Triacylglycerols dietary

Dietary triacylglycerols are degraded to a small extent (via fatty acid release) by lipases in the low-pH environment of the stomach, but mostly pass untouched into the duodenum. Alkaline pancreatic juice secreted into the... 

Figure 7.4 Fate of triacylglycerol that is present in fuel blood after secretion by the intestine. The dietary triacylglycerol in the intestine is hydrolysed to long-chain fatty acids and monoacyl-glycerol, both of which are taken up by the enterocytes in which they are then re-esterified. The triacylglycerol is released in the form of chylomicrons into the blood, from where it is hydrolysed to fatty acids and glycerol by the enzyme lipoprotein lipase in specific tissues (Figure 7.3). The fatty acids are taken up by adipocytes, muscle fibres and secretory cells in the mammary gland.

Trans fatty acids The phospholipids in the plasma and in membranes of all cells contain long-chain polynnsatnrated fatty acids (PUFA). During periods of growth and development of organs, PUFAs are reqnired for phospholipid synthesis. The PUFAs are, of conrse, obtained from dietary triacylglycerol and phospholipids. The donble bonds in most natural fatty acids are cis not trans Nonetheless trans fatty acids do occur in dietary fats. If the diet contains trans fatty acids, they might be incorporated into the phospholipids along with the cis fatty acids and hence into membranes. The presence of these abnormal fatty acids will modify the stmctnre of the phospholipids which conld impair the fnnction of the membrane. There are two main sonrces of trans fatty acids in the diet foods produced from ruminants contain trans fatty... 

The fatty acids of triacylglycerols furnish a large fraction of the oxidative energy in animals. Dietary triacylglycerols are emulsified in the small intestine by bile salts, hydrolyzed by intestinal lipases, absorbed by intestinal epithelial cells, reconverted into triacylglycerols, then formed into chylomicrons by combination with specific apolipoproteins. 

Chylomicrons, discussed in Chapter 17 in connection with the movement of dietary triacylglycerols from the intestine to other tissues, are the largest of the lipoproteins and the least dense, containing a high... 

The dietary triacylglycerol, cholesteryl esters, and phospholipids are enzymically degraded ("digested") by pancreatic enzymes, whose secretion is hormonally controlled. 

A. Dietary triacylglycerol is partially hydrolyzed and absorbed as free fatty acids and monoacylglycerol. 

B. Dietary triacylglycerol must be completely hydrolyzed to free fatty acids and glycerol before absorption. 

Correct answer = A. Pancreatic lipase hydrolyzes dietary triacylglycerol primarily to 2-monoacylglycerol plus two fatty acids. These products of hydrolysis can be absorbed by the intestinal mucosal cells. Bile salts do not inhibit release of fatty acids from triacylglycerol, but rather are necessary for the proper solubilization and hydrolysis of dietary triacylglycerol in the small intestine. Short- and medium-chain length fatty acids enter the portal circulation after absorption from the small intestine. Synthesis of apolipoproteins, especially apo B-48, is essential for the assembly and secretion of chylomicrons. 

Chylomicrons are assembled in intestinal mucosal cells and cany dietary triacylglycerol, cholesterol, fat-soluble vitamins, and cholesteryl esters (plus additional lipids made in these cells) to the peripheral tissues (Figure 18.16). 

Chylomicrons transport dietary triacylglycerol and cholesteryl ester from the intestine to other tissues in the body. Very-low-density lipoprotein functions in a manner similar to the transport of endogenously made lipid from the liver to other tissues. These two types of triacylglycerol-rich particles are initially degraded by the action of lipoprotein lipase, an extracellular enzyme that is most active within the capillaries of adipose tissue, cardiac and skeletal muscle, and the lactating mammary gland. Lipoprotein lipase catalyzes the hydrolysis of triacylglycerols (see fig. 18.3). The enzyme is specifically activated by apoprotein C-II, which... 

Chylomicrons are synthesized in the intestine and transport dietary triacylglycerols to skeletal muscle and adipose tissue, and dietary cholesterol to the liver. At these target tissues the triacylglycerols are hydrolyzed by lipoprotein lipase on the surface of the cells and the released fatty acids are taken up either for metabolism to generate energy or for storage. The resulting cholesterol-rich chylomicron remnants are transported in the blood to the liver where they are taken up by receptor-mediated endocytosis. 

Lipid digestion is accomplished in the small intestine by the action of hydrolytic enzymes, called lipases and phospholipases, which act on dietary triacylglycerol and phospholipids, respectively. 

Ri, R2, and R3 are the hydrocarbon chains of fatty acids. Ester bonds between a fatty acid and glycerol are hydrolyzed. In the lumen of the intestine, the complete action of pancreatic lipase on dietary triacylglycerol produces the following ... 

The transport of lipids and cholesterol is accomplished by packaging them into lipoprotein complexes this is undertaken by both hepatocytes and enterocytes. Dietary triacylglycerols and cholesterol are packaged by enterocytes into chylomicrons, whereas de novo synthesised triacylglycerols in the liver are packaged in very low-density lipoproteins (VLDLs). 

EFFECTS OF STEREOSPECIFIC STRUCTURE OF DIETARY TRIACYLGLYCEROLS ON THEIR HEALTH-RELATED NUTRITIONAL EFFECTS... 

Pancreatic lipase is one of the mammalian key digestive enzymes. It completes the dietary triacylglycerol breakdown initiated by preduode-nal lipases, including lingual and gastric enzymes, (see below). The enzyme is inhibited in the intestine by bile salts, but the activity is restored in the presence of colipase (CLP), a relatively short (95 residues) heat-stable polypeptide secreted by the pancreas (Semeriva and Desnuelle, 1979 Borgstrbm and Erlanson-Albertsson, 1984). The structural details of the interaction of colipase with lipase are described in Section III,C. 

