Lipids are a concentrated form of energy. They provide 30 to 40% of the daily caloric intake in the Western diet. Lipids are also essential for normal body functions, as they form part of cellular membranes and are precursors of bile acids, steroid hormones, prostaglandins, and leukotrienes. The human body is capable of synthesizing most of the lipids it requires with the exception of the essential fatty acids linoleic acid (C 18:2, an 18-carbon long fatty acid with two double bonds) and arachidonic acid (C 20:4). Both of these acids belong to the family of omega-6 fatty acids. Recently, researchers have provided convincing evidence that eicosapentaenoic acid (C 20:5) and docosahexaenoic acid (C 22:6) are also essential for the normal development of vision in newborns. Both of these acids are omega-3 fatty acids and are abundant in seafood and algae.
Dietary lipids. A, A triglyceride molecule. R1, R2, and R3 belong to different fatty acids. B, A phospholipid molecule. The fatty acid occupying the first position (R1) is usually a saturated fatty acid and that in the second position (R2) is usually an unsaturated or polyunsaturated fatty acid. The third position after the phosphate group is occupied by a nitrogenous base (N), such as choline or ethanolamine.
gestion of dietary triglyceride by gastric lipase and the churning action of the stomach produce a suspension of oil droplets (an emulsion) that help increase the area of the oil-water interface. Pancreatic juice also contains the peptide colipase, which is necessary for the normal digestion of fat by pancreatic lipase. Colipase binds lipase at a molar ratio of 1:1, thereby allowing the lipase to bind to the oil-water interface where lipolysis takes place. Colipase also counteracts the inhibition of lipolysis by bile salt, which, despite its importance in intestinal fat absorption, prevents the attachment of pancreatic lipase to the oil-water interface.
Phospholipase A2 is the major pancreatic enzyme for digesting phospholipids, forming lysophospholipids and fatty acids. For instance, phosphatidylcholine (lecithin) is hydrolyzed to form lysophosphatidylcholine (lysolecithin) and fatty acid (see Fig. 27.24).
Dietary cholesterol is presented as a free sterol or as a sterol ester (cholesterol ester). The hydrolysis of cholesterol ester is catalyzed by the pancreatic enzyme car-boxylester hydrolase, also called cholesterol esterase (see Fig. 27.24). The digestion of cholesterol ester is important because cholesterol can be absorbed only as the free sterol.
Bile Salt Plays an Important Role in Lipid Absorption
A layer of poorly stirred fluid called the unstirred water layer coats the surface of the intestinal villi (Fig. 27.25A). The unstirred water layer reduces the absorption of lipid digestion products because they are poorly soluble in water. They are rendered water-soluble by micellar solubilization by bile salts in the small intestinal lumen. This mechanism greatly enhances the concentration of these products in the unstirred water layer (Fig. 27.25B). The lipid digestion products are then absorbed by enterocytes, mainly by passive diffusion. Fatty acid and monoglyceride molecules are taken up individually. Similar mechanisms seem to operate for cholesterol and lysolecithin.
Bile salts are derived from cholesterol, but they are different from cholesterol in that they are water-soluble. They are essentially detergents—molecules that possess both hy-drophilic and hydrophobic properties. Because bile salts are polar molecules, they penetrate cell membranes poorly. This is significant because it ensures their minimal absorption by the jejunum where most fat absorption takes place. At or above a certain concentration of bile salts, the critical micellar concentration, they aggregate to form micelles, the concentration of luminal bile salts is usually well above the critical micellar concentration. When bile salts alone are present in the micelle, it is called a simple micelle. Simple micelles incorporate the lipid digestion products— monoglyceride and fatty acids—to form mixed micelles. This renders the lipid digestion products water-soluble by incorporation into mixed micelles. Mixed micelles diffuse across the unstirred water layer and deliver lipid digestion products to the enterocytes for absorption.
Enterocytes Process Absorbed Lipid to Form Lipoproteins
After entering the enterocytes, the fatty acids and mono-glycerides migrate to the smooth ER. A fatty acid-binding protein may be involved in the intracellular transport of fatty acids, but whether or not a protein carrier is involved in the intracellular transport of monoglycerides is unknown. In the smooth ER, monoglycerides and fatty acids
, The digestion of dietary lipids by pancreatic enzymes in the small intestine. Solid circles represent oxygen atoms.
^IGUREIHHIH^ The micellar solubilization of lipids. Micel-TBammH^rnmmr lar solubilization enhances the delivery of lipid to the brush border membrane. A, In the absence of bile salts. B, In the presence of bile salts.
Fatty acid Lysolecithin Cholesterol
Fatty acid Lysolecithin Cholesterol
Cholesterol ester Free cholesterol
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