming. He kept repeating the words over and over: “Ei-co… ei-co-sa… eicosapentaenoic acid. Docosahexaenoic acid.” He’d never heard them before.