The dietary triacylglycerols, which are water-insoluble, are digested in the small intestine to fatty acids and 2-monoacylglycerols. These digestive products are resynthesized to triacylglycerols in intestinal epithelial cells and are secreted in chylomicrons via the lymph into the blood. 

A. Dietary triacylglycerols are digested in the small intestine by a process... 

C. The pancreas produces bicarbonate (which neutralizes stomach acid) and digestive enzymes (including the lipase that degrades dietary lipids). Decreased bicarbonate will lead to a decrease of intestinal pH. Decreased digestion of dietary triacylglycerols will lead to formation of fewer bile salt micelles. Intestinal cells will have less substrate for chylomicron formation, and less fat-soluble vitamins will be absorbed. More dietary fat will be excreted in the feces. 

B. VLDL is produced mainly from dietary carbohydrate, LDL from VLDL, and chylomicrons from dietary triacylglycerol. Elevated HDL levels are desirable and not considered to be a lipid disorder. 

D. A palmitate residue attached to carbon 1 of a dietary triacylglycerol is released by pancreatic lipase and carried from the intestinal lumen to the gut epithelial cell in a bile salt micelle. 

A. Pancreatic lipase catalyzes the breakdown of dietary triacylglycerols into free fatty acids and 2-monoacylglycerols, an essential step in the digestion of dietary lipids. Since prostaglandins are produced from linoleate, an essential fatty add, a deficiency of pancreatic lipase would eventually cause a prostaglandin defidency. 

At least three acyl-CoA synthases, each specific for a particular size of fatty acid, exist acetyl-CoA synthase acts on acetate and other low-molecular-weight carboxylic acids, medium-chain acyl-CoA synthase on fatty acids with 4-11 carbon atoms, and acyl-CoA synthase on fatty acids with 6-20 carbon atoms. The activity of acetyl-CoA synthase in muscle is restricted to the mitochondrial matrix. Medium-chain acyl-CoA synthase occurs only in liver mitochondria, where medium-chain fatty acids obtained from digestion of dietary triacylglycerols and transported by the portal blood are metabolized. Acyl-CoA synthase, the major activating enzyme, occurs on the outer mitochondrial membrane surface and in endoplasmic reticulum. The overall reaction of activation is as follows ... 

Dietary triacylglycerols are hydrolyzed in the intestine by lipases to form fatty acids, glycerol, and file IIIO.I... 20and%20Settings/Bariski/Desktop/26134 9780471022053 9780585329482 0471022055/files/page 205.html[12/10/2009 10 14 20]... 

The principal functions of the lipoprotein classes are determined by their apolipoprotein (apo) and lipid components. The CM are synthesized in the intestines for the transport of dietary triacylglycerols to various tissues (Chapter 19). VLDL are synthesized in the liver for the export of endogenous triacylglycerols (Chapter 19), while LDL arise from the metabolic transformation of VLDL in circulation (Chapter 20). The function of LDL is to deliver CE to peripheral tissues and to the liver. HDL are synthesized and assembled in the liver and intestine or are formed from metabolic transformations of other lipoproteins in circulation, and from cellular lipids at the cell membranes (see Chapter 20). HDL remove excess cholesterol from cells and transport it to liver and steroidogenic tissues for metabolism and excretion. 

Two types of lipoproteins, chylomicrons and VLDL, are produced in the fed state. The major function of these lipoproteins is to provide a blood transport system for triacylglycerols, which are very insoluble in water. However, these lipoproteins also contain the lipid cholesterol, which is also somewhat insoluble in water. The triacylglycerols of chylomicrons are formed in intestinal epithelial cells from the products of digestion of dietary triacylglycerols. The triacylglycerols of VLDL are synthesized in the liver. 

C) dietary triacylglycerols are transported in the portal vein to the liver. 

Fat constitutes approximately 38% of the calories in the average North American diet. Of this, more than 95% of the calories are present as triacylglycerols (3 fatty acids esterified to a glycerol backbone). During ingestion and absorption, dietary triacylglycerols are broken down into their constituents and then reassembled for transport to adipose tissue in chylomicrons (see Chapter 2). Thus, the fatty acid composition of adipose triacylglycerols varies with the type of food consumed. 

The major enzyme that digests dietary triacylglycerols is a lipase produced in the pancreas. Pancreatic lipase is secreted along with another protein, colipase, along with bicarbonate, which neutralizes the acid that enters the intestine with partially digested food from the stomach. Bicarbonate raises the pH of the contents of the intestinal lumen into a range (pH 6) that is optimal for the action of all of the digestive enzymes of the intestine. Bicarbonate secretion from the pancreas is stimulated by the hormone secretin, which is released from the intestine when acid enters the duodenum. 

Bile salts, which are produced in the liver from cholesterol obtained from the blood lipoproteins or synthesized from acetyl CoA, are secreted into the bile. They are stored in the gallbladder and released into the intestine during a meal. The bile salts emulsify dietary triacylglycerols, thus aiding in digestion. The digestive products are absorbed by intestinal epithelial cells from bile salt micelles, tiny microdroplets that contain bile salts at their water interface. After the contents of the micelles are absorbed, most of the bile salts travel to the ileum, where they are resorbed and recycled by the liver. Less than 5% of the bile salts that enter the lumen of the small intestine are eventually excreted in the feces. 

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