Normal Lipid Metabolism

Lipids are a heterogeneous group of substances soluble in organic solvents but insoluble in water. They are largely intracellular but circulate in blood as lipoprotein particles. There are four general functions for lipids:

• Structural components of membranes

• Storage forms of metabolic fuel

• Transport forms of metabolic fuel

• Protective functions as an outer coating of the organism

Lipids consist of cholesterol and its derivatives, fatty acids, triacylglycerols, phospholipids, and apolipoproteins. The lipoprotein particle has a core of neutral lipids (cholesterol esters and triacylgly-cerol) and a surface coat of polar lipids (unesterified cholesterol and phospholipids) and apolipoproteins. They are classified in terms of density. The following are the main lipoproteins:

• Chylomicrons

• Very low-density lipoprotein (VLDL)

• Immediate-density lipoprotein (IDL)

• Low-density lipoprotein (LDL)

• High-density lipoprotein (HDL)

Synthesis of lipoproteins occurs in the intestine or liver. They are then modified by enzymes and taken up by cell surface receptors in processes largely regulated by the apolipoproteins. A series of receptors, transporters, and enzymes are important in lipoprotein metabolism and function as detailed later. The physicochemical characteristics of the main lipoprotein classes are shown in Table 1.

Interest in lipids lies in circulating lipid concentrations and their relationship to atherosclerosis, particularly coronary heart disease, stroke, and peripheral vascular disease.

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