Digestion, Mobilization, and Transport of Fats:- Dietary Fats Are Absorbed in the Small Intestine
In vertebrates, before ingested triacylglycerols can be absorbed through the intestinal wall they must be con verted from insoluble macroscopic fat particles to finely dispersed microscopic micelles. This solubilization is carried out by bile salts, such as taurocholic acid (p. 355), which are synthesized from cholesterol in the liver, stored in the gallbladder, and released into the small intestine after ingestion of a fatty meal. Bile salts are amphipathic compounds that act as biological deter gents, converting dietary fats into mixed micelles of bile salts and triacylglycerols (Fig. 17–1, step 1).
Micelle formation enormously increases the fraction of lipid molecules accessible to the action of water-soluble lipases in the intestine, and lipase action converts triacylglycerols to monoacylglycerols (monoglycerides) and diacylglycerols (diglycerides), free fatty acids, and glycerol (step 2). These products of lipase action diffuse into the epithelial cells lining the intestinal surface (the intestinal mucosa) (step 3), where they are recon verted to triacylglycerols and packaged with dietary cholesterol and specific proteins into lipoprotein aggregates called chylomicrons (Fig. 17–2; see also Fig. 17–1, step 4).
Apolipoproteins are lipid-binding proteins in the blood, responsible for the transport of triacylglycerols, phospholipids, cholesterol, and cholesteryl esters between organs. Apolipoproteins (“apo” means “detached” or “separate,” designating the protein in its lipid-free form) combine with lipids to form several classes of lipoprotein particles, spherical aggregates with hydrophobic lipids at the core and hydrophilic protein side chains and lipid head groups at the surface. Various combinations of lipid and protein produce particles of different densities, ranging from chylomicrons and very low-density lipoproteins (VLDL) to very-high-density lipoproteins (VHDL), which can be separated by ultra centrifugation. The structures of these lipoprotein par ticles and their roles in lipid transport are detailed in Chapter 21. The protein moieties of lipoproteins are recognized by receptors on cell surfaces. In lipid uptake from the intestine, chylomicrons, which contain apolipoprotein C-II (apoC-II), move from the intestinal mucosa into the lymphatic system, and then enter the blood, which carries them to muscle and adipose tissue (Fig. 17–1, step 5). In the capillaries of these tissues, the extracellular enzyme lipoprotein lipase, activated byapoC-II, hydrolyzes triacylglycerols to fatty acids and glycerol (step 6), which are taken up by cells in the target tissues (step 7). In muscle, the fatty acids are oxidized for en ergy; in adipose tissue, they are reesterified for storage as triacylglycerols (step 8). The remnants of chylomicrons, depleted of most of their triacylglycerols but still containing cholesterol and apolipoproteins, travel in the blood to the liver, where they are taken up by endocytosis, mediated by recep tors for their apolipoproteins. Triacylglycerols that en ter the liver by this route may be oxidized to provide energy or to provide precursors for the synthesis of ke tone bodies, as described in Section 17.3. When the diet contains more fatty acids than are needed immediately for fuel or as precursors, the liver converts them to triacylglycerols, which are packaged with specific apolipoproteins into VLDLs. The VLDLs are transported in the blood to adipose tissues, where the triacylglycerols are removed and stored in lipid droplets within adipocytes.
FIGURE 17–1 Processing of dietary lipids in vertebrates. Digestion and absorption of dietary lipids occur in the small intestine, and the fatty acids released from triacylglycerols are packaged and delivered to muscle and adipose tissues. The eight steps are discussed in the text.
FIGURE 17–2 Molecular structure of a chylomicron. The surface is a layer of phospholipids, with head groups facing the aqueous phase. Triacylglycerols sequestered in the interior (yellow) make up more than 80% of the mass. Several apolipoproteins that protrude from the sur face (B-48, C-III, C-II) act as signals in the uptake and metabolism of chylomicron contents. The diameter of chylomicrons ranges from about 100 to 500 nm.
