Lipid Metabolism during Infection and Endotoxemia
ARMY MEDICAL RESEARCH INST OF INFECTIOUS DISEASES FORT DETRICK MD
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Of three major nutrients, only lipid can be stored in large quantities in most vertebrates. This is due in part to the fact that the body contains specialized mesenchymal cells, adipocytes, that are devoted solely to the function of storing fat. Therefore, lipid in the form of triglyceride is the major storage form of energy in the human body. In addition to being the major storage form of energy and metabolic fuel, lipids function as structural components of cell membranes, emulsifying agents, and precursors for the synthesis of sterols, four vitamins and prostaglandins. Since lipids are organic compounds that are poorly soluble in water, a complex system has been developed for the absorption and transport of lipids throughout the body. A number of excellent reviews have been written on various aspects of lipid metabolism therefore, only a brief description of the subject will be covered in this chapter. Ordinary dietary fats are emulsified by the bile secretions conjugated bile acids, phosphatidylcholine and cholesterol into mixed micelles in the duodenum of the small intestine. In the micellar form the lipids can be hydrolyzed by enzymes secreted from the exocrine pancreas to 2-monoglycerides, fatty acids and cholesterol. These hydrolyzed products are absorbed by passive diffusion into the mucosal cells of the intestine. Within these cells the absorbed fatty acids and 2-monoglycerides are resynthesized into triglycerides. These triglycerides are utilized to form two micellar lipoproteins, chylomicrons and very low-density lipoproteins VLDL which are secreted into the lymph and pass from it into venous blood. The triglyceride content of these lipoproteins is removed by the action of a hydrolytic enzyme, lipoprotein lipase, and the released fatty acids are taken up by various tissues of the body. This enzyme is present in capillary walls and in such tissues as adipose tissue, mammary gland and heart. However, the majority of the absorbed dietary...
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